EARLY SEA MORTALITY OF CHUM SALMON JUVENILES IN THE JAPAN SEA COAST

We examined the early mortality of juvenile chum salmon to clarify population dynamics of hatchery-reared chum salmon in the Japan Sea coast. We calculated the survival rate using mark-and-recapture experiments during 10-50 days after release from the hatcheries along the Japan Sea coast of Honshu. Estimated daily survival rate during early sea life was 76.9 approx. 100% day . Mean survival during the whole sea life was 0.258% in 1988-1997. Early sea mortality of chum salmon accounted for 97.4% of the whole sea mortality. Results suggest that early sea mortality soon after release from hatcheries will determined a population level of returning adults of a cohort of chum salmon. INTRODUCTION In anadromous Pacific salmon, mortality is often extensively high soon after the entering into the sea (Pearcy 1992). Hatchery-reared chum salmon juveniles reach coastal waters within several days after release (Mayama et al. 1983; Kaeriyama 1986). Early sea mortality is much higher than subsequent stages in the life history of hatchery-reared chum salmon, thus it may determine adult population level (Bax 1983). Early sea distribution of chum salmon is restricted in nearshore regions in the Japan Sea coastal water (Fukuwaka and Suzuki In press). To clarify the population dynamics of hatchery-reared chum salmon in the Japan Sea coast, we examined early sea mortality of juvenile chum salmon using mark-and-recapture experiments in the coastal waters. METHODS Mark-and-recapture data sets for chum salmon juveniles were provided from the prefectural fisheries experimental stations along the Japan Sea coast of Honshu in 1992-1997 (Hokkaido Salmon Hatchery 1993-1997; National Salmon Resources Center 1998). Over 3 million fin-clipped juveniles were released from several private or prefectural hatcheries in 11 marking experiments in 1992-1997 (Table 1; Fig. 1). Surveys in coastal waters operated by the fisheries experimental stations of Yamagata, Niigata, Toyama, and Ishikawa Prefectures. Fisheries Agency of Japan financially supported these mark-and-recapture experiments. In 1992-1997, 1663 marked juveniles were recaptured by the prefectural fisheries experimental stations or commercial fishermen. Daily survival rate was estimated using the equation: ln (mt / nt)= ln (M / N0) + (t – 0.5) ln S,