Pop-up satellite archival tags reveal evidence of intense predation on large immature chinook salmon (Oncorhynchus tshawytscha) 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.