Survival and Development of Lake Trout Eggs and Fry in Eastern Lake Ontario — in situ Incubation, Yorkshire Bar, 1989–1993

J. Great Lakes Res. 21 (Supplement 1);384-399 Internat. Assoc. Great Lakes Res., 1995

Survival and Development of Lake Trout Eggs and Fry in Eastern Lake Ontario - in situ Incubation, Yorkshire Bar, 1989-1993 John M. Casselman Ontario Ministry ofNatural Resources Research, Science, and Technology Branch Aquatic Ecosystems Research Section Glenora Fisheries Station R. R. 4, Picton, Ontario KOK 2TO

ABSTRACT. Restricted production of lake trout fry in eastern Lake Ontario limits recruitment and the establishment of a self-sustaining population. To examine the survival and development of eggs and fry, incubation studies were conducted from 1989 to 1993 in situ on Yorkshire Bar and in the laboratory with untreated surface water from Lake on the Mountain. Eggs were collected at spawning time (16 October-9 November, median 24 October) from Lakes Ontario and Manitou, fertilized, and held individually for an average of 162 days in 290 50-cell Plexiglas incubation chambers. On Yorkshire Bar, significantly more eggs died (P < 0.001, 1.65x mean absolute difference (Ltx) = 14%), and regardless of egg source, there were fewer hatched fry than in the laboratory (P < 0.001, 0.63x, L1:x = 29%); 75% of the fry that hatched died early in the incubation period, and 42% of these completely decomposed and disappeared by the end of the incubation period, leaving empty incubation cells. Significantly fewer live fry were produced on Yorkshire Bar than in the laboratory (11.2% versus 69.2%, L1:x = 58%). Abundance of live fry was inversely related to cumulative thermal units (CTU). Increased exposure on the bar from 370 to 690 CTU decreased survival from 25% to O. In eastern Lake Ontario, spawning has been observed at a mean water temperature of 11.5°C (29 October) but ranged between 12. 7°C (19 October) and 8.SOC (14 November); fry survival on 1 May from these dates and temperatures would be 10%, 0%, and 21%, respectively. If the mean temperature at spawning were 2°C lower (9.5°C, 9 days later), production offry on 1 May would be almost double (19% vs.l0%). Spawning at lower temperatures would increase fry production on shoals like Yorkshire Bar. Temperature inversely affects fry survival, especially if the spawning substrate is degraded by organic sedimentation, which causes increased biological oxygen demand and reduces oxygen concentrations. INDEX WORDS:

Lake trout, fish eggs, fry, incubation, temperature, mortality.

ern basin of Lake Ontario on historic spawning sites such as Yorkshire Bar (Goodyear et al. 1981) and deposit large numbers of eggs. A high percentage of these eggs (50%-60%) are either unfertilized or dead at the end of the spawning period in mid-November (Casselman 1991, unpublished data). At the Stony Island site, naturally produced fry have been captured consistently in emergent traps each spring from 1986 to the present (Marsden et al. 1988, Marsden and Krueger 1991, Perkins and Krueger 1995). Fry production, however, appears to be restricted: a similar type of emergent trapping on Yorkshire Bar in early spring in 5 of the 6 years since 1988 has caught only two fry, one in 1989 and the other in 1990 (Casselman, unpublished data). Fry production in eastern Lake Ontario is limited geographically and in numbers. In recent years, indexing programs have collected only a few "unmarked" lake trout (not finclipped or coded-wire tagged) that are yearling or older. Since 1988, 40 to 50 of these unmarked

INTRODUCTION Native lake trout disappeared from Lake Ontario in the mid-1950s (Christie 1973). To rehabilitate this important large keystone predator, large numbers of hatchery fish have been stocked in Lake Ontario in recent years (Schneider et al. 1983). Intensive stocking commenced in both United States and Canadian waters in the mid-1970s (Elrod et al. 1995); by 1992, 10.7 million yearlings had been stocked in Ontario waters (Bowlby and LeTendre 1993). Since 1984, stocking has produced a large standing stock of mature hatchery lake trout in the eastern outlet basin of Lake Ontario. In 1986, naturally deposited lake trout eggs were collected from artificial substrate placed in the cobbles and boulders at the north tip of Yorkshire and Stony islands (Sly 1988). Although spawning habitat in Lake Ontario may not be plentiful (Thibodeau and Kelso 1990), mature hatchery lake trout now congregate at spawning time in the east-

384

Survival and Development of Lake Trout Eggs and Fry

385

In 1988, I began a set of in situ incubation studies to determine what factors were affecting the survival and development of naturally deposited eggs and fry in eastern Lake Ontario. I chose Yorkshire Bar for this study because it was a well-documented historic spawning site in the east end of Lake Ontario (Fig. 1); I interviewed commercial fishermen who described in detail spawning lake trout on Yorkshire Bar when they fished the bar commercially as late as 1943 (Casselman, unpublished data). Since 1975, also, the Ontario Ministry of Natural Resources Lake Ontario Research Unit had monitored the establishment of a spawning population of hatchery fish on the bar. Intense mark-recapture studies were conducted in 1987 and 1989. Time of spawning and egg deposition were studied in 1988-1992. The in situ incubation studies reported here were conducted in 1989-1993. This study reports the individual

fish have been collected annually in sampling programs done by the Ontario Ministry of Natural Resources (Casselman 1991). Techniques developed by Casselman (1986) were used to examine the otoliths, scales, and fins of these lake trout to determine whether they were indigenous. From 1988 to 1991, only 7 indigenous trout were recognized. Of these, 2 each came from the 1984 and 1987 spawning seasons (Casselman 1990, 1991). In Lake Ontario at present, indigenous lake trout that are yearling and older are rare. The number of unmarked lake trout < 250 mm TL caught in U.S. trawling programs from 1987 to 1992 was also not different than would be expected from hatchery fish that had either lost their marks or been inadvertently released unmarked (Elrod et al. 1995). These observations confirm that at present in Lake Ontario, natural reproduction and recruitment are very limited.

ONTARIO

YORKSHIRE BAR o r\

J

MAIN DUCK--IS.

44" 00'

IS.

STONY IS.

t/..O

YORKSHIRE/?

LAKE ONTARIO

t N

10 km

FIG. 1. Locations in eastern Lake Ontario where incubation studies were conducted, 1989-93, to examine egg andfry survival and development. In situ incubation was conducted in the lake on Yorkshire Bar on the north tip of Yorkshire Island. Incubators were also held at the Glenora Fisheries Station of the Ontario Ministry of Natural Resources, Prince Edward County, Ontario, in untreated water from a gravity-fed surface supply from Lake on the Mountain.

386

John M. Casselman

incubation of fertilized eggs in incubators that were installed for an uninterrupted period that lasted until early spring and the preswim-up and emergent-fry stage. The purpose of this research was: (1) to evaluate and compare the importance of the source of gametes-the Lake Ontario population from Yorkshire Bar versus the Lake Manitou population, Manitoulin Island; (2) to quantify and compare the effects of location-in situ on Yorkshire Bar versus laboratory conditions at the Glenora Fisheries Station that used untreated water from Lake on the Mountain; and (3) to quantify and evaluate the effects of temperature and time of fertilization on survival and development of eggs and fry.

MATERIALS AND METHODS From the fall of 1989 to the spring of 1993, egg-fry incubation was studied on Yorkshire Bar (43°55'N lat., 76°31'W long.) in the middle of the eastern basin of Lake Ontario and in the laboratory at the Glenora Fisheries Station of the Ontario Ministry of Natural Resources, Prince Edward County, Ontario (Fig. 1).

Incubation Sites Yorkshire Bar is a cobble-boulder spit that extends north from the bedrock of Yorkshire Island on the Main Duck Sill in the eastern basin of Lake Ontario. The spit was formed by down-drift deposition beginning shortly after the low-level Admiralty phase of Lake Ontario (Sly 1988). The east side of the spit, over a range of peak depths of 2 to 6 m, has a moderately steep slope (15°-30°) 5 to 10 m wide (Fig. 2), but the west side at these depths slopes more gently. Closer to shore on the west side, however, is a steep roll-down slope. Ice scours the bar to 10m. The lack of periphyton on the substrate and more recently the absence of zebra mussels in the shallows suggest that wave action moves the cobblegravel substrate down to 2 m. Zebra mussels were first seen on the bar in the fall of 1991 and in the spring of 1993 were at a density ranging from 13 to 31O/m 2 over most of the bar (Casselman, unpublished data). In the 4- to 5-m depth range on the peak and the exposed east slope, the substrate rocks range in diameter from 20 to 140 mm (Fig. 3). Average diameters were calculated from the shortest and the longest circumferences of the rocks removed from 50-cm2 quadrats excavated to a depth of 15 em. This included all particles shallower than 15 em and, depending upon size, usually included two or three layers of cobbles and boulders. Substrate on the peak (4 m) was slightly smaller than that on the steeper east slope (Fig. 3) where the incubators were installed (dark shaded area, Fig. 2) and in the deeper water at the bottom of the slope (5 m, Fig. 3). The size of the substrate over this area varied somewhat but was generally classified as approximately 75% cobbles and 25% boulders (Table 1). The rock is subrounded and mainly limestone (90%), with some granite (10%). The

FIG. 2. Bathymetric map of Yorkshire Bar, north tip of Yorkshire Island in the outlet basin of Lake Ontario. Depth contours are at I-m intervals. Prepared from data provided by the Canadian Hydrographic Service, Fisheries and Oceans Canada, Bayfield Institute, Burlington. The survey was conducted 26-27 July 1988. The gravel-cobble beach and depth < 0.2 mare depicted at the tip of the island. Light shaded area on the bar indicates the location where naturally deposited lake trout eggs were observed consistently (> 101m 2 ) during SCUBA diving surveys to measure egg densities. Dark shaded rectangular area depicts the location on the bar (depth, 4-5 m) where chains and incubators were installed during the 4 years ofthe study.

Survival and Development of Lake Trout Eggs and Fry

35

A

30

SITE 1 DEPTH - 4.0 m

25

20

15

10

10

20 30

40

50

60

70

80

B

~

SITE 2 DEPTH - 4.5 m

20

Z

W ::J 15

a

w

a:

LL. 10

5

10

20

30

40

50

60

70

80

90 100 110 120 130 140 150

35

c

30

SITE 3 DEPTH - 5.0 m

25

20

15

10

5

10

20 30

40

50

60

70

80

limestone was derived from the fracturing and erosion of the local and regional bedrock. The volume of substrate removed from the quadrats (Table 1) indicated that there is a large amount of interstitial space. Since very little sand is present or produced as the result of weathering of the limestone, the substrate down to two or three cobbles or boulders deep remains largely devoid of inorganic surface infilling. In the laboratory, incubators were held in untreated water from a gravity-fed shoreline supply (2 m deep) from Lake on the Mountain, a 97-hectare sink-hole lake 29.9 m deep (Terasmae and Mirynech 1964) that provides hatchery-quality water similar to that of Lake Ontario. Rate of flow in the troughs was low but provided ambient temperature conditions similar to those in Lake on the Mountain.

90 100 110 120 130 140 150

35

30

387

90 100 110 120 130 140 150

SUBSTRATE DIAMETER (mm)

FIG. 3. Frequency distributions of the diameter (mean of length and width) of the cobble-boulder substrate at three sites 4 m apart on a transect due east across the contours at the peak of the bar (4 m), in the area used for in situ incubation (4.5 m), and at the bottom ofthe steep east slope of the bar (5 m).

Environmental Conditions Temperature was continuously recorded every 2 hr with electronic thermistors located at the surface of the cobble-boulder substrate on Yorkshire Bar and in the incubation troughs in the laboratory. Temperature was recorded from before spawning to the end of the egg-fry incubation period in April. In March 1991, the recorder on Yorkshire Bar was dislodged and drifted away. It was subsequently recovered on the U.S. shore-missing temperature data for the remaining period were simulated from the relation between air temperature and water temperature measured during the other 3 years of the study. Mean daily water temperature was accumulated to calculate the number of cumulative thermal units (CTU, expressed as degree-days-numerically equal to DC > 0) for each incubator in the study; the calculation of CTU considered different examination times and, in the case of incubators recovered from Yorkshire Bar, laboratory exposure before and during examination. Temperatures reported here are daily means except those for Lake Manitou, which are single measurements. In the second to fourth years of the study, sediment traps composed of four polyvinyl chloride tubes (52 mm inside diameter and 360 mm high) were placed in the corners of a rigid plastic carton (40 x 40 cm) and buried in the cobble-boulder substrate at the ends of the incubator chains so that the tubes were just above the surface of the substrate. These samplers and the procedure were recommended by the Great Lakes Fishery Commission ad hoc committee that examined bioassay procedures for lake trout habitat-degradation research (Eshenroder 1988); these procedures were tested by Manny et al. (1989). Several of these replicate samplers were installed by SCUBA divers either just before or when incubation began-24 October 1990, 16 October 1991, 1 October 1992. They were retrieved at different times during and after the spawning period and when the incubators were finally removed in April. Sedimentation was measured as grams of wet sediment that accumulated per cm 2 . The contents of each tube were drained through filter paper

388

John M. Casselman

TABLE 1. Particle size and type of rock substrate on Yorkshire Bar where the incubators were located in all 4 years of the in situ egg-fry incubation studies. Three sites are indicated: site 1, on the peak of the bar; site 2, on the steep east slope in the middle of the incubation area; site 3, at the bottom of the slope. Size was classified into three categories, based on mean diameter, and is presented as percent frequency of occurrence. Total volume of the substrate was measured to a depth of 15 cm; this included all particles that were shallower than 15 cm and, depending upon size, usually included two or three layers ofcobbles or boulders. Particle Size

Type

Depth

Boulder

Cobble

Pebble

Limestone

Granite

Volume

(m)

(> 100 mm)

(25-100 mm)

(l2-25mm)

(%)

(%)

(Llm 2)

1

4.0

11.6

86.0

2.3

86.0

14.0

30.9

2

4.5

30.7

69.2

0.0

92.3

7.7

21.6

3

5.0

28.6

71.4

0.0

92.6

7.1

26.3

23.6

75.5

0.8

90.3

9.6

26.3

Site

Combined

and weighed after 24 hr. Two mean values were available for 1990-1991 and 1991-1992, and although three were available for 1992-1993, there was no value for April 1993 because ice scouring caused by strong north winds on 14 March removed the traps. Oxygen concentration was measured at the surface of the substrate at least twice during the incubation period in the fall and when the incubators were removed in the spring, and in the laboratory several times throughout the incubation period. Egg Sources Gametes were obtained from two sources. In Lake Ontario, they were collected, when logistically possible, from lake trout captured at spawning time on Yorkshire Bar in the immediate vicinity of the incubation site from short-term early-evening gillnet sets. Netting studies indicate that spawning congregations of lake trout were densest on the peak of the bar over the 2- to 6-m depth range (Casselman, unpublished data). During the years of the incubation studies, naturally deposited eggs were densest over the same general area on the peak and the steeper east slope of the bar, where postspawning densities of 10 to 2,500/m2 were measured (light shaded area, Fig. 2). Mean egg deposition over this area usually ranged from 20 to 21O/m2, and some eggs were as deep as 30 em in the substrate (Casselman 1990, 1991, unpublished data). For Lake Ontario, eggs were usually obtained from 4 to 6 females and were fertilized with sperm from several males taken at the same time (Table 2). Egg collection and fertilization followed routine hatchery procedures (Dibbits 1989), but the eggs were not disinfected. After water hardening, eggs that were obviously opaque and unfertilized were removed « 0.5%). The eggs were then pooled. In all 4 years, eggs were obtained from a large number of females from Lake Manitou on Manitoulin Island

(Table 2). They were supplied from the Ontario Ministry of Natural Resources lake trout egg-taking operations for provincial hatchery production, and fertilization and handling were similar to that for Yorkshire Bar. Eggs were obtained from Lake Manitou at a time when it was thought that comparable collections could be made from Lake Ontario. From year to year, however, dates and temperatures of egg collections varied somewhat (Table 2). With only one exception, fertilized eggs were loaded in the incubators and divers installed the incubators the day after fertilization. Gale-force winds in the fall of 1990 delayed by I day the installation of fertilized eggs from the Lake Manitou source. Incubation Procedures The incubation chambers used in the study were recommended by the ad hoc committee of the Great Lakes Fishery Commission (Eshenroder et al. 1988) and were developed by Kennedy (1980), modified by Gunn and Keller (1984), and tested by Manny et al. (1989). Each Plexiglas incubation chamber held 50 eggs in individual compartments enclosed on both sides by 2-mm mesh Nitex plastic screening. Incubators were installed on chains that were permanently anchored on the bar in 1989 (dark shaded area, Fig. 2). Two chains were placed parallel to each other and to the peak of the bar, near the top and the middle of the steep east slope. Each chain had 44 I-m side chains located opposite each other on each side of the main chain and spaced 1 m apart. These side chains allowed for the placement of 88 incubators. Adjacent side chains on each of the main chains were 1-3 m apart. The incubators were, therefore, installed in an area on the bar ranging in depth from 4 to 5 m (midpoint depth 4.5 m), encompassing an area of approximately 150 m 2 . SCUBA divers buried the incubators on their edge in the substrate so that the upper edge was just visible but not protruding above the substrate. This exposed the

Survival and Development of Lake Trout Eggs and Fry

389

TABLE 2. Numbers offemale and male lake trout used to obtain gametes for the egg-fry incubation studies, separated by year and egg source. Specific dates and water temperatures when egg collections were made are provided. Temperatures and dates when fertilized egg lots, pooled by source, were loaded into incubators and initially installed on Yorkshire Bar and in the laboratory are included. All water temperatures are daily means, except those for Lake Manitou, which were taken when the females and eggs were collected. Installation of incubators

Egg collection Numbers used

Temperature (DC)

Temperature

Source

Females

Males

Date

(0C)

Date

Yorkshire Bar

1989-90

Manitou Ontario

> 20 4

>20 6

22 Oct 89 7 Nov 89

10.0 8.8

23 Oct 89 8 Nov 89

11.2 8.7

1990-91

Manitou Ontario

16 6

26 12

22 Oct 90 24 Oct 90

11.0 12.5

24 Oct 90 25 Oct 90

12.5 12.5

11.5 11.3

1991-92

Manitou

27

> 27

15 Oct 91

11.0

16 Oct 91

13.1

12.1

1992-93

Manitou Ontario

6 5

9 17

27 Oct 92 8 Nov 92

9.0 10.0

28 Oct 92 9 Nov 92

11.8 9.9

10.2 6.4

Year

eggs and fry to ambient conditions in the shallow interstitial waters. Depending on the size of the substrate, the eggs in the incubators ranged in depth from the surface of the upper layer to the bottom of the second layer of substrate. Since there was some thermal stratification in the troughs in the laboratory, the incubators were placed horizontally near the bottom so that they would be exposed to cooler and more uniform temperatures.

Examination of Incubators, Collection and Analysis of Data At the end of the incubation period, which varied slightly during the 4 years of the study (162 ± 2.7 days, Table 3), the contents of each cell were examined by immersing the unopened incubator in water in transmitted light under a dissecting microscope. The contents were described in detail, and the incubators were then opened to remove the live fry for further experimentation. The remains in each cell were then dissected to identify the stage of development and approximate time when death occurred. Although fungus was often present on the remains, its appearance differed between eggs and fry. However, it could be scraped away to determine the specific stage of development at time of death. The contents of each cell were classified into general categories. All categories of dead eggs were combined. Fry were classified as dead or alive. A few live fry died during handling; these were quite apparent by coloration and transparency and were classified as having been alive at the end of the incubation period. Dead fry were categorized as having died early in the incubation period, oikn-at-thetim~lJf hatching or shortly afterward, oraled late in the period. Fry that died early were less

Laboratory

developed, more heavily decomposed (usually only the more persistent parts, such as head and eyes, remained), and the yolk sacs were relatively large. Fry that died later were more heavily pigmented, more advanced in development, had smaller yolk sacs, and were decomposed only slightly. Live fry and all categories of dead fry were combined to obtain the total number of hatched fry. Deformed fry were found, both dead and alive. These were categorized independent of the type of deformity. Empty cells were common in some incubators removed from in situ condition on Yorkshire Bar. These were carefully examined for any evidence that could explain the cause. All categories were totalled for each incubator and were described as percent frequency of occurrence, originating from the 50 fertilized eggs that were initially loaded. Means were calculated for each treatment, egg source, and location, and were tested by means of twosample t-tests, which considered equality of group variance (Snedecor and Cochran 1980). Results were separated by year but combined for those years when comparable treatments were applied. Regression, correlation, and covariance analyses were also done. Unless otherwise indicated, the value following the mean in Table 3 is the 95% confidence interval. Of the four incubation periods, the first and third did not include incubators deployed at both locations or eggs from both sources. In 1989-1990, laboratory controls were not included, although both egg sources were used (Table 3). In 1991-1992, both locations were studied, but only eggs from Lake Manitou were used. Eggs were not obtained from Lake Ontario that fall because heavy winds and cold temperatures caused Yorkshire Bar to cool very rapidly; temperatures associated with spawning lasted only 1 week-they normally extend over a 3-

390

John M. Casselman

TABLE 3. Mean duration of egg-fry incubation periods for the various treatments by year, combined where egg sources were comparable. Mean number of cumulative thermal units (degree-days) and mean water temperatures during the incubation periods are provided. In all cases 95% conjuJence intervals (C.I.) are included. Treatments were combined for years when both locations and both egg sources were included-1990-91 and 1992-93. Numbers of incubators examined are indicated by year except for 1990-91 and 1992-93, when 16 to 24 and 13 to 23, respectively, were used. Duration (days) Location 1989-90 Yorkshire Bar 1990-91, 1992-93 Yorkshire Bar Laboratory 1991-92 Yorkshire Bar Laboratory

Cumulative thermal units

Source

N

Mean

95% C.l.

Mean

Ontario Manitou

41 43

164 180

1.0 1.0

382.4 544.2

Ontario Manitou Ontario Manitou

39 37 36 39

171 177 122 133

1.9 2.3 4.7 7.2

Manitou Manitou

35 20

183 164

290

162

Combined

week period. In 1990-1991 and 1992-1993, both egg sources and locations were studied (Table 3), so analyses were combined. On 22 March 1990 and 14 March 1993, ice scouring dislodged some incubators on Yorkshire Bar. Results for incubators that were disturbed were not used in these analyses; even so, the number of incubators for comparable paired treatments was usually greater from Yorkshire Bar than from the laboratory. To compensate for such circumstances, more incubators were installed on the bar than were held in the laboratory.

RESULTS Environmental Conditions In the laboratory, the egg-fry incubation period was slightly shorter because it was logistically necessary to examine these incubators before those from Yorkshire Bar (Table 3). This reduced exposure occurred at the end of the incubation period, and the accumulation of thermal units at the beginning of the period was unaffected. Over the years, temperatures on Yorkshire Bar at the beginning of the incubation period averaged 2.0°C warmer than Lake Manitou and 1.0°C warmer than the laboratory (Table 2). Not only were initial temperatures in the laboratory cooler, but also the rate of cooling of Lake on the Mountain was faster than that of eastern Lake Ontario. Nevertheless, mean temperatures in the laboratory were slightly warmer (Table 3);

95% C.I.

Temperature CC) Mean

95% C.I.

1.29 1.32

2.3 3.0

0.01 0.01

529.1 589.5 434.7 495.2

21.64 4.47 13.32 7.20

3.1 3.3 3.7 3.8

0.12 0.04 0.32 0.25

1.0 0.9

720.4 658.3

2.90 1.94

3.9 4.0

0.21 0.12

2.7

534.0

12.47

3.3

0.18

because Lake on the Mountain was insulated by complete ice cover, and, temperatures were slightly warmer than in the eastern basin where there was extensive open water during those winters. Over the years, eggs were incubated on Yorkshire Bar over a rather broad range of dates and temperatures--commencing between 16 October (13.1 "C) and 8 November (8.7°C) and continuing until 3 to 27 April. The various treatments were, therefore, exposed to slightly different thermal units (Table 3). During all years of the study, the relative oxygen concentration was always > 82% air saturation at the surface of the substrate on the bar and> 92% saturation in the laboratory. Sedimentation on the bar during the incubation periods was moderately high (Fig. 4). For the first few weeks of incubation, up to 1 g of wet sediment accumulated per cm 2 . At the end of the period, 10 times this amount had accumulated. Less sediment accumulated during the 1992-199Tiricubation period than in the other years. Incubation 1989-1990 In 1989, the Manitou eggs were installed on Yorkshire Bar 16 days earlier and in water that was 2.5°C warmer than when the Ontario eggs were put down. Over the incubation period, Manitou eggs and fry accumulated 42% more thermal units than did-ffiOsefronrL~io (Table 3). Significantly more Ontario eggs were dead at

391

Survival and Development of Lake Trout Eggs and Fry

were deformed; these differences were not significant between the two egg sources (Table 5).

10.0 9.0

~

8.0

!2.

Cl 7.0 I

~

'iii

1990~91

.......

1991 -92

___

Incubation 1991-1992

1992 - 93

60

~

Q)

5.0

.!

!Z w :!: o

30

(/)

2.0

w

40

1.0 0.0

L.....L~AL.':.!..m.:....LlL.l----L-L.ll.:....LJ.J.-...J..L..J.J.-..L...l~~...LL~--L.J

1

14 28 OCT

14 28 NOV

14 28 DEC

14 28 JAN

14 28 FEB

14 28 MAR

14 28 APR

FIG. 4. Wet weight of sediment accumulated per unit area in sediment traps installed just above the surface of the boulder-cobble substrate at the ends of the incubator chains, 1990-93. The months are delineated by long, dark ticks; shorter, lighter ticks mark 7-day intervals. Symbols indicate means, and vertical lines represent 95% C.I. Deposition periods commenced on 24 October 1990, 16 October 1991, and 1 October 1992.

the end of the incubation period in April (1.25x mean absolute difference (~X) = 9%; Table 4). Egg source did not affect the percentage of fry that hatched, but the incubators that had longer incubating periods had more than twice as many empty cells (2.5x ~x = 13%). The production of fry on Yorkshire Bar was different for the two egg sources. In direct contrast to the number of empty cells and the duration of exposure, Ontario eggs produced almost twice as many live fry (1.99x ~x = 13%; Table 5), and most of the Manitou fry that hatched died early in the incubation period (1.56x ~x = 9%). Only a few fry died late in the period, and very few

Only Lake Manitou eggs were used in 1991-1992they were installed in both locations at the same time (Table 2), but exposure times were slightly different. The eggs and fry on Yorkshire Bar accumulated 9.4% more thermal units, but 55% of these units were acquired because the bar was warmer by 1.0o C at the time of installation (Table 2) and cooled more slowly than Lake on the Mountain. Incubators from Yorkshire Bar had more than twice as many dead eggs as those in the laboratory (2.4x ~x = 25%; Table 6), which had many more hatched fry 03.0x ~x = 79%). As in 1989-1990, a high percentage of the cells in the incubators from Yorkshire Bar were empty-this was not seen in the laboratory (Table 6). After this early installation, few fry of any type were left in the incubators from Yorkshire Bar, although large numbers of live fry were produced in the laboratory (Table 7). Indeed, of the few remaining fry on the bar, more were dead than alive, and the dead ones had died early in the period. In the laboratory, the occurrence of the two types of dead fry was not significant (Table 7). As in 1989-1990, the occurrence of deformities was not significant. Location did not effect deformities in fry from Lake Manitou eggs. Incubations 1990-1991 and 1992-1993 !In the second and fourth years, eggs from both soll;ces were studied at both locations, so results were cobbined. Approximately 25% of the eggs died; but there were no appreciable differences between either egg sources or locations (Table 8). In these years, as in 1989-1990, the percentages of fry that hatched from the two egg sources were not significantly different. But, as in 1991-1992, a much higher percentage of hatched fry was found in incubators in the laboratory; the difference was more than twice that on the bar (2.1x ~x = 79%; Table 8). Egg source did not affect the occurrence of

TABLE 4. Percent of dead eggs, hatched fry, and empty cells observed in incubators at the end of the egg-fry incubation period in 1989-90. Eggs were installed only on Yorkshire Bar but were from both sources, Lakes Ontario and Manitou. Means and 95% c.l. are provided, along with the probability levels for t-tests, which considered equality of group variance. Values that are significantly greater are italicized. Dead eggs (%)

Hatched fry (%)

Empty cells (%)

Source

N

Mean

95% c.1.

Mean

95% c.1.

Mean

95% c.1.

Ontario Manitou

41 43

46.3 37.1

4.19 3.33

44.4 40.3

4.26 4.94

9.2 22.6

2.50 3.59

p

0.001

0.203

< 0.001

392

John M. Casselman TABLE 5. Percent of hatched fry in various conditions that were observed at the end of the egg-fry incubation period in 1989-90. Fry were classified as alive, died early in the incubation period, died late in the incubation period, and deformed (both dead and alive). Treatment, analysis, and presentation are the same as in Table 4. Alive (%) Source

N

Mean

95% c.1.

Mean

Ontario Manitou

41 43

26.6

5.63 4.59

16.9 26.4

13.4

p

Died late (%)

Died early (%)

95% c.1. Mean 3.59 3.74

95% c.1. Mean

1.0 0.5

< 0.001

< 0.001

Deformed (%)

0.77 0.42

95% c.1.

0.1 0.1

0.245

0.10 0.09 0.973

TABLE 6. Percent of dead eggs, hatched fry, and empty cells observed in incubators at the end of the egg-fry incubation period in 1991-92. Eggs were from only one source, Lake Manitou, but were both installed on Yorkshire Bar and held in the laboratory. Means and 95% c.l. are provided, along with the probability levels for t-tests, which considered equality of group variance. Values that are significantly greater are italicized. Dead eggs (%) Source

Hatched fry (%)

Empty cells (%)

N

Mean

95% c.1.

Mean

95% c.1.

Mean

95% C.1.

Yorkshire Bar 35 Laboratory 20

43.4 18.0

6.24 2.89

2.7 81.9

1.15 2.94

53.9 0.3

6.63 0.31

p

< 0.001

< 0.001

< 0.001

TABLE 7. Percent of hatched fry in various conditions that were observed at the end of the egg-fry incubation period in 1991-92. Fry were classified as alive, died early in the incubation period, died late in the incubation period, and deformed (both dead and alive). Treatment, analysis, and presentation are the same as in Table 6, except two incubators removed from Yorkshire Bar contained no fry in any ofthese conditions. Alive (%)

Died early (%)

Location

N

Mean

95% c.1.

Mean

Yorkshire Bar Laboratory

33 20

0.3 67.0

0.33 3.54

1.3 8.3

P

< 0.001

empty cells, but, as in 1991-1992, there were many more empty cells in the incubators removed from Yorkshire Bar (L1x = 36%). Egg source had virtually no effect on the production of live fry, although in the laboratory there was some indication that eggs from Lake Ontario produced more live fry (1.20x L1x = 13%; Table 9). The obvious difference in these years, however, which was independent of egg source, was that eggs in the incubators in the laboratory produced many more live fry than did those on Yorkshire Bar (lO.4x.1:X = 63%). In the laboratory, also, eggs

Died late (%)

95% C.1. Mean 0.58 3.06

0.001

0.0 6.6

Deformed (% )

95% c.1. Mean 4.33

0.1 0.2

95% c.1. 0.11 0.29

0.349

from Lake Manitou produced a higher percentage of fry that died early than did eggs from Lake Ontario (4.9x L1x = 7%; Table 9). These differences between egg sources were associated with the fact that more live fry were produced from incubations that started later (lower temperature) and more fry died early in those that started earlier (higher temperature). As in 1991-1992, a higher percentage of the fry that hatched were alive in the laboratory than on Yorkshire Bar, but on the bar, a higher percentage of the hatched fry died early in the incubation period than in the laboratory (5.1x L1x = 24%; Table 9). As in

Survival and Development of Lake Trout Eggs and Fry

393

TABLE 8. Percent of dead eggs, hatched fry, and empty cells observed in incubators at the end of the egg-fry incubation periods when the 1990-91 and 1992-93 studies were combined. During these two studies, eggs from both sources were both installed on Yorkshire Bar and held in the laboratory. To facilitate comparisons, columns are staggered by egg source and location. Results for combined egg sources and locations are also included. Probability levels for t-tests, which considered equality ofgroup variance, were used to examine differences between egg sources within the same location (P-source) and locations with egg sources combined (P-locations). Values that are significantly greater are italicized. Dead eggs (%) Location

c.1.

Hatched fry (%)

c.1.

Empty cells (%) 95%

c.1.

Source

N

Mean

Combined

Combined

151

24.9

1.93

56.5

2.84

18.6

3.48

Yorkshire Bar

Combined Ontario Manitou

76 39 37

26.7 26.9 26.5

2.75 4.34 3.53

36.9 36.1 37.7

3.43 4.50 5.43

36.4 37.0 35.8

3.88 4.77 5.43

P (Source)

Laboratory

Combined Ontario Manitou P (Source)

P (Location)

95%

Mean

0.890 75 36 39

23.0 20.1 25.8

95%

Mean

0.752

0.640 2.70 3.95 3.63

76.3 78.9 73.9

2.66 3.86 3.68

0.24 0.42 0.27

0.5 0.7 0.3

0.033

0.064

0.100

0.061

< 0.001

< 0.001

other years, very few fry « 1%) died late in the incubation period, and this was not related to egg source or location. There were very few deformed fry, but more deformed fry were produced in the laboratory, where survival was high, than on Yorkshire Bar. Eggs from Lake Ontario produced significantly more deformed fry than those from Lake Manitou (2.8x LlX = 1%; Table 9), although the numbers were small (approximately 2%).

Analysis of covariance indicated that the slopes for Yorkshire Bar and the laboratory were not significantly different but that the elevations were highly significantly different (P < 0.001; Fig. 5). That incubation on the bar directly affected the survival of eggs and fry is apparent. Independent of egg source, over the entire study the bar produced only 1/5 as many live fry as the laboratory, an absolute difference of 58%.

Cumulative Thermal Units, Temperature, and Associated Correlations When all results for the incubators were combined (N = 290) and compared, some highly significant correlations were apparent. The occurrence of dead eggs was directly related to the mean incubation temperature; this relationship was stronger if only dead eyed eggs were compared to the number of cumulative thermal units (r = 0.39). The occurrence of hatched fry was inversely related to the number of cumulative thermal units (r = -0.48). The percentage of fry that were deformed was directly related to the incubation temperature (r = 0.35). The number of thermal units directly affected the occurrence of empty cells (r = 0.57) and the number of fry that died early in the incubation period (r = 0.38). When all treatments were considered, the abundance of live fry was inversely related to the number of thermal units that were accumulated (r = -0.40). The regressions for the two locations were greatly different (Fig. 5).

DISCUSSION Incubation procedures similar to those of this study have been used in the other Great Lakes. Production of live fry in the spring from the eggs of locally captured lake trout was somewhat variable: Lake Superior-18% ± 5.3% (Manny et at. 1995); Lake Huron-lO% ± 7.8% to 24% ± 6.0% (Manny et al. 1989); Lake Michigan-49% ± 5.0% (Edsall et ai. 1992). Survival to the fry stage, ranging from 40% ± 16.6% to 88% ± 4.5%, was higher in incubators held in the laboratory. Survival in the lake relative to that in the laboratory was: Lake Michigan79%; Lake Huron-43%; Lake Superior-20%. Overall, production of live fry from 1989 to 1993 for Yorkshire Bar (11.2% ± 2.2%) and the laboratory (69.3% ± 2.8%) was within the range for the other Great Lakes, but in situ survival was low and, relative to the laboratory, only 17%. Since the accumulation of thermal units had a strong negative effect on survival, it is difficult to compare these results with any precision without

394

John M. Casselman

TABLE 9. Percent of hatched fry in various conditions that were observed at the end ofthe erg-fry incubation periods when the 1990-91 and 1992-93 studies were combined. Fry were classified as alive, dWearly in the incubation period, died late in the incubation period, and deformed (alive and dead). Treatments" analyses, and presentations are the same as in Table 8. Alive (%) Location

Source

Combined Yorkshire Bar

Mean

95% c.1.

Combined

151

38.1

5.49

17.8

2.72

0.5

0.31

0.6

0.24

Combined Ontario Manitou

76 39 37

6.7 5.3 8.2

2.32 2.44 4.08

29.8 30.0 29.5

2.97 4.10 4.50

0.4 0.8 0.1

0.42 0.81 0.11

0.2 0.1 0.2

0.12 0.15 0.21

0.844

0.213

Combined Ontario Manitou

69.9 76.6 63.7

75 36 39

100

3.42 4.09 4.73

-

=

50

UJ

>

0.430

< 0.001

P < 0.001

10

~

~

~

~

~

~

~

0.46 0.81 0.42

0.019

20

0 300

1.1 1.7 0.6

< 0.001

=

- - - Y = 53.2 - O.077X N = 194 r = 0.56

30

0.45 0.36 0.83

0.024

YORKSHIRE BAR

40

::i

0.7 0.4 0.9 0.234

70

u.

2.48 0.84 4.51

0.373

0.002

Y = 101.3 - 0.063X 0.44 P < 0.001 N 94 r

80

5.8 1.9 9.3

0.084

< 0.001

LABORATORY

90

60

Mean 95% C.I. Mean 95% C.1.

95% c.1.

P (Location)

>0::

Deformed (%)

Mean

P (Source)

0~

Died late (%)

N

P (Source)

Laboratory

Died early (%)

~

~

CUMULATIVE THERMAL UNITS (degree-days)

FIG. 5. Relationships between percent of the fry that were alive at the end of the egg-fry incubation period (162 ± 3 days) and number of thermal units that had accumulated, expressed as degree-days (OC > 0). Regression lines and 95% confidence limits (light dashed lines) are illustrated over the range of values obtained for Yorkshire Bar and the laboratory. Data are combined for all four years of the study; number of incubators involved, correlation coefficients, and P values are provided.

detailed information on thermal history. Perkins and Krueger (1995) used similar incubation techniques and obtained results in 199 I-92 and 1992-93 at another physically similar cobble-boulder spit in eastern Lake Ontario, Stony Island Reef-36.9% ± 7.8% to 40.7% ± 12.9%; this was 3.5x the survival on Yorkshire Bar in the same years. Incubation on Stony Island Reef was begun at a later time and lower temperature (8 and I 1 November, 9°C) than on Yorkshire Bar. This probably accounts for some of the difference, because when survival was estimated for comparable exposure by using the cumulative thermal exposure-fry survival relationship (Fig. 5), an estimated 22.6% of the fry would have survived on Yorkshire Bar. With comparable exposure, therefore, the difference in survival for the two sites would have been reduced to less than half (1.72x). For Lake Michigan (Edsall et ai. 1992), as for the present study, source of the eggs made no significant difference, except that Lake Ontario eggs produced significantly more deformed live fry, although the numbers were small (approx. 2%). It is not known whether these deformities were related to contaminants (Mac and Edsall 1991); nevertheless, their occurrence was directly related to mean incubation temperature.

Time and Stage of Mortality In similar in situ incubation studies in the other Great Lakes, wave action dislodged incubators and was consid-

Survival and Development of Lake Trout Eggs and Fry ered to have caused mortality (Manny et al. 1989, Edsall et al. 1992, Manny et al. 1995). Wave action did not dislodge incubators at the two sites in eastern Lake Ontario; fetches were much less than at the other Great Lakes sites. Nevertheless, wave action and wind-driven currents during storms could still create physical shock at 4 to 6 m and affect survival in the cells of incubators installed in the shallow substrate at these two sites. Perkins and Krueger (1995) considered that wave action reduced survival at Stony Island by as much as 42 to 49%. This effect was very much less on Yorkshire Bar but it might explain why, overall, 14% more fertilized eggs (36.1 % vs. 21.9%) died early (before the eyed stage) than in the laboratory. Nevertheless, wave action was not the major factor affecting survival and development of eggs and fry on Yorkshire Bar. The greatest mortality on Yorkshire Bar occurred when fry died early in the incubation period (21.4%). Associated with this, the occurrence of empty cells was substantially different on Yorkshire Bar than in the laboratory. This phenomenon was common on Yorkshire Bar but rare in the laboratory, which suggests that the frequency of empty cells on the bar did not result from initial loading error or escapement. Careful examination of these "empty" cells, which were usually quite clean, revealed that some contained mucoid remains of highly decomposed fry. Under high magnification, many live ciliates were visible in the remains and decomposing fry. Incubators removed from the substrate on Yorkshire Bar contained an abundance of invertebrate fauna, ranging from ciliates, hydra, and oligochaetes to Gammarus and Asellus; incubators in the laboratory were almost devoid of these invertebrates. Periodic examination and reciprocal transfer of large numbers of incubators back and forth between the two locations (N = 304) showed that half (50.3%) of the empty cells observed in these "uninterrupted" in situ installations had contained fry that hatched (prematurely), died, and were completely decomposed or were consumed during the remaining incubation period (Casselman, unpublished data). On Yorkshire Bar, therefore, 42% of the fry that hatched early completely decomposed and disappeared. When these were combined with those that remained, mortality at this stage and time on the bar was even more substantial (36.8%). Overall, the single greatest mortality occurred in newly hatched fry; they accounted for 67% of the relative difference in survival between Yorkshire Bar and the laboratory.

395

time in the fall when water temperatures were high than in winter or even spring, when water temperatures were relatively low (Fig. 6). Since the incubation period was terminated when daily water temperatures were relatively low (1.2°C ± 0.23°C), the major differences in the accumulation of units during the incubation periods were mainly caused by the time and temperature of the initiation in the fall. Since the early accumulation of additional thermal units adversely affects production of live fry (Fig. 5), time and temperature of spawning and egg deposition in the fall could greatly influence fry production the following spring. To quantify and evaluate the magnitude of this effect, this relation for Yorkshire Bar was applied to the times and temperatures when lake trout spawned in the eastern basin of Lake Ontario. From 1987 to 1993, the seasonal dynamics of lake trout maturity and spawning was studied at six sites in the Canadian waters of the outlet basin (Casselman, unpublished data). Naturally deposited and viable eggs were collected from 19 October at a mean daily water temperature of 12.7°C to 13 November at 8.8°C, with a mean spawning date and temperature of 29 October and 11.5°C. Although egg deposition may continue later at some spawning sites in Lake Ontario (e.g., Stony Island until late November, Marsden and Krueger 1991), this was not seen in the middle and northeast regions of the outlet basin. To examine the effect of various spawning times and temperatures on survival, I calculated the thermal units that would be accumulated by using average temperature conditions measured on Yorkshire Bar (Fig. 6) and applying them to the relation (Fig. 5) between percent live fry and CTUs for Yorkshire Bar. No fry would be alive on 1 May from eggs fertilized on the earliest spawning date and at the highest temperature (Fig. 6), but survival from the latest date and lowest temperature would be 21 %, with mean conditions producing 10% survival. If the mean spawning temperature were 2°C lower (9.5°C, 9 days later), fry production on 1 May would be almost double (19% versus 10%; Fig. 6). It has generally been assumed that lake trout spawning begins when falling water temperatures reach lOoC (Martin and Olver 1980). A high percentage (approx. 82%) of the lake trout in eastern Lake Ontario spawn at higher temperatures than might be considered typical for the species. This would substantially affect the number of fry that reach the swim-up stage and emerge during the first 2 weeks of May (Marsden et al. 1988). Environmental Conditions

Cumulative Thermal Exposure and Survival

Time and temperature when the incubation studies were initiated directly affected the frequency of dead eyed eggs, fry that died early, and empty cells, and inversely affected survival (Fig. 5). When thermal exposure on the bar increased over the range observed, from 370 to 690 CTUs, absolute survival decreased from 25% to O. Thermal units accumulated more rapidly per unit

Since a large portion of the fry mortality occurred early in the incubation period, interstitial water quality could be implicated. Siltation of the spawning ground can detrimentally affect lake trout recruitment (Martin and Olver 1980). Qualitative evidence exists that sediment is prevalent in the substrate on the historic spawning sites in eastern Lake Ontario (Sly and Schneider 1984, Sly 1988, Edsall 1990) and has increased since na-

John M. Casselman

396

14

PRESWIM-UP

SPAWNING

1 MAY LIVE FRY:

12

0%:

800 700

( /)

>.

/,

ro

"0

600

) 10

I

Q)

~

OJ

-!:: Q)

0'

~

w

8

~~

w

--~

_____________

~

::::>

~

----------

19 %, ~

,"

21

,

500

/.

;0.. : ,

~~

"0 Cf)

z 400 ::::> ...J

6

0-

m4 I-

/ /

2

«

-----------

,, , , ,, , , , , ,,,,,, ,, ,,,,,,

300

a

200

14 28

100

I

OCT

14 28 NOV

IUJ

>

L......L...J.....ll---"-,-...L.....ll---l-....l...I-L.......L......l........l..l-.J...--I....-J-.L.L-'--""---'---'--=---""'.J..-J-"--'--'---l.--'-'--'--'

1

~

W I

I ,

:E

14 28

14 28

DEC

JAN

14 28 14 28 FEB

MAR

14 28

a

i=

« ...J ::J ~ ::J

o

APR

FIG. 6. Mean daily water temperatures, 1989-93, at the surface of the bouldercobble substrate (4.5 m), where the incubators were located on Yorkshire Bar from the beginning of the spawning period to the end of April the following spring. The months are delineated by long, dark ticks; shorter, lighter ticks mark 7-day intervals. Includes the beginning (16 October to 9 November, median = 25 October) and the end (3 April to 27 April, median = 19 April) of the in situ incubation period. Mean daily water temperatures associated with the lake trout spawning period in eastern Lake Ontario are delineated by vertical solid lines falling on the appropriate dates: earliest, highest temperature; mean; later, lower temperature-9.5°C; latest, lowest temperature. The dates when these temperatures are reached mark potentially important times when incubation would begin for naturally deposited and fertilized lake trout eggs in eastern Lake Ontario. Curves (dashes) illustrate the cumulative thermal units commencing on the respective dates (19 October, 29 October, 7 November, and 13 November). Percent survival to the preswim-up stage, extrapolatedfrom these dates to 1 May (vertical dotted line), is also shown.

tive lake trout reproduced on reefs such as Yorkshire Bar (Cecil Lobb, commercial fisherman, Prince Edward County, pers. comm. 1987). In the fall of 1986, Sly (1988) used sedimentation boxes to measure sediment loading at the same general location on Yorkshire Bar. Although his collection methods were slightly different, sedimentation rates were less, one to two orders of magnitude lower, than those measured in this study in 1990-93. Sedimentation on Yorkshire Bar was very high (Fig. 4) and no doubt the method underestimated flux; nevertheless, under natural conditions, it could be high enough to physically encumber or, by late winter, even to smother fry that are in the deep interstices. During in situ incubation, however, the

cells of the incubators, which were installed in the upper interstitial water, would have protected the eggs and fry and probably almost eliminated smothering. Although oxygen concentrations measured at the surface of the substrate in the incubation area were always > 82% air saturation, the biological oxygen demand of this highly organic sediment (Sly 1988; Casselman, unpublished data) undoubtedly caused localized oxygen deficiencies in the deeper interstitial water. Reduced interstitial oxygen levels had already been measured on Yorkshire Bar. In 1986, Sly (1988) installed dialysis chambers in the substrate in this section of the bar and showed that in late October at 11.5°C, the average dissolved oxygen was only 53% air saturation. Later, in

Survival and Development of Lake Trout Eggs and Fry early November, when temperature was lower (9°C), the average oxygen saturation increased to 65%. This provided direct evidence that later in the spawning and incubation period, as temperature on Yorkshire Bar decreased, eggs and developing fry would be exposed to relatively higher concentrations of dissolved oxygen. Sly concluded that the lower dissolved oxygen measurements made by these chambers, which in some locations were as low as 13 to 32% air saturation, better reflected the interstitial conditions within the substrate, because the higher values more reflected the chemistry of the overlying lake water. Since sedimentation rates on Yorkshire Bar were higher in 1990-93 than in 1986, the relative oxygen concentration in the interstitial water during the in situ incubation studies could have been as low as, and probably lower than, that measured by Sly in 1986. Garside (1959) and Carlson and Siefert (1974) showed that when the oxygen concentration is reduced to some of the localized levels (13-32%) measured on Yorkshire Bar by Sly (1988), survival of eggs and fry would be affected and that this adverse effect would increase with increasing temperature. Carlson and Siefert showed that at 7°C, survival was reduced if air saturation was depressed to 36%, and at 26% and less, there was no survival. At a higher temperature of 10°C, the adverse effect was greater; it commenced when the 50% level was reached, and at 40% and less, there was no survival. Garside (1959) and Balon (1980) have documented that hatching is earlier and even premature at high temperatures and low oxygen tensions. Low oxygen can induce the release of proteolytic enzymes that stimulate hatching (Balon 1980). Temperature and reduced interstitial oxygen conditions on spawning habitat such as Yorkshire Bar early in the incubation period would directly affect fry survival. If fertilization occurs early and at relatively high temperatures, it would stimulate premature development and even hatching and place eggs and fry at a disadvantage, especially if the interstitial water were degraded by organic matter. High temperature would coincidentally stimulate both metabolic rate and development of the embryo, as well as organic decomposition and associated biological oxygen demand in the interstitial water. If the organic degradation were severe enough, the oxygen concentration could be critically depressed just when the oxygen requirements of the embryo were increasing. Low oxygen saturation levels on Yorkshire Bar, as measured by Sly (1988), associated with high temperatures at the beginning of the incubation period, probably explain why 76% of the fry that hatched on Yorkshire Bar died early, while only 8% died at this stage in the laboratory. This oxygen depression would probably be less critical if the embryos were less developed, if the temperature were lower, or if the substrate were less degraded or possibly even more thoroughly purged by fall winds. Later spawning at lower temperature, especially in degraded environments like Yorkshire Bar, would increase the production of fry in late spring.

397

Comparative Survival-Eastern Lake Ontario Stony Island Reef and Yorkshire Bar, major spawning sites in the east end of Lake Ontario, are physically quite similar. Some aspects of spawning and fry production are, however, very different. The Stony Island site annually produces substantial numbers of pre-swim-up and emergent fry (Perkins and Krueger 1995), but emergent fry are rarely captured on Yorkshire Bar. Since 1986, the Seneca strain of lake trout has consistently been the major parental contributor of fry at Stony Island (Marsden et al. 1988, Grewe 1991, Marsden et al. 1993). Genetic analysis of naturally deposited eggs in 1989 indicated that the contribution of the various stocks that use the two reefs was different (Grewe 1991). For the Stony Island sample, 73% of the parental contribution was of Seneca origin, but that stock accounted for only 12% of the sample from Yorkshire Bar. The original stocking location probably influenced this difference. Since fry from the Seneca stock successfully emerge at the Stony Island site, some fry emergence from the Seneca eggs deposited on Yorkshire Bar was expected the following spring (Grewe 1991). This apparently did not happen, because intensive trapping in 1990 captured only one fry. Advanced development may have negatively affected survival, because eggs collected from Yorkshire Bar contained embryos that were much more developed: 72% of the eggs collected on Yorkshire Bar on 27 November 1989 (6°C) were eyed, but 9 days later (3°C), only 8% of those from Stony Island had reached this stage of development (P. Grewe, Cornell University, pers. comm., 1990). Since water temperatures at Stony Island during the incubation period (Perkins and Krueger 1995) seem similar to those of Yorkshire Bar, the eggs from Yorkshire Bar probably were deposited much earlier. These late-fall egg collections provided further evidence that early deposition and development negatively affect survival. Time of spawning and egg deposition for these two reefs appear to be substantially different. Even though egg deposition commenced about the same time at both sites, it was essentially finished by 13 November (8.8°C) on Yorkshire Bar (Casselman 1990, 1991, unpublished data), whereas at Stony Island, it continued much later; substantial numbers of eggs have been collected in stationary traps up to the end of November (Marsden and Krueger 1991). If we assume that environmental conditions on these physically similar spits are generally similar, then differences in deposition times expose the developing eggs and fry to substantially different temperatures and relative oxygen concentrations. If the relation between the accumulation of thermal units and fry survival is considered, much of the difference in fry production between these two reefs may be explained by differences in the time of spawning and egg deposition. If lake trout spawned on Yorkshire Bar as late as has been observed on Stony Island (20 November-7.6°C) then survival estimates calculated for I May for Yorkshire Bar (26.2% ± 6.8%) would not be signifi-

398

John M. Casselman

cantly different from those found at the same time on Stony Island (32.5% ± approx. 10%, interpolated estimate from Perkins and Krueger 1995).

Factors Affecting Time and Temperature of Spawning, Fry Survival, and Production Spawning time affects the rate at which thermal units are accumulated by the eggs and embryos at the beginning of the incubation period; this inversely affects survival and emergence in the spring. High temperature accentuates this effect if the interstitial water is degraded and the relative oxygen concentration is reduced-conditions that detrimentally affect developing embryos and newly hatched fry. Spawning time is critically important; late spawning increases fry survival and production. Many factors other than environmental conditions affect time and temperature of spawning. Older females spawn later (Martin and Olver 1980), and this applies to hatchery lake trout in eastern Lake Ontario (Casselman, unpublished data). As the age of the spawning females increases, therefore, so would the survival and production of fry. This, in part, explains why older female lake trout are more effective spawners. Currently, a large portion of the spawning population in Lake Ontario consists of fish that are mature for the first time (Elrod et ai. 1995); a substantial portion of the population, therefore, spawns early. Obviously, this negatively affects fry survival and production. In Seneca Lake, the Seneca stock spawns in deep water (Sly and Widmer 1984), and spawning at subthermocline depths begins at a relatively low temperature, 9°C (from Fig. 4, Sly 1988). At Stony Island Reef in eastern Lake Ontario, where this stock successfully produces emergent fry, some spawning and egg deposition is very late and at approximately the same water temperature as in Seneca Lake. This probably has an important positive effect on fry survival and production. Why late spawning occurs at the Stony Island site is unknown but should be determined. It could be genetically related or simply the age of the spawning females. Whether hatchery practices and artificial culture affect time of spawning and hatching should be determined. Early egg-collection practices could result in inadvertent artificial selection for early spawning. Also, through culture and artificial feeding, fry that hatch and start to feed earlier may be larger at age and have greater survival when stocked in the natural environment. The result could be artificial selection for rapid development and early hatching. Indeed, some evidence exists that the eggs of hatchery lake trout from eastern Lake Ontario require fewer cumulative thermal units to hatching (x = 12%-Casselman, unpublished data) than has been reported in the literature (Embody 1934, Balon 1980). If fry production is to be increased on spawning sites such as Yorkshire Bar, then egg deposition and survival must increase. This would occur if spawning fish were older or if the stocked fish were later spawning or if their eggs and fry had slower embryological development.

In 1988, an incubation study done with similar procedures to those in this study provided preliminary evidence that late spawning at low temperature (7.6°C) and deep-water incubation could result in survival as high as in the laboratory (78% ± 3.8%-Casselman and Fitzsimons, unpublished data). These results and the shallowwater alewife predation on emerging lake trout fry reported by Krueger et al. (1995), suggest strongly that experimental stocking of a late, deep-water spawning stock should be done in eastern Lake Ontario. The stock from Green Lake, Wisconsin, which apparently originated from a native deep-water stock from southern Lake Michigan (Krueger et ai. 1983, Kincaid et ai. 1993), should be considered because it spawns both deep and late (peak spawning 28 November, surface water temperature 6.7°C, Hacker 1956). Even if this stock spawned in shallow water-as long as it occurred late in the season- fry survival and production would be increased. ACKNOWLEDGMENTS I would like to thank the Operations staff at the Glenora Fisheries Station, Ontario Ministry of Natural Resources (OMNR), who helped deliver this research program, which was accomplished under adverse and sometimes dangerous fall conditions on the lake. Members of this group are: David Jeffrey, Dawn Walsh, Wayne Miller, Dale Dewey, Steve Lawrence, Alan McIntosh, Tim Shannon, Chuck Wood, and Joe Dibbits. Steve Lawrence and Joe Dibbits examined most of the incubators and collected the observational data that form the basis of this study. Their careful interpretations were extremely consistent; this was important and essential to this study. David Brown's help with the initial compilation of the data and computer graphics is much appreciated. The capable assistance of the divers of Lower Lakes Diving of Peterborough is appreciated. Gary Clark of the OMNR Blue Jay Fish Hatchery on Manitoulin Island helped arrange for eggs from Lake Mantiou lake trout. I am endebted to Randy Eshenroder of the Great Lakes Fishery Commission and Bruce Manny of the U.S. Fish and Wildlife Service for their encouragement during this study. Constructive reviews were provided by M. Bozek, C. Krueger, B. Shuter, P. Sly, R. Steedman, and one anonymous reviewer. This manuscript is contribution No. 95-01 of the Aquatic Ecosystems Research Section of the Research, Science, and Technology Branch of the Ontario Ministry of Natural Resources. REFERENCES Balon, E. K. 1980. Early ontogeny of the lake chaIT, Salvelinus (Cristivomer) namaycush. In Charrs: Salmonid fishes of the genus Salvelinus, E. K. Balon (ed.), pp. 485-562. The Hague, The Netherlands: Dr. W. Junk bv Publishers. Bowlby, J., and LeTendre, G. C. 1993. Lake Ontario stocking and marking program for 1992. In Lake Ontario Fisheries Unit 1992 Annual Report, Ontario Ministry of Natural Resources, Glenora, Ontario. Carlson, A. R., and Siefert, R. E. 1974. Effects of reduced oxy-

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