Optimum timing and size at release of chum salmon: Improving survival by modifying hatchery practices

An existing conceptual model that assessed the timing of and size at release of juvenile Japanese chum salmon in Japanese waters is modified using data obtained from young salmon collected in coastal waters off Kombumori, eastern Hokkaido, Japan. The early life histories of Tokachi River late-migrating chum salmon are estimated based on microstructural analysis of thermally marked otoliths from recaptured fish. A revised model is presented that reveals the optimal timing of and size at release of chum salmon juveniles to be dependent upon their size at sea entry and sea surface temperature (SST) near Tokachi River mouth. Juvenile salmon are assumed to grow at a fixed rate after sea entry and to reach a target fork length upon arrival in Kombumori waters when SST conditions are within a specific optimal range. However, because of annual variation in SST, the size at and timing of juvenile chum salmon sea entry following their release from hatcheries represents a moving target, which flexible hatchery release practices are required to meet. Return rates of adult chum salmon to Tokachi River correlate positively with the number of days elapsed between the period when SST is suitable near the river mouth where they enter the sea, and SST conditions in Kombumori waters, which suggests that in years when SST increases rapidly the survival of salmon tends to decline. Windows of optimal times of and sizes at release are proposed for juvenile chum salmon to improve their survival and hatchery success.