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.     

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.     

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 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.     

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.     

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.     

Diet of lake trout and burbot in Northern Lake Michigan during spring: Evidence of ecological interaction

We used analyses of burbot (Lota lota) and lake trout (Salvelinus namaycush) diets taken during spring gill-net surveys in northern Lake Michigan in 2006–2008 to investigate the potential for competition and predator–prey interactions between these two species. We also compared our results to historical data from 1932. During 2006–2008, lake trout diet consisted mainly of alewives (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax), whereas burbot utilized a much wider prey base including round goby (Neogobius melanostomus), rainbow smelt, alewives, and sculpins. Using the Schoener's diet overlap index, we found a higher potential for interspecific competition in 1932 than in 2006–2008, though diet overlap was not significant in either time period. No evidence of cannibalism by lake trout or lake trout predation on burbot was found in either time period. In 2006–2008, however, lake trout composed 5.4% (by weight) of burbot diet. To determine whether this predation could be having an impact on lake trout rehabilitation efforts in northern Lake Michigan, we developed a bioenergetic-based consumption estimate for burbot on Boulder Reef (a representative reef within the Northern Refuge) and found that burbot alone can consume a considerable proportion of the yearling lake trout stocked annually, depending on burbot density. Overall, we conclude that predation, rather than competition, is the more important ecological interaction between burbot and lake trout, and burbot predation may be contributing to the failed lake trout rehabilitation efforts in Lake Michigan.     

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.     

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.     

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.     

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.     

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.     

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) 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.     

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.     

Exotic species in Lake Champlain

The Lake Champlain basin contains substantially fewer exotic species (N = 48) than the Great Lakes (N > 180), in part due to its isolation from commercial traffic. Exotic species have been introduced by authorized and unauthorized stocking, bait buckets, use of ornamental plants, and through the Champlain and Chambly canals that link the lake to the Hudson River, Mohawk River, Erie Canal, and the Great Lakes. Several species, such as water chestnut and zebra mussels, have had severe ecological, economic, and nuisance effects. The rate of appearance of new species increased in the 1990s, potentially as a result of increasing activity in the basin, improved water quality in the Champlain Canal, and increased sampling. Efforts to slow the introduction of new species have focused on public education and legislation to reduce bait bucket introductions and quarantine undesirable plants; however, the major remaining vector for introductions is the Champlain Canal. An estimated 20 species have entered the lake via canals, of which at least 12 used the Champlain Canal, and numerous species in the connected drainage systems could still enter via this route; some are already in the Erie Canal. Most recently (2008), the Asian clam was discovered two locks below Lake Champlain. The Lake Champlain canals also function as a conduit for exotic species exchange between the Hudson River, St. Lawrence River, and Great Lakes. The potential for future introductions could be reduced by a biological barrier on the Champlain Canal, and additional emphasis on public education.     

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.     

Tracking the Surface Flow in Lake Champlain

Understanding the hydrodynamics of Lake Champlain is a basic requirement for developing forecasting tools to address the lake‘s environmental issues. In 2003 through 2005, surface drifting buoys were used to help characterize the circulation of the main body and northeast region (Inland Sea) of the lake. Progressive vector diagrams of over-lake winds when compared to drifter trajectories suggest the presence of gyre-like circulation patterns. Drifter statistics suggest average current speeds of 10 cm s⁻¹ and were predominantly northward (+ V) due to northerly-directed winds and lake geometry. Singleparticle eddy diffusivities on the order of 10⁶ cm² s⁻¹ were calculated which is consistent with results from the Great Lakes and in some oceanic regions. However, the Lagrangian length and time scales, a measure of flow decorrelation scales, were in general smaller than seen in the Great Lakes, which is a natural consequence of the smaller basin size of Lake Champlain relative to the Great Lakes.     

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.     

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.