Thomas A. Edsall

Lake Trout Spawning Habitat in the Great Lakes — a Review of Current Knowledge

Abstract We review existing information on lake trout spawning habitat, which might indicate whether habitat is now a limiting factor in lake trout reproductive success. Lake trout spawning habitat quality is defined by the presence or absence of olfactory cues for homing, reef location with respect to the shoreline, water depth, proximity to nursery areas, reef size, contour, substrate size and shape, depth of interstitial spaces, water temperature at spawning time, water quality in interstitial spaces, and the presence of egg and fry predators. Data on factors which attracted native spawners to spawning reefs are lacking, due to the absence of historic data on egg deposition. No direct evidence of egg deposition has been collected from sites deeper than 18 m. Interstitial space and, therefore, substrate size and shape, appear to be critical for both site selection by adults and protection of eggs and fry. Water quality is clearly important for egg incubation, but the critical parameters which define water quality have not yet been well determined in the field. Exposure to wave energy, dictated in part by reef location, may maintain high water quality but may also damage or dislodge eggs. The importance of olfactory cues, water temperature, and proximity to nursery habitat to spawning trout is unclear. Limited data suggest that egg and fry predators, particularly exotic species, may critically affect fry production and survival. Although availability of physical spawning habitat is probably not limiting lake trout reproduction, changes in water quality and species composition may negatively affect early life stages. This review of habitat factors that affect early life stages of lake trout suggests several priorities for research and management.

Availability of Lake Trout Reproductive Habitat in the Great Lakes

Abstract A decades-long program to reestablish self-sustaining stocks of lake trout (Salvelinus namaycush) in the four lower Great Lakes produced excellent fisheries supported by stocked fish. These fish spawned widely and small numbers of their offspring were collected intermittently from Lakes Michigan, Huron, and Ontario, but no self-sustaining stocks were established. In this paper we address habitat sufficiency as a factor in the failure of stocked lake trout to established self-sustaining populations in the four lower Great Lakes. We present the previously unpublished results of lake trout spawning habitat surveys conducted at seven sites in the Great Lakes since 1987 and we compare them with the published results of similar surveys conducted at 24 other sites in the four lower lakes since 1981. Our evaluation indicates all but two of these sites can support the production of viable fry from spawnings by the shallow-water strains of lake trout that are stocked in the Great Lakes. However, some of the best spawning, egg, and fry habitat in the lower Great Lakes seems to be at deeper offshore sites that may be unattractive to these shallow-water strains. Thus, we suggest also stocking the lower four lakes with strains from Lake Superior that might more fully exploit the best spawning habitat at these deeper, offshore sites.

Burrowing mayflies ( Hexagenia ) as indicators of ecosystem health

Three State of the Lakes Ecosystem Conferences have been held since 1996 to encourage the development of Great Lakes indicators of ecosystem health for use in reporting on progress in restoring and maintaining the chemical, physical and biological integrity of the Great Lakes ecosystem. Here we report on the development of an indicator based on burrowing mayflies , Hexagenia (Ephemeroptera: Ephemeridae), using production and biomass as the indicator metrics. Burrowing mayflies were selected because they (1) were historically abundant in unpolluted, soft-bottomed mesotrophic habitats throughout the Great Lakes, (2) are intolerant of and were extirpated by pollution in most of those habitats during the 1940s to1950s, (3) have shown the ability to recover in one of those habitats following pollution abatement, (4) are ecologically important as bioturbators of lakebed sediments and as trophic integrators that link detrital energy resources directly to fishes that feed preferentially on them, and (5) have highly visible mating flights, which carry the message directly to an informed public that the source water body is healthy. In addition, their annual production can be estimated from their mean annual biomass by the sizefrequency method. Productivity and biomass can also could be estimated with a cohort-direct method, using the biomass of mature nymphs collected in May or early June from the cohort that is about to emerge as subimagos in late June or early July. Although both the size-frequency and cohort-direct methods provide reliable estimates of productivity and biomass, the latter method greatly reduces sample collection and processing effort and thus makes it feasible to use Hexagenia as an indicator of ecosystem health in surveys requiring the collection of large numbers of samples.

The growth-temperature relation of Juvenile Lake whitefish

Abstract The lake whitefish Coregonus clupeaformis supports major commercial fisheries in Lakes Superior, Huron, and Michigan, where it is managed on a sustained-yield basis; it also supports a recreational hook-and-line fishery in some Great Lakes embayments and nearshore areas. To better understand habitat use by juvenile lake whitefish in the Great Lakes, we acclimated groups of test fish in the laboratory to 5, 10, 15, 18, 21, and 24°C and fed them to excess twice daily for 55 d. The test fish increased in length and weight at all of the test temperatures and at the end of the study were heaviest and longest at 18.1°C. A curve fitted to the specific growth rate data indicated that the optimum temperature for growth was 18.5°C and, thus, that the fundamental thermal niche for juvenile lake whitefish is 15.5–19.5°C. Our results support the limited, published information on thermal ecology of wild, free-ranging juvenile lake whitefish in the Great Lakes.

The Effect of Temperature and Ration Size on the Growth, Body Composition, and Energy Content of Juvenile Coho Salmon

Juvenile (post-smolt) coho salmon (Oncorhynchus kitsuch) were held in fresh water in the laboratory at 5, 10, 15, and 18°C for 8 weeks and fed freshly thawed, juvenile alewives (Alosa pseudoharengus) at rates equal to 1 and 2 % of their wet body weight/day, and also at the ad libitum or unrestricted ration rate. Most rapid growth in weight (1.2% wet body weight/day) occurred among fish fed the ad libitum ration at 15°C; growth was most rapid at about 10°C for fish fed the 2% ration (0.7% /day), and the 1% ration (0.1% /day). Gross conversion efficiency was highest at 10°C for all three ration levels. Gross body constituents and energy content of the test fish changed with temperature and ration during the study. Growth rate was positively related to lipid, energy content, and ration; lipid and energy content were positively related to water temperature; lipid, energy content, growth rate, ration, and water temperature were negatively related to water content; and protein was not related to any of the test variables. At the end of the study, water (68.7 to 76.4%) and lipid (3.5 to 10.4%) content were more variable than ash (1.8 to 3.1%), carbohydrate (0.1 to 1.9%), and protein (16.9 to 19.4%) content. Energy content of the fish increased with ration and was highest for each ration level at 15°C.

An Evaluation of Lake Trout Reproductive Habitat on Clay Banks Reef, Northwestern Lake Michigan

Abstract The extinction of the native populations of lake trout (Salvelinus namaycush) in Lake Michigan in about 1956 has been followed by a decades-long attempt to reestablish self-sustaining populations of this valuable species in habitats it formerly occupied throughout the lake. One of the most recent management strategies designed to facilitate recovery was to make a primary management objective the establishment of sanctuaries where stocked lake trout could be protected and self-sustaining populations reestablished. In the present study we employed habitat survey and mapping techniques, field and laboratory bioassays, egg traps, sediment traps, and gill nets to examine the potential for successful natural reproduction by stocked lake trout on Clay Banks Reef in the Door-Kewaunee sanctuary in Wisconsin waters of Lake Michigan. Our study revealed (1) there was suitable habitat on the reef to support the production of viable fry, (2) spawner abundance on the reef was the highest recorded in the Great Lakes, and (3) eggs taken from spawners on the reef and held on the reef in Plexiglas incubators hatched and produced fry that survived through swim-up. We conclude that Clay Banks Reef has the potential to support successful natural reproduction by stocked lake trout.

Survival of Lake Trout Eggs on Reputed Spawning Grounds in Lakes Huron and Superior: in situ Incubation, 1987–1988

Lake trout reproduce widely in Lake Superior but little in Lake Huron. We examined whether survival of lake trout eggs and fry in either lake was reduced by physical disturbances and swim-up mortality. Eggs were collected from feral lake trout in Lake Superior and placed in 108 plastic incubators. A total of 48 incubators was set at Partridge Island Reef in southern Lake Superior, 48 were set at Port Austin Reef in southern Lake Huron, and 12 were held as controls inflowing well water at a laboratory. Survival-to-hatching of these eggs at Partridge Island Reef (18%) was significantly different from that at Port Austin Reef (43%) and significantly different in the laboratory (88%) from that at either reef (P < 0.05). During egg-fry incubation from 28 October 1987 to 5 May 1988, 11–18 cm of sediment accumulated in sediment traps placed on the reefs but < 1 cm of sediment was present on each reef in May 1988. Analysis showed that 44% of the eggs at Port Austin Reef and 28% of those at Partridge Island Reef were buried and killed by sediments. During the first week after deployment, mean wave energy was 90% higher at Partridge Island Reef and significantly different from that at Port Austin Reef. Wave energy may be a habitat condition that makes Partridge Island Reef less suitable than Port Austin Reef for incubation of lake trout eggs. Fry from eggs incubated at all three sites experienced no swim-up mortality. We conclude that in 1987–88 habitat conditions required for survival of lake trout eggs were more suitable at Port Austin Reef than at Partridge Island Reef.

Optimum Temperature for Growth and Preferred Temperatures of Age-0 Lake Trout

Abstract This study was performed to determine the thermal preferences and optimum temperature for growth of age-0 lake trout Salvelinus namaycush to help predict the thermal habitat they select when they leave the spawning grounds and to assess the risk posed to them in the Great Lakes by piscivorus, nonnative fishes whose thermal habitat preferences are known. The test fish were hatched in the laboratory from eggs taken from wild fish, acclimated to 5, 10, 15, and 18°C, and fed to excess with commercial trout food for 47 d. The test fish grew at all of the temperatures, and the specific growth rate was highest at about 12.5°C (3.8% wet body weight/d). Fish used in the growth study were also tested in a vertical thermal gradient tank and had a final thermal preferendum between 10.1°C and 10.2°C. These results, which generally agreed with those of an earlier laboratory study of the temperature preference of age-1 lake trout and the limited information on thermal habitat use by age-0 lake trout in the Grea...

Burrowing mayflies as indicators of ecosystem health: Status of populations in western Lake Erie, Saginaw Bay and Green Bay

Abstract The U.S. Environmental Protection Agency and Environment Canada are supporting the development of indicators of ecosystem health that can be used to report on progress in restoring and maintaining the Great Lakes ecosystem, as called for in the Great Lakes Water Quality Agreement between the United States and Canada. One indicator under development is based on burrowing mayflies (Hexagenia: Ephemeroptera: Ephemeridae). We sampled in western Lake Erie, Saginaw Bay (Lake Huron), and Green Bay (Lake Michigan) in spring 2001 at 117 stations covering about 1,870 km2 of lake bed, to determine the status of nymphal populations of Hexagenia, and to provide information that would further the technical development of an indicator of ecosystem health based on Hexagenia. In western Lake Erie, density and biomass of nymphs were generally highest on fine-grained substrate in offshore waters and were lower on coarser substrates in near shore waters. Nymphs were virtually absent from Saginaw Bay, where only one nymph was collected at 28 stations. Nymphs were collected at only 6 of 48 stations in Green Bay, and density and biomass were highest at the northern end of the bay. Polluted sediments are likely responsible for the absence or low density and biomass of nymphs observed on fine-grained substrates in western Lake Erie, Saginaw Bay, and Green Bay, all of which historically supported abundant populations.

Starvation Resistance in Lake Trout Fry

Newly hatched fry were acclimated to 7 or 12°C and either fed daily (controls) or denied food for varying lengths of time and then fed daily until the end of the study (day 91 at 7°C and day 43 at 12°C). Growth was reduced by delays in the onset of feeding of 27 or more days at 7°C and 7 or more days at 12°C. Mortality of fry unfed for more than 34 days at 7°C, or more than 21 days at 12°C, was higher than among controls. Daily mortality increased with the length of the food deprivation period and did not cease immediately when food was made available, but reached zero by the end of the study. Mortality among unfed fry reached 50% in about 59 days at 7°C and 32 days at 12°C. Study results permitted calculation of the “point-of-no-return” (PNR) mortality, which included the mortality that occurred during the period of food deprivation, and also the delayed component of mortality that was directly attributable to starvation and that occurred after food was made available. The PNR for 50% mortality for food-deprived fry occurred after 52 days at 7°C and 24 days at 12°C. Thus, both measures of mortality indicate that lake trout fry would be highly resistant to death by starvation in the thermal habitat they would be expected to occupy in the Great Lakes. We conclude that a more likely adverse effect of reduced food availability would result from a reduction in growth rate that extends the length of time fry remain small and vulnerable to predation by adult alewives and other non-native fishes with which they associate.