Size-Dependent Overwinter Mortality of Young-of-the-Year Yellow Perch (Perca flavescens): Laboratory, In Situ Enclosure, and Field Experiments

In laboratory experiments, fed and starved young-of-the-year yellow perch (Perca flavescens) lost weight when kept under simulated overwinter temperature and photoperiod regimes, small fish losing a greater proportion of their mass than larger ones. Perch in fed and starved treatments suffered 1 and 46% mortality, respectively, mortality being higher among the smaller individuals. Winter duration was an important determinant of both total mortality and the intensity of size-selective mortality. Size-selective mortality also occurred in yellow perch from the same stock kept overwinter in an in situ lake enclosure with natural food. Fall and spring population estimates for two cohorts of young-of-the-year yellow perch from Lake St. George also indicated the occurence of size-selective overwinter mortality. We present a quantitative technique for identification of size-dependent mortality and size-dependent growth from sequential length frequency distributions. This technique allowed identification of overwinter size-selective mortality for five natural cohorts sampled in the field. Results from a stochastic simulation model, incorporating observed variability in both first year growth and winter duration suggest that overwinter starvation mortality can cause substantial variability in year-class strength that is independent of adult stock size.