Spatial variation in reproductive measures of female bluegill Lepomis macrochirus Rafinesque in a small lake with diverse thermal conditions

Abstract

Our previous studies have revealed that bluegill Lepomis macrochirus Rafinesque in Lake Biwa exhibit spatial variations in growth, maturation, and reproduction. The topographical characteristics of Lake Biwa produce a wide range of thermal conditions, which seem to be responsible for those findings. Despite its small size, Lake Yogo is characterized by diverse thermal conditions owing to the presence of a small, shallow pool in the northern part of the lake and the inflow of water from Lake Biwa into the southern part of the lake. Whether bluegill in such a small lake also exhibit spatially varying physiological activity remains uncertain. The maturation status of female bluegill in Lake Yogo was investigated from late April to early June 2011 and in late May 2012. The gonadosomatic index (GSI) of the fish in the pool significantly exceeded those in the main lake in late April and mid-May 2011 before the onset of spawning. The spawning of bluegill had probably already begun by late May 2012, and the fish in the pool and along the northwestern edge of the lake had smaller GSIs than did the fish in the southern part of the lake. The relationship between oocyte diameter and GSI could be described by a saturation function. The plot of GSI/oocyte diameter vs. GSI did not vary significantly among the sites. The maturation status of bluegill varied among sites in Lake Yogo, likely to be due to the diverse thermal conditions of the lake. Higher temperatures in the pool during the prespawning season favored the rapid maturation of ovaries of the fish therein. The inflowing colder water from Lake Biwa seemed to be responsible for delayed ovary maturation and, thereby, the delay in the spawning season of bluegill in the southern part of the lake.