The Subtlety of Lake Superior Ciscoe Recruitment Dynamics

Objective: Lake Superior whitefish [Salmonidae: Coregoninae] have undergone high annual variability in survival to age-1 over the last 50-years, particularly within the dominant ciscoe flock of Bloater Coregonus hoyi, Cisco C. artedi, and Kiyi C. Kiyi. This work sought to determine if this variance was associated with hydrometeorological attributes that have also varied considerably over the same period. Methods: Forty-seven years of age-1 Bloater and Cisco (1978-2024) and 12-years of age-1 Kiyi (2011-2024) density estimates and 42-years (1982-2024) of satellite derived lakewide ice cover, surface water temperature, air temperature, cloud cover, wind speed, and wind direction data were evaluated using Classification and Regression Tree models and correlation analyses. A decade of larval ciscoe data (2014-2024) were also incorporated into the evaluation to identify the critical period for their survival in Lake Superior. Results: Lake Superior ciscoe year–class strength was highest when June water temperature was <4.8°C, January ice cover was >19%, and winds were calmer than average during the November to January spawning season. There was a threshold relationship where maximum survival to age-1 occurred when mean June surface water temperature was <4.8°C, and survival was often near zero when mean June water temperature was >4.8°C. This was true for Bloater, Cisco, and Kiyi. The critical period for survival appeared to be a few weeks post–hatching, which likely coincided with the initiation of exogenous feeding. Conclusion: Calm winds during the spawning season may serve to reduce embryo disturbance and drift to unfavorable habitats and promote early ice formation. Development of early ice can foster more ice cover throughout the winter, leading to later ice breakup and reduced spring water temperature. Cold spring water temperature reduces the metabolic demands of larval deepwater ciscoes during a time of low zooplankton prey abundance.