Migration timing is an important factor in determining conditions experienced along migratory corridors, which influence en route and latent mortality. This was the case for threatened spring-summer Chinook salmon Oncorhynchus tshawytscha populations in the Columbia River Basin during 2010– 2015, as earlier migrants experienced higher mortlaity from the river mouth to fish ladders in Bonneville Dam (RKm 233). In 2013–2015, survival was especially low for early migrants, corresponding with a period of increased California sea lion Zalophus californianus presence in the estuary. Chinook salmon populations exhibit a range of migration timings, adapted in response to the phenology of migration, spawning and juvenile rearing habitat conditions, and past hatchery practices. Therefore, we examined the role of migration timing in determining population- and year-specific survival rates from the mouth of the Columbia River to Bonneville Dam. We estimated population- and year-specific river-entry timing by combining one model of detections of tagged fish that were identifiable to populations of origin in Bonneville Dam ladders, with another model of elapsed time between the river mouth and the fish ladders. We estimated population- and year-specific survival rates using both a previously developed model of survival as a function of river-entry date and our river-entry timing model.
Results/Conclusions
As expected, individual populations exhibited a range of arrival-timing distributions at the fish ladders, and returned in roughly the same order across years. Migration timing of all populations shifted similarly among years, in association with environmental conditions within the river and ocean. Early-migrating populations experienced a 17% reduction in survival in 2013-2015 relative to a baseline period of 2010 to 2012. Survival of later-migrating populations declined by only 4%, showing that the trend of decreasing survival disproportionately affected early-migrating populations. If the trend of decreased survival continues, it could have a significant effect on the viability of these populations. Past hatchery practices introduced out-of-population fish into many of these populations, which likely influenced their migration timings and thus en route survivals. Further investigation of the causes of en route mortality, modeling of predator-prey interactions and dynamics, and salmon population modeling would help to evaluate the effects of sea lions on survival and the viability of different populations. This could be used to evaluate the need for actions designed to offset the increase in mortality.