Linkages between coastal and open-ocean habitats and dynamics of Japanese stocks of chum salmon and Japanese sardine

Abstract

Abstract Coastal-ocean–open-ocean migrations, prey–predator relations and long-term population dynamics of chum salmon (Oncorhynchus keta) and Japanese sardine (Sardinops melanostictus), associated with large-scale climate and oceanographic conditions, are reviewed. After early marine life in coastal waters in northern Japan, chum salmon of Japanese origin spend their first summer in the Okhotsk Sea, then move to the Western Subarctic Gyre for the first winter at sea. Thereafter, they migrate between summer feeding grounds in the Bering Sea and wintering grounds in the Alaskan Gyre for a period of usually up to four years, and finally return to their natal rivers to spawn. Carrying capacity (K) for chum salmon at an unfished equilibrium level was estimated from a Ricker spawner–recruitment curve, and the residual carrying capacity ( RCC = ( K - abundance ) K - 1 ). was positively correlated with body size at age 4, and negatively correlated with age at maturity. Marine survival of Hokkaido chum populations was affected by body size at release, but neither by Aleutian low pressure activity nor sea-surface temperature (SST) around coastal Hokkaido in spring, although there is some correlation between survival rate and coastal SST. Juveniles of the Pacific stock of Japanese sardine become broadly distributed in the Kuroshio Extension (KE) as far east as 180° longitude during spring. Adults disperse as far as the central Pacific and the southern areas of the Okhotsk Sea and Western Subarctic Gyre in years of high abundance. Somatic growth and age at maturation of sardine are density-dependent. We used catch, biomass and residuals of observed recruitment numbers from a Ricker curve (LNRR) as a measure of sardine population dynamics. LNRR was highly correlated with SST of KE in winter, which shifted in 1970 and 1988. Recent biomass and catch remain at extremely low levels due to a combination of adverse environmental conditions and intensive fishing. We suggest that Japanese populations of chum salmon and Japanese sardine have a broader geographic range than North American populations, perhaps because of differences between western and eastern boundary current systems, and associated larval drift (sardine) or avoidance of intra-species competition (chum salmon).