Competition with odd-year pink salmon in the ocean affects natural populations of chum salmon from Washington

Abstract

Over the last 5 decades, natural populations of pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon were the most abundant salmon species returning to Washington, USA. Pink salmon predominantly returned in odd years, and chum salmon stocks that interacted with pink salmon exhibited strong even- and odd-year variations in abundance, size, age-at-maturity, and productivity (recruits-per-spawner). We investigated the effects of competition between pink and chum salmon originating from Washington during different life-history phases. Overall, chum salmon returns were 34% lower in pink salmon (odd) years compared to non-pink salmon (even) years. Chum salmon productivity tended to be below average for odd broods, especially along the Washington coast where there are no pink salmon populations, suggesting that competition during overlapping marine periods established their distinct even- and odd-year patterns. We evaluated long-term trends in chum (and pink) salmon productivity using correlation and time series analysis and examined the influence of ocean indicators on those trends. In general, chum salmon productivity increased between brood years 1968 and 1989 and was positively related to summertime Pacific Decadal Oscillation (PDO), but declined between brood years 1990 and 2012 and was negatively related to the PDO. Pink salmon productivity had no directional trend, but exhibited declines over the last decade and was negatively related to the PDO over the entire time series. Chum salmon productivity was also negatively related to pink salmon abundance, supporting the conclusion that ocean conditions and competition accounted for brood-year differences in chum salmon age-at-maturity and lower returns in odd versus even years.