First-year overwinter mortality in Eurasian perch (Perca fluviatilis L.) -Results from a field study and a simulation experiment

The importance of overwinter mortality of 0+ perch (Perca fluviatilis L.) i n central European lakes was estimated using a stochastic simulation model. The probability of overwinter mortality of a perch cohort was calculated b y using a model developed for yellow perch (P. flavescens Mitchill). Winter duration from a long-term data set and the length of perch at the end of t he first year from five lakes were used as input data. After 1000 simulatio n runs, the total extinction of a cohort in the lakes studied was never pre dicted. Mortality rates of more than 0.5 were only predicted in two of the five lakes, and rates of more than 0.3 in these two lakes were predicted in approximately 10% of all cases. For two consecutive winters differing in d uration, the length-frequency distributions of Of perch in the autumn and f ollowing spring were compared by a graphical method. No significant size-de pendent mortality of smaller individuals could be detected in any of the po pulations studied. Simulated spring length-frequencies were derived from ob served autumn length distributions by the same model that was used for the stochastic simulation. These simulated and the empirical spring length-freq uency distributions were not identical. The differences between the two dis tributions were attributed to growth, which occurred between the sampling d ates. The results from the simulation and the analysis of the empirical dat a suggest that high overwinter mortality caused by starvation is rare in ce ntral European lakes.