Elevated stream temperature, origin, and individual size influence Chinook salmon prespawn mortality across the Columbia River Basin

Abstract

Conservation and restoration efforts for Pacific salmon Oncorhynchus spp. can be hampered by prespawn mortality, when adult fish reach reproductive sites but die before spawning. We examined annual estimates of female Chinook salmon O. tshawytscha prespawn mortality relative to individual fish traits (77,707 individual females) and reach-scale variables in 49 study reaches from 41 streams throughout the interior Columbia River Basin. Mean annual prespawn mortality estimates across 14 years ranged from 0 % to 65 %. For spring-run Chinook salmon, the probability of prespawn mortality decreased over the spawning period, was positively associated with mean August stream temperature and individual fish length, and was higher for hatchery-origin than natural-origin fish. Based on the basin-wide statistical model and future stream temperature predictions, average spring-run Chinook salmon prespawn mortality rates in 2040 were predicted to increase by 0–17 % for fish of natural origin and 1–17 % for fish of hatchery origin. Climate change is likely to exacerbate conditions that lead to prespawn mortality, particularly in low elevation stream reaches, for larger fish, and for those of hatchery origin.