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Lake Trout Spawning at Five Sites in Ontario Waters of Lake Superior
J. Great Lakes Res. 21 (Supplement 1):202-211 Internat. Assoc. Great Lakes Res., 1995
Lake Trout Spawning at Five Sites in Ontario Waters of Lake Superior John R. M. Kelso,! Wayne R. MacCallum,2 and Marla L. Thibodeau! 1Department
ofFisheries & Oceans, Great Lakes Lab. for Fisheries & Aquatic Sciences, 1 Canal Drive, Sault Ste. Marie, Ontario P6A 6W4
2Lake Superior Management Unit, Ontario Ministry ofNatural Resources, 435 James St. N., Thunder Bay, Ontario P7C 5G6
ABSTRACT. To assess lake trout, Salvelinus namaycush, spawning on natural substrates, we studied five sites in Canadian waters of Lake Superior between 1987 and 1990. Lake trout were reported to spawn historically at three of these sites-Welcome Island, Hare Island, and near the Agawa River. No lake trout eggs were found at the Welcome Island site in 1987 or 1989. Eggs were found at the Agawa site in 1989 but not in 1988. Spawning occurred at Hare Island in 1987 and 1989 but egg deposition rates were < 20 eggs m-2 . Two other sites-Sinclair Cove and No Name Shoal-were studied because lake trout catches were high in gil/nets set during autumn and substrate that might be suitable for spawning was nearby. Lake trout eggs were deposited at rates up to 370 eggs m-2 in Sinclair Cove. No eggs were found at No Name Shoal. Egg deposition greater than 20 eggs m-2 occurred in pebble, cobble and boulder substrates that had interstices extending between 20 and 120 cm below the substrate surface. Eggs were not found in substrates < 2 mm in diameter. Lake trout abundance, inferred from gil/net catches, at these five sites did not vary directly with egg-deposition rates. The site with the highest lake trout egg-deposition, Sinclair Cove A, was comprised of mobile substrate and its location, interstitial depth, and area changed among years presumably in response to wave action. INDEX WORDS:
Lake trout, fish eggs, spawning, habitat, Lake Superior.
lake trout and substrate presumed suitable for spawning. Our objectives were to 1) evaluate current use of three historic lake trout spawning sites, 2) evaluate spawning at two sites with high CPUE of lake trout during the spawning season, 3) evaluate the reliability of gillnet surveys and substrate assessment as tools to identify lake trout spawning sites, and 4) estimate lake trout egg deposition rates (number m-2) in natural Lake Superior habitats.
INTRODUCTION Summaries of historical lake trout spawning sites in the Great Lakes (Goodier 1981a, b; Goodyear et al. 1982) are based largely on anecdotal information. Only 31 or 3.8% of the documented spawning sites in the Great Lakes have been confirmed by detection of reproductive products or direct observation of spawning by either native or stocked lake trout (Thibodeau and Kelso 1990). Krueger et al. (1986) indicated that lake trout abundance was low during spawning season at historically important spawning reefs in Lake Superior and suggested that reefs that did not have remnant native populations were not used for spawning by lake trout. Peck (1986) confirmed spawning in Lake Superior and documented reproduction by hatchery-origin lake trout on a man-made spawning reef in Marquette Harbour. Schreiner et al. (1995) reported trapping lake trout eggs on two small man-made structures in Minnesota waters, but none were trapped on five reputed natural spawning areas. To assess current spawning and to determine characteristics of natural sites used for lake trout spawning in Lake Superior, we studied five sites in Ontario waters. Three sites were historical spawning sites (Goodier 1981a, b) and two sites had high catch-per-unit effort (CPUE) of
Study Site Background Three spawning sites were historically importantHare Island and Welcome Island in Thunder Bay and Agawa in eastern Lake Superior (Fig. 1). Spawning by lake trout occurred historically at Hare Island and Welcome Island, and fishing stations, operated by the Hudson Bay Company, existed on both islands during the autumn from the 1820s until about 1890 (Goodier 1982). Several lake trout "varieties" spawned at both island sites (Goodier 1981b, 1982). Lake trout were reported to spawn later at Hare Island than at other sites in Thunder Bay but spawning was thought to peak in early October (Goodier 1981b). Lake trout may have spawned both north and south of the mouth of the Agawa River (Goodier 1981b), but these fish were not identified as a river-
202
203
Lake Trout Spawning in Ontario Waters of Lake Superior
~o
3
RiV:J
Agawa
1:6480000
4 1:60628
1 WELCOME ISLAND Thunder Bay
2 HARE ISLAND 3 SINCLAIR COVE 4 NN SHOAL
()o
1
o FIG. 1.
o
5AGAWA 2
1:685714
Location of sites where lake trout spawning was studied in Lake Superior.
spawning population by Loftus (1958) who studied riverspawning populations of lake trout in eastern Lake Superior. Lean lake trout were reported to spawn in the Agawa River area, and spawning was presumed to be complete by the end of October (Goodier 198Ia). The other two sites (Fig. I) had high CPUE of lake trout during the spawning season and substrate presumed suitable for spawning within 100 m of the netting site. One site was near the entry to Sinclair Cove; the other site, No Name (NN) Shoal, was in a small bay about 2 km north of the Agawa River. Netting corroborated Goodier (I98Ia, b) in that lake trout probably spawned to the north of the Agawa River.
METHODS Substrate Assessment SCUBA divers first located substrates> 4 mm in diameter at each site. A marked and weighted line was then placed on the lake bottom to guide SCUBA divers, and a swath of the lake bottom at least I m wide, along transects, was taped with a Sony HVC-20800 camcorder (Fig. 2). The marked line (1.3-cm diameter with weights spaced I m apart) served as a reference when substrate size was estimated from the film. To complete the areal estimation of substrate type, divers were towed on the surface in water depths between I and 5 m or swam on
204
Kelso et ale
NN SHOAL N
A 200m
I
SINCLAIR COVE
A,S
HARE
N
ISLAND
A
N
A
N
A ~
[S]
D U ~
§ E§ [ill]
BEDROCK I BOUlDERS
COURSE GRAVEL
~
D
EXPOSED SHOAL/ SHORE UNE
IIOULDER.cosaLE WITH LIGHT SAND rNOR!AY
BOULDER / COBBLE WITH PEB8LE .. FU
COBBlE • VERY EVEN
veGETATION
SAND/SILT
BOUlDER WITH UTTlE COIIIll.E FI.L
f2J
PEB8LE WITH VERY SHALLOW COBSLE
E;]
COBBlE FILLED IN WITH SAND ANlIolUll
.. D
BOULDERS
IIOULDER / COBBLE
FIG. 2. Substrate type, transect locations (solid lines), and sampling areas (A's, B's, and transect lines, see text) for five lake trout-spawning study areas, Lake Superior.
Lake Trout Spawning in Ontario Waters of Lake Superior the lake bottom in depths> 5 m. We surveyed substrates to water depths of 25 m at Welcome Island, Hare Island, and Agawa. At Sinclair Cove and NN Shoal we mapped substrates> 0.6 mm in diameter. These methods to survey substrate conditions approximated those of Nester and Poe (1987) except that taping and extrapolation were assisted by SCUBA divers. Substrate was classified as boulder (> 256), cobble (255-64), pebble (63-4), granular (3.9-2.0), sand (1.9-0.6), or fine materials « 0.6 mm). Substrate size was measured either from the video tape by using the marked line as a reference or by divers using calipers. Substrate classes estimated from the videotape were checked against those measured by divers. The depth to which interstices existed in the substrate was measured by divers. Divers excavated 140 sites at Welcome Island, 80 at Hare Island, 123 at Agawa River, 24 at NN Shoal, and 48 at Sinclair Cove and measured the depth to which interstices occurred with aT-square. Areas occupied by substrates> 2 mm were measured with a surveyor's tape measure at Hare Island, Agawa River, NN Shoal, and Sinclair Cove and measured with a planimeter from our map of substrates at Welcome Island. Our assessment of substrate at spawning sites is an approximation because of our classification which is based on the frequency of excavations to determine depth of interstices, errors in positioning and measurement, and wind-induced changes to the substrate between days of sampling.
Lake Trout Assessment Lake trout were sampled from gillnets set by commercial fishers and ourselves between September and December from 1985 to 1988. Our gillnets (joined panels of stretched-mesh sizes between 6.3 and 13.3 cm) were set on the lake bottom in < 12 m of water (most 2-8 m). Nets were placed over substrate of the type assumed to be suitable for spawning by lake trout from descriptions by Martin and Olver (1980), Wagner (1982), and Thibodeau and Kelso (1990). These substrates were clean, between 4 and 260 mm in diameter, and in wind-exposed areas near deep water. We also sampled lake trout from nets set by commercial fishers near Hare and Welcome islands in waters 10- to 15-m deep and occasionally to 20 m. We examined each lake trout for fin clips or other marks and measured the fork length. All healthy fish were released. Sex of live fish was determined by gentle expulsion of reproductive products, and sex and condition of gonads was reported only when sexual condition was obvious. Sex of dead fish was determined by dissection. Egg Sampling Egg-trap pails, egg-collection trays, and an air-lift sampler were used to collect lake trout eggs. Egg-trap pails (Stauffer 1981, Peck 1986) with 540-cm2 openings were used at Hare Island A and Band Welcome Island A
205
in 1986 and 1987. The egg-trap pails were made from 15L plastic industrial pails with 6.5-cm-diameter holes cut in the sides and bottoms. Hardware cloth (3-mm mesh) was glued over each hole to allow water circulation and to permit drainage when the egg-trap pail was removed. Pairs of egg-trap pails were placed in excavations, filled, and covered with 10-15 cm of excavated substrate. Because pails were 45-cm deep, they were installed where interstices extended to a depth of at least 65 cm. We used egg-collection trays and an air-lift sampler at all sites and in several years. The egg-collection trays were 30x30xlO-cm-deep plastic containers (the bottom segment of industrial milk crates) and were lined with hardware cloth (1.3-mm mesh) attached with black silicone glue. Each tray was filled and covered with 5-15 cm of excavated substrate. Thereafter, egg-collection trays were buried at the five sites in substrates with interstices 15 cm and greater. The air-lift sampler was a modification of those used by MacKay (1972) and Dorr et al. (1981). Compressed air from a SCUBA bottle was injected into a nozzle with a 2.5-cm opening attached to a flexible 7.5-cm-diameter hose. The effluent from the flexible hose was discharged on a 1-mm screen. The sampler was dismantled and residual eggs and debris washed free between samplings. For each air-lift sample, 0.25 m 2 of lake bottom was vacuumed as the substrate was gently removed by a diver. To determine the efficiency of the air-lift sampler, 5-mm-diameter plastic beads, similar in color and size to lake trout eggs (Scott and Crossman 1973) and with a slight negative buoyancy were seeded by a diver in a 0.5 x 0.5-m quadrat. Between 40 and 800 beads were placed within 12 quadrats oIi the lake bottom. Substrate size in the quadrats ranged from 5 to 256 mm with interstices extending from 5 to 60 cm. Average recovery by another diver was 97.3% (N = 12; S.D. = 4) when interstices were < 35 cm. Recovery declined when interstices exceeded > 35 cm because beads moved laterally out of the quadrat as the substrate was removed (fish eggs were seen later to behave similarly). We used the air-lift sampler for substrates where interstices were < 30 cm at Hare Island B and at Sinclair Cove B (Fig. 2). Pail and tray samplers were installed in mid September and removed in early to mid November after spawning was presumed to be complete (CPUEs had declined and spent females were captured). Air-lift samples were also taken after spawning was presumed to be complete. Collected eggs were counted on site, preserved in 10% formalin, and each sample was recounted in the laboratory. Estimates of egg density are only from buried samplers that were not exposed and contained eggs. All egg-trap pails installed at Hare and Welcome islands in 1986 were exposed. Estimates of egg density from air-lift samples are from substrates > 2-mm diameter with interstices 2 cm and greater. Divers haphazardly sampled substrates < 2 mm at each site for at least 0.5 h between depths of 4 and 25 m and did not retrieve any lake trout eggs.
206
Kelso et al. RESULTS
Substrate Conditions Although substrate assessment at Welcome Island indicated that 70.2 ha might be suitable (Fig. 3) for lake trout spawning, the interstices of substrate in waters> 4m deep were filled with sand and fine materials. Interstices among the coarse gravel and boulder located inshore were also filled with fine materials. Only 490 m 2 of the lake bottom had interstices> 10 cm and these were either small areas between 5 and 20 m 2 in crevices in the bedrock to the south or in one larger area (415 m2) to the south of the smallest island (Table I, A in Fig. 2). All egg-trap pails and egg-collection trays were installed at Welcome Island A (Figs. 2, 3). About 6,000 m 2 of habitat near Hare Island (Fig. 2) seemed to be suitable for lake trout spawning and was comprised of clean, rounded material (4- to 260-mm diameter) in water depths of 2 to 5 m (Table 2). Two areas (Fig. 2, Hare Island A, B) had interstices extending to 1.1 m (Fig. 3). Adjacent areas and most of the substrate in waters < 4-m deep had interstices> 10 and < 50 cm. Substrate in waters> 5-m deep was either sand or fine materials. To the north of the Agawa River, an 0.8-km length of large-boulder (> 1 m) shoreline dropped to water depths of 10 m within 5 to 30 m from shore (Figs. 2, 3). Distribution of substrates was heterogeneous as boulders occurred throughout, but the materials between boulders included sand, pebble, and cobble (Table 1). Near shore, the spaces between the large boulders were filled with sand but, when bottom slopes exceeded approximately 15 interstices extended up to 50 cm (Fig. 3). Depth of interstices was variable locally. Repeated visual inspection at biweekly intervals between mid August and the end of November 1987 and 1988 indicated that substrate size and depth of interstices changed at four marked sites at water depths between 3 and 5 m. High winds could fill interstices with sand and silt. Both filling and evacuation of interstices occurred within days depending upon the intensity and direction of prevailing winds. At NN Shoal, four approximately equal-sized areas (Fig. 2) were made up of substrate> 4 cm with interstices extending from 5 to 15 cm (Fig. 3, Table 1). The remainder of this exposed embayment (Fig. 1) consisted of cobble and boulders but depth of interstices were 5-cm or less. At Sinclair Cove, two distinct areas (Sinclair A and B, Fig. 2, Table 1) with similarly clean, rounded pebble, cobble, and boulder substrate (4-260 cm) appeared suitable for spawning by lake trout (Fig. 3). Area A was a flattened shoal with a layer of cobble 1 to 2-m deep on bedrock with interstices extending to 60 cm in the center of the area. Area B (Fig. 2, Fig. 3) was a rounded shoal of similar materials perched on bedrock between two bedrock islands. Interstices extended to 150 cm near the centre of area B, and, near the edge of the shoal, extended to the depth of the cobble. The location of area B 0
,
changed in 1987-1988 and 1988-1989 (Fig. 2) but area A was stable. Divers could hear the substrate at Sinclair Cove B moving during surges that occurred even when winds were light. These five sites were all composed of coarse substrate (> 4 mm) that sloped between 5 and 30 where interstices were deepest (Table I, Fig. 2, Fig. 3). Maximum depths of interstices for these substrates ranged from 15 cm at NN Shoal to at least 150 cm at Sinclair Cove B. Substrate was rounded (Sinclair Cove A, B; NN Shoal; Hare Island) or of mixed rounded and angular materials (Welcome Island, Agawa). Only substrates at Welcome Island showed evidence of obvious colonization by epiphytes, at least during September and October. Water depths over these apparently suitable substrates ranged between 1 and 6.3 m but> 75% of the substrates were at depths < 3.5 m. 0
Lake Trout Assessment In total, 629 lake trout were captured near the five study sites in Lake Superior between 1985 and 1988 (Table 2). The average water depth of capture was 5.4 m and all fish were captured within 30 cm of the gillnet lead line. More males (297) were captured than females (105) and the resulting ratio of males to females was almost 3: 1. Spent females were captured from mid to late October each year but made up only 1.6% (9 fish) of the total catch. Overall, more stocked than native lake trout were captured but that ratio differed from about 0.5 stocked to 1.0 native at the Welcome and Hare islands and to about 1.3 stocked to 1.0 native at the three sites in eastern Lake Superior. Although the Hare Island and Welcome Island areas were sampled intensively only in 1985 (Table 2), we calculated the CPUE for each sampling date between late August and early November for all sites regardless of collection year. The CPUE remained consistently low « 6 lake trout per 100 m of gillnet) near Welcome and Hare islands for the duration of sampling. Conversely, catches on the Agawa site, the NN Shoal, and the Sinclair Cove site increased in early October, reached a maximum of between 10 and 20 lake trout per 100 m of gillnet between 13 and 25 October, and declined thereafter until mid November. Lake Trout Eggs in Lake Superior Substrates Lake trout eggs were found at Hare Island, Agawa, and Sinclair Cove (Table 3). No eggs were found at Welcome Island or NN Shoal in egg-trap pails, egg-collection trays, or air-lift samples. The Hare Island, Welcome Island, Agawa, and Sinclair Cove sites were subject to high winds, wave action, and strong currents as the 10 to 15 cm of substrate we used to cover pail and tray samplers was often gone. Up to 75% of the samplers were exposed or missing. Fine material, sand, and gravel were trapped in egg-collection trays at NN Shoal and Agawa;
Lake Trout Spawning in Ontario Waters of Lake Superior
FIG. 3. Substrates at five sites where lake trout spawning was studied in Lake Superior: Sinclair Cove B (substrate top left; top of shoal, top right), NN Shoal (center left), Agawa River (center right), Hare Island B (bottom left) and Welcome Island A (bottom right), Lake Superior. Scale in photographs is 46-cm long, and cobble in bottom photographs are 80 to 220 mm.
207
208
Kelso et al.
TABLE 1. Area of suitable substrate, maximum water depth over suitable substrate, substrate description, and slope at five sites where lake trout spawning was studied in Lake Superior. Area of suitable substrate* m 2
Maximum water depth over suitable substrate
490
4.9
mixed rounded and jagged cobble epiphyte covered
rounded shoal, variable 5-20
Hare Is.
6,000
6.3
rounded clean cobble variable interstitial depth substrate mobile
15-22
Sinclair Cove
1,950
5.4
two distinct areas of suitable su!;.;;!rate rounded cobble area B
Location Welcome Is.
Incline, degrees
Substrate Description
rounded shoal 5-15 flattened shoal 0-7
(Fig. 2) mobile Agawa
3,760
3.8
mixed rounded cobble, jagged boulders, some infilling with sand
variable shore incline 9-30
150
3.6
four small areas with smooth and jagged cobble
flattened shoal 0-5
NN Shoal
*Suitable substrate was material> 4 mm with interstitial spaces> 10 cm.
TABLE 2. Gill netting effort, mean CPUE, percent female, and origin (% wild) for lake trout caught between early September and mid November during 1985 and 1988 at five sites where lake trout spawning was studied in Lake Superior.
Location
Year
Effort (m)
Hare Island
1985
6,000
Welcome Island
1985
Agawa
Mean CPUE (sd)
Female
Wild
(%)
(%)
5.6 (0.6)
63
40
6,000
1.5 (1.5)
81
69
1988
1,300
4.8 (3.8)
16
40
Sinclair Cove A and B
1988
1,300
6.4
24
48
NN Shoal
1988
1,300
7.8 (7.1)
28
46
thus, it is likely that the depth of interstices varied with weather conditions. Substrate at both Hare Island A and B and Sinclair Cove B was mobile. This movement of substrate resulted in exposure or loss of samplers and changed position of the areas among years. This change
was mapped for Sinclair Cove B but not for Hare Island (Fig. 2). Most eggs were collected at the Sinclair Cove B site, followed by Hare Island, Sinclair Cove A, and Agawa (Table 3). The CPUE indicated that lake trout were most
209
Lake Trout Spawning in Ontario Waters of Lake Superior TABLE 3. 1990.
Method of sampling and mean number of lake trout eggs found at five study sites, Lake Superior, 1987-
Location
Year
Method
Welcome Island
1987 1989
pails trays air-lift
09111 08/11 08/11
12 (8)* 10 (6)* 18
12 10 18
Hare Island
1987 1989
pails trays air-lift
07/11 09111 09/11
23 (7)* 7 (7)* 8
21 3 4
NN Shoal
1988 1989
trays trays air-lift
03111 12/11 12/11
10 (2) 8 16
10+ 8 16
Agawa
1988
trays air-lift trays air-lift
03/11 03/11 11/11 13/11
18 (4)* 32 (2) 18 (4)* 10
18 30+ 18+ 10
trays air-lift air-lift air-lift
04/11 04/11 12/11 09111
12 (2) 10 12 13
11 8 10 10
2 1 2 2
03/11 11/11 12/11 05/11 08/11
16 12 (2)* 10 11 (3)* 8
13 9 7 6 5
18(1-27) 27(3-48) 19(1-34) 144(2-370) « 110(1-277)
1989 Sinclair Cove A
1988 1989 1990
Sinclair CoveB
+
« *
1988 1989
air-lift trays air-lift 1990 trays air-lift tray bottoms covered with up to 5 cm of sand an underestimate as recovery incomplete some samplers partly exposed
abundant at Sinclair Cove, followed by NN Shoal, Agawa River, Hare Island, and Welcome Island. The order of intensity of spawning indicated by our estimates of lake trout egg deposition was not reflected in the order of abundance provided by CPUE. Deposition of lake trout eggs was generally variable both among sites and within spawning areas and ranged from 0 to 370 eggs m-2 (Table 3). Ninety-four percent of all lake trout eggs were fertilized and were in their first to third week of development (Balon 1980). The estimate of lake trout egg deposition rates for Sinclair Cove B in 1990 was an underestimate because we used the air-lift sampler in substrates with interstices > 30 cm and eggs were observed emigrating from the quadrat. When eggcollection trays and the air-lift sampler were used to collect lake trout eggs, the range in estimates of egg densities was generally similar between methods. Differences in egg deposition rates were apparent
No. of samples (no. lost)
Samples without eggs
Eggs m-2 in samples with eggs (range)
Day/month of sampling or sampler retrieval
4 (1-7) 36(4-68) « 25(3-74)
2(1-3)
among years as well as among methods and among sites; however, this phenomenon could result from sampling at different times during the spawning period. We collected more eggs in 1990 than in 1988 or 1989 at Sinclair Cove B. We also collected more eggs at Hare Island in 1989 than in 1987. Few eggs were deposited at Sinclair Cove A during each of 3 years of study, and lake trout eggs were found at the Agawa site only in 1988. In general, lake trout egg deposition exceeded 20 eggs m-2 only in areas with cobble and boulder substrates exposed to high winds and strong currents. These substrates were mobile and the area of highest egg deposition varied annually, probably as the substrate changed location. No eggs were found on pebble, sand, or finer materials. All eggs were found at water depths between 0.5 and 4.5 m although we sampled to depths of 25 m at each site. We could not determine whether egg deposition resulted from spawning by native or stocked lake trout.
Kelso et al.
210 DISCUSSION AND CONCLUSIONS
No lake trout eggs were found at Welcome Island and Agawa in 1987 and 1989. Spawning occurred at Hare Island but egg deposition rates were low. The historic accounts (Goodier 1981 a, b; 1982) of spawning at Welcome Island and Agawa may have been incorrect. However, the stocks using these sites may have been affected by the overall decline in lake trout abundance in the 1950s (Lawrie and Rahrer 1972). Some, perhaps large, proportion of the 260 reputed lake trout spawning areas identified in Lake Superior (Thibodeau and Kelso 1990) are currently not used or were never used for spawning. Other undocumented areas are, however, currently used for spawning by lake trout. Lake trout eggs have been found in varying densities up to ==1,500 eggs m-2 (Martin and Olver 1980, Kelso 1993) in inland lakes and up to 3,572 eggs m-2 in Lake Ontario (Perkins and Krueger 1995). Peck (1986) found up to 518 lake trout eggs m-2 over three spawning seasons in Marquette Harbor, Lake Superior. We found lake trout eggs at densities ranging up to 370 eggs m-2 . The spawning Area B in Sinclair Cove was only 420 m2 and approximately 55% of the total area had egg deposition rates> 100 lake trout eggs m-2 . The majority of our samples from Lake Superior, however, had few eggs and we estimate that, for all sites, there was ==7,000 m2 of substrate with 1 to 50 lake trout eggs m- 2 . Approximately 5,000 m 2 of habitat at these study sites was comprised of substrate > 4 mm in diameter with interstices > 10 cm, was exposed to wind, was near deep water, and was without lake trout eggs. We caught few spent female lake trout in our samples and are uncertain when most lake trout spawn. Goodier (1981b) suggested that spawning activity peaked in early October. Our CPUE peaked in late October, but protracted spawning by lake trout will clearly affect any estimate of egg deposition rates. High gillnet CPUEs occurred at NN Shoal where no spawning was detected. While this result may seem anomalous, in 1990 lake trout eggs were collected from crevices between boulders 40 to 200-cm in diameter at water depths between 1 and 3.6 m about 400 m north of the area we studied. These large substrates would be almost impossible to sample other than qualitatively. Peck (1975, 1977, 1979) and Wagner (1982) recognized the problems associated with using gillnet catches to direct searches for lake trout spawning areas. Unfortunately, confirmation by direct observation of spawning or recovery of reproductive products is required as neither high gillnet CPUEs nor the presence of apparently suitable habitat are definitive. Egg density was low and occurred in a variety of substrates at historical sites (Hare Island and, to a lesser extent, Agawa) and a previously undocumented lake trout spawning site. The area where we found highest egg deposition rates was comprised of mobile substrate. If survival of eggs in mobile substrate is low, any
management technique to enhance congregation of spawning lake trout (Krueger et al. 1986) should include selection of sites where egg survival is optimized.
ACKNOWLEDGMENTS The help of L. Golden, R. Luik, G. Agawa, J. Lipsit, and K. Bray in the field is appreciated. The editorial advice of J. Peck and anonymous reviewers was helpful.
REFERENCES Balon, E.K. 1980. Early ontogeny of the lake charr Salvelinus (Cristivomer) namaycush. In Charrs: salmonid fishes of the genus Salvelinus, ed. E.K. Balon, pp. 485-562. The Hague, The Netherlands: Dr. W. Junk Pub!. Dorr, J.A. III, O'Connor, D.V., Foster, N.R., and Jude, D.J. 1981. Substrate conditions and abundance of lake trout eggs in a traditional spawning area in southeastern Lake Michigan. N. Am. J. Fish. Manage. 1:165-172. Eshenroder, R.L., Poe, T.P., and Olver, C.H. 1984. Strategies for rehabilitation of lake trout in the Great Lakes. In Proceedings of a conference on lake trout research (CLAR), August, 1983. Great Lakes Fish. Comm. Tech. Rep. No. 40. Goodier, J.L. 1981a. Native lake trout (Salvelinus namaycush) stocks in the Canadian waters of Lake Superior prior to 1955. Can. J. Fish. Aquat. Sci. 38:1724-1737. _ _ _. 1981b. Native lake trout (Salvelinus namaycush) stocks in the Canadian waters of Lake Superior prior to 1955. M.Sc. thesis. University of Toronto, Toronto, Ontario. _ _ _. 1982. The fish and fisheries of Canadian Lake Superior. Instit. for Env. Studies, Univ.of Toronto. Goodyear, C.S., Edsall, T.A., Ormsby-Dempsey, D.M., Ross, G.D., and Polanski, P.E. 1982. Atlas of the spawning and nursery areas of Great Lakes fishes. Vols. 1-13. U.S. Fish. Wild!. Service, Wash. D.C. FWS/OBS-82/52. Kelso, J.R.M. 1995. The relation between reproductive capacity of a lake trout population and the apparent availability of spawning habitat in Megisan Lake, Ontario. J. Great Lakes Res. 21(Supplement 1):212-217. Krueger, C.c., Swanson, B.L., and Selgeby, J.H. 1986. Evaluation of hatchery-reared lake trout for reestablishment of populations in the Apostle Islands region of Lake Superior, 1960--84. In Fish Culture in Fisheries Management, ed. R. H. Strand, pp. 93-107. Bethesda: American Fisheries Society. Lawrie, A.H., and Rahrer, J.F. 1972. Lake Superior: effects of exploitation and introductions on the salmonid community. J. Fish. Res. Board Can. 29:765-776. Loftus, K.H. 1958. Studies on river spawning lake trout in eastern Lake Superior. Trans. Am. Fish. Soc. 87:259-277. MacKljY, A.P. 1972. An air lift for sampling freshwater benthos. OIKOS 23:413-415. Martin, N.V., and Olver, C.H. 1980. The lake charr, Salvelinus namaycush. In Charrs: salmonidfishes of the genus Salvelinus, ed. E.K. Balon, pp. 205-277. The Hague, The Netherlands: Dr. W. Junk Pub!. Nester, R.T. and Poe, T.P. 1987. Visual observations of histori-
Lake Trout Spawning in Ontario Waters of Lake Superior cal lake trout spawning grounds in western Lake Huron. N. Am. J. Fish. Manage. 7:418--424. Peck, J.W. 1975. Location of lake trout spawning in Lake Superior and Lake Michigan. Michigan Dept. of Natural Resources Annual Prog. Rep. Dingell-Johnson Proj. No. F-35-R-1. Study No.5, pp. 171-179. _ _ _. 1977. Location of lake trout spawning in Lake Superior and Lake Michigan. Michigan Dept. of Natural Resources Annual Prog. Rep. Dingell-Johnson Proj. No. F-35-R-3, pp.137-150. ____. 1979. Lake trout reproduction on a man-made spawning reef Michigan Dept. of Natural Resources Fish. Res. Rep. No. 1871. _ _ _. 1986. Dynamics of reproduction by hatchery lake trout on a man-made spawning reef. J. Great Lakes Res. 12:293-303. Perkins, D.L., and Krueger, e.C. 1995. Dynamics of reproduc-
211
tion by hatchery-origin lake trout (Salvelinus namaycush) at Stony Island Reef, Lake Ontario. J. Great Lakes Res. 21 (Supplement 1):400--417. Schreiner, D. R., Bronte, C. R., Payne, N. R., Fitzsimons, J. D., and Casselman, J. M. 1995. Use of egg traps to investigate lake trout spawning in the Great Lakes. J. Great Lakes Res. 21 (Supplement 1):433--444. Scott, W.B., and Crossman, E.J. 1973. Freshwater fishes of Canada. Fish. Res. Board of Can. Bull. No. 184. Stauffer, T.M. 1981. Collecting gear for lake trout eggs and fry. Prog. Fish. Cult. 43: 186-193. Thibodeau, M.L., and Kelso, J.R.M. 1990. An evaluation of putative lake trout (Salvelinue namaycush) spawning sites in the Great Lakes. Can. Tech. Rep. Fish. Aquat. Sci. 1739. Wagner, W.e. 1982. Lake trout spawning habitat in the Great Lakes. Michigan Dept. of Natural Resources Fish. Div. Fish. Res. Rep. 1904.
Lake Trout Spawning at Five Sites in Ontario Waters of Lake Superior John R. M. Kelso,! Wayne R. MacCallum,2 and Marla L. Thibodeau! 1Department
ofFisheries & Oceans, Great Lakes Lab. for Fisheries & Aquatic Sciences, 1 Canal Drive, Sault Ste. Marie, Ontario P6A 6W4
2Lake Superior Management Unit, Ontario Ministry ofNatural Resources, 435 James St. N., Thunder Bay, Ontario P7C 5G6
ABSTRACT. To assess lake trout, Salvelinus namaycush, spawning on natural substrates, we studied five sites in Canadian waters of Lake Superior between 1987 and 1990. Lake trout were reported to spawn historically at three of these sites-Welcome Island, Hare Island, and near the Agawa River. No lake trout eggs were found at the Welcome Island site in 1987 or 1989. Eggs were found at the Agawa site in 1989 but not in 1988. Spawning occurred at Hare Island in 1987 and 1989 but egg deposition rates were < 20 eggs m-2 . Two other sites-Sinclair Cove and No Name Shoal-were studied because lake trout catches were high in gil/nets set during autumn and substrate that might be suitable for spawning was nearby. Lake trout eggs were deposited at rates up to 370 eggs m-2 in Sinclair Cove. No eggs were found at No Name Shoal. Egg deposition greater than 20 eggs m-2 occurred in pebble, cobble and boulder substrates that had interstices extending between 20 and 120 cm below the substrate surface. Eggs were not found in substrates < 2 mm in diameter. Lake trout abundance, inferred from gil/net catches, at these five sites did not vary directly with egg-deposition rates. The site with the highest lake trout egg-deposition, Sinclair Cove A, was comprised of mobile substrate and its location, interstitial depth, and area changed among years presumably in response to wave action. INDEX WORDS:
Lake trout, fish eggs, spawning, habitat, Lake Superior.
lake trout and substrate presumed suitable for spawning. Our objectives were to 1) evaluate current use of three historic lake trout spawning sites, 2) evaluate spawning at two sites with high CPUE of lake trout during the spawning season, 3) evaluate the reliability of gillnet surveys and substrate assessment as tools to identify lake trout spawning sites, and 4) estimate lake trout egg deposition rates (number m-2) in natural Lake Superior habitats.
INTRODUCTION Summaries of historical lake trout spawning sites in the Great Lakes (Goodier 1981a, b; Goodyear et al. 1982) are based largely on anecdotal information. Only 31 or 3.8% of the documented spawning sites in the Great Lakes have been confirmed by detection of reproductive products or direct observation of spawning by either native or stocked lake trout (Thibodeau and Kelso 1990). Krueger et al. (1986) indicated that lake trout abundance was low during spawning season at historically important spawning reefs in Lake Superior and suggested that reefs that did not have remnant native populations were not used for spawning by lake trout. Peck (1986) confirmed spawning in Lake Superior and documented reproduction by hatchery-origin lake trout on a man-made spawning reef in Marquette Harbour. Schreiner et al. (1995) reported trapping lake trout eggs on two small man-made structures in Minnesota waters, but none were trapped on five reputed natural spawning areas. To assess current spawning and to determine characteristics of natural sites used for lake trout spawning in Lake Superior, we studied five sites in Ontario waters. Three sites were historical spawning sites (Goodier 1981a, b) and two sites had high catch-per-unit effort (CPUE) of
Study Site Background Three spawning sites were historically importantHare Island and Welcome Island in Thunder Bay and Agawa in eastern Lake Superior (Fig. 1). Spawning by lake trout occurred historically at Hare Island and Welcome Island, and fishing stations, operated by the Hudson Bay Company, existed on both islands during the autumn from the 1820s until about 1890 (Goodier 1982). Several lake trout "varieties" spawned at both island sites (Goodier 1981b, 1982). Lake trout were reported to spawn later at Hare Island than at other sites in Thunder Bay but spawning was thought to peak in early October (Goodier 1981b). Lake trout may have spawned both north and south of the mouth of the Agawa River (Goodier 1981b), but these fish were not identified as a river-
202
203
Lake Trout Spawning in Ontario Waters of Lake Superior
~o
3
RiV:J
Agawa
1:6480000
4 1:60628
1 WELCOME ISLAND Thunder Bay
2 HARE ISLAND 3 SINCLAIR COVE 4 NN SHOAL
()o
1
o FIG. 1.
o
5AGAWA 2
1:685714
Location of sites where lake trout spawning was studied in Lake Superior.
spawning population by Loftus (1958) who studied riverspawning populations of lake trout in eastern Lake Superior. Lean lake trout were reported to spawn in the Agawa River area, and spawning was presumed to be complete by the end of October (Goodier 198Ia). The other two sites (Fig. I) had high CPUE of lake trout during the spawning season and substrate presumed suitable for spawning within 100 m of the netting site. One site was near the entry to Sinclair Cove; the other site, No Name (NN) Shoal, was in a small bay about 2 km north of the Agawa River. Netting corroborated Goodier (I98Ia, b) in that lake trout probably spawned to the north of the Agawa River.
METHODS Substrate Assessment SCUBA divers first located substrates> 4 mm in diameter at each site. A marked and weighted line was then placed on the lake bottom to guide SCUBA divers, and a swath of the lake bottom at least I m wide, along transects, was taped with a Sony HVC-20800 camcorder (Fig. 2). The marked line (1.3-cm diameter with weights spaced I m apart) served as a reference when substrate size was estimated from the film. To complete the areal estimation of substrate type, divers were towed on the surface in water depths between I and 5 m or swam on
204
Kelso et ale
NN SHOAL N
A 200m
I
SINCLAIR COVE
A,S
HARE
N
ISLAND
A
N
A
N
A ~
[S]
D U ~
§ E§ [ill]
BEDROCK I BOUlDERS
COURSE GRAVEL
~
D
EXPOSED SHOAL/ SHORE UNE
IIOULDER.cosaLE WITH LIGHT SAND rNOR!AY
BOULDER / COBBLE WITH PEB8LE .. FU
COBBlE • VERY EVEN
veGETATION
SAND/SILT
BOUlDER WITH UTTlE COIIIll.E FI.L
f2J
PEB8LE WITH VERY SHALLOW COBSLE
E;]
COBBlE FILLED IN WITH SAND ANlIolUll
.. D
BOULDERS
IIOULDER / COBBLE
FIG. 2. Substrate type, transect locations (solid lines), and sampling areas (A's, B's, and transect lines, see text) for five lake trout-spawning study areas, Lake Superior.
Lake Trout Spawning in Ontario Waters of Lake Superior the lake bottom in depths> 5 m. We surveyed substrates to water depths of 25 m at Welcome Island, Hare Island, and Agawa. At Sinclair Cove and NN Shoal we mapped substrates> 0.6 mm in diameter. These methods to survey substrate conditions approximated those of Nester and Poe (1987) except that taping and extrapolation were assisted by SCUBA divers. Substrate was classified as boulder (> 256), cobble (255-64), pebble (63-4), granular (3.9-2.0), sand (1.9-0.6), or fine materials « 0.6 mm). Substrate size was measured either from the video tape by using the marked line as a reference or by divers using calipers. Substrate classes estimated from the videotape were checked against those measured by divers. The depth to which interstices existed in the substrate was measured by divers. Divers excavated 140 sites at Welcome Island, 80 at Hare Island, 123 at Agawa River, 24 at NN Shoal, and 48 at Sinclair Cove and measured the depth to which interstices occurred with aT-square. Areas occupied by substrates> 2 mm were measured with a surveyor's tape measure at Hare Island, Agawa River, NN Shoal, and Sinclair Cove and measured with a planimeter from our map of substrates at Welcome Island. Our assessment of substrate at spawning sites is an approximation because of our classification which is based on the frequency of excavations to determine depth of interstices, errors in positioning and measurement, and wind-induced changes to the substrate between days of sampling.
Lake Trout Assessment Lake trout were sampled from gillnets set by commercial fishers and ourselves between September and December from 1985 to 1988. Our gillnets (joined panels of stretched-mesh sizes between 6.3 and 13.3 cm) were set on the lake bottom in < 12 m of water (most 2-8 m). Nets were placed over substrate of the type assumed to be suitable for spawning by lake trout from descriptions by Martin and Olver (1980), Wagner (1982), and Thibodeau and Kelso (1990). These substrates were clean, between 4 and 260 mm in diameter, and in wind-exposed areas near deep water. We also sampled lake trout from nets set by commercial fishers near Hare and Welcome islands in waters 10- to 15-m deep and occasionally to 20 m. We examined each lake trout for fin clips or other marks and measured the fork length. All healthy fish were released. Sex of live fish was determined by gentle expulsion of reproductive products, and sex and condition of gonads was reported only when sexual condition was obvious. Sex of dead fish was determined by dissection. Egg Sampling Egg-trap pails, egg-collection trays, and an air-lift sampler were used to collect lake trout eggs. Egg-trap pails (Stauffer 1981, Peck 1986) with 540-cm2 openings were used at Hare Island A and Band Welcome Island A
205
in 1986 and 1987. The egg-trap pails were made from 15L plastic industrial pails with 6.5-cm-diameter holes cut in the sides and bottoms. Hardware cloth (3-mm mesh) was glued over each hole to allow water circulation and to permit drainage when the egg-trap pail was removed. Pairs of egg-trap pails were placed in excavations, filled, and covered with 10-15 cm of excavated substrate. Because pails were 45-cm deep, they were installed where interstices extended to a depth of at least 65 cm. We used egg-collection trays and an air-lift sampler at all sites and in several years. The egg-collection trays were 30x30xlO-cm-deep plastic containers (the bottom segment of industrial milk crates) and were lined with hardware cloth (1.3-mm mesh) attached with black silicone glue. Each tray was filled and covered with 5-15 cm of excavated substrate. Thereafter, egg-collection trays were buried at the five sites in substrates with interstices 15 cm and greater. The air-lift sampler was a modification of those used by MacKay (1972) and Dorr et al. (1981). Compressed air from a SCUBA bottle was injected into a nozzle with a 2.5-cm opening attached to a flexible 7.5-cm-diameter hose. The effluent from the flexible hose was discharged on a 1-mm screen. The sampler was dismantled and residual eggs and debris washed free between samplings. For each air-lift sample, 0.25 m 2 of lake bottom was vacuumed as the substrate was gently removed by a diver. To determine the efficiency of the air-lift sampler, 5-mm-diameter plastic beads, similar in color and size to lake trout eggs (Scott and Crossman 1973) and with a slight negative buoyancy were seeded by a diver in a 0.5 x 0.5-m quadrat. Between 40 and 800 beads were placed within 12 quadrats oIi the lake bottom. Substrate size in the quadrats ranged from 5 to 256 mm with interstices extending from 5 to 60 cm. Average recovery by another diver was 97.3% (N = 12; S.D. = 4) when interstices were < 35 cm. Recovery declined when interstices exceeded > 35 cm because beads moved laterally out of the quadrat as the substrate was removed (fish eggs were seen later to behave similarly). We used the air-lift sampler for substrates where interstices were < 30 cm at Hare Island B and at Sinclair Cove B (Fig. 2). Pail and tray samplers were installed in mid September and removed in early to mid November after spawning was presumed to be complete (CPUEs had declined and spent females were captured). Air-lift samples were also taken after spawning was presumed to be complete. Collected eggs were counted on site, preserved in 10% formalin, and each sample was recounted in the laboratory. Estimates of egg density are only from buried samplers that were not exposed and contained eggs. All egg-trap pails installed at Hare and Welcome islands in 1986 were exposed. Estimates of egg density from air-lift samples are from substrates > 2-mm diameter with interstices 2 cm and greater. Divers haphazardly sampled substrates < 2 mm at each site for at least 0.5 h between depths of 4 and 25 m and did not retrieve any lake trout eggs.
206
Kelso et al. RESULTS
Substrate Conditions Although substrate assessment at Welcome Island indicated that 70.2 ha might be suitable (Fig. 3) for lake trout spawning, the interstices of substrate in waters> 4m deep were filled with sand and fine materials. Interstices among the coarse gravel and boulder located inshore were also filled with fine materials. Only 490 m 2 of the lake bottom had interstices> 10 cm and these were either small areas between 5 and 20 m 2 in crevices in the bedrock to the south or in one larger area (415 m2) to the south of the smallest island (Table I, A in Fig. 2). All egg-trap pails and egg-collection trays were installed at Welcome Island A (Figs. 2, 3). About 6,000 m 2 of habitat near Hare Island (Fig. 2) seemed to be suitable for lake trout spawning and was comprised of clean, rounded material (4- to 260-mm diameter) in water depths of 2 to 5 m (Table 2). Two areas (Fig. 2, Hare Island A, B) had interstices extending to 1.1 m (Fig. 3). Adjacent areas and most of the substrate in waters < 4-m deep had interstices> 10 and < 50 cm. Substrate in waters> 5-m deep was either sand or fine materials. To the north of the Agawa River, an 0.8-km length of large-boulder (> 1 m) shoreline dropped to water depths of 10 m within 5 to 30 m from shore (Figs. 2, 3). Distribution of substrates was heterogeneous as boulders occurred throughout, but the materials between boulders included sand, pebble, and cobble (Table 1). Near shore, the spaces between the large boulders were filled with sand but, when bottom slopes exceeded approximately 15 interstices extended up to 50 cm (Fig. 3). Depth of interstices was variable locally. Repeated visual inspection at biweekly intervals between mid August and the end of November 1987 and 1988 indicated that substrate size and depth of interstices changed at four marked sites at water depths between 3 and 5 m. High winds could fill interstices with sand and silt. Both filling and evacuation of interstices occurred within days depending upon the intensity and direction of prevailing winds. At NN Shoal, four approximately equal-sized areas (Fig. 2) were made up of substrate> 4 cm with interstices extending from 5 to 15 cm (Fig. 3, Table 1). The remainder of this exposed embayment (Fig. 1) consisted of cobble and boulders but depth of interstices were 5-cm or less. At Sinclair Cove, two distinct areas (Sinclair A and B, Fig. 2, Table 1) with similarly clean, rounded pebble, cobble, and boulder substrate (4-260 cm) appeared suitable for spawning by lake trout (Fig. 3). Area A was a flattened shoal with a layer of cobble 1 to 2-m deep on bedrock with interstices extending to 60 cm in the center of the area. Area B (Fig. 2, Fig. 3) was a rounded shoal of similar materials perched on bedrock between two bedrock islands. Interstices extended to 150 cm near the centre of area B, and, near the edge of the shoal, extended to the depth of the cobble. The location of area B 0
,
changed in 1987-1988 and 1988-1989 (Fig. 2) but area A was stable. Divers could hear the substrate at Sinclair Cove B moving during surges that occurred even when winds were light. These five sites were all composed of coarse substrate (> 4 mm) that sloped between 5 and 30 where interstices were deepest (Table I, Fig. 2, Fig. 3). Maximum depths of interstices for these substrates ranged from 15 cm at NN Shoal to at least 150 cm at Sinclair Cove B. Substrate was rounded (Sinclair Cove A, B; NN Shoal; Hare Island) or of mixed rounded and angular materials (Welcome Island, Agawa). Only substrates at Welcome Island showed evidence of obvious colonization by epiphytes, at least during September and October. Water depths over these apparently suitable substrates ranged between 1 and 6.3 m but> 75% of the substrates were at depths < 3.5 m. 0
Lake Trout Assessment In total, 629 lake trout were captured near the five study sites in Lake Superior between 1985 and 1988 (Table 2). The average water depth of capture was 5.4 m and all fish were captured within 30 cm of the gillnet lead line. More males (297) were captured than females (105) and the resulting ratio of males to females was almost 3: 1. Spent females were captured from mid to late October each year but made up only 1.6% (9 fish) of the total catch. Overall, more stocked than native lake trout were captured but that ratio differed from about 0.5 stocked to 1.0 native at the Welcome and Hare islands and to about 1.3 stocked to 1.0 native at the three sites in eastern Lake Superior. Although the Hare Island and Welcome Island areas were sampled intensively only in 1985 (Table 2), we calculated the CPUE for each sampling date between late August and early November for all sites regardless of collection year. The CPUE remained consistently low « 6 lake trout per 100 m of gillnet) near Welcome and Hare islands for the duration of sampling. Conversely, catches on the Agawa site, the NN Shoal, and the Sinclair Cove site increased in early October, reached a maximum of between 10 and 20 lake trout per 100 m of gillnet between 13 and 25 October, and declined thereafter until mid November. Lake Trout Eggs in Lake Superior Substrates Lake trout eggs were found at Hare Island, Agawa, and Sinclair Cove (Table 3). No eggs were found at Welcome Island or NN Shoal in egg-trap pails, egg-collection trays, or air-lift samples. The Hare Island, Welcome Island, Agawa, and Sinclair Cove sites were subject to high winds, wave action, and strong currents as the 10 to 15 cm of substrate we used to cover pail and tray samplers was often gone. Up to 75% of the samplers were exposed or missing. Fine material, sand, and gravel were trapped in egg-collection trays at NN Shoal and Agawa;
Lake Trout Spawning in Ontario Waters of Lake Superior
FIG. 3. Substrates at five sites where lake trout spawning was studied in Lake Superior: Sinclair Cove B (substrate top left; top of shoal, top right), NN Shoal (center left), Agawa River (center right), Hare Island B (bottom left) and Welcome Island A (bottom right), Lake Superior. Scale in photographs is 46-cm long, and cobble in bottom photographs are 80 to 220 mm.
207
208
Kelso et al.
TABLE 1. Area of suitable substrate, maximum water depth over suitable substrate, substrate description, and slope at five sites where lake trout spawning was studied in Lake Superior. Area of suitable substrate* m 2
Maximum water depth over suitable substrate
490
4.9
mixed rounded and jagged cobble epiphyte covered
rounded shoal, variable 5-20
Hare Is.
6,000
6.3
rounded clean cobble variable interstitial depth substrate mobile
15-22
Sinclair Cove
1,950
5.4
two distinct areas of suitable su!;.;;!rate rounded cobble area B
Location Welcome Is.
Incline, degrees
Substrate Description
rounded shoal 5-15 flattened shoal 0-7
(Fig. 2) mobile Agawa
3,760
3.8
mixed rounded cobble, jagged boulders, some infilling with sand
variable shore incline 9-30
150
3.6
four small areas with smooth and jagged cobble
flattened shoal 0-5
NN Shoal
*Suitable substrate was material> 4 mm with interstitial spaces> 10 cm.
TABLE 2. Gill netting effort, mean CPUE, percent female, and origin (% wild) for lake trout caught between early September and mid November during 1985 and 1988 at five sites where lake trout spawning was studied in Lake Superior.
Location
Year
Effort (m)
Hare Island
1985
6,000
Welcome Island
1985
Agawa
Mean CPUE (sd)
Female
Wild
(%)
(%)
5.6 (0.6)
63
40
6,000
1.5 (1.5)
81
69
1988
1,300
4.8 (3.8)
16
40
Sinclair Cove A and B
1988
1,300
6.4
24
48
NN Shoal
1988
1,300
7.8 (7.1)
28
46
thus, it is likely that the depth of interstices varied with weather conditions. Substrate at both Hare Island A and B and Sinclair Cove B was mobile. This movement of substrate resulted in exposure or loss of samplers and changed position of the areas among years. This change
was mapped for Sinclair Cove B but not for Hare Island (Fig. 2). Most eggs were collected at the Sinclair Cove B site, followed by Hare Island, Sinclair Cove A, and Agawa (Table 3). The CPUE indicated that lake trout were most
209
Lake Trout Spawning in Ontario Waters of Lake Superior TABLE 3. 1990.
Method of sampling and mean number of lake trout eggs found at five study sites, Lake Superior, 1987-
Location
Year
Method
Welcome Island
1987 1989
pails trays air-lift
09111 08/11 08/11
12 (8)* 10 (6)* 18
12 10 18
Hare Island
1987 1989
pails trays air-lift
07/11 09111 09/11
23 (7)* 7 (7)* 8
21 3 4
NN Shoal
1988 1989
trays trays air-lift
03111 12/11 12/11
10 (2) 8 16
10+ 8 16
Agawa
1988
trays air-lift trays air-lift
03/11 03/11 11/11 13/11
18 (4)* 32 (2) 18 (4)* 10
18 30+ 18+ 10
trays air-lift air-lift air-lift
04/11 04/11 12/11 09111
12 (2) 10 12 13
11 8 10 10
2 1 2 2
03/11 11/11 12/11 05/11 08/11
16 12 (2)* 10 11 (3)* 8
13 9 7 6 5
18(1-27) 27(3-48) 19(1-34) 144(2-370) « 110(1-277)
1989 Sinclair Cove A
1988 1989 1990
Sinclair CoveB
+
« *
1988 1989
air-lift trays air-lift 1990 trays air-lift tray bottoms covered with up to 5 cm of sand an underestimate as recovery incomplete some samplers partly exposed
abundant at Sinclair Cove, followed by NN Shoal, Agawa River, Hare Island, and Welcome Island. The order of intensity of spawning indicated by our estimates of lake trout egg deposition was not reflected in the order of abundance provided by CPUE. Deposition of lake trout eggs was generally variable both among sites and within spawning areas and ranged from 0 to 370 eggs m-2 (Table 3). Ninety-four percent of all lake trout eggs were fertilized and were in their first to third week of development (Balon 1980). The estimate of lake trout egg deposition rates for Sinclair Cove B in 1990 was an underestimate because we used the air-lift sampler in substrates with interstices > 30 cm and eggs were observed emigrating from the quadrat. When eggcollection trays and the air-lift sampler were used to collect lake trout eggs, the range in estimates of egg densities was generally similar between methods. Differences in egg deposition rates were apparent
No. of samples (no. lost)
Samples without eggs
Eggs m-2 in samples with eggs (range)
Day/month of sampling or sampler retrieval
4 (1-7) 36(4-68) « 25(3-74)
2(1-3)
among years as well as among methods and among sites; however, this phenomenon could result from sampling at different times during the spawning period. We collected more eggs in 1990 than in 1988 or 1989 at Sinclair Cove B. We also collected more eggs at Hare Island in 1989 than in 1987. Few eggs were deposited at Sinclair Cove A during each of 3 years of study, and lake trout eggs were found at the Agawa site only in 1988. In general, lake trout egg deposition exceeded 20 eggs m-2 only in areas with cobble and boulder substrates exposed to high winds and strong currents. These substrates were mobile and the area of highest egg deposition varied annually, probably as the substrate changed location. No eggs were found on pebble, sand, or finer materials. All eggs were found at water depths between 0.5 and 4.5 m although we sampled to depths of 25 m at each site. We could not determine whether egg deposition resulted from spawning by native or stocked lake trout.
Kelso et al.
210 DISCUSSION AND CONCLUSIONS
No lake trout eggs were found at Welcome Island and Agawa in 1987 and 1989. Spawning occurred at Hare Island but egg deposition rates were low. The historic accounts (Goodier 1981 a, b; 1982) of spawning at Welcome Island and Agawa may have been incorrect. However, the stocks using these sites may have been affected by the overall decline in lake trout abundance in the 1950s (Lawrie and Rahrer 1972). Some, perhaps large, proportion of the 260 reputed lake trout spawning areas identified in Lake Superior (Thibodeau and Kelso 1990) are currently not used or were never used for spawning. Other undocumented areas are, however, currently used for spawning by lake trout. Lake trout eggs have been found in varying densities up to ==1,500 eggs m-2 (Martin and Olver 1980, Kelso 1993) in inland lakes and up to 3,572 eggs m-2 in Lake Ontario (Perkins and Krueger 1995). Peck (1986) found up to 518 lake trout eggs m-2 over three spawning seasons in Marquette Harbor, Lake Superior. We found lake trout eggs at densities ranging up to 370 eggs m-2 . The spawning Area B in Sinclair Cove was only 420 m2 and approximately 55% of the total area had egg deposition rates> 100 lake trout eggs m-2 . The majority of our samples from Lake Superior, however, had few eggs and we estimate that, for all sites, there was ==7,000 m2 of substrate with 1 to 50 lake trout eggs m- 2 . Approximately 5,000 m 2 of habitat at these study sites was comprised of substrate > 4 mm in diameter with interstices > 10 cm, was exposed to wind, was near deep water, and was without lake trout eggs. We caught few spent female lake trout in our samples and are uncertain when most lake trout spawn. Goodier (1981b) suggested that spawning activity peaked in early October. Our CPUE peaked in late October, but protracted spawning by lake trout will clearly affect any estimate of egg deposition rates. High gillnet CPUEs occurred at NN Shoal where no spawning was detected. While this result may seem anomalous, in 1990 lake trout eggs were collected from crevices between boulders 40 to 200-cm in diameter at water depths between 1 and 3.6 m about 400 m north of the area we studied. These large substrates would be almost impossible to sample other than qualitatively. Peck (1975, 1977, 1979) and Wagner (1982) recognized the problems associated with using gillnet catches to direct searches for lake trout spawning areas. Unfortunately, confirmation by direct observation of spawning or recovery of reproductive products is required as neither high gillnet CPUEs nor the presence of apparently suitable habitat are definitive. Egg density was low and occurred in a variety of substrates at historical sites (Hare Island and, to a lesser extent, Agawa) and a previously undocumented lake trout spawning site. The area where we found highest egg deposition rates was comprised of mobile substrate. If survival of eggs in mobile substrate is low, any
management technique to enhance congregation of spawning lake trout (Krueger et al. 1986) should include selection of sites where egg survival is optimized.
ACKNOWLEDGMENTS The help of L. Golden, R. Luik, G. Agawa, J. Lipsit, and K. Bray in the field is appreciated. The editorial advice of J. Peck and anonymous reviewers was helpful.
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