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.     

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.     

Spatial genetic structure and recruitment dynamics of burbot (Lota lota) in Eastern Lake Michigan and Michigan tributaries

Burbot (Lota lota) are the only freshwater member of the Cod like (Lotidae) family that have a circumpolar distribution and occupy the widest geographic distribution of all Laurentian Great Lakes fish species. Information regarding burbot spatial genetic structure and recruitment dynamics is critical for the development of effective management strategies. Although burbot are a species of conservation concern throughout their range, little demographic or behavioral information exists. We estimated levels of genetic diversity within, and the degree of spatial population structure between samples collected from Lake Michigan and tributaries of the Manistee River, MI. Measures of genetic diversity across 10 microsatellite loci were moderately high. Disparities between adult groups sampled in Lake Michigan and the Manistee River were notable for observed heterozygosity (0.662 vs 0.488) and allelic richness (11.7 vs 6.6). Significant levels of inter-population variance in microsatellite allele frequencies (F_ST 0.154 to 0.208) were detected between Lake Michigan and the Manistee River samples. Results indicate reproductive isolation between what plausibly may be riverine and lacustrine spawning life history types. Pedigree analyses for three cohorts sampled in the Manistee River revealed that a sizeable number of adults contributed reproductively to multiple cohorts, indicating spawning philopatry. While data were collected from restricted areas in lacustrine and river habitats, analyses revealing microgeographic genetic structuring, potentially attributed to life history polymorphisms, have significant implications for burbot management in the Great Lakes.     

Spatial and seasonal comparisons of growth of wild and stocked juvenile lake trout in Lake Champlain

After 42 years of stocking in Lake Champlain, recruitment of wild juvenile lake trout (Salvelinus namaycush) was first observed in 2015. Abundance of wild lake trout juveniles was spatially heterogeneous. Recruitment of wild fish to age-1 and subsequent survival are likely related to growth including overwinter growth. We hypothesized that growth potential or growth-related mortality of wild and stocked fish may explain spatial differences in abundance. We collected juvenile (age-0 to 3) lake trout by bottom trawling in the central, north, and south Main Lake every 2–4 weeks during the ice-free season, 2015–2018. The percentage of wild juveniles increased from 27.8% of the total catch in 2015 to 65.7% in 2018. Rates of growth in length and change in condition were compared in wild versus stocked lake trout, among sampling areas, and between seasons (sampling season relative to winter). Wild juveniles grew equally or faster in length than stocked juveniles at the same age, but changed more slowly in condition. There was a higher percentage of wild juveniles in the central sampling area than the north and south, but no differences in growth among sampling areas. Wild and stocked fish grew in length over winter, but most cohorts (6 of 7) maintained or increased condition. Results indicate high growth potential of wild juvenile lake trout and progress toward population restoration.     

Differences in seasonal distribution of wild and stocked juvenile lake trout by depth and temperature in Lake Champlain

Lake trout (Salvelinus namaycush) reared in hatcheries are exposed to an environment and feeding regime that is different from wild lake trout, and are stocked at substantially larger sizes with higher lipid reserves. In addition to differences in diet and growth, this early experience may alter habitat use compared to the wild cohort. We used seasonal data on the depth and temperature distribution of wild and stocked juvenile lake trout to test for differences in habitat use and inform sampling strategies to evaluate annual recruitment. Bottom trawling was conducted from 2015 to 2019 in the central basin of Lake Champlain every two to four weeks during the ice-free season. Differences in distribution of wild and stocked lake trout were most pronounced during thermal stratification, when wild juveniles were more abundant than stocked juveniles at shallower depths and warmer temperatures and stocked juveniles were more abundant at deeper depths and colder temperatures. Temperature preferences may be a consequence of different early rearing environments; wild lake trout are acclimated to lake temperatures and forage, whereas stocked fish entered the lake with high lipid content and little foraging experience. Unbiased assessment of the proportion of wild lake trout and growth and survival of the entire juvenile lake trout population using bottom trawl sampling should either take place in the pre- and post-stratification seasons when wild and stocked fish are at the same depths, or include the full range of depths and temperatures that wild and stocked fish occupy during the stratified period.     

Thiamine concentrations in lake trout and Atlantic salmon eggs during 14 years following the invasion of alewife in Lake Champlain

Thiamine (vitamin B₁) deficiency in Great Lakes salmonines has been linked to consumption of alewife Alosa pseudoharengus. Thiamine deficiency has been recognized as a possible impediment to lake trout Salvelinus namaycush recruitment in the Great Lakes and Atlantic salmon Salmo salar recruitment in the Finger Lakes and Baltic Sea. Alewife invaded Lake Champlain in 2003 which provided an opportunity to investigate changes in thiamine concentrations in salmonine predators during an alewife invasion. We monitored egg unphosphorylated and total thiamine concentrations in lake trout and Atlantic salmon in 2004 and 2007–2019, assessed whether concentrations were associated with mortality, and examined thiaminase activity in alewife. Total thiamine concentrations in lake trout and Atlantic salmon were significantly lower than in 2004 for seven of the ten collection years for lake trout and for nine of the 12 collection years for Atlantic salmon. Mortality and signs of thiamine deficiency were observed in laboratory-reared Atlantic salmon free embryos but not in lake trout. Average thiaminase activity in adult alewife declined from 5200 pmol/g/min in 2006 to 1500 pmol/g/min in 2012. Our results provide further evidence that a diet that includes alewife reduces egg thiamine concentrations in salmonines. This effect was observed within four years of the invasion of alewife.     

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.     

Reevaluation of lake trout and lake whitefish bioenergetics models

Using a corrected algorithm for balancing the energy budget, we reevaluated the Wisconsin bioenergetics model for lake trout (Salvelinus namaycush) in the laboratory and for lake whitefish (Coregonus clupeaformis) in the laboratory and in the field. For lake trout, results showed that the bioenergetics model slightly overestimated food consumption by the lake trout when they were fed low and intermediate rations, whereas the model predicted food consumption by lake trout fed ad libitum without any detectable bias. The slight bias in model predictions for lake trout on restricted rations may have been an artifact of the feeding schedule for these fish, and we would therefore recommend application of the Wisconsin lake trout bioenergetics model to lake trout populations in the field without any revisions to the model. Use of the Wisconsin bioenergetics model for coregonids resulted in overestimation of food consumption by lake whitefish both in the laboratory and in the field by between 20 and 30%, on average. This overestimation of food consumption was most likely due to overestimation of respiration rate. We therefore adjusted the respiration component of the bioenergetics model to obtain a good fit to the observed consumption in our laboratory tanks. The adjusted model predicted the consumption in the laboratory and the field without any detectable bias. Until a detailed lake whitefish respiration study can be conducted, we recommend application of our adjusted version of the Wisconsin generalized coregonid bioenergetics model to lake whitefish populations in the field.     

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.     

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.     

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.     

Recruitment of lake trout in Lake Champlain

Lake trout were extirpated from Lake Champlain by 1900, and are currently the focus of intensive efforts to restore a self-sustaining population. Stocking of yearling lake trout since 1972 has re-established adult populations, spawning occurs at multiple sites lake-wide, and fry production at several sites is very high. However, little to no recruitment past age-0 has occurred, as evidenced by the absence of adults without hatchery fin clips in fall assessments; no regular sampling for juveniles is conducted. We began focused sampling for juvenile lake trout in fall, 2015, in the Main Lake using bottom trawling, and expanded sampling to sites in the north and south of the lake in 2016. In 2015 we collected 303 lake trout < 350 mm total length, of which 23.8% were unclipped. Based on non-overlapping length modes, these wild fish comprised at least three age classes (young-of-year, age-1, and age-2). In 2016, we collected 1215 lake trout < 350 mm, including a fourth wild year class (2016 young-of-year). Forty-nine percent of juvenile lake trout from the Main Lake were unclipped; however, only 20% from the north lake and 9% from the south lake were unclipped. The absence of older unclipped fish indicates that recruitment of wild fish began recently. We discuss several hypotheses to explain this sudden, substantial recruitment success, and factors that may be affecting lake trout restoration in Lake Champlain and the Great Lakes.     

Foredune dynamics at a Lake Michigan site during rising and high lake levels

On Great Lakes dunes, the link between foredune dynamics and coastal processes is seen in dune responses to changing lake levels. This paper investigates foredune dynamics during a recent period of rising and high lake levels. The study location was an active foredune in P.J. Hoffmaster State Park on the east coast of Lake Michigan, where field data were collected from 2000 through the final destruction of the foredune by wave removal in November 2019. Foredune dynamics were studied with erosion pins, direct observations, photographs, mapping, and on-site wind measurements. Regional climate and lake-level data were obtained from established data collection programs. The response of the foredune to rising lake levels was compared to several models of foredune behavior. During the study, the Lake Michigan-Huron level rose 1.89 m from January 2013 to July 2020. After an early transitional period, foredune activity was characterized by scarp retreat (4–19 m per year) and dune narrowing from 2014 to 2019. When the foredune completely disappeared in November 2019, erosion/scarping began on the next landward dune. The foredune activity fits Olson’s (1958) model for foredune growth and erosion through lake-level cycles. The foredune migration predicted by the revised Davidson-Arnott (2021) model of foredune response to relative water level rise did not occur, most likely because the rate of lake-level rise was too high. The six years of foredune narrowing before wave erosion started affecting the next landward dune represent a time-lag in Lake Michigan dune history models of increased dune activity during high lake-level stands.     

Decline in bloater fecundity in Southern Lake Michigan after decline of Diporeia

Population fecundity can vary through time, sometimes owing to changes in adult condition. Consideration of these fecundity changes can improve understanding of recruitment variation. Herein, we estimated fecundity of Lake Michigan bloater Coregonus hoyi during December 2005 and February 2006. Bloater recruitment has been highly variable from 1962 to present, and consistently poor since 1992. We compared our fecundity vs. weight regression to a previously published regression that used fish sampled in October 1969. We wanted to develop a new regression for two reasons. First, it should be more accurate because it uses fish collected closer to spawning, thus minimizing the potential for atresia (egg reabsorption) which could bias fecundity high. Second, we hypothesized that fecundity would be lower in 2006 because adult condition was 41% lower in 2006 compared to 1969, likely owing to the decline of Diporeia spp, a primary prey for bloater. Although the slope of the fecundity versus weight regression was similar between the years, fecundity was 24% lower in 2006 than in 1969 for bloater weighing between 70 and 240 g. Whether this was the result of the difference in sampling time prior to spawning or of differences in condition is unknown. We also found no relationship between maternal size and mature oocyte size. Incorporating our updated fecundity regression into a stock/recruit model failed to improve the model fit, indicating that the low bloater recruitment that has been observed since the early 1990s is not solely the result of reduced fecundity.     

Diet composition and overlap for adult walleye,lake whitefish,and yellow perch in Green Bay,Lake Michigan

Interspecific interactions among walleye Sander vitreus, lake whitefish Coregonus clupeaformis, and yellow perch Perca flavescens in Green Bay could influence the population status of each species, but potential trophic interactions are poorly understood. Our objectives were to determine if diet assemblages for each species and diet overlap among species varied spatially and temporally within Green Bay. Adult walleye (≥381 mm total length (TL); N = 981), lake whitefish (≥432 mm TL; N = 1507), and yellow perch (≥150 mm TL; N = 1174) were collected during May-October of 2018 and 2019 from multiple locations in southern and northern Green Bay. Diet assemblages of each species varied between northern and southern Green Bay, but walleye diets were more temporally variable (among months within zones and between years) than diets of lake whitefish or yellow perch. Lake whitefish represented a seasonally important prey item for walleye in southern Green Bay, composing 10 % and 41 % of walleye diets by weight in May and June, respectively. Yellow perch generally composed <15 % of walleye diets by weight but were consumed at a broader spatiotemporal scale than lake whitefish. Diet overlap between walleye and both lake whitefish and yellow perch was generally weak or moderate, whereas diet overlap between whitefish and perch was generally strong. Our assessment of adult trophic interactions suggests that changes in the population status of one species could influence fisheries for all three, and we identify additional research questions to address potential population-level effects of these trophic interactions.     

Evaluating the growth potential of sea lampreys (Petromyzon marinus) feeding on siscowet lake trout (Salvelinus namaycush) in Lake Superior

Differences in the preferred thermal habitat of Lake Superior lake trout morphotypes create alternative growth scenarios for parasitic sea lamprey (Petromyzon marinus) attached to lake trout hosts. Siscowet lake trout (Salvelinus namaycush) inhabit deep, consistently cold water (4-6 °C) and are more abundant than lean lake trout (Salvelinus namaycush) which occupy temperatures between 8 and 12 °C during summer thermal stratification. Using bioenergetics models we contrasted the growth potential of sea lampreys attached to siscowet and lean lake trout to determine how host temperature influences the growth and ultimate size of adult sea lamprey. Sea lampreys simulated under the thermal regime of siscowets are capable of reaching sizes within the range of adult sea lamprey sizes observed in Lake Superior tributaries. High lamprey wounding rates on siscowets suggest siscowets are important lamprey hosts. In addition, siscowets have higher survival rates from lamprey attacks than those observed for lean lake trout which raises the prospect that siscowets serve as a buffer to predation on more commercially desirable hosts such as lean lake trout, and could serve to subsidize lamprey growth.     

Predation by alewife on lake trout fry emerging from laboratory reefs: Estimation of fry survival and assessment of predation potential

Alewife (Alosa pseudoharengus) predation may be an important mortality source on lake trout fry (Salvelinus namaycush), and could affect the success of lake trout restoration in the Great Lakes. This study tested the prediction that fry showing typical swimming and avoidance behavior over artificial reefs will differ in survival when alewives are present versus when alewives are absent. Six tanks with cobble substrate were each stocked with 153 lake trout fry (density = 131 m^− 2), a density comparable to that recorded at Stony Island reef, Lake Ontario during the early 1990s. Four treatment tanks each contained ten alewives (density = 8 m^− 2) and two control tanks contained no alewives. After 12 days, mean recovery of fry was less in treatment tanks (31.5 fry per tank) than in control tanks (150 fry per tank; P < 0.009). Fry mortality in control tanks was about 2% in contrast to 46 to 91% mortality in tanks containing alewives. Alewife predation effects were evident early in the experiment as the mean daily capture of fry by traps set in each tank was always lower after day two in treatment tanks than in control tanks. The rate of consumption of lake trout fry by alewives ranged from 0.57 to 1.16 fry alewife^− 1 day^− 1 (mean = 0.99 ± 0.141; median = 1.12). The results of this study support the hypothesis that predation by alewives could cause a high level of lake trout fry mortality, and thus affect natural recruitment of lake trout and the success of population rehabilitation.     

Initial insights on the thermal ecology of lake whitefish in northwestern Lake Michigan

Lake whitefish Coregonus clupeaformis are a native coldwater species supporting important recreational and commercial fisheries in the Laurentian Great Lakes. Climate-related changes in water temperature may have important implications for the future sustainability of these fisheries. However, projecting future habitat availability is difficult because limited information is available on lake whitefish thermal ecology in the region. In this study, archival temperature loggers were implanted into 400 lake whitefish from northwestern Lake Michigan, including Green Bay, during October–November 2017. Loggers recorded temperature for 11 months at 4-hr intervals. Thirteen recovered temperature loggers were used in analyses. In winter (1 December–31 March), temperatures occupied by lake whitefish ranged from 0 to 8.0 °C, while in spring (1 April–31 May) temperatures ranged from 0 to 20.0 °C. In summer (1 June–15 September) and fall (16 September–7 November), lake whitefish occupied temperatures of 4–21.5 and 4–21.0 °C, respectively. Average temperatures in summer (10.8 °C) were within the previously proposed optimal temperature range (10–14 °C) and broad thermal niche (7–17 °C); however, 58% of observations were outside the optimal temperature range and 11% of observations were outside the broad thermal niche. Our results suggest that lake whitefish from northwestern Lake Michigan inhabit temperatures both above and below previously reported expected temperature ranges. This study provides initial insights on lake whitefish thermal ecology in Lake Michigan and can be used as a baseline for future work aimed at determining how lake whitefish habitat availability may change in the future.     

Evidence of spawning by lake trout Salvelinus namaycush on substrates at the base of large boulders in northern Lake Huron

Identification of lake trout spawning sites has focused on cobble substrates associated with bathymetric relief (e.g., ‘contour’ or ‘slope’ along reefs), but this ‘model’ may be narrow in scope. Previous telemetry work conducted near Drummond Island, USA, Lake Huron, identified egg presence in substrates at the base of large boulders (>1 m diameter); however, the extent of this phenomenon was unknown. Telemetry data paired with multi-beam bathymetry identified a 0.63 km² area used by lake trout characterized by low bathymetric relief and numerous (~269) large boulders (>1 m diameter) with small-diameter substrates at their bases. Diver surveys revealed egg presence at all 40 boulders surveyed, exclusively associated with clean gravel-cobble (0.6–42 cm) substrates in undercut areas beneath overhanging edges of boulders and in narrow spaces between adjacent boulders. Egg presence was not associated with boulder or substrate physical characteristics which highlighted the possible importance of interstitial currents. Successful incubation in these habitats was inferred by capture of free embryos and post-embryos the following spring using traps and an electrofishing ROV although at lower densities than at popular spawning habitats nearby (1–3 km away). Free embryos and post-embryos were also caught where eggs were not observed the previous fall including unexpectedly on top of boulders which suggested that post-hatch stages may move more than previously thought. Extensive use of boulder-associated habitats for spawning, egg incubation, and early growth suggested this undescribed habitat type may provide an unanticipated contribution to total available lake trout spawning habitat and recruitment in the Great Lakes.     

Evidence of successful river spawning by lake trout (Salvelinus namaycush) in the Lower Niagara River,Lake Ontario

Restoration of a wild-produced lake trout Salvelinus namaycush population in Lake Ontario has not been successful despite the adult population often meeting or exceeding restoration targets. Lack of high-quality spawning habitat in Lake Ontario is suggested as one impediment to recruitment of wild lake trout, although the quantity and location of spawning habitat is poorly understood. If high-quality spawning habitat is limited in Lake Ontario, lake trout may be using uncommon spawning locations such as rivers. Anecdotal angler accounts point to the Niagara River as a lake trout spawning location. To better understand the potential of the Niagara River as a spawning location, egg and juvenile fish collections were conducted 12–14 river kilometers from the mouth of the Niagara River from 2010 to 2012; and mature female lake trout with surgically implanted acoustic tags were monitored from 2015 to 2019. Genetic analyses confirmed 60% of collected eggs and 93% of collected post-hatch juvenile fish in the Niagara River were lake trout. Tagged female lake trout returned to the Niagara River over consecutive years during the spawning season. The short duration of lake trout presence in the river (mean = 56 days/year) suggests female lake trout use the Niagara River primarily for spawning. Diversity in spawning locations may provide lake trout population’s resilience against environmental variability through a portfolio effect. Improved identification of riverine spawning locations, including their overall contribution to wild recruitment, may be a useful tool for managers to restore a wild-produced population of lake trout in Lake Ontario.