Lars G. Rudstam

Coregonine fishes, including lake whitefish ( Coregonus clupeaformis ) and cisco ( C. artedi ), are socioecologically important in the Laurentian Great Lakes and of conservation concern, but the processes driving recruitment variability are unclear. In Lake Ontario, cisco and lake whitefish exhibit similar spawning behaviours and early life histories, but population trajectories are diverging. One hypothesis is that sympatric cisco and lake whitefish larvae occupy distinct habitats and experience dissimilar local environmental conditions, despite co‐occurrence within nursery areas. We described the spatiotemporal distributions of larval cisco and lake whitefish among multiple Lake Ontario embayment nursery areas, characterised physical habitat features associated with their distributions, determined the degree of spatial habitat partitioning between species and evaluated how habitat niche divergence occurred along an ontogenetic progression. Both species were widely distributed across larval nursery areas, though lake whitefish were less abundant and more narrowly distributed than cisco. Within the yolk sac stage, lake whitefish occupied more nearshore, shallower and colder waters than cisco, indicating potential habitat niche partitioning between congeners. However, distributional differences were subtle and likely driven by differential hatch timing and staggered ontogenetic habitat shifts. Combined, our results illustrate similar habitat use between cisco and lake whitefish through the larval stage and demonstrate that ontogeny and species‐specific phenology influence habitat use for these species. This study provides additional evidence that the early life histories of cisco and lake whitefish are highly similar and does not support the hypothesis that larval habitat use differences are a major driver of differential recruitment success for these species.

Bythotrephes longimanus is a predatory zooplankton native to Eurasia known for its large caudal spine. This species can decrease Daphnia abundance and may decrease growth rates of native planktivorous fish including yellow perch ( Perca flavescens ). Bythotrephes was first documented in Oneida Lake, USA in 2019 and reached high densities in 2020 and 2021. This recent invasion created an opportunity to investigate at what length yellow perch feed on Bythotrephes , if age‐0 yellow perch select for Bythotrephes , and if yellow perch first‐year growth decreased. We found Bythotrephes in the stomachs of 23% (2020) and 35% (2021) of age‐1 and older yellow perch caught from June through September and 54% of the age‐0 yellow perch caught from June through October (2021). Age‐0 yellow perch started feeding on Bythotrephes at 27 mm and selected for Bythotrephes from June 24 through the autumn of 2021. Bythotrephes accounted for 54% of age‐0 diets by dry weight over the summer and fall. We used the extensive water quality and biological monitoring data from Oneida Lake to develop a multivariate linear regression model for age‐0 yellow perch October length from 1993 to 2021. Significant variables included were age‐0 yellow perch abundance (negative effect) and temperature, Daphnia biomass, and Bythotrephes presence, all positive effects. Contrary to our expectations, Bythotrephes were selected by age‐0 yellow perch from end of June through October, and contributed a substantial proportion of ingested mass by age‐0 perch, without negatively affecting age‐0 yellow perch growth.