Competitive Interactions between Natural Populations of Pink and Chum Salmon from Puget Sound and Coastal Washington, USA

Natural populations of pink (Oncorhynchus gorbuscha) and chum salmon (O. keta) make up the highest returns of Pacific salmon to Washington, USA and many stocks spawn in the same watersheds, leading to density dependent interactions that persist across all life stages (Fig. 1). From 1967–2017, annual returns of pink salmon used in this study averaged 2,210,481 (range: 413,269 to 7,813,504) and from 1968–2017, returns of chum salmon averaged 1,010,333 (range: 174,334 to 2,662,673; Fig. 2). In this region, pink salmon predominantly return in odd years and are associated with reduced survival of chum salmon, but also other species, including Chinook salmon (O. tshawytscha, Ruggerone and Goetz 2004) and southern resident killer whales (Orcinus orca, Ruggerone et al. 2019). For chum salmon, stocks interacting with pink salmon exhibit strong biennial patterns in abundance (Gallagher 1979) with regular evenand odd-year variations in size (Pratt 1974), age-at-maturity (Smoker 1984), and productivity (Ruggerone and Neilsen 2004). However, there has not been a comprehensive review of these biological attributes in decades. Competition between pink and chum salmon in Washington can occur during any life history stage, except in coastal populations where there are no pink salmon producing systems. Interestingly, coastal chum salmon populations also exhibit strong inter-annual variations in adult abundance and size, suggesting that competition during the overlapping marine period may be most critical for establishing the distinct evenand odd-year patterns.