Pop-up satellite archival tags reveal evidence of intense predation on large immature Chinook salmon (<i>Oncorhynchus tshawytscha</i>) in the North Pacific Ocean

Abstract

Throughout the northern extent of its range, Chinook salmon (Oncorhynchus tshawytscha) adult returns have been in decline for over a decade, leading to severe harvest restrictions on subsistence, commercial, and recreational fisheries. In addition to these overall declines in abundance, changes in size structure and age structure, including a proportional decrease of older age classes returning to spawn, suggest that late-stage marine mortality for this species may be more frequent than currently assumed. To examine this late-stage mortality hypothesis, we examined diagnostic evidence of predation on large (57–100 cm fork length) Chinook salmon (n = 33) from depth, temperature, and light records collected during recent satellite tagging research. Satellite tags provided evidence of predation on tagged Chinook salmon by salmon sharks (Lamna ditropis) (n = 14), marine mammals (n = 2), ectothermic fish(es) (n = 3), and unidentified predators (n = 5) in the Bering Sea and Gulf of Alaska. High mortality rates in this study suggest that fisheries scientists should consider that late-stage mortality by marine apex predators may be shaping this species’ abundance and demographics.