Using cumulative degree-days to refine sampling periods for early life stages of Lake Whitefish

ABSTRACT Objective Evaluating recruitment bottlenecks for Lake Whitefish Coregonus clupeaformis in the Great Lakes and inland lakes requires effective sampling of early life stages. The objective of this study was to determine whether targeted sampling of Lake Whitefish informed by a simple model relating total length to cumulative degree-days (CDDs; based on air temperature) could improve the precision of CPUE estimates by focusing sampling on specific sizes of larval and early juvenile fish. Methods Early life stages of Lake Whitefish were collected with neuston nets (larvae) or beach seines (juveniles) from southeast Lake Michigan during 2014–2022. Simple models were developed separately for larval and early juvenile Lake Whitefish to explain total length as a function of CDDs, day of year, or in situ water temperature. The CDD model was then used to determine when sampling should have occurred to target specific fish lengths for each life stage. Results Cumulative degree-days were the best predictor of fish total length for both larval fish in the neuston net (R2 = 0.71) and juvenile fish in the beach seine (R2 = 0.76). Restricting catches to the periods when fish lengths were in the target range resulted in a 29% and 26% lower average CV in CPUE for the neuston net and beach seine, respectively. Conclusions As climate change affects fish phenology, a simple model based on CDDs can focus sampling on specific fish lengths and improve precision of catch estimates for both larval and early juvenile Lake Whitefish with less overall sampling effort.