Fig. 2. Frequency distributions of the gonadosomatic index (GSI) for pre-migratory male and female chum salmon in the central Bering Sea, 2001–2003. The GSIs were determined from fish caught during summer (June and July, upper panel) and autumn (September, lower panel). Fitted curves were obtained using a Gaussian distribution with multiple peaks. The ages of fish were estimated from their scales. Data collected during summer show a sizable component of fish with a high GSI, whereas fish with a high GSI are nearly absent in September.