Biomass and mortality of chum salmon in the pelagic Bering Sea

Abstract

To provide a key piece of information for understanding the functioning of the Bering Sea pelagic ecosystem, we estimated the biomass of immature chum salmon in the Bering Sea basin in autumn using a return-at-age analysis incorporated with the results of genetic stock identification studies. The estimated biomass was 742 000 t in 2002 and 617 000 t in 2003, which is possibly an underestimate because of the uncertainty of trawl selectivity parameters used in the estimation. Although chum salmon has been the dominant nekton species in the pelagic ecosystem of the Bering Sea basin in recent years, biomass estimates for the species were smaller than one-tenth of the maximum biomass of walleye pollock in the 1980s. Previous studies have highlighted the importance of the huge biomass of mesopelagic planktivores, such as myctophid fishes and gonatid squid, to the ecosystem function of the pelagic Bering Sea. Planktivorous Pacific salmon including chum and pink salmon may also play a significant role as competitors and predators of small planktivores, thus affecting the dynamics of ecosystem function and nekton community structure in the pelagic Bering Sea.KEY WORDS: Pelagic ecosystem · Planktivores · Biomass estimate · Salmon · Catch-at-age analysis · Genetic stock identification · Markov chain Monte Carlo Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 403: [219][220][221][222][223][224][225][226][227][228][229][230] 2010 9.3 million (17 000 t) in 1972 (Fukuwaka & Morita 2008). The high-seas salmon gillnet fisheries were closed when the Convention for the Conservation of Anadromous Stocks in the North Pacific Ocean (NPAFC Convention) was enforced in 1993. However, the density index, i.e. number of chum salmon caught by research gillnets, increased in the 1980s and in the early 2000s reached levels 3-to 8-fold of those in 1972 (Fukuwaka et al. 2007). A biomass estimation of chum salmon in the Bering Sea would provide key information to aid in understanding the functioning of the Bering Sea pelagic ecosystem.Chum salmon originating from rivers in Pacific Rim nations migrate to the North Pacific and adjacent seas and finally return to their natal rivers to spawn after 2 to 5 yr of ocean life (Groot & Margolis 1991). From summer to autumn, chum salmon originating from many regions enter and mix in the Bering Sea (Seeb et al. 2004). For the international management of highseas salmon fisheries, stock identification techniques have been well developed using tagging experiments, scale pattern analysis, otolith marking, and genetic techniques (e.g. Tanaka et al. 1969, Myers et al. 1996, Urawa et al. 2000, Seeb et al. 2004. After the closure of high-seas gillnet fisheries, the oceanic abundance of immature chum salmon could be estimated from the number of returns-at-age with an assumed or estimated value of natural mortality. However, return-atage data are not always available for all chum stocks around the North Pacific. The basic idea of the present study was th...

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