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Evidence of Lake Trout Reproduction at Lake Michigan's Mid-lake Reef Complex
J. Great Lakes Res. 32:749–763 Internat. Assoc. Great Lakes Res., 2006
Evidence of Lake Trout Reproduction at Lake Michigan’s Mid-lake Reef Complex John Janssen1,*, David J. Jude2, Thomas A. Edsall3,†, Robert W. Paddock1, Nigel Wattrus4, Mike Toneys5,†, and Pat McKee5 1Great
Lakes WATER Institute University of Wisconsin-Milwaukee 600 East Greenfield Avenue Milwaukee, Wisconsin 53204 2School
of Natural Resources and Environment University of Michigan 440 Church St. Ann Arbor, Michigan 48109
3United
States Geological Survey Great Lakes Science Center 1451 Green Road Ann Arbor, Michigan 48105 4Large
Lakes Observatory University of Minnesota-Duluth Duluth, Minnesota 55812. 5Wisconsin
Department of Natural Resources Sturgeon Bay, Wisconsin 54235
ABSTRACT. The Mid-Lake Reef Complex (MLRC), a large area of deep (> 40 m) reefs, was a major site where indigenous lake trout (Salvelinus namaycush) in Lake Michigan aggregated during spawning. As part of an effort to restore Lake Michigan’s lake trout, which were extirpated in the 1950s, yearling lake trout have been released over the MLRC since the mid-1980s and fall gill net censuses began to show large numbers of lake trout in spawning condition beginning about 1999. We report the first evidence of viable egg deposition and successful lake trout fry production at these deep reefs. Because the area’s existing bathymetry and habitat were too poorly known for a priori selection of sampling sites, we used hydroacoustics to locate concentrations of large fish in the fall; fish were congregating around slopes and ridges. Subsequent observations via unmanned submersible confirmed the large fish to be lake trout. Our technological objectives were driven by biological objectives of locating where lake trout spawn, where lake trout fry were produced, and what fishes ate lake trout eggs and fry. The unmanned submersibles were equipped with a suction sampler and electroshocker to sample eggs deposited on the reef, draw out and occasionally catch emergent fry, and collect egg predators (slimy sculpin Cottus cognatus). We observed slimy sculpin to eat unusually high numbers of lake trout eggs. Our qualitative approaches are a first step toward quantitative assessments of the importance of lake trout spawning on the MLRC. INDEX WORDS:
*Corresponding
Great Lakes, ROV, submersible, Salvelinus namaycush, spawning.
author. E-mail: [email protected]
†Retired
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INTRODUCTION Lake trout (Salvelinus namaycush) were extirpated from Lake Michigan in the 1950s as a result of overfishing exacerbated by sea lamprey (Petromyzon marinus) predation (Holey et al. 1995). Efforts to restore self-sustaining lake trout populations have not been successful, although there has been some evidence of limited wild recruitment (e.g., Jude et al. 1981). The restoration of lake trout in the Great Lakes has been geographically restricted; the only restorations have occurred in Lake Superior and Parry Sound (Reid et al. 2001), an area of Lake Huron where a surviving remnant population occurred. In Lake Michigan, one of the most important lake trout spawning areas, based on historical records of fish in or near spawning condition, is a series of large reefs almost entirely deeper than 40 m that separate the northern and southern basins, called the Mid-Lake Reef Complex (MLRC; Coberly and Horrall 1980, Dawson et al. 1997). This area reportedly had a population of lake trout that was behaviorally and physiologically specialized (Brown et al. 1981). Unfortunately little is known about the MLRC, either physically or biologically. Eshenroder et al. (1995) considered the MLRC as extreme habitat, in contrast to typical lake trout spawning areas, for two reasons. First, the MLRC is near the southern edge of the species’ distribution (Scott and Crossman 1973) suggesting environmental constraints may exist on lake trout distribution farther south. Second, spawning at the MLRC would have to occur in unusually deep water. Lake trout in Ontario lakes typically spawn along shores with wind-driven waves in a few meters of water where loose rock forms interstitial spaces for egg incubation (Gunn 1995). Eshenroder et al. (1995) argued that this behavior of lake trout may be responsible for the strong tendency for stocked lake trout to attempt to spawn along shorelines of the Great Lakes. The MLRC is nearly unexplored biologically or physically. It has a total surface area of 2,859 km2 (Holey et al. 1995) and the bedrock is limestone, presumably of Devonian age, that was largely resistant to Pleistocene glaciations. There would be subsequent weathering during a period when various parts were terrestrial (Chrzastowski and Thompson 1994). Prominent MLRC features include three cuestas, i.e., elevated features with a steep drop-off on one side and slight gradient approximating a plateau on the other. The cuestas are 40–60 m deep at their
various summits. Two of the summits, named East Reef and Sheboygan Reef, were the focus of our study. Previous habitat mapping of part of Sheboyogan Reef indicated a spatially heterogeneous substrate that included cobble, bedrock, clay, and sand (Edsall and Kennedy 1995). Habitat suitable for lake trout reproduction may exist only in small patches. Small patches of cobble sustain the lake trout population at Parry Sound (Marsden et al. 2005). Restoration of deep-water lake trout requires an understanding of original physical and biological conditions, and this knowledge is extremely vague for the MLRC because it is based primarily on commercial fishers’ reports (Coberly and Horrall 1980, Brown et al. 1981, Dawson et al. 1997). The potential importance of locating viable spawning habitat is evident from the recent lake trout restoration success at Devil’s Island Shoal, Lake Superior (Bronte et al. 2002). Part of the successful strategy at this location was to incubate eggs at sites physically similar to where lake trout were known to spawn successfully elsewhere. Hypotheses regarding the physical and biological history of lake trout spawning on deep reefs have been developed in Brown et al. (1981), Eshenroder et al. (1995), and Dawson et al. (1997). If, as Brown et al. (1981) suggest, the MLRC lake trout were a strain specialized for whatever unique conditions exist at the MLRC, then matching characteristics of stocked strains to an extirpated strain may facilitate restoration. A synopsis of MLRC hypotheses includes the following: 1. Brown et al. (1981) integrated the information gathered from interviews with commercial fishers (Coberly and Horrall 1980) with fisheries records to argue that deep-spawning lake trout were likely a discrete stock and strain. 2. Eshenroder et al. (1995) argued that deepspawning lake trout may have begun as shallow spawners at the MLRC during the period when Lake Michigan was about 100 m shallower and much of the area was terrestrial. These shallow spawners would initially have habitat preference similar to those studied by Gunn (1995). A deepwater strain would then evolve from individuals that continued homing to the MLRC as it gradually became “drowned.” This scenario helps explain isolation from shallow spawning strains, fostering strain evolution.
Deep-spawning Lake Trout in Lake Michigan 3. Dawson et al. (1997) pointed out that the geology of the MLRC was most likely Silurian and Devonian limestones so there should be areas of rocky substrate suitable for lake trout spawning. 4. Dawson et al. (1997) presented evidence, based on commercial records, that the highest densities of spawning lake trout in Lake Michigan were at the MLRC and that these fish spawned in late October. The current state of information on the MLRC is inadequate to estimate the likelihood of recruitment of lake trout or factors that may limit recruitment. Therefore, the best guidelines for evaluation are those proposed for lake trout spawning on shallow reefs elsewhere. These guidelines include estimates of minimum spawner density (Selgeby et al. 1995), evaluation of spawning habitat (Marsden et al. 1995a), and elucidation of the impacts of predation during the fall through spring incubation (Jones et al. 1995). Existing approaches to evaluating lake trout spawning success mainly use scuba (e.g., Jonas et al. 2005, Marsden et al. 2005). The depth of the MLRC severely limits the use of conventional scuba-based techniques. Successful use of scuba also requires knowledge of potential lake trout spawning habitat locations. Such sites and even lake trout can often be seen from the surface (Jonas et al. 2005), but this is not feasible for the MLRC. An alternative approach, using a tethered unmanned submersible (ROV) with a suction sampler, had been used by Marsden and Janssen (1997) to collect several lake trout eggs and several slimy sculpin (Cottus cognatus) from Lake Michigan’s Julian’s Reef. Julian’s Reef is a few km2 and about 30 m deep and the work was facilitated by an existing habitat map. While the ROV was limited to qualitative information, Marsden and Janssen (1997) were able to establish for the first time that egg deposition was occurring on a Great Lakes deep reef and justify a concerted effort on the much larger MLRC. Initiation of this effort was justified by indications that large numbers of ripe and nearripe lake trout were appearing at the MLRC; results are briefly reported here. The biological goals of this project were to answer the basic qualitative questions of whether lake trout eggs were being deposited at the MLRC, whether the deposited eggs were viable, whether fry were being produced, and what fishes may be egg or fry predators. This project also had technological
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goals, which were to enhance ROV capabilities to meet specific sampling needs for deep-water reefs in the Great Lakes. We expect that these methods will continue to evolve, much as scuba-based sampling has. METHODS General Approach Our approach to studying the MLRC followed recommendations of Marsden and Janssen (1997). We used hydroacoustics to find locations where targets, likely to be lake trout, aggregated during the spawning season. At these locations we sampled for eggs and egg predators using an ROV. During spring, we searched for evidence of fry production via ROV. Preliminary successes justified bathymetry mapping, which led to better success, including collecting sufficient eggs to permit a conservative estimation of viability to swim-up. Sampling Locations East Reef and Sheboygan Reef are located in central Lake Michigan (Fig. 1). Initial locations for sampling of eggs and egg predators were chosen by hydroacoustic detection of large fish during October and November 2001–2003 as an indicator of the presence of lake trout. The sonar was a Furuno FCV-582L (aboard the R/V Neeskay, University of
FIG. 1. General location of East Reef and Sheboygan Reef, Lake Michigan.
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Wisconsin-Milwaukee) operating simultaneously at 50 and 200 kHz. We recorded GPS coordinates of sites that appeared to have high concentrations of relatively large fish near bottom (based on the sonar’s color code and oscillograph). Sampling site selection prior to fall 2003 was completed without use of bathymetry maps reported here. Available maps had 2-km spatial resolution and were insufficient for locating sampling sites. Once available (fall 2003), the East Reef bathymetry map (methods described below) was used to locate sonar transects for subsequent sampling of eggs and egg predators (also fall 2003). These sites were also sampled for lake trout fry and fry predators in spring, 2004. The East Reef ROV dive sites were near the top of the dropoff (henceforth Horrall Slope), defined as the slope along the westerly flank from about 50 m to 100 m. Replay of the sonar video showed apparently large fish near the bottom, slightly downslope of the reef crest. Dive sites in fall 2003 and spring 2004 for collecting eggs were based on GPS locations where large fish were present on the bathymetry map of East Reef. We did not use the bathymetry map for Sheboygan Reef to locate sampling sites because all biological data (primarily slimy sculpin diets) for Sheboygan Reef pre-date our mapping. We chose what the sonar suggested was a rocky hump (subsequently found to be a ridge) that consistently showed what appeared to be large fish near bottom. Bathymetry Mapping The purpose of bathymetry mapping was to facilitate locating ROV sampling sites as well as to examine the bathymetry at sites where we had found lake trout eggs and fry during the earlier stages of this project. Detailed bathymetry data of East Reef and Sheboygan Reef sampling sites were measured from 27 to 29 June 2003 with a Reson Seabat 8101 multibeam sonar mounted in the hull of the R/V Blue Heron (University of Minnesota). Survey lines were spaced at about 2.5 × minimum water depth (100 m wide for Sheboygan Reef and 125 m wide for East Reef ) to ensure approximately 20% overlap between adjacent survey lines and therefore 100% coverage of the lake floor in the survey area. The Reson Seabat 8101 multibeam sonar is a 240 kHz multibeam that uses 101 1.5-degree beams to measure the bathymetry of the lake floor. Width of the swath “illuminated” on the lake floor is approximately 7.5 times the water depth in water less than 70 m deep. The system has a range resolution of
1.25 cm (Reson Inc, Goleta, CA). To achieve this resolution, vessel motion (roll, pitch, and heading) was measured to an accuracy of less than ± 0.05 degrees, and the boat’s position was measured with an accuracy of less than 1 m horizontally and 25 cm vertically by a TSS POS MV/320 motion sensing and positioning system. The POS MV/320 uses a system of accelerometers, gyroscopes, and multiple survey-grade Differential GPS units to provide consistently high accuracy motion and position information under most conditions (TSS America, Houston, TX). Variations in the speed of sound in the water column were measured periodically to correct for refraction by a SVPlus sound velocimeter to measure the sound speed profile of the water column. This information was used in post-acquisition processing to correct the multibeam data. Post-acquisition processing of the multibeam data was performed using CARIS HIPS software (Universal Systems Ltd., Fredericton, New Brunswick, Canada), which integrates GPS coordinates with the bathymetry data. The processing removed outliers from the data, and applied appropriate corrections for vessel motion and ray refraction due to sound velocity variations. Lake Trout Spawner Assessment The Wisconsin Department of Natural Resources initiated sampling at the MLRC for lake trout of spawning size during fall 1983 to ascertain whether sexually mature stocked lake trout may be spawning. Sampling gear consisted of multifilament, multimesh gill nets (11.4-, 12.7-, 14.0-, 15.2-cm stretch, equal lengths per mesh size, 1.8 m deep, and usually 240 m total length). Sheboygan Reef was sampled from 1983 to 2000 and East Reef was sampled from 1997 to 2002. Gill net sets were overnight and locations were targeted for the vicinity of 43°20′N, 87°11′W to 43°20′N, 87°10′W at Sheboygan Reef and 43°04′N, 87°23′ W to 43°04′N, 87°22′W at East Reef, but actual locations varied with wind and current. Lake trout were measured (total length) and scales removed for ageing. Further details of sampling and aging of lake trout are available in McKee et al. (2004). Fish of age 7 or older were considered to be mature adults (Scott and Crossman 1973). Results from Madenjian et al. (1998) suggest a somewhat greater age for MLRC lake trout, but age 7 was used at the initiation of the survey.
Deep-spawning Lake Trout in Lake Michigan ROV Configurations The biological purpose of ROV sampling was to verify that lake trout were present where hydroacoustics suggested there were large fish, locate areas of egg deposition, collect egg predators (sculpins), and sample for lake trout fry. The focus of our efforts evolved to some extent based on experience. Two Benthos MiniRover MK IIs, one from Eastern Oceanics (Groton, CT; November 2001) and one from the National Underseas Research Center (NURC), University of Connecticut, were used from the support vessel, the R/V Neeskay. Each ROV was outfitted with an electroshocker and suction sampler. The ROV tether was attached to a weight on a winch cable and the weight was lowered to near bottom. The range of the ROV (length of tether between the weight and vehicle) relative to the vessel was a radius of approximately 30 m; representing an area of about 3,826 m2. This area does not indicate the area actually covered during a dive, which was unknown due to a lack of georeferencing capability. The electroshocker was an AbP-2 Dual Channel, battery-powered, pulsed-DC, backpack electroshocker designed at the University of WisconsinMadison’s Engineering Technical Services. The power supply was a sealed 12-V, 7.5-amp battery. The control unit was housed in a 38- × 25- × 13-cm waterproof case. Power to the electrodes was carried from the ship’s lab via a 300-m long, AWG 182 wire. The electroshocking unit produced a pulsed-DC current (1–1,000 Hz and up to 250 V). We varied voltages between about 100 to 160 v with no detectable difference in performance. Pulse rate was between 60 and 100 Hz initially, with no obvious difference in effectiveness. Experience and evolving objectives resulted in changes in the physical configurations of the suction sampler and electrodes; these are discussed in Results. The suction sampler for the Eastern Oceanics ROV had two collection chambers, each with a vertically mobile nozzle tube (50-mm diameter, clear acrylic) designed and constructed by Tusting and Tietze (1995). For the NURC ROV we designed a single collection chamber and nozzle (50mm diameter, clear acrylic). This ROV had a movable “gripper” that we altered so that the nozzle could be moved for some configurations. The pump for both ROVs was a MiniRover thruster. All dives were videotaped (Sony EV-C100 Hi-8). Videotapes from spring sampling were replayed,
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frame by frame where necessary, to search for and count lake trout fry. Two observers made independent observations and compared results. ROV Sampling Locations Seventeen ROV dives were made on six cruises to Sheboygan Reef and 24 dives on 11 cruises to East Reef (Table 1). The cruise schedule was entirely opportunistic in the fall because days calm enough to launch the ROV were rare. Duration of dives was determined by a combination of factors including (1) whether rocky habitat was encountered during the dive, (2) occasional system adjustments (trim, nozzle positioning) and system failures, (3) approach of high winds and seas, and (4) onset of darkness. In general, each dive had components of sampling, exploration, and testing of equipment configurations. ROV Samples and Observations We collected slimy sculpin to determine whether they were consuming lake trout eggs or fry. Slimy sculpins were collected via ROV electroshocking and suction sampling and euthanized in an overdose of MS-222. Fish were either measured and had their stomachs removed and preserved immediately or the whole fish was preserved (70% ethanol) and its stomach removed later. All prey were identified and counted in the laboratory. Egg numbers per stomach were compared among sites using an analysis of covariance with log (egg number +1) as the dependent variable, log (total length (TL)) as the covariate, and site as the group variable to determine whether the number of eggs in stomachs varied with slimy sculpin size. Eggs were collected to verify egg deposition at sampling sites. We expected to only count lake trout eggs collected from the substrate, anticipating that only a few were likely to be collected (as in Marsden and Janssen 1997) and that these would likely be too damaged to survive. However, large numbers of eggs were collected in fall 2003, most appeared undamaged, and, therefore, arrangements were made to incubate the eggs. Eggs were incubated initially in trays in a refrigerator (water temperature range 4.1–6.1°C) with the water changed daily. When eggs began to hatch they were transferred to a 1.5- m-diameter, 0.5-m deep, fiberglass tank with a constant flow of dechlorinated water. Temperatures ranged from about 4°C at hatching to 7°C at swim-up. The first 98 eggs to develop eye
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TABLE 1. Dive dates, locations, and duration of lake trout studies on Lake Michigan’s Mid Lake Reefs. The ROVs were either from the National Underseas Research Center (NURC) or Eastern Oceanics (EO). Electrode and nozzle configurations are discussed in the text. For electrodes +/- means there were two electrodes (rod or spring) each with opposing polarities; -/+/- means the two springs were (-) and the nozzle tip was (+). Sheboygan Reef 2001 Dive Date S1 11 Sep S2 11 Sep S3 11 Sep S4 11 Sep S5 30 Sep S6 30 Sep S7 30 Sep S8 30 Sep S9 16 Nov S10 16 Nov
Latitude (N) 43°20.65′ 43°20.65′ 43°20.68′ 43°20.60′ 43°20.55′ 43°20.71′ 43°20.61′ 43°20.57′ 43°20.60′ 43°20.60′
Longitude (W) 87°09.58′ 87°09.58′ 87°09.68′ 87°09.65′ 87°09.15′ 87°09.70′ 87°08.82′ 87°09.99′ 87°09.25′ 87°09.25′
Duration 40 min 55 min 15 min 55 min 40 min 46 min 20 min 75 min 79 min 93 min
ROV NURC NURC NURC NURC NURC NURC NURC NURC EO EO
Electrodes rod +/rod +/rod +/rod +/rod +/rod +/rod +/rod +/rod +/rod +/-
Nozzle diagonal single diagonal single diagonal single diagonal single diagonal single diagonal single diagonal single diagonal single vertical double vertical double
Sheboygan Reef 2002 Dive Date S11 06 Apr S12 06 Apr S13 06 Apr S14 06 Apr S15 23 Apr S16 14 May S17 14 May
Latitude (N) 43°20.52′ 43°20.58′ 43°20.56′ 43°20.63′ 43°20.58′ 43°20.60′ 43°20.60′
Longitude (W) 87°09.09′ 87°09.17′ 87°09.11′ 87°09.10′ 87°09.16′ 87°09.25′ 87°09.25′
Duration 80 min 50 min 50 min 50 min 200 min 41 min 77 min
ROV NURC NURC NURC NURC NURC NURC NURC
Electrodes rod +/rod +/rod +/rod +/rod +/rod +/rod +/-
Nozzle vertical single vertical single vertical single vertical single vertical single vertical single vertical single
East Reef 2002 Dive Date E1 23 Apr E2 14 May E3 5 Nov E4 5 Nov E5 20 Nov E6 20 Nov E7 20 Nov
Latitude (N) 43°02.90′ 43°02.83′ 43°02.84′ 43°02.84′ 43°02.79′ 43°02.85′ 43°02.76′
Longitude (W) 87°23.29′ 87°23.21′ 87°23.34′ 87°23.15′ 87°23.15′ 87°23.20′ 87°23.20′
Duration 92 min 155 min 43 min 38 min 38 min 87 min 106 min
ROV NURC NURC NURC NURC NURC NURC NURC
Electrodes rod +/rod +/spring +/spring +/spring +/spring +/spring +/-
Nozzle vertical single vertical single vertical single vertical single vertical single vertical single vertical single
East Reef 2003 Dive Date E8 10 Apr E9 10 Apr E10 10 Apr E11 13 May E12 13 May E13 8 Nov E14 11 Nov E15 12 Nov
Latitude (N) 43°02.79′ 43°02.79′ 43°02.79′ 43°02.84′ 43°02.84′ 43°02.95′ 43°04.30′ 43°01.21′
Longitude (W) 87°23.28′ 87°23.21′ 87°23.21′ 87°23.16′ 87°23.16′ 87°23.21′ 87°25.10′ 87°21.11′
Duration 72 min 50 min 60 min 95 min 66 min 124 min 145 min 91 min
ROV NURC NURC NURC NURC NURC NURC NURC NURC
Electrodes spring +/spring +/spring +/spring +/spring +/spring +/spring +/spring +/-
Nozzle vertical single vertical single vertical single vertical single vertical single vertical single vertical single vertical single
East Reef 2004 Dive Date E16 6 May E17 6 May E18 6 May E19 10 May E20 10 May E21 13 May E22 13 May E23 13 May
Latitude (N) 43°01.24′ 43°01.37′ 43°01.37′ 43°01.24′ 43°01.27′ 43°01.22′ 43°01.22′ 43°01.23′
Longitude (W) 87°21.11′ 87°21.21′ 87°21.21′ 87°21.20′ 87°21.17′ 87°21.11′ 87°21.11′ 87°21.09′
Duration 138 min 51 min 61 min 28 min 189 min 105 min 128 min 93 min
ROV NURC NURC NURC NURC NURC NURC NURC NURC
Electrodes spring +/spring +/spring +/spring +/spring +/spring +/spring +/spring +/-
Nozzle vertical single vertical single vertical single vertical single vertical single vertical single vertical single vertical single
Deep-spawning Lake Trout in Lake Michigan pigment were harvested for genetic analyses at the time the pigment was obvious to the naked eye (DeKoning et al. 2006). When remaining eggs began to hatch, they were transferred to a large flow-through tank that gradually warmed from about 4°C to 7°C at swim-up. Swim-up fry were fed Daphnia from a laboratory culture. Changes in ROV Configurations ROV dives during which the most eggs were collected occurred during the last several trips in fall, 2003 (dives E13–15, Table 1, detailed below) and the most lake trout fry seen on video occurred during the last spring (2004, dives E20–E24, detailed below). Sculpin collecting was successful throughout the course of this study, except when there was a mechanical failure. Improvement in performance was a result of changes in ROV configuration, operator experience, and production of bathymetric maps. Changes in nozzle configurations used for collecting sculpins and lake trout fry for ROVs were the result of accruing experience (Table 1). ROVs are idiosyncratic in their components and configurations, so details of our modifications are not presented. The ROV gripper originally moved its “thumb” horizontally. This was changed to diagonal (fall 2001), then vertical motions (spring 2002 and subsequently). The vertical nozzle had enough lateral flex to allow the nozzle tip to slip between rocks as the nozzle was lowered. For collections targeting lake trout eggs, beginning fall, 2003 (Table 1), we used a configuration similar to that used by Marsden and Janssen (1997). A 50-mm PVC tube was rigidly mounted horizontally upstream of the collection chamber; its nozzle pointed down via a 90o angle connector. The nozzle was rammed into rocks or hooked over rocks to turn them while simultaneously suction sampling (see Marsden and Janssen 1997). This configuration was also useful for collecting electroshocked sculpins, but the rigid configuration limited our ability to slip into crevices to remove stunned sculpins via suction. Electroshocker configurations also varied (Table 1). Rigid, stainless-steel rod electrodes (Table 1) were used initially; these were 37 cm long. These projected into the video viewing space of the ROV. The rigid rods limited maneuverability, so in fall 2002 they were replaced with flexible spring electrodes (37 cm long), which greatly improved our ability to maneuver close to rocks. The separation
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between rods or springs was 35 cm. Initially one of the rods or springs was an anode (+) and the other a cathode (–). In spring 2003 we changed the configuration so that the two springs were cathodes and the anode was a hose clamp that encircled the tip of the nozzle. This electrode configuration, with the anode now at the tip of the rigid nozzle, was also used for the egg collection configuration in fall 2003. The intent of this design was to attract lake trout fry and sculpins toward the suction sampler’s nozzle. RESULTS Bathymetry Mapping, Description of Habitats, and Bioacoustics Bathymetry of 36 km2 of Sheboygan Reef (Fig. 2a) indicated a ridge about 1.5 km long atop the plateau (Fig. 3a). The ridge extended from southeast to northwest with an orientation of about 108o relative to North. The height was uneven and varied from about 1.5 to 3 m. Areas of steep dropoff were to the east and south of the mapped area, but not investigated via ROV. At the ridge (Fig. 3a) the ROV video showed loose cobble with occasional large boulders (> 1 m high) (ROV dives S9–S17, Table 1). Other dives (dives S1–S8) revealed areas of bedrock, patches of sand, cobble, and boulders. The sand was generally rippled. Sonar indicated that apparently large fish echoes (i.e., strongest echoes as ascertained via sonar color coding and oscillograph) were near bottom, associated with the Sheboygan Reef ridge. No large fish echos were associated with the dropoff areas, but our recordings were limited to the southern dropoff. Bathymetry of 48 km 2 of East Reef (Fig. 2b) showed that the Horrall Slope was variable with three areas of relatively steep dropoffs (Figs. 3b, c, and d) separated by two areas of more gradual dropoffs. The three areas of the Horrall Slope examined by ROV were distinctly different. The North Site (Fig. 3b) had a “staircase” of rock strata, with each stratum about 1 m thick (dive E14, Table 1). Steps close to the top had areas of cobble that were mostly clear of silt. Lower steps had greater accumulations of cobble and silt. The Middle Site (Fig. 3c) had a 3–4-m thick slab of bedrock and loose flagstone cobble down slope (dives E1–E13). The vertical wall of bedrock was very uneven in its horizontal extent, with small points extending about 2–3 m. The South Site (Fig. 3d) had a low (< 1 m) limestone cap summit with a vertical face and
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FIG. 2a. Multibeam sonar-generated bathymetry of part (34 km 2 ) of Sheboygan Reef, southern Lake Michigan. The ridge (arrow) is in greater detail in Figure 3. Contours are in m.
loose, globular cobble down slope (dives E15–E24). This cobble, which had abundant interstitial space for egg incubation, had large boulders farther down the slope that may have kept it from slumping. The plateau atop the Horrall Slope was explored with the ROV at Sites North, Middle, and South whenever we encountered it. The plateau’s edge was generally clean of sand and silt but proceeding east onto the plateau there was coarse sand, then rippled finer sand easily suspended by the ROV thrusters. Sonar showed (presumed) large fish along the Horrall Slope at the North, Middle, and South Site during fall. Lake trout were always encountered during fall ROV dives at these sites. Lake Trout Spawner Assessment Beginning around 1998 there was a dramatic increase in the CPE (catch per effort, corrected to number per 300 m of net) of mature lake trout (Fig. 4) at Sheboygan Reef. The CPE for East Reef indicates CPE comparable to the last few years of Sheboygan Reef data; data prior to 1998 were not available (Fig. 4).
FIG. 2b. Multibeam sonar-generated bathymetry of part (48 km 2 ) of East Reef, southern Lake Michigan. The Horrall Slope is the area of dropoff from about 50 m to 100 m. The three areas of the Horrall Slope investigated (arrows) are shown in more detail in Figure 3. Contours are in m.
Slimy Sculpin Diets Slimy sculpin diets provided the first evidence that lake trout eggs were being deposited at Sheboygan Reef (fall 2001, dives S9–S10, Table 2). Similar evidence was found for East Reef the next year (dives E4, E6–7, Table 2). Overall, stomachs contained from 0 to 14 eggs or egg chorions. Where chorions were present there were masses of congealed, yolk-colored material. In fall the mean number of eggs per sculpin stomach varied with site/date (2.2 for East Reef slope in 2002, 1.1 for Sheboygan Reef in 2001, 4.6 for East Reef N in 2003, 4.3 for East Reef S in 2003, and 2.4 for East Reef M in 2003). Analysis of covariance of egg number vs. fish TL and collection site/date indicated that the number of eggs in the stomach increased with total length of the slimy sculpin (F1,56 = 41.3, P < 0.0001), but the exact relationship var-
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FIG. 3. Detail from Figure 2 showing ROV dive sites and local bathymetry in southern Lake Michigan where lake trout eggs, fry, and slimy sculpins that had eaten lake trout eggs were collected and/or videotaped. Areas of the circles approximate the range of the ROV, 30 m or 2,826 m2, see Table 1 for location descriptions. These circles represent the area of uncertainty in ROV position, not that the area was completely explored. A: Detail of Sheboygan Reef ridge and vicinity. Dives S9 and S10, where slimy sculpins with lake trout eggs in their stomachs were collected. B: East Reef Site N: Dive E14, where lake trout eggs and slimy sculpin that had eaten lake trout eggs were collected. C: East Reef Site M: Dives E5–E7, where lake trout eggs and slimy sculpin that had eaten lake trout eggs were collected. Dive E11, E12, where one lake trout fry was collected and others were seen on video. D: East Reef Site S: Dive E15, where lake trout eggs and slimy sculpin that had eaten lake trout eggs were collected. Dives E20-E24, where three lake trout fry were collected and others were seen on video.
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FIG. 4. Spawning lake trout gill net catch-pereffort (CPE) data for Sheboygan Reef and East Reef, southern Lake Michigan. Figure 4a shows CPE data for Sheboygan Reef and Figure 4b shows CPE data for East Reef. Catch per effort has been corrected for a 300 m net.
ied with site/date (F5,56 = 8.7, P < 0.0001) (Fig. 5). The curve superimposed on Figure 5 is an estimated maximum number of eggs per stomach based on ad libitum laboratory feeding of the morphologically similar mottled sculpin (Cottus bairdi; Biga et al. 1998). Most slimy sculpin stomachs had fewer eggs than this maximum. We found no evidence that electroshocking caused slimy sculpin to regurgitate eggs. We found only two eggs that were not in stomachs from 72 slimy sculpins collected during fall. These eggs were clear, and thus not digested. Moreover, we could see each of the fish after being electroshocked and often prior to electroshocking and none showed abdominal or oral movements characteristic of regurgitation. During November slimy sculpins seldom ate prey other than lake trout eggs (Table 2). On the East Reef Plateau (2002, dive E5) ten slimy sculpins, that had no eggs in their stomachs, had eaten Mysis and Diporeia, averaging about one prey item per fish. From the other East Reef dives in fall 2002 and 2003, most slimy sculpin stomachs contained only lake trout eggs; one fish’s stomach contained no eggs and three Mysis, and another’s stomach contained six Diporeia. At Sheboygan Reef (2001) three of six slimy sculpin stomachs contained one to three prey items consisting of Diporeia, Mysis, isopods, Pleurocera, or Bythotrephes. Slimy sculpin (> 40 mm TL) diets outside of November were variable (Table 3) and we found no evidence of predation on lake trout fry or juveniles. On 11 and 30 September 2001 (dives S1–S8, Table 1) and 6 April 2002 (dives S11–S16) stomachs contained mostly Mysis. Other prey were mainly midge larvae. For 23 April 2002 (dive S15), at Sheboygan Reef, only Diporeia and zebra mussels (Dreissena
TABLE 2. Number of slimy sculpins collected, number of slimy sculpins that had eaten lake trout eggs, and mean (range) number of eggs per slimy sculpin for various dates and sites on the mid-lake reefs in southern Lake Michigan (see Figs. 1, 2, and 3). The table includes only fish > 50 mm TL. Date 16 Nov 2001 20 Nov 2002 20 Nov 2002 8 Nov 2003 11 Nov 2003 12 Nov 2003
Reef Sheboygan East M, plateau East M, slope East M East N East S
Number of sculpins 21 10 12 10 12 7
Number with eggs 12 0 9 7 12 7
Mean (range) number of eggs 1.1 (0–8) 0 (0) 2.2 (0–6) 2.4 (1–6) 4.6 (1–14) 4.2 (1–8)
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TABLE 4. Lake trout eggs collected during 2001–2003 with an ROV on East Reef, Lake Michigan and their survival for the three East Reef sites sampled in fall, 2003 (Fig. 3). Live eggs had no opaque areas 1 day after collection and were subsequently incubated. Eyed eggs had embryos with pigmented eyes visible to the naked eye. Frozen eggs were intentionally harvested for DNA analysis (DeKoning et al. 2006). Hatched eggs are those that became sac-fry; “died” are individuals that were eyed but not harvested and died prior to hatching. After hatch the sac-fry from the three samples were either frozen or combined and raised until they began feeding (see text). FIG. 5. Number of lake trout eggs and chorions (ruptured egg membranes) in stomachs of slimy sculpins collected during 2001 (Sheboygan Reef), 2002 (East Reef), and 2003 East Reef sites north (N), middle (M), and south (S), Lake Michigan. The “predicted” line is based on maximum stomach capacity for mottled sculpins (Biga et al. 1998). polymorpha) were found. On the same date at East Reef (dive E1) Diporeia was the major prey item. Diporeia were mainly males; of 24 with intact antennae, 20 had the characteristically long antennae of mature males, which are free-swimming near bottom. Suction Sampling for Eggs and Egg Development We first targeted lake trout eggs by suction sampling in fall 2003. Prior to that only two eggs were
Site North Middle South
Number Collected 66 30 386
Live eggs 18 24 203
Hatched Eyed Harvested (died) 16 12 3 (1) 24 14 10 (0) 196 72 109 (15)
collected (Dive E7, 20 November 2002) as by-catch while collecting sculpins. Most eggs collected were at East Reef South site (386 eggs, Table 4, dive E15) with only 30 eggs collected at the East Reef Middle site (dive E13) and 66 eggs at the East Reef North site (dive E14). The East Reef South site dive was shorter, but more productive than the other two dives, but, because there were no replicates (i.e., multiple dives), no statistical analysis can justify a conclusion that eggs are at greater densities at the East Reef South site. Multiple spawning appears to have occurred since multiple stages of embryological development
TABLE 3. Diets of slimy sculpins (> 40 mm TL) collected when lake trout eggs were not present. Shown are number of fish, number with food, total number of Mysis among all slimy sculpin from a dive and total number of Diporeia among slimy sculpin from a dive. Other includes midge larvae, snails, and Bythotrephes. S.R. is Sheboygan Reef and E. R. is East Reef in the vicinity of Site M in southern Lake Michigan (see Fig. 1). Other includes chironomid larvae, snails, and Bythotrephes.
Date 30 Sep, 2001 6 Apr 2002 14 May 2002 14 May 2002
Site S.R. E.R. S.R. E.R.
Number of fish 6 26 35 69
Number with food 3 10 8 21
Number of Mysis 3 25 0 1
Diporeia 0 6 4 58
Number of Other 0 0 4 0
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were present. For example, East Reef South eggs showed eye pigmentation (“eyed-up”) from 29 November to 28 December. First eye-up stages occurred from 8 to 22 December for East Reef North and 2–16 December for East Reef Middle. Sac fry also hatched from 20 January to 20 February 2004. After hatching, sac-fry were transferred to a communal tank and most of these produced swim-up fry. Their swimming was sufficiently coordinated that all swim-up fry were feeding on Daphnia. Of the 122 embryos that hatched (Table 4), 92 reached swim-up, 10 died without swimming up, and the remaining 20 died while still in the chorion. The 10 that died lay on their sides and displayed erratic swimming, suggestive of Early Mortality Syndrome (Fitzsimons 1995a). Lake Trout Fry The first lake trout fry, which had a thin yolk sac, was collected on 13 May 2003 at the East Reef Middle (dive E11, Table 1). Examination of the video from dives E11 and E12 revealed an additional ten fry that responded to the electroshocker but were not collected, including one that apparently either escaped the collection chamber or never made it into the chamber and swam from the collection tube. During spring 2004, we only sampled at the East Reef South site. We collected one lake trout fry (28 mm) on 10 May (dive E21) and an additional two on 13 May (27 and 29 mm, dive E22), none of which had yolk. Video analysis from these same dives showed four additional lake trout fry on 10 May (dives E20–E21) and 13 on 13 May (dives E11–E12) that were not collected. Other Observations Both Sheboygan Reef and East Reef are becoming colonized with quagga mussels (Dreissena bugensis). None was observed or collected in fall, 2001 at Sheboygan Reef, although a few zebra mussels were collected. By spring 2002 small quagga mussels were detectable by video at both East Reef and Sheboygan Reef and some of these were gathered as incidental catch with the ROV suction sampler. DISCUSSION This study documented the first evidence of lake trout spawning at the MLRC and the deepest collection (40–50 m) of lake trout eggs and fry in the
Great Lakes. This means that other steps, such as growth of stocked fish, their mating, and their selection of suitable spawning substrate at deep reefs for incubation can be accomplished with lake trout of hatchery origin. Whether any of the fry survive to later stages is not yet known. It appears that restoration of a spawning population at the MLRC is potentially possible, but whether there are limiting constraints on successful restoration is not yet known. Substantial numbers of eggs survived suction sampling. Estimates of viability must be considered conservative because some eggs (an unknown fraction) were damaged. It is encouraging that the percentage of individuals that died with symptoms of Early Mortality Syndrome, about 10%, was relatively low. Fitzsimons et al. (2005) reported about 30% mortality of fry from Early Mortality Syndrome from Keuka Lake, which has a self-sustaining lake trout population. Comparisons with other studies must be made with caution since there is little comparable methodology with regards to the ROV-based work. Moreover, results may vary with ROV type, configuration, and pilot, particularly for egg sampling, where rocks need to be moved for egg collection. For example, Marsden and Janssen (1997) used a larger ROV (Phantom S2) that, compared with the MiniRovers used in the present study, more easily moved rocks to sample eggs. Marsden and Janssen collected only five eggs, and these were collected during the last dive, after the ROV had been reconfigured several times. While the ROVs in the present study were smaller and less able to move rocks, they were much more maneuverable among rocks, which could be more important than moving rocks. We do not know if the difference between the Marsden and Janssen study and our improved results (by the end of this study) are due to relative egg deposition or ROV type and configuration. One difference between the MLRC and shallow reefs is the lower diversity of potential egg and fry predators. Jones et al. (1995) listed 16 species of egg and fry predators but the suite of predators varied with location. For the MLRC, the list includes only slimy sculpin confirmed as an interstitial predator. Lake trout egg predation by slimy sculpins at the MLRC indicated stomachs were nearly full for some sites, based on comparison with the curve from Biga et al. (1998). The numbers per stomach appear to be relatively high relative to other sites. The numbers of eggs per stomach for slimy sculpin
Deep-spawning Lake Trout in Lake Michigan we collected (means of 1.1 to 4.3 depending on date/site) exceeded those reported by Fitzsimons et al. (2002) for shallow reefs. They reported means less than 1.0 for all sites and sculpin sizes except one. This suggests that either there are more eggs deposited at the MLRC, or it may mean eggs are less well protected there. However, interpretation must be made with caution because Fitzsimons et al. collected sculpins in egg bags retrieved 1–2 mo after egg deposition. We collected sculpins by a different means and may have been collecting them before egg deposition was complete. The number of eggs present in sculpin stomachs is likely to decrease quickly after egg deposition is complete as the most vulnerable eggs disappear. Except for depth, the physical and biological spawning conditions where we found lake trout eggs and fry at the MLRC are qualitatively similar to those operating at shallow Great Lakes spawning reefs. In Lakes Ontario, Michigan, Huron, and Champlain, lake trout spawn at sites with loose rock adjacent to steep drop-offs that may enhance currents that sweep the substrate clear of sediments (Marsden et al. 1995a,b; 2005). Sculpins are important predators on shallow reefs, but other predators such as crayfish are also present (Jonas et al. 2005). While we can say that lake trout are spawning at slope areas, we cannot quantitatively state that lake trout prefer these sites at the MLRC. Our observations provide evidence that restoration of lake trout at the MLRC may be feasible, or even occurring. Verification of that will require evidence that naturally spawned lake trout are surviving to maturity and spawning at the MLRC. Given that lake trout take a long time to mature (Madenjian et al. 1998), confirmation is unlikely to occur soon. The strategy we used to locate deepwater spawning habitat was that proposed by Marsden and Janssen (1997); use of fish sonar during the spawning season to locate relatively large echoes likely to be lake trout followed by deployment of the ROV. Updating that strategy, we suggest that studies of deep spawning lake trout begin with adequate mapping, followed by fish sonar surveys (preferentially with quantitative sonar), and finally deployment of ROVs. Once we obtained high-resolution, multibeam sonar maps we were able to much more easily locate potential habitat physically similar to the ridges and dropoffs where lake trout spawn in shallow water. In shallow water lake trout and habitat can often be seen from the surface (Jonas et al.
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2005); acoustics are, at present, the best substitute for visual reconnaissance. Our important technological advances included modification of the ROVs for collection of large numbers of eggs and egg predators, and first use of ROV-based electroshocking to collect and observe lake trout fry. We know of no other examples of combining ROV-based electroshocking and suction sampling to collect fishes. Lake trout fry electroshocking and suction sampling technology may have the most potential for generating quantitative data. This would require ground-truthing and probably more work regarding optimizing visualization and capture of fish. Collecting lake trout fry and sculpins by electroshocking among rocky habitat is unlikely to be 100% effective, but this problem is not unique to electroshocking. Quantitative data are necessary for comparing the impact of factors potentially limiting lake trout reproduction. For shallow-water lake trout, scuba techniques were used by Jonas et al. (2005) and Marsden et al. (2005) to compare shallow sites in Lake Michigan and Lake Champlain, where there has been no evidence of substantial recruitment of lake trout, with those in Parry Sound (Lake Huron), where lake trout have been fully restored (Reid et al. 2001). A dynamic modeling approach (Jones et al. 1995, Savino et al. 1999) to what limits lake trout reproduction is more technologically challenging because it requires data of sufficient quality to estimate rates of egg deposition and egg/fry predation, etc., to estimate the attrition of developing embryos. Our results demonstrate the need for development of quantitative techniques for sampling for lake trout eggs, fry, and predators on lake trout eggs and fry in deep water. Scuba-based methods suitable for shallow water have evolved since the work of Marsden et al. (1988), who used fry emergence traps to detect lake trout fry emergence. The quantitative method of choice for egg sampling is scuba diver-deployed egg bags (Perkins and Krueger 1994, Fitzsimons 1995b, Fitzsimons et al. 2005, Jonas et al. 2005). Existing technologies, not available for this study, would greatly improve ROV sampling efficiency. A support vessel with dynamic positioning, which allows the vessel to hold the desired station or slowly follow a course with the ROV, would eliminate the time needed to set and recover the anchor. When working from anchor it is difficult to get the ship to a desired position over a deep bottom or remain there when the wind or current shifts.
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We commonly set anchor more than once before deploying the ROV or had to abandon an anchorage to reset anchor after a wind shift. A second technology, which was available for the Marsden and Janssen (1997) study, was GPS tracking of the ROV position. For the present study, our resolution of position was a circle with a radius equal to the length of the ROV tether from its down-weight (30 m or an area of 2,827 m2). Because techniques used to evaluate shallow reefs depend mainly on conventional scuba techniques (e.g., Jonas et al. 2005), it is likely that further study of deep-spawning lake trout or comparisons between lake trout spawning at shallow reefs vs. deep reefs will require novel techniques for both shallow and deep water. It may be that highly technical, mixed-gas/decompression diving is required for deployment and recovery of quantitative gear, but there will continue to be limitations to the use of human divers because lake trout eggs incubate during the harsh weather of fall and winter when access to spawning sites is difficult. Nevertheless, acoustic and submersible techniques we used will provide the foundation for deployment of quantitative sampling gear at deepwater sites because of the vast area that needs to be explored. ACKNOWLEDGMENTS This work was funded by the University of Wisconsin Sea Grant Institute under grants from the National Sea Grant College Program, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, and from the State of Wisconsin, Federal grant number NA16RG2257, project number R/LR-89. Mapping was funded by the United States Fish and Wildlife Service Restoration Program, NOAA EXPLORE Program, and the Great Lakes Fishery Trust. Additional grant support and ROV support were provided by the National Underseas Research Program. We thank the crews of the R/V Neeskay (University of Wisconsin-Milwaukee) and R/V Blue Heron (University of Minnesota-Duluth) for their exemplary hospitality. Dave Lovalvo owns and operates the Eastern Oceanics ROV and was very inventive. We also appreciate the help of numerous assistants and volunteers for braving the gales of November. In particular, we appreciate the dedication of Kirby Wolfe and Michelle Luebke, Carolina Penalva, and Steve Hensler. Chris Houghton and Tom Hanson helped draft figures. This paper is dedicated to the
late Ross Horrall, who showed the deep reefs to Janssen in 1976. REFERENCES Biga, H., Janssen, J., and Marsden, J.E. 1998. Effect of substrate size on lake trout egg predation by mottled sculpin. J. Great Lakes Res. 24:464–473. Bronte, C.R., Schram, S.T., Selgeby, J.H., and Swanson, B.L. 2002. Reestablishing a spawning population of lake trout in Lake Superior with fertilized eggs in artificial turf incubators. N. Am. J. Fish. Manage. 22:796–805. Brown, E.H., Eck, G.W., Forster, N.R., Horrall, R.M., and Coberly, C.E. 1981. Historical evidence for discrete stocks of lake trout (Salvelinus namaycsh) in Lake Michigan. Can. J. Fish. Aquat. Sci. 38:1747–1758. Chrzastowski, M.J., and Thompson, T.A. 1994. Late Wisconsonian and Holocene geologic history of the Illinois-Indiana coast of Lake Michigan. J. Great Lakes Res. 20:9–26. Coberly, C.E., and Horrall, R.M. 1980. Fish spawning ground in Wisconsin waters of the Great Lakes. University of Wisconsin Sea Grant Institute, Madison, WI. Dawson, K.A., Eshenroder, R.L, Holey, M.E., and Ward, C. 1997. Quantification of historic lake trout (Salvelinus namaycush) spawning aggregations in Lake Michigan. Can. J. Fish Aquat. Sci. 54:2290–2302. DeKoning, J., Noakes, M., Keatley, K., Phillips, R., and Janssen, J. 2006. Genetic analysis of wild lake trout embryos recovered from Lake Michigan. Trans. Am. Fish. Soc. 135:399–407. Edsall, T.A., and Kennedy, G.W. 1995. Availability of lake trout reproductive habitat in the Great Lakes. J. Great Lakes Res. 21(Suppl. 1):290–301. Eshenroder, R.L., Crossman, E.J., Meffe, G.K., Olver, C. H., and Pister, E. P. 1995. Lake trout rehabilitation in the Great Lakes: an evolutionary, ecological, and ethical perspective. J. Great Lakes Res. 21:518–530. Fitzsimons, J.D. 1995a. The effect of B-vitamins on swim-up syndrome in Lake Ontario lake trout. J. Great Lakes Res. 21(Suppl. 1):337–347. ——— . 1995b. Assessment of lake trout spawning habitat and egg deposition in Lake Ontario. J. Great Lakes Res. 21(Suppl. 1):337–347. ——— , Perkins, D.L., and Krueger, C.C. 2002. Sculpins and crayfish in lake trout spawning areas in Lake Ontario: estimates of abundance and egg predation on lake trout eggs. J. Great Lakes Res. 28:421–436. ——— , Fodor, G., Williston, B., Don, V., Gray, B., Benner, M., Breedon, T., and Gilroy, D. 2005. Deepwater spawning by lake trout (Salvelinus namaycush) in Keuka Lake, New York. J. Great Lakes Res. 31:1–10. Gunn, J.M. 1995. Spawning behavior of lake trout:
Deep-spawning Lake Trout in Lake Michigan effects on colonization ability. J. Great Lakes Res. 21(Suppl. 1):323–329. Holey, M.E., Rybicki, R.W., Eck, G.W., Brown Jr., E.H., Marsden, J.E., Lavis, D.S., Toneys, M.L., Trudeau, T.M., and Horrall, R.M. 1995. Progress toward lake trout restoration in Lake Michigan. J. Great Lakes Res. 21 (Suppl. 1):128–151. Jonas, J.L., Claramunt, R.M., Fitzsimons, J.D., Marsden, J.E., and Ellrott, B.J. 2005. Estimates of egg deposition and effects of lake trout (Salvelinus namaycush) egg predators in three regions of the Great Lakes. Can. J. Fish. Aquat. Sci. 62:2254–2264. Jones, M.L.K., Eck, G.W., Evans, D.O., Fabrizio, M.C., Hoff, M., Hudson, P., Janssen, J., Jude, D.J., O’Gorman, R., and Savino, J. 1995. Limitations to lake trout (Salvelinus namaycush) rehabilitation in the Great Lakes imposed by biotic interactions occurring at early life stages. J. Great Lakes Res. 21 (Suppl. 1): 505–517. Jude, D.J., Klinger, S.A., and Enk, M.D. 1981. Evidence of natural reproduction by planted lake trout in Lake Michigan. N. Am. J. Fish. Manage. 1:165–172. Madenjian, C.P., DeSorcie, T.J., and Stedman, R.M. 1998. Maturity schedules of lake trout in Lake Michigan. J. Great Lakes Res. 24:40–410. Marsden, J.E., and. Janssen, J. 1997. Evidence of lake trout spawning on a deep reef in Lake Michigan using an ROV-based egg collector. J. Great Lakes Res. 23:450–457. ——— , Krueger, C.C., and Schneider, C.P. 1988. Evidence of natural reproduction by stocked lake trout in Lake Ontario. J. Great Lakes Res. 14:3–8. ——— , Casselmann, J.M., Edsall, T.A., Elliott, R.F., Fitzsimons, J.D., Horns, W.H., Manny, B.A., McAughey, S.C., Sly, P., and Swanson, B.L. 1995a. Lake trout spawning habitat in the Great Lakes: a review of current knowledge. J. Great Lakes Res. 21:330–336. ——— , Perkins, D.L., and Krueger, C.C. 1995b. Recog-
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nition of spawning areas by lake trout: deposition and survival of eggs on small, man-made rock piles. J. Great Lakes Res. 21:487–497. ——— , Claramunt, R.M., Ellrot, B.J., Fitzsimons, J.D., and Jonas, J.L. 2005. A comparison of lake trout spawning, fry emergence, and habitat use in Lakes Michigan, Huron, and Champlain. J. Great Lakes Res. 31:492–508. McKee, P.M., Toneys, M.L., Hansen, M.J., and Holey, M.E. 2004. Performance of lake trout stocked in the Midlake Refuge of Lake Michigan. N. Amer. J. Fish. Manage. 24: 1101–1111. Perkins, D.L., and Krueger, C.C. 1994. Design and use of mesh bags to study egg deposition and embryo survival in cobble substrate. N. Am. J. Fish. Manage. 14:866–869. Reid, D.M., Anderson, D.M., and Henderson, B.A. 2001. Restoration of lake trout in Parry Sound, Lake Huron. N. Am. J. Fish. Manage. 21:156–169. Savino, J.F., Hudson, P.L., Fabrizio, M.C., and Bowen II, C.A. 1999. Predation on lake trout eggs and fry: a modeling approach. J. Great Lakes Res. 25:36–44. Scott, W.B., and Crossman, E.J. 1973. Freshwater fishes of Canada. Fisheries Research Board of Canada, Bulletin No 184. Selgeby, J.H., Bronte, C.R., Brown, E.H., Hansen, M.J., Holey, M., VanAmberg, J.P., Muth, K.M., Makauskas, D.B., McKee, P., Anderson, D.M., Ferreri, C.P., and Schram, S.T. 1995. Lake trout restoration in the Great Lakes: stock-size criteria for natural reproduction. J. Great Lakes Res. 21:498–504. Tusting, R.F., and Tietze, T.C. 1995. Two-bucket stand alone suction sampler (TB-SSASS) user’s information manual. Harbor Branch Oceanographic Institution Inc., Tech. Note No. 111, Fort Pierce, FL. Submitted: 18 October 2004 Accepted: 26 July 2006 Editorial handling: Stephen B. Brandt
Evidence of Lake Trout Reproduction at Lake Michigan’s Mid-lake Reef Complex John Janssen1,*, David J. Jude2, Thomas A. Edsall3,†, Robert W. Paddock1, Nigel Wattrus4, Mike Toneys5,†, and Pat McKee5 1Great
Lakes WATER Institute University of Wisconsin-Milwaukee 600 East Greenfield Avenue Milwaukee, Wisconsin 53204 2School
of Natural Resources and Environment University of Michigan 440 Church St. Ann Arbor, Michigan 48109
3United
States Geological Survey Great Lakes Science Center 1451 Green Road Ann Arbor, Michigan 48105 4Large
Lakes Observatory University of Minnesota-Duluth Duluth, Minnesota 55812. 5Wisconsin
Department of Natural Resources Sturgeon Bay, Wisconsin 54235
ABSTRACT. The Mid-Lake Reef Complex (MLRC), a large area of deep (> 40 m) reefs, was a major site where indigenous lake trout (Salvelinus namaycush) in Lake Michigan aggregated during spawning. As part of an effort to restore Lake Michigan’s lake trout, which were extirpated in the 1950s, yearling lake trout have been released over the MLRC since the mid-1980s and fall gill net censuses began to show large numbers of lake trout in spawning condition beginning about 1999. We report the first evidence of viable egg deposition and successful lake trout fry production at these deep reefs. Because the area’s existing bathymetry and habitat were too poorly known for a priori selection of sampling sites, we used hydroacoustics to locate concentrations of large fish in the fall; fish were congregating around slopes and ridges. Subsequent observations via unmanned submersible confirmed the large fish to be lake trout. Our technological objectives were driven by biological objectives of locating where lake trout spawn, where lake trout fry were produced, and what fishes ate lake trout eggs and fry. The unmanned submersibles were equipped with a suction sampler and electroshocker to sample eggs deposited on the reef, draw out and occasionally catch emergent fry, and collect egg predators (slimy sculpin Cottus cognatus). We observed slimy sculpin to eat unusually high numbers of lake trout eggs. Our qualitative approaches are a first step toward quantitative assessments of the importance of lake trout spawning on the MLRC. INDEX WORDS:
*Corresponding
Great Lakes, ROV, submersible, Salvelinus namaycush, spawning.
author. E-mail: [email protected]
†Retired
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INTRODUCTION Lake trout (Salvelinus namaycush) were extirpated from Lake Michigan in the 1950s as a result of overfishing exacerbated by sea lamprey (Petromyzon marinus) predation (Holey et al. 1995). Efforts to restore self-sustaining lake trout populations have not been successful, although there has been some evidence of limited wild recruitment (e.g., Jude et al. 1981). The restoration of lake trout in the Great Lakes has been geographically restricted; the only restorations have occurred in Lake Superior and Parry Sound (Reid et al. 2001), an area of Lake Huron where a surviving remnant population occurred. In Lake Michigan, one of the most important lake trout spawning areas, based on historical records of fish in or near spawning condition, is a series of large reefs almost entirely deeper than 40 m that separate the northern and southern basins, called the Mid-Lake Reef Complex (MLRC; Coberly and Horrall 1980, Dawson et al. 1997). This area reportedly had a population of lake trout that was behaviorally and physiologically specialized (Brown et al. 1981). Unfortunately little is known about the MLRC, either physically or biologically. Eshenroder et al. (1995) considered the MLRC as extreme habitat, in contrast to typical lake trout spawning areas, for two reasons. First, the MLRC is near the southern edge of the species’ distribution (Scott and Crossman 1973) suggesting environmental constraints may exist on lake trout distribution farther south. Second, spawning at the MLRC would have to occur in unusually deep water. Lake trout in Ontario lakes typically spawn along shores with wind-driven waves in a few meters of water where loose rock forms interstitial spaces for egg incubation (Gunn 1995). Eshenroder et al. (1995) argued that this behavior of lake trout may be responsible for the strong tendency for stocked lake trout to attempt to spawn along shorelines of the Great Lakes. The MLRC is nearly unexplored biologically or physically. It has a total surface area of 2,859 km2 (Holey et al. 1995) and the bedrock is limestone, presumably of Devonian age, that was largely resistant to Pleistocene glaciations. There would be subsequent weathering during a period when various parts were terrestrial (Chrzastowski and Thompson 1994). Prominent MLRC features include three cuestas, i.e., elevated features with a steep drop-off on one side and slight gradient approximating a plateau on the other. The cuestas are 40–60 m deep at their
various summits. Two of the summits, named East Reef and Sheboygan Reef, were the focus of our study. Previous habitat mapping of part of Sheboyogan Reef indicated a spatially heterogeneous substrate that included cobble, bedrock, clay, and sand (Edsall and Kennedy 1995). Habitat suitable for lake trout reproduction may exist only in small patches. Small patches of cobble sustain the lake trout population at Parry Sound (Marsden et al. 2005). Restoration of deep-water lake trout requires an understanding of original physical and biological conditions, and this knowledge is extremely vague for the MLRC because it is based primarily on commercial fishers’ reports (Coberly and Horrall 1980, Brown et al. 1981, Dawson et al. 1997). The potential importance of locating viable spawning habitat is evident from the recent lake trout restoration success at Devil’s Island Shoal, Lake Superior (Bronte et al. 2002). Part of the successful strategy at this location was to incubate eggs at sites physically similar to where lake trout were known to spawn successfully elsewhere. Hypotheses regarding the physical and biological history of lake trout spawning on deep reefs have been developed in Brown et al. (1981), Eshenroder et al. (1995), and Dawson et al. (1997). If, as Brown et al. (1981) suggest, the MLRC lake trout were a strain specialized for whatever unique conditions exist at the MLRC, then matching characteristics of stocked strains to an extirpated strain may facilitate restoration. A synopsis of MLRC hypotheses includes the following: 1. Brown et al. (1981) integrated the information gathered from interviews with commercial fishers (Coberly and Horrall 1980) with fisheries records to argue that deep-spawning lake trout were likely a discrete stock and strain. 2. Eshenroder et al. (1995) argued that deepspawning lake trout may have begun as shallow spawners at the MLRC during the period when Lake Michigan was about 100 m shallower and much of the area was terrestrial. These shallow spawners would initially have habitat preference similar to those studied by Gunn (1995). A deepwater strain would then evolve from individuals that continued homing to the MLRC as it gradually became “drowned.” This scenario helps explain isolation from shallow spawning strains, fostering strain evolution.
Deep-spawning Lake Trout in Lake Michigan 3. Dawson et al. (1997) pointed out that the geology of the MLRC was most likely Silurian and Devonian limestones so there should be areas of rocky substrate suitable for lake trout spawning. 4. Dawson et al. (1997) presented evidence, based on commercial records, that the highest densities of spawning lake trout in Lake Michigan were at the MLRC and that these fish spawned in late October. The current state of information on the MLRC is inadequate to estimate the likelihood of recruitment of lake trout or factors that may limit recruitment. Therefore, the best guidelines for evaluation are those proposed for lake trout spawning on shallow reefs elsewhere. These guidelines include estimates of minimum spawner density (Selgeby et al. 1995), evaluation of spawning habitat (Marsden et al. 1995a), and elucidation of the impacts of predation during the fall through spring incubation (Jones et al. 1995). Existing approaches to evaluating lake trout spawning success mainly use scuba (e.g., Jonas et al. 2005, Marsden et al. 2005). The depth of the MLRC severely limits the use of conventional scuba-based techniques. Successful use of scuba also requires knowledge of potential lake trout spawning habitat locations. Such sites and even lake trout can often be seen from the surface (Jonas et al. 2005), but this is not feasible for the MLRC. An alternative approach, using a tethered unmanned submersible (ROV) with a suction sampler, had been used by Marsden and Janssen (1997) to collect several lake trout eggs and several slimy sculpin (Cottus cognatus) from Lake Michigan’s Julian’s Reef. Julian’s Reef is a few km2 and about 30 m deep and the work was facilitated by an existing habitat map. While the ROV was limited to qualitative information, Marsden and Janssen (1997) were able to establish for the first time that egg deposition was occurring on a Great Lakes deep reef and justify a concerted effort on the much larger MLRC. Initiation of this effort was justified by indications that large numbers of ripe and nearripe lake trout were appearing at the MLRC; results are briefly reported here. The biological goals of this project were to answer the basic qualitative questions of whether lake trout eggs were being deposited at the MLRC, whether the deposited eggs were viable, whether fry were being produced, and what fishes may be egg or fry predators. This project also had technological
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goals, which were to enhance ROV capabilities to meet specific sampling needs for deep-water reefs in the Great Lakes. We expect that these methods will continue to evolve, much as scuba-based sampling has. METHODS General Approach Our approach to studying the MLRC followed recommendations of Marsden and Janssen (1997). We used hydroacoustics to find locations where targets, likely to be lake trout, aggregated during the spawning season. At these locations we sampled for eggs and egg predators using an ROV. During spring, we searched for evidence of fry production via ROV. Preliminary successes justified bathymetry mapping, which led to better success, including collecting sufficient eggs to permit a conservative estimation of viability to swim-up. Sampling Locations East Reef and Sheboygan Reef are located in central Lake Michigan (Fig. 1). Initial locations for sampling of eggs and egg predators were chosen by hydroacoustic detection of large fish during October and November 2001–2003 as an indicator of the presence of lake trout. The sonar was a Furuno FCV-582L (aboard the R/V Neeskay, University of
FIG. 1. General location of East Reef and Sheboygan Reef, Lake Michigan.
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Wisconsin-Milwaukee) operating simultaneously at 50 and 200 kHz. We recorded GPS coordinates of sites that appeared to have high concentrations of relatively large fish near bottom (based on the sonar’s color code and oscillograph). Sampling site selection prior to fall 2003 was completed without use of bathymetry maps reported here. Available maps had 2-km spatial resolution and were insufficient for locating sampling sites. Once available (fall 2003), the East Reef bathymetry map (methods described below) was used to locate sonar transects for subsequent sampling of eggs and egg predators (also fall 2003). These sites were also sampled for lake trout fry and fry predators in spring, 2004. The East Reef ROV dive sites were near the top of the dropoff (henceforth Horrall Slope), defined as the slope along the westerly flank from about 50 m to 100 m. Replay of the sonar video showed apparently large fish near the bottom, slightly downslope of the reef crest. Dive sites in fall 2003 and spring 2004 for collecting eggs were based on GPS locations where large fish were present on the bathymetry map of East Reef. We did not use the bathymetry map for Sheboygan Reef to locate sampling sites because all biological data (primarily slimy sculpin diets) for Sheboygan Reef pre-date our mapping. We chose what the sonar suggested was a rocky hump (subsequently found to be a ridge) that consistently showed what appeared to be large fish near bottom. Bathymetry Mapping The purpose of bathymetry mapping was to facilitate locating ROV sampling sites as well as to examine the bathymetry at sites where we had found lake trout eggs and fry during the earlier stages of this project. Detailed bathymetry data of East Reef and Sheboygan Reef sampling sites were measured from 27 to 29 June 2003 with a Reson Seabat 8101 multibeam sonar mounted in the hull of the R/V Blue Heron (University of Minnesota). Survey lines were spaced at about 2.5 × minimum water depth (100 m wide for Sheboygan Reef and 125 m wide for East Reef ) to ensure approximately 20% overlap between adjacent survey lines and therefore 100% coverage of the lake floor in the survey area. The Reson Seabat 8101 multibeam sonar is a 240 kHz multibeam that uses 101 1.5-degree beams to measure the bathymetry of the lake floor. Width of the swath “illuminated” on the lake floor is approximately 7.5 times the water depth in water less than 70 m deep. The system has a range resolution of
1.25 cm (Reson Inc, Goleta, CA). To achieve this resolution, vessel motion (roll, pitch, and heading) was measured to an accuracy of less than ± 0.05 degrees, and the boat’s position was measured with an accuracy of less than 1 m horizontally and 25 cm vertically by a TSS POS MV/320 motion sensing and positioning system. The POS MV/320 uses a system of accelerometers, gyroscopes, and multiple survey-grade Differential GPS units to provide consistently high accuracy motion and position information under most conditions (TSS America, Houston, TX). Variations in the speed of sound in the water column were measured periodically to correct for refraction by a SVPlus sound velocimeter to measure the sound speed profile of the water column. This information was used in post-acquisition processing to correct the multibeam data. Post-acquisition processing of the multibeam data was performed using CARIS HIPS software (Universal Systems Ltd., Fredericton, New Brunswick, Canada), which integrates GPS coordinates with the bathymetry data. The processing removed outliers from the data, and applied appropriate corrections for vessel motion and ray refraction due to sound velocity variations. Lake Trout Spawner Assessment The Wisconsin Department of Natural Resources initiated sampling at the MLRC for lake trout of spawning size during fall 1983 to ascertain whether sexually mature stocked lake trout may be spawning. Sampling gear consisted of multifilament, multimesh gill nets (11.4-, 12.7-, 14.0-, 15.2-cm stretch, equal lengths per mesh size, 1.8 m deep, and usually 240 m total length). Sheboygan Reef was sampled from 1983 to 2000 and East Reef was sampled from 1997 to 2002. Gill net sets were overnight and locations were targeted for the vicinity of 43°20′N, 87°11′W to 43°20′N, 87°10′W at Sheboygan Reef and 43°04′N, 87°23′ W to 43°04′N, 87°22′W at East Reef, but actual locations varied with wind and current. Lake trout were measured (total length) and scales removed for ageing. Further details of sampling and aging of lake trout are available in McKee et al. (2004). Fish of age 7 or older were considered to be mature adults (Scott and Crossman 1973). Results from Madenjian et al. (1998) suggest a somewhat greater age for MLRC lake trout, but age 7 was used at the initiation of the survey.
Deep-spawning Lake Trout in Lake Michigan ROV Configurations The biological purpose of ROV sampling was to verify that lake trout were present where hydroacoustics suggested there were large fish, locate areas of egg deposition, collect egg predators (sculpins), and sample for lake trout fry. The focus of our efforts evolved to some extent based on experience. Two Benthos MiniRover MK IIs, one from Eastern Oceanics (Groton, CT; November 2001) and one from the National Underseas Research Center (NURC), University of Connecticut, were used from the support vessel, the R/V Neeskay. Each ROV was outfitted with an electroshocker and suction sampler. The ROV tether was attached to a weight on a winch cable and the weight was lowered to near bottom. The range of the ROV (length of tether between the weight and vehicle) relative to the vessel was a radius of approximately 30 m; representing an area of about 3,826 m2. This area does not indicate the area actually covered during a dive, which was unknown due to a lack of georeferencing capability. The electroshocker was an AbP-2 Dual Channel, battery-powered, pulsed-DC, backpack electroshocker designed at the University of WisconsinMadison’s Engineering Technical Services. The power supply was a sealed 12-V, 7.5-amp battery. The control unit was housed in a 38- × 25- × 13-cm waterproof case. Power to the electrodes was carried from the ship’s lab via a 300-m long, AWG 182 wire. The electroshocking unit produced a pulsed-DC current (1–1,000 Hz and up to 250 V). We varied voltages between about 100 to 160 v with no detectable difference in performance. Pulse rate was between 60 and 100 Hz initially, with no obvious difference in effectiveness. Experience and evolving objectives resulted in changes in the physical configurations of the suction sampler and electrodes; these are discussed in Results. The suction sampler for the Eastern Oceanics ROV had two collection chambers, each with a vertically mobile nozzle tube (50-mm diameter, clear acrylic) designed and constructed by Tusting and Tietze (1995). For the NURC ROV we designed a single collection chamber and nozzle (50mm diameter, clear acrylic). This ROV had a movable “gripper” that we altered so that the nozzle could be moved for some configurations. The pump for both ROVs was a MiniRover thruster. All dives were videotaped (Sony EV-C100 Hi-8). Videotapes from spring sampling were replayed,
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frame by frame where necessary, to search for and count lake trout fry. Two observers made independent observations and compared results. ROV Sampling Locations Seventeen ROV dives were made on six cruises to Sheboygan Reef and 24 dives on 11 cruises to East Reef (Table 1). The cruise schedule was entirely opportunistic in the fall because days calm enough to launch the ROV were rare. Duration of dives was determined by a combination of factors including (1) whether rocky habitat was encountered during the dive, (2) occasional system adjustments (trim, nozzle positioning) and system failures, (3) approach of high winds and seas, and (4) onset of darkness. In general, each dive had components of sampling, exploration, and testing of equipment configurations. ROV Samples and Observations We collected slimy sculpin to determine whether they were consuming lake trout eggs or fry. Slimy sculpins were collected via ROV electroshocking and suction sampling and euthanized in an overdose of MS-222. Fish were either measured and had their stomachs removed and preserved immediately or the whole fish was preserved (70% ethanol) and its stomach removed later. All prey were identified and counted in the laboratory. Egg numbers per stomach were compared among sites using an analysis of covariance with log (egg number +1) as the dependent variable, log (total length (TL)) as the covariate, and site as the group variable to determine whether the number of eggs in stomachs varied with slimy sculpin size. Eggs were collected to verify egg deposition at sampling sites. We expected to only count lake trout eggs collected from the substrate, anticipating that only a few were likely to be collected (as in Marsden and Janssen 1997) and that these would likely be too damaged to survive. However, large numbers of eggs were collected in fall 2003, most appeared undamaged, and, therefore, arrangements were made to incubate the eggs. Eggs were incubated initially in trays in a refrigerator (water temperature range 4.1–6.1°C) with the water changed daily. When eggs began to hatch they were transferred to a 1.5- m-diameter, 0.5-m deep, fiberglass tank with a constant flow of dechlorinated water. Temperatures ranged from about 4°C at hatching to 7°C at swim-up. The first 98 eggs to develop eye
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TABLE 1. Dive dates, locations, and duration of lake trout studies on Lake Michigan’s Mid Lake Reefs. The ROVs were either from the National Underseas Research Center (NURC) or Eastern Oceanics (EO). Electrode and nozzle configurations are discussed in the text. For electrodes +/- means there were two electrodes (rod or spring) each with opposing polarities; -/+/- means the two springs were (-) and the nozzle tip was (+). Sheboygan Reef 2001 Dive Date S1 11 Sep S2 11 Sep S3 11 Sep S4 11 Sep S5 30 Sep S6 30 Sep S7 30 Sep S8 30 Sep S9 16 Nov S10 16 Nov
Latitude (N) 43°20.65′ 43°20.65′ 43°20.68′ 43°20.60′ 43°20.55′ 43°20.71′ 43°20.61′ 43°20.57′ 43°20.60′ 43°20.60′
Longitude (W) 87°09.58′ 87°09.58′ 87°09.68′ 87°09.65′ 87°09.15′ 87°09.70′ 87°08.82′ 87°09.99′ 87°09.25′ 87°09.25′
Duration 40 min 55 min 15 min 55 min 40 min 46 min 20 min 75 min 79 min 93 min
ROV NURC NURC NURC NURC NURC NURC NURC NURC EO EO
Electrodes rod +/rod +/rod +/rod +/rod +/rod +/rod +/rod +/rod +/rod +/-
Nozzle diagonal single diagonal single diagonal single diagonal single diagonal single diagonal single diagonal single diagonal single vertical double vertical double
Sheboygan Reef 2002 Dive Date S11 06 Apr S12 06 Apr S13 06 Apr S14 06 Apr S15 23 Apr S16 14 May S17 14 May
Latitude (N) 43°20.52′ 43°20.58′ 43°20.56′ 43°20.63′ 43°20.58′ 43°20.60′ 43°20.60′
Longitude (W) 87°09.09′ 87°09.17′ 87°09.11′ 87°09.10′ 87°09.16′ 87°09.25′ 87°09.25′
Duration 80 min 50 min 50 min 50 min 200 min 41 min 77 min
ROV NURC NURC NURC NURC NURC NURC NURC
Electrodes rod +/rod +/rod +/rod +/rod +/rod +/rod +/-
Nozzle vertical single vertical single vertical single vertical single vertical single vertical single vertical single
East Reef 2002 Dive Date E1 23 Apr E2 14 May E3 5 Nov E4 5 Nov E5 20 Nov E6 20 Nov E7 20 Nov
Latitude (N) 43°02.90′ 43°02.83′ 43°02.84′ 43°02.84′ 43°02.79′ 43°02.85′ 43°02.76′
Longitude (W) 87°23.29′ 87°23.21′ 87°23.34′ 87°23.15′ 87°23.15′ 87°23.20′ 87°23.20′
Duration 92 min 155 min 43 min 38 min 38 min 87 min 106 min
ROV NURC NURC NURC NURC NURC NURC NURC
Electrodes rod +/rod +/spring +/spring +/spring +/spring +/spring +/-
Nozzle vertical single vertical single vertical single vertical single vertical single vertical single vertical single
East Reef 2003 Dive Date E8 10 Apr E9 10 Apr E10 10 Apr E11 13 May E12 13 May E13 8 Nov E14 11 Nov E15 12 Nov
Latitude (N) 43°02.79′ 43°02.79′ 43°02.79′ 43°02.84′ 43°02.84′ 43°02.95′ 43°04.30′ 43°01.21′
Longitude (W) 87°23.28′ 87°23.21′ 87°23.21′ 87°23.16′ 87°23.16′ 87°23.21′ 87°25.10′ 87°21.11′
Duration 72 min 50 min 60 min 95 min 66 min 124 min 145 min 91 min
ROV NURC NURC NURC NURC NURC NURC NURC NURC
Electrodes spring +/spring +/spring +/spring +/spring +/spring +/spring +/spring +/-
Nozzle vertical single vertical single vertical single vertical single vertical single vertical single vertical single vertical single
East Reef 2004 Dive Date E16 6 May E17 6 May E18 6 May E19 10 May E20 10 May E21 13 May E22 13 May E23 13 May
Latitude (N) 43°01.24′ 43°01.37′ 43°01.37′ 43°01.24′ 43°01.27′ 43°01.22′ 43°01.22′ 43°01.23′
Longitude (W) 87°21.11′ 87°21.21′ 87°21.21′ 87°21.20′ 87°21.17′ 87°21.11′ 87°21.11′ 87°21.09′
Duration 138 min 51 min 61 min 28 min 189 min 105 min 128 min 93 min
ROV NURC NURC NURC NURC NURC NURC NURC NURC
Electrodes spring +/spring +/spring +/spring +/spring +/spring +/spring +/spring +/-
Nozzle vertical single vertical single vertical single vertical single vertical single vertical single vertical single vertical single
Deep-spawning Lake Trout in Lake Michigan pigment were harvested for genetic analyses at the time the pigment was obvious to the naked eye (DeKoning et al. 2006). When remaining eggs began to hatch, they were transferred to a large flow-through tank that gradually warmed from about 4°C to 7°C at swim-up. Swim-up fry were fed Daphnia from a laboratory culture. Changes in ROV Configurations ROV dives during which the most eggs were collected occurred during the last several trips in fall, 2003 (dives E13–15, Table 1, detailed below) and the most lake trout fry seen on video occurred during the last spring (2004, dives E20–E24, detailed below). Sculpin collecting was successful throughout the course of this study, except when there was a mechanical failure. Improvement in performance was a result of changes in ROV configuration, operator experience, and production of bathymetric maps. Changes in nozzle configurations used for collecting sculpins and lake trout fry for ROVs were the result of accruing experience (Table 1). ROVs are idiosyncratic in their components and configurations, so details of our modifications are not presented. The ROV gripper originally moved its “thumb” horizontally. This was changed to diagonal (fall 2001), then vertical motions (spring 2002 and subsequently). The vertical nozzle had enough lateral flex to allow the nozzle tip to slip between rocks as the nozzle was lowered. For collections targeting lake trout eggs, beginning fall, 2003 (Table 1), we used a configuration similar to that used by Marsden and Janssen (1997). A 50-mm PVC tube was rigidly mounted horizontally upstream of the collection chamber; its nozzle pointed down via a 90o angle connector. The nozzle was rammed into rocks or hooked over rocks to turn them while simultaneously suction sampling (see Marsden and Janssen 1997). This configuration was also useful for collecting electroshocked sculpins, but the rigid configuration limited our ability to slip into crevices to remove stunned sculpins via suction. Electroshocker configurations also varied (Table 1). Rigid, stainless-steel rod electrodes (Table 1) were used initially; these were 37 cm long. These projected into the video viewing space of the ROV. The rigid rods limited maneuverability, so in fall 2002 they were replaced with flexible spring electrodes (37 cm long), which greatly improved our ability to maneuver close to rocks. The separation
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between rods or springs was 35 cm. Initially one of the rods or springs was an anode (+) and the other a cathode (–). In spring 2003 we changed the configuration so that the two springs were cathodes and the anode was a hose clamp that encircled the tip of the nozzle. This electrode configuration, with the anode now at the tip of the rigid nozzle, was also used for the egg collection configuration in fall 2003. The intent of this design was to attract lake trout fry and sculpins toward the suction sampler’s nozzle. RESULTS Bathymetry Mapping, Description of Habitats, and Bioacoustics Bathymetry of 36 km2 of Sheboygan Reef (Fig. 2a) indicated a ridge about 1.5 km long atop the plateau (Fig. 3a). The ridge extended from southeast to northwest with an orientation of about 108o relative to North. The height was uneven and varied from about 1.5 to 3 m. Areas of steep dropoff were to the east and south of the mapped area, but not investigated via ROV. At the ridge (Fig. 3a) the ROV video showed loose cobble with occasional large boulders (> 1 m high) (ROV dives S9–S17, Table 1). Other dives (dives S1–S8) revealed areas of bedrock, patches of sand, cobble, and boulders. The sand was generally rippled. Sonar indicated that apparently large fish echoes (i.e., strongest echoes as ascertained via sonar color coding and oscillograph) were near bottom, associated with the Sheboygan Reef ridge. No large fish echos were associated with the dropoff areas, but our recordings were limited to the southern dropoff. Bathymetry of 48 km 2 of East Reef (Fig. 2b) showed that the Horrall Slope was variable with three areas of relatively steep dropoffs (Figs. 3b, c, and d) separated by two areas of more gradual dropoffs. The three areas of the Horrall Slope examined by ROV were distinctly different. The North Site (Fig. 3b) had a “staircase” of rock strata, with each stratum about 1 m thick (dive E14, Table 1). Steps close to the top had areas of cobble that were mostly clear of silt. Lower steps had greater accumulations of cobble and silt. The Middle Site (Fig. 3c) had a 3–4-m thick slab of bedrock and loose flagstone cobble down slope (dives E1–E13). The vertical wall of bedrock was very uneven in its horizontal extent, with small points extending about 2–3 m. The South Site (Fig. 3d) had a low (< 1 m) limestone cap summit with a vertical face and
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FIG. 2a. Multibeam sonar-generated bathymetry of part (34 km 2 ) of Sheboygan Reef, southern Lake Michigan. The ridge (arrow) is in greater detail in Figure 3. Contours are in m.
loose, globular cobble down slope (dives E15–E24). This cobble, which had abundant interstitial space for egg incubation, had large boulders farther down the slope that may have kept it from slumping. The plateau atop the Horrall Slope was explored with the ROV at Sites North, Middle, and South whenever we encountered it. The plateau’s edge was generally clean of sand and silt but proceeding east onto the plateau there was coarse sand, then rippled finer sand easily suspended by the ROV thrusters. Sonar showed (presumed) large fish along the Horrall Slope at the North, Middle, and South Site during fall. Lake trout were always encountered during fall ROV dives at these sites. Lake Trout Spawner Assessment Beginning around 1998 there was a dramatic increase in the CPE (catch per effort, corrected to number per 300 m of net) of mature lake trout (Fig. 4) at Sheboygan Reef. The CPE for East Reef indicates CPE comparable to the last few years of Sheboygan Reef data; data prior to 1998 were not available (Fig. 4).
FIG. 2b. Multibeam sonar-generated bathymetry of part (48 km 2 ) of East Reef, southern Lake Michigan. The Horrall Slope is the area of dropoff from about 50 m to 100 m. The three areas of the Horrall Slope investigated (arrows) are shown in more detail in Figure 3. Contours are in m.
Slimy Sculpin Diets Slimy sculpin diets provided the first evidence that lake trout eggs were being deposited at Sheboygan Reef (fall 2001, dives S9–S10, Table 2). Similar evidence was found for East Reef the next year (dives E4, E6–7, Table 2). Overall, stomachs contained from 0 to 14 eggs or egg chorions. Where chorions were present there were masses of congealed, yolk-colored material. In fall the mean number of eggs per sculpin stomach varied with site/date (2.2 for East Reef slope in 2002, 1.1 for Sheboygan Reef in 2001, 4.6 for East Reef N in 2003, 4.3 for East Reef S in 2003, and 2.4 for East Reef M in 2003). Analysis of covariance of egg number vs. fish TL and collection site/date indicated that the number of eggs in the stomach increased with total length of the slimy sculpin (F1,56 = 41.3, P < 0.0001), but the exact relationship var-
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FIG. 3. Detail from Figure 2 showing ROV dive sites and local bathymetry in southern Lake Michigan where lake trout eggs, fry, and slimy sculpins that had eaten lake trout eggs were collected and/or videotaped. Areas of the circles approximate the range of the ROV, 30 m or 2,826 m2, see Table 1 for location descriptions. These circles represent the area of uncertainty in ROV position, not that the area was completely explored. A: Detail of Sheboygan Reef ridge and vicinity. Dives S9 and S10, where slimy sculpins with lake trout eggs in their stomachs were collected. B: East Reef Site N: Dive E14, where lake trout eggs and slimy sculpin that had eaten lake trout eggs were collected. C: East Reef Site M: Dives E5–E7, where lake trout eggs and slimy sculpin that had eaten lake trout eggs were collected. Dive E11, E12, where one lake trout fry was collected and others were seen on video. D: East Reef Site S: Dive E15, where lake trout eggs and slimy sculpin that had eaten lake trout eggs were collected. Dives E20-E24, where three lake trout fry were collected and others were seen on video.
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FIG. 4. Spawning lake trout gill net catch-pereffort (CPE) data for Sheboygan Reef and East Reef, southern Lake Michigan. Figure 4a shows CPE data for Sheboygan Reef and Figure 4b shows CPE data for East Reef. Catch per effort has been corrected for a 300 m net.
ied with site/date (F5,56 = 8.7, P < 0.0001) (Fig. 5). The curve superimposed on Figure 5 is an estimated maximum number of eggs per stomach based on ad libitum laboratory feeding of the morphologically similar mottled sculpin (Cottus bairdi; Biga et al. 1998). Most slimy sculpin stomachs had fewer eggs than this maximum. We found no evidence that electroshocking caused slimy sculpin to regurgitate eggs. We found only two eggs that were not in stomachs from 72 slimy sculpins collected during fall. These eggs were clear, and thus not digested. Moreover, we could see each of the fish after being electroshocked and often prior to electroshocking and none showed abdominal or oral movements characteristic of regurgitation. During November slimy sculpins seldom ate prey other than lake trout eggs (Table 2). On the East Reef Plateau (2002, dive E5) ten slimy sculpins, that had no eggs in their stomachs, had eaten Mysis and Diporeia, averaging about one prey item per fish. From the other East Reef dives in fall 2002 and 2003, most slimy sculpin stomachs contained only lake trout eggs; one fish’s stomach contained no eggs and three Mysis, and another’s stomach contained six Diporeia. At Sheboygan Reef (2001) three of six slimy sculpin stomachs contained one to three prey items consisting of Diporeia, Mysis, isopods, Pleurocera, or Bythotrephes. Slimy sculpin (> 40 mm TL) diets outside of November were variable (Table 3) and we found no evidence of predation on lake trout fry or juveniles. On 11 and 30 September 2001 (dives S1–S8, Table 1) and 6 April 2002 (dives S11–S16) stomachs contained mostly Mysis. Other prey were mainly midge larvae. For 23 April 2002 (dive S15), at Sheboygan Reef, only Diporeia and zebra mussels (Dreissena
TABLE 2. Number of slimy sculpins collected, number of slimy sculpins that had eaten lake trout eggs, and mean (range) number of eggs per slimy sculpin for various dates and sites on the mid-lake reefs in southern Lake Michigan (see Figs. 1, 2, and 3). The table includes only fish > 50 mm TL. Date 16 Nov 2001 20 Nov 2002 20 Nov 2002 8 Nov 2003 11 Nov 2003 12 Nov 2003
Reef Sheboygan East M, plateau East M, slope East M East N East S
Number of sculpins 21 10 12 10 12 7
Number with eggs 12 0 9 7 12 7
Mean (range) number of eggs 1.1 (0–8) 0 (0) 2.2 (0–6) 2.4 (1–6) 4.6 (1–14) 4.2 (1–8)
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TABLE 4. Lake trout eggs collected during 2001–2003 with an ROV on East Reef, Lake Michigan and their survival for the three East Reef sites sampled in fall, 2003 (Fig. 3). Live eggs had no opaque areas 1 day after collection and were subsequently incubated. Eyed eggs had embryos with pigmented eyes visible to the naked eye. Frozen eggs were intentionally harvested for DNA analysis (DeKoning et al. 2006). Hatched eggs are those that became sac-fry; “died” are individuals that were eyed but not harvested and died prior to hatching. After hatch the sac-fry from the three samples were either frozen or combined and raised until they began feeding (see text). FIG. 5. Number of lake trout eggs and chorions (ruptured egg membranes) in stomachs of slimy sculpins collected during 2001 (Sheboygan Reef), 2002 (East Reef), and 2003 East Reef sites north (N), middle (M), and south (S), Lake Michigan. The “predicted” line is based on maximum stomach capacity for mottled sculpins (Biga et al. 1998). polymorpha) were found. On the same date at East Reef (dive E1) Diporeia was the major prey item. Diporeia were mainly males; of 24 with intact antennae, 20 had the characteristically long antennae of mature males, which are free-swimming near bottom. Suction Sampling for Eggs and Egg Development We first targeted lake trout eggs by suction sampling in fall 2003. Prior to that only two eggs were
Site North Middle South
Number Collected 66 30 386
Live eggs 18 24 203
Hatched Eyed Harvested (died) 16 12 3 (1) 24 14 10 (0) 196 72 109 (15)
collected (Dive E7, 20 November 2002) as by-catch while collecting sculpins. Most eggs collected were at East Reef South site (386 eggs, Table 4, dive E15) with only 30 eggs collected at the East Reef Middle site (dive E13) and 66 eggs at the East Reef North site (dive E14). The East Reef South site dive was shorter, but more productive than the other two dives, but, because there were no replicates (i.e., multiple dives), no statistical analysis can justify a conclusion that eggs are at greater densities at the East Reef South site. Multiple spawning appears to have occurred since multiple stages of embryological development
TABLE 3. Diets of slimy sculpins (> 40 mm TL) collected when lake trout eggs were not present. Shown are number of fish, number with food, total number of Mysis among all slimy sculpin from a dive and total number of Diporeia among slimy sculpin from a dive. Other includes midge larvae, snails, and Bythotrephes. S.R. is Sheboygan Reef and E. R. is East Reef in the vicinity of Site M in southern Lake Michigan (see Fig. 1). Other includes chironomid larvae, snails, and Bythotrephes.
Date 30 Sep, 2001 6 Apr 2002 14 May 2002 14 May 2002
Site S.R. E.R. S.R. E.R.
Number of fish 6 26 35 69
Number with food 3 10 8 21
Number of Mysis 3 25 0 1
Diporeia 0 6 4 58
Number of Other 0 0 4 0
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were present. For example, East Reef South eggs showed eye pigmentation (“eyed-up”) from 29 November to 28 December. First eye-up stages occurred from 8 to 22 December for East Reef North and 2–16 December for East Reef Middle. Sac fry also hatched from 20 January to 20 February 2004. After hatching, sac-fry were transferred to a communal tank and most of these produced swim-up fry. Their swimming was sufficiently coordinated that all swim-up fry were feeding on Daphnia. Of the 122 embryos that hatched (Table 4), 92 reached swim-up, 10 died without swimming up, and the remaining 20 died while still in the chorion. The 10 that died lay on their sides and displayed erratic swimming, suggestive of Early Mortality Syndrome (Fitzsimons 1995a). Lake Trout Fry The first lake trout fry, which had a thin yolk sac, was collected on 13 May 2003 at the East Reef Middle (dive E11, Table 1). Examination of the video from dives E11 and E12 revealed an additional ten fry that responded to the electroshocker but were not collected, including one that apparently either escaped the collection chamber or never made it into the chamber and swam from the collection tube. During spring 2004, we only sampled at the East Reef South site. We collected one lake trout fry (28 mm) on 10 May (dive E21) and an additional two on 13 May (27 and 29 mm, dive E22), none of which had yolk. Video analysis from these same dives showed four additional lake trout fry on 10 May (dives E20–E21) and 13 on 13 May (dives E11–E12) that were not collected. Other Observations Both Sheboygan Reef and East Reef are becoming colonized with quagga mussels (Dreissena bugensis). None was observed or collected in fall, 2001 at Sheboygan Reef, although a few zebra mussels were collected. By spring 2002 small quagga mussels were detectable by video at both East Reef and Sheboygan Reef and some of these were gathered as incidental catch with the ROV suction sampler. DISCUSSION This study documented the first evidence of lake trout spawning at the MLRC and the deepest collection (40–50 m) of lake trout eggs and fry in the
Great Lakes. This means that other steps, such as growth of stocked fish, their mating, and their selection of suitable spawning substrate at deep reefs for incubation can be accomplished with lake trout of hatchery origin. Whether any of the fry survive to later stages is not yet known. It appears that restoration of a spawning population at the MLRC is potentially possible, but whether there are limiting constraints on successful restoration is not yet known. Substantial numbers of eggs survived suction sampling. Estimates of viability must be considered conservative because some eggs (an unknown fraction) were damaged. It is encouraging that the percentage of individuals that died with symptoms of Early Mortality Syndrome, about 10%, was relatively low. Fitzsimons et al. (2005) reported about 30% mortality of fry from Early Mortality Syndrome from Keuka Lake, which has a self-sustaining lake trout population. Comparisons with other studies must be made with caution since there is little comparable methodology with regards to the ROV-based work. Moreover, results may vary with ROV type, configuration, and pilot, particularly for egg sampling, where rocks need to be moved for egg collection. For example, Marsden and Janssen (1997) used a larger ROV (Phantom S2) that, compared with the MiniRovers used in the present study, more easily moved rocks to sample eggs. Marsden and Janssen collected only five eggs, and these were collected during the last dive, after the ROV had been reconfigured several times. While the ROVs in the present study were smaller and less able to move rocks, they were much more maneuverable among rocks, which could be more important than moving rocks. We do not know if the difference between the Marsden and Janssen study and our improved results (by the end of this study) are due to relative egg deposition or ROV type and configuration. One difference between the MLRC and shallow reefs is the lower diversity of potential egg and fry predators. Jones et al. (1995) listed 16 species of egg and fry predators but the suite of predators varied with location. For the MLRC, the list includes only slimy sculpin confirmed as an interstitial predator. Lake trout egg predation by slimy sculpins at the MLRC indicated stomachs were nearly full for some sites, based on comparison with the curve from Biga et al. (1998). The numbers per stomach appear to be relatively high relative to other sites. The numbers of eggs per stomach for slimy sculpin
Deep-spawning Lake Trout in Lake Michigan we collected (means of 1.1 to 4.3 depending on date/site) exceeded those reported by Fitzsimons et al. (2002) for shallow reefs. They reported means less than 1.0 for all sites and sculpin sizes except one. This suggests that either there are more eggs deposited at the MLRC, or it may mean eggs are less well protected there. However, interpretation must be made with caution because Fitzsimons et al. collected sculpins in egg bags retrieved 1–2 mo after egg deposition. We collected sculpins by a different means and may have been collecting them before egg deposition was complete. The number of eggs present in sculpin stomachs is likely to decrease quickly after egg deposition is complete as the most vulnerable eggs disappear. Except for depth, the physical and biological spawning conditions where we found lake trout eggs and fry at the MLRC are qualitatively similar to those operating at shallow Great Lakes spawning reefs. In Lakes Ontario, Michigan, Huron, and Champlain, lake trout spawn at sites with loose rock adjacent to steep drop-offs that may enhance currents that sweep the substrate clear of sediments (Marsden et al. 1995a,b; 2005). Sculpins are important predators on shallow reefs, but other predators such as crayfish are also present (Jonas et al. 2005). While we can say that lake trout are spawning at slope areas, we cannot quantitatively state that lake trout prefer these sites at the MLRC. Our observations provide evidence that restoration of lake trout at the MLRC may be feasible, or even occurring. Verification of that will require evidence that naturally spawned lake trout are surviving to maturity and spawning at the MLRC. Given that lake trout take a long time to mature (Madenjian et al. 1998), confirmation is unlikely to occur soon. The strategy we used to locate deepwater spawning habitat was that proposed by Marsden and Janssen (1997); use of fish sonar during the spawning season to locate relatively large echoes likely to be lake trout followed by deployment of the ROV. Updating that strategy, we suggest that studies of deep spawning lake trout begin with adequate mapping, followed by fish sonar surveys (preferentially with quantitative sonar), and finally deployment of ROVs. Once we obtained high-resolution, multibeam sonar maps we were able to much more easily locate potential habitat physically similar to the ridges and dropoffs where lake trout spawn in shallow water. In shallow water lake trout and habitat can often be seen from the surface (Jonas et al.
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2005); acoustics are, at present, the best substitute for visual reconnaissance. Our important technological advances included modification of the ROVs for collection of large numbers of eggs and egg predators, and first use of ROV-based electroshocking to collect and observe lake trout fry. We know of no other examples of combining ROV-based electroshocking and suction sampling to collect fishes. Lake trout fry electroshocking and suction sampling technology may have the most potential for generating quantitative data. This would require ground-truthing and probably more work regarding optimizing visualization and capture of fish. Collecting lake trout fry and sculpins by electroshocking among rocky habitat is unlikely to be 100% effective, but this problem is not unique to electroshocking. Quantitative data are necessary for comparing the impact of factors potentially limiting lake trout reproduction. For shallow-water lake trout, scuba techniques were used by Jonas et al. (2005) and Marsden et al. (2005) to compare shallow sites in Lake Michigan and Lake Champlain, where there has been no evidence of substantial recruitment of lake trout, with those in Parry Sound (Lake Huron), where lake trout have been fully restored (Reid et al. 2001). A dynamic modeling approach (Jones et al. 1995, Savino et al. 1999) to what limits lake trout reproduction is more technologically challenging because it requires data of sufficient quality to estimate rates of egg deposition and egg/fry predation, etc., to estimate the attrition of developing embryos. Our results demonstrate the need for development of quantitative techniques for sampling for lake trout eggs, fry, and predators on lake trout eggs and fry in deep water. Scuba-based methods suitable for shallow water have evolved since the work of Marsden et al. (1988), who used fry emergence traps to detect lake trout fry emergence. The quantitative method of choice for egg sampling is scuba diver-deployed egg bags (Perkins and Krueger 1994, Fitzsimons 1995b, Fitzsimons et al. 2005, Jonas et al. 2005). Existing technologies, not available for this study, would greatly improve ROV sampling efficiency. A support vessel with dynamic positioning, which allows the vessel to hold the desired station or slowly follow a course with the ROV, would eliminate the time needed to set and recover the anchor. When working from anchor it is difficult to get the ship to a desired position over a deep bottom or remain there when the wind or current shifts.
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We commonly set anchor more than once before deploying the ROV or had to abandon an anchorage to reset anchor after a wind shift. A second technology, which was available for the Marsden and Janssen (1997) study, was GPS tracking of the ROV position. For the present study, our resolution of position was a circle with a radius equal to the length of the ROV tether from its down-weight (30 m or an area of 2,827 m2). Because techniques used to evaluate shallow reefs depend mainly on conventional scuba techniques (e.g., Jonas et al. 2005), it is likely that further study of deep-spawning lake trout or comparisons between lake trout spawning at shallow reefs vs. deep reefs will require novel techniques for both shallow and deep water. It may be that highly technical, mixed-gas/decompression diving is required for deployment and recovery of quantitative gear, but there will continue to be limitations to the use of human divers because lake trout eggs incubate during the harsh weather of fall and winter when access to spawning sites is difficult. Nevertheless, acoustic and submersible techniques we used will provide the foundation for deployment of quantitative sampling gear at deepwater sites because of the vast area that needs to be explored. ACKNOWLEDGMENTS This work was funded by the University of Wisconsin Sea Grant Institute under grants from the National Sea Grant College Program, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, and from the State of Wisconsin, Federal grant number NA16RG2257, project number R/LR-89. Mapping was funded by the United States Fish and Wildlife Service Restoration Program, NOAA EXPLORE Program, and the Great Lakes Fishery Trust. Additional grant support and ROV support were provided by the National Underseas Research Program. We thank the crews of the R/V Neeskay (University of Wisconsin-Milwaukee) and R/V Blue Heron (University of Minnesota-Duluth) for their exemplary hospitality. Dave Lovalvo owns and operates the Eastern Oceanics ROV and was very inventive. We also appreciate the help of numerous assistants and volunteers for braving the gales of November. In particular, we appreciate the dedication of Kirby Wolfe and Michelle Luebke, Carolina Penalva, and Steve Hensler. Chris Houghton and Tom Hanson helped draft figures. This paper is dedicated to the
late Ross Horrall, who showed the deep reefs to Janssen in 1976. REFERENCES Biga, H., Janssen, J., and Marsden, J.E. 1998. Effect of substrate size on lake trout egg predation by mottled sculpin. J. Great Lakes Res. 24:464–473. Bronte, C.R., Schram, S.T., Selgeby, J.H., and Swanson, B.L. 2002. Reestablishing a spawning population of lake trout in Lake Superior with fertilized eggs in artificial turf incubators. N. Am. J. Fish. Manage. 22:796–805. Brown, E.H., Eck, G.W., Forster, N.R., Horrall, R.M., and Coberly, C.E. 1981. Historical evidence for discrete stocks of lake trout (Salvelinus namaycsh) in Lake Michigan. Can. J. Fish. Aquat. Sci. 38:1747–1758. Chrzastowski, M.J., and Thompson, T.A. 1994. Late Wisconsonian and Holocene geologic history of the Illinois-Indiana coast of Lake Michigan. J. Great Lakes Res. 20:9–26. Coberly, C.E., and Horrall, R.M. 1980. Fish spawning ground in Wisconsin waters of the Great Lakes. University of Wisconsin Sea Grant Institute, Madison, WI. Dawson, K.A., Eshenroder, R.L, Holey, M.E., and Ward, C. 1997. Quantification of historic lake trout (Salvelinus namaycush) spawning aggregations in Lake Michigan. Can. J. Fish Aquat. Sci. 54:2290–2302. DeKoning, J., Noakes, M., Keatley, K., Phillips, R., and Janssen, J. 2006. Genetic analysis of wild lake trout embryos recovered from Lake Michigan. Trans. Am. Fish. Soc. 135:399–407. Edsall, T.A., and Kennedy, G.W. 1995. Availability of lake trout reproductive habitat in the Great Lakes. J. Great Lakes Res. 21(Suppl. 1):290–301. Eshenroder, R.L., Crossman, E.J., Meffe, G.K., Olver, C. H., and Pister, E. P. 1995. Lake trout rehabilitation in the Great Lakes: an evolutionary, ecological, and ethical perspective. J. Great Lakes Res. 21:518–530. Fitzsimons, J.D. 1995a. The effect of B-vitamins on swim-up syndrome in Lake Ontario lake trout. J. Great Lakes Res. 21(Suppl. 1):337–347. ——— . 1995b. Assessment of lake trout spawning habitat and egg deposition in Lake Ontario. J. Great Lakes Res. 21(Suppl. 1):337–347. ——— , Perkins, D.L., and Krueger, C.C. 2002. Sculpins and crayfish in lake trout spawning areas in Lake Ontario: estimates of abundance and egg predation on lake trout eggs. J. Great Lakes Res. 28:421–436. ——— , Fodor, G., Williston, B., Don, V., Gray, B., Benner, M., Breedon, T., and Gilroy, D. 2005. Deepwater spawning by lake trout (Salvelinus namaycush) in Keuka Lake, New York. J. Great Lakes Res. 31:1–10. Gunn, J.M. 1995. Spawning behavior of lake trout:
Deep-spawning Lake Trout in Lake Michigan effects on colonization ability. J. Great Lakes Res. 21(Suppl. 1):323–329. Holey, M.E., Rybicki, R.W., Eck, G.W., Brown Jr., E.H., Marsden, J.E., Lavis, D.S., Toneys, M.L., Trudeau, T.M., and Horrall, R.M. 1995. Progress toward lake trout restoration in Lake Michigan. J. Great Lakes Res. 21 (Suppl. 1):128–151. Jonas, J.L., Claramunt, R.M., Fitzsimons, J.D., Marsden, J.E., and Ellrott, B.J. 2005. Estimates of egg deposition and effects of lake trout (Salvelinus namaycush) egg predators in three regions of the Great Lakes. Can. J. Fish. Aquat. Sci. 62:2254–2264. Jones, M.L.K., Eck, G.W., Evans, D.O., Fabrizio, M.C., Hoff, M., Hudson, P., Janssen, J., Jude, D.J., O’Gorman, R., and Savino, J. 1995. Limitations to lake trout (Salvelinus namaycush) rehabilitation in the Great Lakes imposed by biotic interactions occurring at early life stages. J. Great Lakes Res. 21 (Suppl. 1): 505–517. Jude, D.J., Klinger, S.A., and Enk, M.D. 1981. Evidence of natural reproduction by planted lake trout in Lake Michigan. N. Am. J. Fish. Manage. 1:165–172. Madenjian, C.P., DeSorcie, T.J., and Stedman, R.M. 1998. Maturity schedules of lake trout in Lake Michigan. J. Great Lakes Res. 24:40–410. Marsden, J.E., and. Janssen, J. 1997. Evidence of lake trout spawning on a deep reef in Lake Michigan using an ROV-based egg collector. J. Great Lakes Res. 23:450–457. ——— , Krueger, C.C., and Schneider, C.P. 1988. Evidence of natural reproduction by stocked lake trout in Lake Ontario. J. Great Lakes Res. 14:3–8. ——— , Casselmann, J.M., Edsall, T.A., Elliott, R.F., Fitzsimons, J.D., Horns, W.H., Manny, B.A., McAughey, S.C., Sly, P., and Swanson, B.L. 1995a. Lake trout spawning habitat in the Great Lakes: a review of current knowledge. J. Great Lakes Res. 21:330–336. ——— , Perkins, D.L., and Krueger, C.C. 1995b. Recog-
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nition of spawning areas by lake trout: deposition and survival of eggs on small, man-made rock piles. J. Great Lakes Res. 21:487–497. ——— , Claramunt, R.M., Ellrot, B.J., Fitzsimons, J.D., and Jonas, J.L. 2005. A comparison of lake trout spawning, fry emergence, and habitat use in Lakes Michigan, Huron, and Champlain. J. Great Lakes Res. 31:492–508. McKee, P.M., Toneys, M.L., Hansen, M.J., and Holey, M.E. 2004. Performance of lake trout stocked in the Midlake Refuge of Lake Michigan. N. Amer. J. Fish. Manage. 24: 1101–1111. Perkins, D.L., and Krueger, C.C. 1994. Design and use of mesh bags to study egg deposition and embryo survival in cobble substrate. N. Am. J. Fish. Manage. 14:866–869. Reid, D.M., Anderson, D.M., and Henderson, B.A. 2001. Restoration of lake trout in Parry Sound, Lake Huron. N. Am. J. Fish. Manage. 21:156–169. Savino, J.F., Hudson, P.L., Fabrizio, M.C., and Bowen II, C.A. 1999. Predation on lake trout eggs and fry: a modeling approach. J. Great Lakes Res. 25:36–44. Scott, W.B., and Crossman, E.J. 1973. Freshwater fishes of Canada. Fisheries Research Board of Canada, Bulletin No 184. Selgeby, J.H., Bronte, C.R., Brown, E.H., Hansen, M.J., Holey, M., VanAmberg, J.P., Muth, K.M., Makauskas, D.B., McKee, P., Anderson, D.M., Ferreri, C.P., and Schram, S.T. 1995. Lake trout restoration in the Great Lakes: stock-size criteria for natural reproduction. J. Great Lakes Res. 21:498–504. Tusting, R.F., and Tietze, T.C. 1995. Two-bucket stand alone suction sampler (TB-SSASS) user’s information manual. Harbor Branch Oceanographic Institution Inc., Tech. Note No. 111, Fort Pierce, FL. Submitted: 18 October 2004 Accepted: 26 July 2006 Editorial handling: Stephen B. Brandt