Predation on emergent lake trout fry in Lake Champlain

The rehabilitation of extirpated lake trout (Salvelinus namaycush) in the Great Lakes and Lake Champlain has been hindered by various biological and physiological impediments. Efforts to restore a lake trout fishery to Lake Champlain include hatchery stocking and sea lamprey control. Despite these management actions, there is little evidence of recruitment of naturally-produced fish in annual fall assessments. Spawning occurs at multiple sites lake-wide in Lake Champlain, with extremely high egg and fry densities, yet sampling for juvenile lake trout has only yielded fin-clipped fish. To investigate this recruitment bottleneck, we assessed predation pressure by epi-benthic fish on emergent fry on two spawning reefs and the subsequent survival and dispersal of fry in potential nursery areas. Epi-benthic predators were sampled with 2-h gillnet sets at two small, shallow sites in Lake Champlain throughout the 24-h cycle, with an emphasis on dusk and dawn hours. In total, we documented seven different species that had consumed fry, with consumption rates from 1 to 17 fry per stomach. Rock bass and yellow perch dominated the near-shore fish community and were the most common fry predators. Predator presence and consumption of fry was highest between 19:00 and 07:00. Predators only consumed fry when fry relative abundance was above a threshold of 1 fry trap^− 1 day^− 1. We used an otter trawl to sample for post-emergent fry adjacent to the reef, but did not capture any age-0 lake trout. Due to the observed predation pressure by multiple littoral, species on shallow spawning reefs, lake trout restoration may be more successful at deep, offshore sites.     

Diet differences between wild and stocked age-0 to age-3 lake trout indicate influence of early rearing environments

Wild lake trout recently began to appear in abundance in Lake Champlain after over 40 years of stocking, providing an opportunity to compare the seasonal diet of wild and stocked juveniles. We sampled 2,349 age-0 to age-3 lake trout collected in bottom trawls from April to November 2015–2018, and examined the relationship between diet and spatial heterogeneity in abundance of wild and stocked juveniles. Stocked fish were, on average, the size of wild fish one year older. Wild juveniles had fewer empty stomachs and more items per stomach than stocked fish at each age. Mysis diluviana dominated the diet of age-0 and age-1 wild lake trout until they began to consume fish in fall at age-1. In contrast, the diet of newly-stocked fish (age-1) comprised rainbow smelt (Osmerus mordax), slimy sculpin (Cottus cognatus), alewife (Alosa pseudoharengus), with Mysis only abundant in summer and fall. Number and composition of diet items varied among geographic areas of the lake but did not explain differences in abundance of wild or stocked fish by area. Diet overlap was high between wild and stocked fish for each age class at each season, except in fall at age-0. Differences in the diet of wild and stocked juveniles likely reflect effects of early rearing experience. Recruitment of wild lake trout depends on availability and abundance of Mysis, but our diet data do not provide insight to explain why recruitment is finally occurring after a protracted delay.     

Evidence of lake trout (Salvelinus namaycush) natural reproduction in Lake Erie

Native lake trout were extirpated from Lake Erie around 1965 and committed restoration efforts began in 1982. In 2021 and 2022, a total of six lake trout (Salvelinus namaycush) in the free embryo or post-embryo life stage were captured in lake trout embryo traps in Lake Erie offshore of Shorehaven Reef, NY. This represents the first conclusive evidence of successful natural reproduction since extirpation. Trapping locations were identified using the results of a fine-scale positioning acoustic telemetry array, visual observations of adult lake trout exhibiting spawning behavior, and underwater cameras to visually identify possible spawning locations. Lake trout utilized a very specific spawning habitat type—the eastern side of shallow offshore humps in 5–8 m of water. These sites were comprised of habitat typically associated with lake trout spawning with slopes of 5–14° and clean rubble-cobble sized rock with visible interstitial spaces. Genetic barcoding was used to identify the post-embryo stage salmonids to species, and microsatellite genotypes assigned strongly to the Seneca strain which comprises the majority of the adult population. These findings represent a significant milestone for lake trout rehabilitation efforts in Lake Erie, confirming that successful reproduction to the post-embryo stage is possible and supporting continued rehabilitation efforts by Lake Erie management agencies.     

Lipid levels and diet compositions in lake charr ecotypes at Isle Royale in northern Lake Superior

Lipid levels have been reported for lake charr Salvelinus namaycush ecotypes from Lake Superior, but have not been investigated for all four extant ecotypes concurrently in the same geographic area. In this study, a Distell fatmeter was used to measure lipid levels in the muscle in lean, humper, redfin, and siscowet lake charr collected seasonally during 2013 and 2014 on the north, east, and west sides of Isle Royale, Lake Superior from two depth strata: <80 and ≥80 m. Diet compositions and dietary similarity of lake charr were also measured. Differences in lipid level were analyzed by ANCOVA with length and age as covariates and ecotype, depth, sex, and season as factors. Lipid levels were found to be higher in lake charr inhabiting waters ≥80 m. In waters <80 m, mean lipid level was highest in large siscowets (≥703 mm; 41.2%) and lowest in old redfins (≥age 32; 11.8%). At depths ≥80 m, mean lipid levels were highest in large siscowets (64.4%) and lowest in young redfins (14.7%). In both depth strata, leans and humpers had intermediate lipid levels. Diet compositions were similar among ecotypes and did not explain the differences in lipid levels. Siscowets have the greatest depth range and vertical movement patterns among lake charr ecotypes in Lake Superior which is likely facilitated by high levels of intramuscular lipid. Lipid levels in leans and siscowets are heritable traits, but heritability of lipid levels in other ecotypes remains unknown.     

Evidence of lake trout (Salvelinus namaycush) spawning and spawning habitat use in the Dog River,Lake Superior

Lake trout spawn primarily in lakes, and the few river-spawning populations that were known in Lake Superior were believed to be extirpated. We confirmed spawning by lake trout in the Dog River, Ontario, during 2013–2016 by the collection of and genetic identification of eggs, and we describe spawning meso- and microhabitat use by spawning fish. Between 2013 and 2016, a total of 277 lake trout eggs were collected from 39 of 137 sampling locations in the river. The majority of eggs (220) were collected at the transition between the estuary and the river channel crossing the beach. Lake trout eggs were most often located near the downstream end of pools in areas characterized by rapid changes in depth or slope, coarse substrates, and increased water velocities, where interstitial flows may occur. Depths in wadeable areas where eggs were found averaged 0.9?m (range: 0.4 to 1.3?m) and substrate sizes consisted of large gravel, cobble, and boulder; comparable to spawning characteristics noted in lakes. Water velocities averaged 0.66?m·s^?1 (range: 0.33 to 1.7?m³·s^?1) at mid-depth. This information on spawning habitat could be used to help locate other remnant river-spawning populations and to restore river-spawning lake trout and their habitat in rivers that previously supported lake trout in Lake Superior. The Dog River population offers a unique opportunity to understand the ecology of a river spawning lake trout population.     

Evidence of a remnant self-sustaining strain of lake trout in the Lake Michigan basin

Managers have long embraced the need to maintain diversity as a requisite condition for population and community sustainability. In the case of Great Lakes lake trout, diversity has been severely compromised. The identification of new gamete sources may be beneficial to lake trout reintroduction efforts, particularly in situations where native stocks have been completely extirpated such as in Lake Michigan. Lake trout from Elk Lake, Michigan, are genetically distinct from domestic hatchery strains and historical forms of lake trout from Lake Michigan. Importantly, Elk Lake fish were genetically distinct from Marquette strain lake trout which were previously stocked into Elk Lake. Elk Lake fish were most similar to Lake Michigan basin-derived Lewis Lake (LLW) and Green Lake (GLW) hatchery strains and to historical Lake Michigan populations from the Charlevoix, Michigan area. While all individuals exhibited characteristics of lean form lake trout, the body shape of lake trout from Elk Lake, stocked lean fish from Lake Michigan and Lake Superior wild lean strains from near Isle Royale differed. Elk Lake fish were more fusiform, elongate, and streamlined with a narrower caudal peduncle compared to hatchery lean strains and wild lean forms from the Isle Royale region of Lake Superior. The lake trout population in Elk Lake is a remnant of a now extirpated native Lake Michigan population that was established either by natural colonization or stocking from historical Lake Michigan populations. Elk Lake lake trout is as genetically diverse as other strains used in Great Lakes reintroduction efforts and likely represent a viable gamete source representing genetic diversity lost from Lake Michigan.     

Differential lipid dynamics in stocked and wild juvenile lake trout

After 45 years of stocking, lake trout in Lake Champlain have started to exhibit strong natural recruitment, suggesting a recent change in limiting factors such as prey availability or overwinter survival. The abundance of juvenile wild lake trout varies among regions of Lake Champlain which suggests the prey base, or foraging success, may vary geographically within the lake. One metric that can indicate differences in resources across regions is lake trout lipid content, which reflects the availability of food and serves as an important energy reserve for overwinter survival. We quantified total lipid content of stocked and wild age-0 to age-3 lake trout among lake regions and seasons. No spatial differences in lipid content were apparent, but wild fish had higher overall mean ± SE percent total lipid content (17.0 ± 0.7% of dry mass) than stocked fish (15.2 ± 0.7%). Lipids in fish stocked in November were high (35.1 ± 0.7% of dry mass) but dropped by spring (14.9 ± 1.3%) and continued to decline through autumn. Wild fish showed seasonal changes with winter depletion in lipids followed by summer increase, and a plateau in autumn. The lipid depletion in stocked fish poses two competing hypotheses: 1) the high lipid concentration is necessary for stocked age-0 fish to transition to foraging in the wild, or 2) the high lipid concentration is difficult to maintain on a wild diet and reduces survival in the first post-stocking year.     

Status and movement of adfluvial brook trout Salvelinus fontinalis at Isle Royale,Michigan, Lake Superior

Adfluvial brook trout Salvelinus fontinalis populations in the Lake Superior basin have suffered declines over the last century due to habitat degradation and exploitation. Natural resources agencies throughout the basin have restored habitat and implemented restrictive angling regulations as tools to protect remnant coaster brook trout populations, but the success of these practices is unknown in some locations. We used electrofishing and passive integrated transponder (PIT) tag movement data from 2008 to 2020 to describe population characteristics, temporal trends, and adfluvial life history of coaster brook trout in Washington Harbor, Isle Royale National Park, Lake Superior after implementation of catch-and-release regulations in 2004. Our results document the presence of an adfluvial coaster brook trout population in Washington Harbor, Isle Royale. Temporal trends indicate that brook trout abundance and distribution in Washington Harbor increased since earlier observations in the late 1990s and early 2000s likely due to enactment of protective regulations, and increased stream flows and water levels observed in recent years in both Washington Creek and Lake Superior, respectively. Annual tag detection of Washington Harbor brook trout in Washington Creek at the antenna arrays varied from 25% in 2009 to 100% in 2011 and 2013, and averaged 60.7% throughout the study. Peak in adfluvial brook trout use of Washington Creek was from August – October with few detections occurring outside of the presumed spawning period. This study provides valuable insight into the population characteristics, movement patterns, and temporal increase in abundance of an adfluvial brook trout population in Lake Superior.     

Heritage strain and diet of wild young of year and yearling lake trout in the main basin of Lake Huron

Restoration of lake trout Salvelinus namaycush stocks in Lake Huron is a fish community objective developed to promote sustainable fish communities in the lake. Between 1985 and 2004, 12.65 million lake trout were stocked into Lake Huron representing eight different genetic strains. Collections of bona fide wild fish in USGS surveys have increased in recent years and this study examined the ancestry and diet of fish collected between 2004 and 2006 to explore the ecological role they occupy in Lake Huron. Analysis of microsatellite DNA revealed that both pure strain and inter-strain hybrids were observed, and the majority of fish were classified as Seneca Lake strain or Seneca Lake hybrids. Diets of 50 wild age-0 lake trout were examined. Mysis, chironomids, and zooplankton were common prey items of wild age-0 lake trout. These results indicate that stocked fish are successfully reproducing in Lake Huron indicating a level of restoration success. However, continued changes to the benthic macroinvertebrate community, particularly declines of Mysis, may limit growth and survival of wild fish and hinder restoration efforts.     

Within-stream release-site fidelity of steelhead trout from Lake Erie hatchery stocks

Straying of salmonids in Lake Erie is not well understood despite the economic importance of these recreational fisheries, which are sustained by stocking approximately 2 million steelhead trout (Oncorhynchus mykiss) yearlings annually. The occurrence of straying in hatchery-reared salmonid populations can be influenced by stocking strategies, such as within-stream stocking location. Conneaut Creek provides a unique opportunity to evaluate the extent of release-site fidelity of adult steelhead trout from Lake Erie, because it is equally stocked by Ohio and Pennsylvania at different distances from the stream mouth. Adult steelhead trout were collected from two Conneaut Creek sites, Conneaut Ohio (2 km from Lake Erie) and Albion Pennsylvania (61 km from Lake Erie), in spring and fall of 2009. Elemental signatures of yearling otoliths measured by laser-ablation-inductively-coupled-plasma-mass-spectrometry were used to identify hatchery stocks. The state-specific hatchery stocks were identified with high confidence using discriminant analysis (Sr and Ba concentrations in nine otolith regions; Ohio 100.0%, Michigan 86.1%, New York 92.4%, and Pennsylvania 93.2% using jackknifed mean correct assignment). Adult steelhead trout (N = 174) collected in spring and fall at Conneaut Ohio included both Ohio and Pennsylvania-stocked fish, but no Ohio-stocked steelhead trout were collected at the Pennsylvania site in either season. Of the classified adult steelhead trout, 13.8% were identified as strays from other states (New York and Michigan). These results confirm strong release-site fidelity between Ohio and Pennsylvania stocked steelhead trout and provides fishery managers with sound scientific data to refine their stocking practices.     

Lake trout in northern Lake Huron spawn on submerged drumlins

Recent observations of spawning lake trout Salvelinus namaycush near Drummond Island in northern Lake Huron indicate that lake trout use drumlins, landforms created in subglacial environments by the action of ice sheets, as a primary spawning habitat. From these observations, we generated a hypothesis that may in part explain locations chosen by lake trout for spawning. Most salmonines spawn in streams where they rely on streamflows to sort and clean sediments to create good spawning habitat. Flows sufficient to sort larger sediment sizes are generally lacking in lakes, but some glacial bedforms contain large pockets of sorted sediments that can provide the interstitial spaces necessary for lake trout egg incubation, particularly if these bedforms are situated such that lake currents can penetrate these sediments. We hypothesize that sediment inclusions from glacial scavenging and sediment sorting that occurred during the creation of bedforms such as drumlins, end moraines, and eskers create suitable conditions for lake trout egg incubation, particularly where these bedforms interact with lake currents to remove fine sediments. Further, these bedforms may provide high-quality lake trout spawning habitat at many locations in the Great Lakes and may be especially important along the southern edge of the range of the species. A better understanding of the role of glacially-derived bedforms in the creation of lake trout spawning habitat may help develop powerful predictors of lake trout spawning locations, provide insight into the evolution of unique spawning behaviors by lake trout, and aid in lake trout restoration in the Great Lakes.     

Temporal and spatial variability of PCB concentrations in lake trout (Salvelinus namaycush) in Lake Superior from 1995 to 2016

Production of polychlorinated biphenyl compounds (PCBs) was banned over 40 years ago in the United States and Canada, but they remain a dominant contaminant in Great Lakes fish. Although PCB concentrations in Great Lakes fish have declined since the 1980s, there is concern that the rate of decline has slowed in recent years. Canadian and U.S. national agencies monitor trends in fish contaminants at only a few sites in each lake, while states and provinces monitor fish at more locations. In this study, we compare fish PCB measurements made by five agencies at multiple locations in order to evaluate spatial as well as temporal patterns in Lake Superior. For several monitoring locations, rapid increases and decreases in concentrations were observed. The wide range of concentrations (up to 1000 ng/g) reported among all stations in any single year is unlikely to result solely from differences in fish preparation or analytical techniques. Recent measurements indicate that spatial variation in fish PCB concentrations exists with peak concentrations 30-fold higher than lowest concentrations. After 1995, statistically significant changes, all declines, in PCB concentrations were observed at only three of seven locations; half-lives in these locations ranged from 9 to 14 years. Differences in diet and food web structure likely contribute both to the spatial variability of concentrations within the lake as well as to the rapid, short-term changes in concentrations at single sites that make long-term trends difficult to discern and to predict.     

Acoustic estimates of abundance and distribution of spawning lake trout on Sheboygan Reef in Lake Michigan

Efforts to restore self-sustaining lake trout (Salvelinus namaycush) populations in the Laurentian Great Lakes have had widespread success in Lake Superior; but in other Great Lakes, populations of lake trout are maintained by stocking. Recruitment bottlenecks may be present at a number of stages of the reproduction process. To study eggs and fry, it is necessary to identify spawning locations, which is difficult in deep water. Acoustic sampling can be used to rapidly locate aggregations of fish (like spawning lake trout), describe their distribution, and estimate their abundance. To assess these capabilities for application to lake trout, we conducted an acoustic survey covering 22 km² at Sheboygan Reef, a deep reef ( < 40 m summit) in southern Lake Michigan during fall 2005. Data collected with remotely operated vehicles (ROV) confirmed that fish were large lake trout, that lake trout were 1–2 m above bottom, and that spawning took place over specific habitat. Lake trout density exhibited a high degree of spatial structure (autocorrelation) up to a range of ∼ 190 m, and highest lake trout and egg densities occurred over rough substrates (rubble and cobble) at the shallowest depths sampled (36–42 m). Mean lake trout density in the area surveyed (∼ 2190 ha) was 5.8 fish/ha and the area surveyed contained an estimated 9500–16,000 large lake trout. Spatial aggregation in lake trout densities, similarity of depths and substrates at which high lake trout and egg densities occurred, and relatively low uncertainty in the lake trout density estimate indicate that acoustic sampling can be a useful complement to other sampling tools used in lake trout restoration research.     

Status of a stocked Atlantic salmon population in Lake Huron

In 2011, the Michigan Department of Natural Resources (MDNR) expanded stocking of Atlantic salmon (Salmo salar) in Lake Huron to enhance fishing opportunities following Chinook salmon (Oncorhynchus tshawytscha) abundance declines. Currently, little is known about the population produced from this stocking. We fit an assessment model to harvest data from Michigan jurisdictional waters to estimate Atlantic salmon population dynamics and abundance. Because of potential biases in MDNR creel survey harvest estimates, a survey was e-mailed to online purchasers of a 2019 Michigan fishing license asking about where and when Atlantic salmon were harvested to correct creel harvest estimates. Anglers were also asked about catch-and-release angling and tested on their ability to identify Lake Huron salmonids. Creel harvest estimates overlapped spatially and temporally with 42% of survey reported Atlantic salmon harvest. After correcting creel harvest estimates, total abundance of Atlantic salmon in 2019 was estimated at approximately 392,000 fish with a peak abundance of approximately 406,000 fish. Anglers released 27% of caught fish and correctly identified Atlantic salmon 28% of the time. To assess the occurrence of food resource competition, differences in condition (i.e., expected weight at length) were evaluated. Condition was higher in later years than in earlier years despite abundance increases. Our results suggested that past stocking established a population of approximately 400,000 Atlantic salmon with evidence suggesting that fish were finding sufficient food resources. Future Atlantic salmon management efforts may be improved by quantifying post-stocking survival rates and other sources (e.g., charter, Canadian) of harvest.     

Use of a thiamine antagonist to evaluate the effects of thiamine deficiency on lake trout embryonic development

To more fully understand the secondary effects of thiamine deficiency on embryo development of lake trout Salvelinus namaycush, we used an immersion protocol to administer graded doses of the thiamine antagonist oxythiamine (OXY) (e.g., 0, 12.5, 25, 50, 100, 200, and 400 mg/L). OXY was administered to eggs of six families at water hardening (fertilization) with the intent that early mortality syndrome (EMS), an acute thiamine deficiency mortality, would only occur at the highest OXY concentration. We assessed the effect of OXY and family on the occurrence of EMS and other embryonic mortality, and alevin growth dynamics up until 2 weeks post emergence. EMS reached only 4% at the highest OXY concentration (400 mg/L) where there was a significant decline in alevin length and yolkless alevin dry weight but not the dry weight of alevins whose yolk-sac was still attached. The dry weight of the yolk-sac as a percentage of total alevin dry weight was lower than controls at low concentrations of OXY (12.5, 25 mg/L), but higher than controls at the highest concentration of OXY (400 mg/L). OXY appeared to exert its effect on growth through reduced yolk-sac utilization although the mechanism(s) involved is unknown. Reduced growth and yolk-sac utilization of alevins resulting from thiamine deficiency may contribute to mortality through decreased ability of affected alevins to secure food and avoid predation. We conclude that even for very low EMS, alevin growth effects resulting from thiamine deficiency may pose a significant impediment to lake trout reproduction.     

Fatty acids in Great Lakes lake trout and whitefish

Fish are an excellent source of lean protein and omega-3 polyunsaturated fatty acids (PUFAs) but there is inadequate information on the levels of PUFAs in freshwater fish and specifically Great Lakes fish. Knowledge of PUFAs is necessary to make informed decisions regarding the balance between the benefits of fish consumption due to these factors versus risks of adverse health effects associated with elevated levels of contaminants known to be present in some Great Lakes fish and linked to increased risk of cancer and adverse neurological effects to both infants and adults. Our goal was to determine the lipid profiles in two species of Great Lakes fish, lake trout and whitefish. Total fat and the percentage of total and omega-3 PUFAs were with one exception significantly higher in lake trout than whitefish. Average concentrations of EPA + DHA were 11.2 and 9.7 g/100 g lipid in lake trout and whitefish, respectively. The concentrations of EPA + DHA in fatty marine fish (22.7, 23.9 and 30.2 g/100 g lipid, respectively) are about double those found in Great Lakes lake trout and whitefish. Nevertheless a 100 g serving of Great Lakes lake trout provides more than 500 mg of EPA + DHA, which is the daily intake level recommended by the American Dietetics Association for the prevention of coronary heart disease.     

Foraging mechanisms of age-0 lake trout (Salvelinus namaycush)

Reaction distances under various light intensities (0-19 uE/m2/s), angles of attack, swimming speeds, and percentage of overall foraging success were measured. Extensive efforts have been invested in restoring lean lake trout (Salvelinus namaycush) populations in the Laurentian Great Lakes, but successful natural recruitment of lake trout continues to be rare outside of Lake Superior and parts of Lake Huron. There is evidence of high mortality during the first several months after eggs hatch in the spring, but little is known about the foraging mechanisms of this age-0 life stage. We developed a foraging model for age-0 lake trout (S. namaycush) in response to amphipods (Hyalella azteca) and mysids (Mysis diluviana) by simulating underwater environmental conditions in the Great Lakes using a temperature-controlled chamber and spectrally matched lighting. Reaction distances under various light intensities (0–19 uE/m²/s), angles of attack, swimming speeds, and percentage of overall foraging success were measured. Intake rates under different light intensities and prey densities were also measured. Age-0 lake trout were non-responsive in the dark, but were equally responsive under all light levels tested. Age-0 lake trout also demonstrated a longer reaction distance in response to moving prey, particularly mysids, which had an escape response that reduced overall foraging success. We determined that prey intake rate (numeric or biomass) could be modeled most accurately as a function of prey density using a Michaelis–Menton equation and that even under low mysid densities (3 individuals/m²), age-0 lake trout could quickly satisfy their energetic demands in a benthic setting.     

Introgression of hatchery rainbow trout in naturalized steelhead populations of western Lake Superior

Fisheries managers and anglers in Minnesota have long been concerned that reproduction in the wild by a hatchery strain of rainbow trout (Oncorhynchus mykiss), called kamloops, could reduce the fitness of naturalized steelhead (migratory rainbow trout) populations in Lake Superior. A previous study found no evidence of kamloops introgression, but a new evaluation is warranted in light of continued stocking and genetic advancements over the past 25 years. We used genotypes from 10 microsatellite DNA loci to assess kamloops ancestry in adult and age-0 rainbow trout samples from 40 streams in Minnesota, one stream in Wisconsin, and two adult broodstocks. We also evaluated genetic population structure for impacts of kamlooops introgression. Overall, the average estimated kamloops ancestry was 8% (range 2–44%) in age-0 juveniles and 4% (1–19%) in adults. The average percentages of kamloops descendants in age-0 and adults were 8% (0–54%) and 5% (0–40%), respectively. Kamloops descendants were found in many Minnesota streams and in Wisconsin’s Bois Brule River, but were most prevalent in streams near the source hatchery, near recent stocking locations, and in lower reaches of longitudinally sampled streams. Feral steelhead broodstock had 9% and 6% kamloops descendants in two years while captive broodstock had no kamloops descendants. Minnesota’s populations showed little spatial genetic structure but were distinct from the Bois Brule population and from kamloops, which indicated that kamloops introgression had not substantially altered structure. Kamloops introgression into Minnesota steelhead populations and impacts on other jurisdictions contributed to the decision to discontinue the kamloops stocking program.     

Differences in pyloric caeca counts between lean and siscowet lake trout (Salvelinus namaycush) in southern Lake Superior

There are four documented morphotypes of lake trout (Salvelinus namaycush) in Lake Superior, with the two dominant forms being the shallow water lean and the deep water siscowet. These morphotypes are differentiated externally by morphometrics and meristics. Pyloric caeca counts have been used to distinguish closely related fish species including the Salvelinus genus. From samples collected in 2009, 2011–2013, and 2017, we counted and measured pyloric caeca from 116 lean and 119 siscowet lake trout from southern Lake Superior. For a subset of 22 leans and 19 siscowets between 575 and 625 mm, we measured individual pyloric caecum basal diameter, length, and estimated individual caecum and total caeca surface areas. Siscowets had significantly fewer and thicker pyloric caeca than leans, but caecum length did not differ between the morphotypes. Mean pyloric caeca count for siscowets and leans was 131 and 153, respectively. Mean individual caecum surface area was 31% higher in siscowets than in leans. When adjusted for mean total number of pyloric caeca, total caecum surface area for siscowets was 12% greater than leans. We postulate that greater pyloric caecum surface area in siscowets may be an adaption for greater lipid uptake because they have substantially higher lipid content than leans. Based on our findings, pyloric caeca counts can be used in addition to other meristic and morphometric characteristics to help distinguish lean and siscowet lake trout.     

Survey of Siscowet Lake Trout at Their Maximum Depth in Lake Superior

The siscowet Salvelinus namaycush is a deepwater morphotype of lake trout in Lake Superior. As part of a standardized lake-wide survey in 2006 to assess siscowet populations, bottom-set, multi-mesh gill nets were fished at 36.6 m depth intervals from near shore areas to the deepest waters in south-central Lake Superior. Siscowet length distributions, diet compositions, and sea lamprey wounding rates were compared for three depth zones: shallow (< 200 m), deep (200–394 m), and deepest (395–399 m). There were 39 siscowets collected in proximity to Lake Superior's greatest recorded depth of 405 m. To our knowledge, this is the greatest depth that fish have been collected in the Great Lakes. Higher proportions of siscowets ≤ 500 mm were caught in the shallow zone compared to deeper zones. Deepwater sculpins were the dominant prey for small siscowets (< 600 mm) across all depth zones. The diet of large siscowets (≥ 600 mm) among all depth zones comprised mostly of coregonines and burbot Lota lota. Terrestrial insects were observed in the diet of siscowets in all depth zones, indicating migration to the surface. Type A sea lamprey wounding rates were higher for large (≥ 600 mm) than small siscowets among all depth zones. The highest wounding rate was observed on large siscowets in the deep zone. Recent work indicates that siscowets are the most abundant lake trout form and this research indicates that siscowets use the maximum depths of Lake Superior.