Seasonal Variation in Gill-Net Sample Indexes for Northern Pike Collected from a Glacial Prairie Lake

We captured 702 northern pike Esox lucius in 416 gill-net sets on 17 monthly occasions from July 1991 to June 1993 to examine seasonal variation in sampling data in Lake Thompson, South Dakota. Mean catch per effort (CPE) of northern pike 35 cm or longer differed significantly among months during both years. Mean CPE was lowest in winter and peaked in spring of both years. Size structure estimates were lowest during early spring and summer when males dominated samples and were highest during winter when females dominated samples. Northern pike 86 cm or longer were caught primarily in winter during ice cover. Relative weight (Wr) of male and female northern pike increased during fall, peaked in late winter, and declined during the spawning period. A similar pattern was found for the gonadosomatic index of female northern pike but not for males. Increases in the gonadosomatic index for males occurred entirely during fall. Gonadosomatic index and Wr were positively correlated for female northern pike, but the correlation was lower for males. We suspect our findings will be most applicable for interpreting northern pike population surveys made during narrow time periods near the southern portion of the speciesˈ range and for populations found in shallow, warmer water bodies.