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Freshwater whitefishes, Salmonidae Coregoninae, are cold stenothermic fishes of ecological and socio-economic importance in northern hemisphere lakes that are warming in response to climate change. To address the effect of warming waters on coregonine reproduction we experimentally evaluated different embryo incubation temperatures on post-hatching survival, growth, and critical thermal maximum of larval cisco (Coregonus artedi) sampled from lakes Superior and Ontario. Embryos were incubated at water temperatures of 2.0, 4.4, 6.9, and 8.9°C to simulate present and increased winter temperatures, and hatched larvae were reared in a common environment. For both populations, larval survival and critical thermal maximum were negatively related to incubation temperature, and larval growth was positively related to incubation temperature. The magnitude of change across incubation temperatures was greater in the population sampled from Lake Superior than Lake Ontario for all traits examined. The more rapid decrease in survival and critical thermal maximum across incubation temperatures for larval cisco in Lake Superior, compared to those from Lake Ontario, suggests that Lake Superior larvae may possess a more limited ability to acclimate to and cope with increasing winter water temperatures. However, the rapid increase in growth rates across incubation temperatures in Lake Superior larvae suggests they could recover better from hatching at a small length induced by warm winters, as compared to Lake Ontario larvae. Our results suggest propagation and restoration programs may want to consider integrating natural habitat preferences and maximizing phenotypic variability to ensure offspring are set up for success upon stocking.

With the exception of lake whitefish (Coregonus clupeaformis), relatively little is known about the early life history of larval coregonines in the Laurentian Great Lakes. For example, our knowledge of the feeding ecology of larval coregonines (excluding lake whitefish) is based on only 900 stomachs reported in the literature. Here, we describe the diets and demographics of larval coregonines from ice-out to late July, and the contemporaneous zooplankton community, in the Apostle Islands region of Lake Superior in 2018. Exogenous feeding was evident among the smallest larvae (down to 6 mm). Percent of larvae with food in their stomachs increased and yolk reserves decreased as larvae grew from 10 to 13 mm. A majority of the diet (58%) was copepod nauplii, with generally positive selection for adult copepods and Holopedium. The patterns in exogenous feeding and yolk sac absorption were similar to observations in Lake Superior in the 1970s. Diets were also generally similar, although Limnocalanus, Holopedium, and zooplankton eggs were more prevalent in 2018 than the 1970s. Demographic data suggested at least two distinct cohorts and/or coregonine species in 2018. Post-hoc genetic testing of larvae in a parallel study suggested our samples comprised a mix of predominantly cisco (C. artedi), kiyi (C. kiyi), and bloater (C. hoyi). Early life history studies, when coupled with emerging genetic techniques that can identify larval coregonines to species, will provide a powerful combination to better understand population dynamics of coregonines at a time of ongoing restoration and rehabilitation efforts throughout the Great Lakes.

After more than 40 years of stocking, lake trout (Salvelinus namaycush) in Lake Champlain have started to exhibit strong, natural recruitment. The abrupt surge in recruitment suggests a change in limiting factors such as prey availability or overwinter survival. The distribution of juvenile wild lake trout varies in relative abundance among regions of Lake Champlain. The differences suggest the prey base, or foraging success, may vary geographically within the lake. Stocked and wild lake trout may differ in their ability to use resources and in overwinter survival. One metric that can indicate differences in resources across regions is lake trout lipid content, which reflects the quality of available food and serves as an important energy reserve for overwinter survival. We quantified total lipid content of stocked and wild juvenile lake trout across spatial (lake regions) and temporal (seasonal) scales. No spatial differences in lipid content were apparent. Wild fish had greater lipid content than stocked fish. Seasonally, stocked fish showed a continuous drop in lipid content from pre-winter levels at stocking to the following autumn. Wild fish showed a cyclical summer increase in lipids following winter depletion, which plateaued by autumn. The high lipid content of hatchery lake trout may be necessary as they acclimate to foraging in the wild. Hatcheries would benefit from evaluating whether post-stocking survival could be improved by altering feeding or rearing regimes.

The greatest response of lakes to climate change has been the increase in water temperatures on a global scale. The responses of many lake fishes to warming water temperatures are projected to be inadequate to counter the speed and magnitude of climate change, leaving some species vulnerable to decline and extinction. We experimentally evaluated the responses of embryos from a group of cold, stenothermic fishes (Salmonidae Coregoninae) - within conspecifics across lake systems, between congeners within the same lake system, and among congeners across lake systems - to a thermal gradient using an incubation method that enabled global comparisons. Study groups included cisco (Coregonus artedi) from lakes Superior and Ontario (USA), and vendace (C. albula) and European whitefish (C. lavaretus) from Lake Southern Konnevesi (Finland). All species spawn in the fall and their embryos incubate over winter before hatching in spring. Embryos were incubated at water temperatures of 2.0, 4.5, 7.0, and 9.0{degrees}C, and the responses to the incubation temperatures were quantified for life-history (i.e., embryo survival and incubation period) and morphological traits (i.e., length-at-hatch and yolk-sac volume). We found contrasting reaction norms to temperature in embryo survival and similar reaction norms to temperature for incubation period, length-at-hatch, and yolk-sac volume in conspecific and congeneric coregonines. For example, congeneric responses differed in embryonic survival in the same system, suggesting species differences in adaptability to warming winter temperatures. Differential levels of parental effects were found within and among study groups and traits suggesting population biodiversity may provide more flexibility for populations to cope with changing inter-annual environmental conditions. Our results suggest coregonines may have a wide range of embryo responses to changing winter conditions as a result of climate change.