Determining the effects of Asian pink and chum salmon on Western Alaska chum salmon growth

Abstract

Increased hatchery production and favorable ocean conditions have resulted in historically high abundances of Pacific salmon (Oncorhynchus spp.) in the North Pacific Ocean. Despite these conditions, chum salmon (O. keta) have experienced reductions in growth, body size, and increases in age at maturity throughout their range. In western Alaska, dramatic declines in the abundance of chum salmon between 1997-2001 resulted in numerous fishery and economic disasters among commercial and subsistence users. Chapter 1 reviews existing data on salmon diet and ocean distribution to address the potential for competition between western Alaska chum salmon and Asian pink (O. gorbuscha) and chum salmon in the Bering Sea. Western Alaska chum salmon reside in the Bering Sea during their summer foraging months where they overlap with abundant populations of Russian pink salmon (primarily wild origin) and Japanese chum salmon (primarily hatchery origin). Chum and pink salmon occupy a similar feeding niche, and during years of high pink salmon abundance chum salmon have been observed to alter their ocean distribution and rely more heavily on gelatinous zooplankton species as a primary food source. This spatial and diet overlap suggests that inter- and intra-specific competition might contribute to reduced growth and increased age at maturity of western Alaska chum salmon. Chapter 2 uses retrospective scale analysis coupled with linear mixed-effects modeling to investigate the potential for such competition between Asian pink and chum salmon abundance and the growth of chum salmon that rear in the Bering Sea. Chum salmon scale samples were collected through in-river fisheries on the Kuskokwim River during 1973-2014 and from incidental catches of chum salmon in the Bering Sea Aleutian Island walleye pollock (Gadus chalcogrammus) fishery during 2001-2016. Linear mixed-effects models demonstrated a strong negative relationship between Bethel chum salmon growth and the abundance of Japanese hatchery chum salmon. Chum salmon intercepted in the Bering Sea did not exhibit increased growth during 2012-2014 despite reductions in Japanese hatchery releases of chum salmon in 2011 as a result of the Tōhoku Earthquake and tsunami. We did not observe a relationship between Bethel chum salmon growth and the abundance of wild Russian pink salmon. Understanding how salmon populations interact while at sea will assist fishery managers in conserving threatened salmon stocks, particularly as Pacific Rim nations consider increasing production of hatchery salmon.