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.     

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.     

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.     

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.     

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.     

Changes in a Population of Exotic Rainbow Smelt in Lake Superior: Boom to Bust, 1974–2005

Changes in a population of rainbow smelt (Osmerus mordax) in the Apostle Islands region of Lake Superior were chronicled over a 32-yr time series, 1974-2005. At the beginning of the time series, rainbow smelt was the predominant prey species, abundance of lake herring (Coregonis artedi) was very low, and the dominant predator was stocked lake trout (Salvelinus namaycush). Following a period of successful lake trout stocking in the 1970s, the rainbow smelt population declined sharply in 1980, largely through mortality of adult fish and subsequent poor recruitment. In the succeeding 4 years, rainbow smelt populations reached historic low levels, resulting in reduced food resources for both wild and stocked lake trout. During 1985–1990 lake herring stocks began a spectacular recovery following the appearance of a very strong 1984 year class and subsequent 1988, 1989, and 1990 year classes. Rainbow smelt benefited from the high abundance of young lake herring as an alternate prey source for lake trout and showed a partial recovery in the late 1980s. However, a growing lake trout population coupled with an 8-yr period of low herring reproduction after 1990 resulted in a diminished rainbow smelt population dominated by age-1 and 2 fish and showing a pattern of alternating recruitment attributed to cannibalism. Low productivity of rainbow smelt and intermittent production of herring over the past decade has left lake trout populations with a diminished prey base. Although lake trout recovery benefited from the presence of rainbow smelt as a prey resource, the Lake Superior fish community was fundamentally altered by the introduction of rainbow smelt.     

Factors influencing egg thiamine concentrations of Lake Ontario lake trout: 2019–2020

In the Great Lakes region, thiamine deficiency is considered a recruitment bottleneck for lake trout Salvelinus namaycush and has been correlated with the consumption of non-native alewife Alosa pseudoharengus. While alewife, the most abundant forage fish in Lake Ontario, are the predominant prey for lake trout, they also consume benthic prey such as round goby Neogobius melanostomus. Because variation in the proportion of alewife in lake trout diets is linked to variation in egg thiamine concentrations, understanding how factors such as region of capture and hatchery-strain of lake trout influence diet, are key to understanding the patterns of variation in egg thiamine concentrations observed in this species. With recent increases in natural recruitment of lake trout being observed in the western region of the lake, understanding if egg thiamine is a potential driver is crucial to the rehabilitation of lake trout. In this study, we evaluated egg thiamine concentrations in lake trout during 2019–2020. We found no significant difference in egg thiamine concentrations among regions. However, a stocked Lake Superior deepwater morphotype (Superior Klondike Wild – SKW) showed significantly higher egg thiamine concentrations compared to the lean morphotype including Seneca (SEN) and Lake Champlain Domestic (LCD) strains. An analysis of fatty acid signatures of each hatchery-strain suggested that the SKW strain consumed a higher proportion of round goby than lean strains. Overall, these results suggest that morphotypic differences in the feeding ecology of lake trout can result in biochemical changes which may influence the effectiveness of restoration efforts.     

A Comparison of Lake Trout Spawning,Fry Emergence,and Habitat Use in Lakes Michigan,Huron, and Champlain

Restoration of self-sustaining populations of lake trout is underway in all of the Great Lakes and Lake Champlain, but restoration has only been achieved in Lake Superior and in Parry Sound, Lake Huron. We evaluated progress toward restoration by comparing spawning habitat availability, spawner abundance, egg and fry density, and egg survival in Parry Sound in Lake Huron, in Lake Michigan, and in Lake Champlain in 2000–2003. Divers surveyed and assessed abundance of spawners at 5 to 15 sites in each lake. Spawning adults were sampled using standardized gill nets, eggs were sampled using egg bags, and fry were sampled using emergent fry traps and egg bags left on spawning reefs overwinter. Spawning habitat was abundant in each lake. Adult lake trout abundance was low in Lake Michigan and Parry Sound, and very high at one site in Lake Champlain. Egg deposition was lowest in Lake Michigan (0.4–154.5 eggs•m⁻², median = 1.7), intermediate in Parry Sound (39–1,027 eggs•m⁻², median = 278), and highest in Lake Champlain (0.001–9,623 eggs•m⁻², median = 652). Fry collections in fry traps followed the same trend: no fry in Lake Michigan, 0.005–0.06 fry•trap⁻¹ day⁻¹ in Parry Sound, and 0.08–3.6 fry•trap⁻¹ in Lake Champlain. Egg survival to hatch in overwinter egg bags was similar in Lake Michigan (7.6%) and Parry Sound (2.3–8.9%) in 2001–02, and varied in Lake Champlain (0.4–1.1% in 2001–02, and 1.8–18.2 in 2002–03). Lake trout restoration appears unlikely in northern Lake Michigan at current adult densities, and failure of restoration in Lake Champlain suggests that there are sources of high mortality that occur after fry emergence.     

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.     

Distinguishing Wild vs. Stocked Lake Trout (Salvelinus namaycush) in Lake Ontario: Evidence from Carbon and Oxygen Stable Isotope Values of Otoliths

We investigated the potential for using carbon and oxygen isotope values of otolith carbonate as a method to distinguish naturally produced (wild) lake trout (Salvelinus namaycush) from hatchery-reared lake trout in Lake Ontario. We determined δ ¹³C_(CaCO3) and δ ¹⁸O_(CaCO3) values of otoliths from juvenile fish taken from two hatcheries, and of otoliths from wild yearlings. Clear differences in isotope values were observed between the three groups. Subsequently we examined otoliths from large marked and unmarked fish captured in the lake, determining isotope values for regions of the otolith corresponding to the first year of life. Marked (i.e., stocked) fish showed isotope ratios similar to one of the hatchery groups, whereas unmarked fish, (wild fish or stocked fish that lost the mark) showed isotope ratios similar either to one of the hatchery groups or to the wild group. We interpret these data to suggest that carbon and oxygen isotope values can be used to determine the origin of lake trout in Lake Ontario, if a catalogue of characteristic isotope values from all candidate years and hatcheries is compiled.     

Evaluation of post-stocking dispersal and mortality of juvenile lake trout Salvelinus namaycush in Lake Ontario using acoustic telemetry

Wild reproduction by stocked lake trout Salvelinus namaycush in Lake Ontario has yet to produce a self-sustaining population, requiring a reliance on stocking. Once released, age-1 juvenile lake trout are not typically surveyed until age-2, creating a gap in knowledge of fine-scale post-release behaviors. A method to track fine-scale movements and estimate mortality of juvenile lake trout could complement standard survey methods and benefit management decisions regarding stocking locations. We used acoustic telemetry to estimate post-stocking mortality and observe fine-scale spatial and temporal movements of 38 hatchery-reared, age-1 lake trout from an offshore stocking site in the eastern basin of Lake Ontario from 2017 to 2018. Cumulative post-stocking mortality was estimated at 5.3%, 10.5%, and 26.3% after one week, one month and one year, respectively. The majority of lake trout (68.4%) emigrated from the stocking location within two months and entered deep water (～50 m) once warm-water incursions at the stocking site exceeded lake trout thermal preferences (15 °C). Lake trout made large movements (i.e., median 1.9 km, maximum 12.4 km straight-line distance) within the first hour post-release and had an average swimming speed of 1.64 km?hr^?1over the first day. There was no statistically significant relationship between total distance traveled and time of day, although distance traveled tended to be greater during crepuscular and dark periods compared to daylight. Our results provide a conservative estimate of post-release mortality and reveal behaviors of hatchery-reared juvenile lake trout that may be helpful when selecting stocking locations beneficial to restoration program goals.     

Contribution of lake trout stocked as fry to an adult population in Lake Superior

Lake trout Salvelinus namaycush fry treated with heated water to create thermal marks in their otoliths were stocked at Sve's Reef in Minnesota waters of Lake Superior in 1994, 1995, and 1996. These fish began to reach maturity in 2000, and were vulnerable to annual assessment gill nets set at several locations along the Minnesota shoreline. Captured fish also included fin-clipped lake trout stocked as yearlings, and naturally reproduced (wild) lake trout. Otoliths from 3106 unclipped lake trout were aged and examined for thermal marks from 2000 to 2007, of which 1152 were from the target year classes (1994–1996). Thermal marks were found in otoliths from 64 fish, or 5.6% of those in the target year classes, demonstrating that stocked fry contributed to the adult lake trout population in Minnesota waters. Although numbers of recaptured fish were too low to demonstrate statistically significant differences, higher recapture rates of marked fish at Sve's Reef in fall and spawning assessments suggest that these fish may have imprinted at the stocking location and homed back to this area to spawn. Wild lake trout populations in Lake Superior may be approaching fully rehabilitated levels, but recovery in the lower Great Lakes has progressed more slowly, and evidence of success with fry stocking could benefit those populations.     

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.     

Movement of Sea Lamprey in the Lake Champlain Basin

Sea lamprey (Petromyzon marinus) are a nuisance aquatic species in the Great Lakes and Lake Champlain that have devastated native fish populations and hampered the restoration of sport fisheries. This study examined inter-basin movement of sea lamprey in Lake Champlain to identify tributaries that contribute parasitic-phase sea lamprey and provide information for prioritizing those tributaries for sea lamprey control. A total of 4,125 recently metamorphosed sea lamprey was captured in tributaries to Lake Champlain and marked using coded wire tags between the fall of 2001 and winter 2003. These sea lamprey migrated to the lake to prey on salmonids and other fishes and returned to tributaries to spawn about 12–18 months after migration. We recaptured 6 tagged sea lamprey from the lake from spring 2002 through winter 2004, and 35 from tributaries in spring 2003 and 2004. We noted no apparent trends in movement among basins. Sea lamprey were collected at distances up to 64 km from their natal tributaries. Tributary contributions of parasites were significantly different from expectations in the 2002 parasitic-phase cohort (χ² = 9.668, p < 0.011, 3 df), suggesting differential survival rates among out-migrating transformers from different tributaries. Estimates of the lake-wide out-migrating transformer population for the 2002 and 2003 parasitic-phase cohorts were 269,139 ± 55,610 (SD) and 111,807 ± 23,511 (SD). Results from this study suggest that sea lamprey movement is not inhibited by causeways dividing sub-basins, but movement among sub-basins is somewhat constrained. This indicates that management efforts to control sea lamprey should continue to treat the lake as a single system.     

Fatty acids reveal salmonine – prey relationships in Lake Michigan

Lake Michigan salmon and trout populations are important species for recreational fisheries and food web management, and are largely supported through stocking efforts, with varying degrees of natural recruitment. Ongoing fisheries management of these salmonine populations is dictated by relationships between predator and prey abundance as well as community structure within the lake. However, while prey fish biomass has declined, and species composition has changed in recent decades, knowledge of prey consumption by the salmonine community has lagged. Herein, we explore trophic relationships using fatty acids profiles, which offer insights into the foraging habits and energy pathways relied on over weeks to months prior to collection. Fatty acids of the prey base for salmonines in Lake Michigan indicate a gradient of foraging habits that range from pelagic (typified by alewife and rainbow smelt) versus benthic (i.e., slimy sculpin and round goby) resource use. Fatty acids implied that there was more variation in foraging habits among individual lake trout and brown trout compared to Chinook salmon, coho salmon and rainbow trout, which appeared to all rely almost exclusively on pelagic prey. Fatty acid profiles also indicated size-based shifts in foraging habits; for example, larger lake trout consuming a greater proportion of benthic prey than smaller individuals. Data herein suggest that Chinook and coho salmon, as well as rainbow trout, are more likely to experience competitive interactions during times of low pelagic prey-fish abundance in Lake Michigan, whereas brown and lake trout are able to utilize benthic resources to a greater degree.     

Fatty acid profiles of lake trout reveal the importance of lipid content for interpreting trophic relationships within and across lakes

Fatty acid profiles increasingly are being used to quantify foraging patterns of consumers, but the associated interpretation may vary with the tissue type and its lipid content. For salmonids, lipid deposits can be found in both dorsal and ventral (“belly flap”) areas of muscle tissues. However, it is uncertain whether belly flap and dorsal muscle fatty acid profiles are similar in natural populations of fish. We examined how fatty acid profiles of belly flap compared to those of dorsal muscle and the consequent impacts on dietary inferences. Fatty acid profiles were derived from lake trout (Salvelinus namaycush) caught in five North American lakes: Champlain, Flathead, Michigan, Ontario, and Swan. Fatty acid profiles were most similar between tissues when lipid content of muscle was > ~10%, the threshold below which similarities decreased and thus increasingly affected dietary inference. Some fatty acids commonly used as trophic indicators preferentially accrued in one tissue over the other depending on lipid content of the tissues. Regardless of tissue type, fatty acid profiles were specific to each lake indicating that food web structures were distinctive over a broad geographic range. Fatty acid profiles of tissues from lakes Michigan and Ontario were highly similar, so were those from Flathead and Swan lakes, whereas those from Lake Champlain were distinct, having comparatively high proportions of 18:1n-9. We conclude that lipid storage areas like belly flaps likely provide a more accurate signal than muscle when using fatty acids to investigate dietary patterns, particularly when muscle lipid levels are low.     

Reproductive Potential and Fecundity of Lake Trout Strains in Southern and Eastern Waters of Lake Ontario, 1977–1994

We assessed the reproductive potential of various genetic strains of hatchery lake trout (Salvelinus namaycush) in southern and eastern Lake Ontario from indices of fecundity and indices of male abundance. Indices were constructed from catches of mature lake trout in gill nets during September 1980 to 1994 after correcting for mortality from sea lampreys (Petromyzon marinus) which occurred between September sampling and late fall spawning. Strain and age were assigned to individual lake trout based on clipped fins and maxillary bones or coded wire tags. Fecundity-length relationships for fish of the same age, determined from mature females collected in 1977 to 1981 and 1994, were not different (P > 0.05) among genetic strains. For all strains combined, fecundity-length relationships in 1977 to 1981 were not different among fish of various ages but in 1994, age-5 and -6 fish had fewer eggs (P < 0.003) than age-7 fish, and age-7 fish had fewer eggs (P < 0.003) than fish of age 8, 9, or 10. Annual indices of fecundity varied 19 fold and indices of mature males varied 11 fold; both indices were low in the early 1980s, increased sharply in the mid 1980s, and peaked in 1993. The strain which dominated fecundity and mature male indices shifted during the study from Seneca Lake strain to Lake Superior strain and then back to Seneca Lake strain. However, changes in either reproductive potential or genotypes do not appear responsible for the abrupt appearance of naturally-produced yearling lake trout throughout southern and eastern Lake Ontario in 1994–1995, the first widespread occurrence of juveniles produced by hatchery lake trout in Lake Ontario.     

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.     

Majority of age-3 Chinook salmon (Oncorhynchus tshawytscha) in Lake Ontario were wild from 1992 to 2005, based on scale pattern analysis

Stocking of hatchery-raised Chinook salmon has been the principal tool utilized by fishery managers for controlling alewives in Lake Ontario and elsewhere in the Great Lakes. Stocked Chinook salmon are also often viewed by anglers as the principal source of maintaining catch rates. Stocking levels are often controversial and set with limited information about the relative contribution of wild fish to lake-wide populations. Recent research documenting large numbers of age-0 fish in tributaries suggested that wild reproduction was increasing and greater than previously thought. Estimating the contribution of wild Chinook salmon is imperative for successful management of this economically important recreational fishery. To differentiate wild from hatchery-derived Chinook salmon, we developed and validated a classification rule from scale pattern analysis of known-origin fish that was based on the area of the scale focus and the distance between the scale focus and the first circulus. We used this technique to determine the annual proportion of angler-caught, age-3 wild Chinook salmon in Lake Ontario from 1992 to 2005. On average over 14 years, the annual proportion of wild age-3 Chinook salmon was 62% (± 13.6%, 95% CI), but has varied between 24% (± 9.4%) and 82% (± 11.2%). Wild fish have been a high proportion of the Chinook salmon population in Lake Ontario since the late 1980s throughout a period when the lake underwent considerable changes, suggesting that wild and hatchery-origin Chinook salmon are both important components for managing the predator–prey dynamics in Lake Ontario and maintaining angler catch rates.