Survival and transit of in-river and transported yearling Chinook salmon in the lower Columbia River and estuary

Abstract

The lower Columbia River and estuary (LRE) is a critically important environment for outmigrating salmonids, yet uncertainties remain about the survival and behavior of barged and in-river migrating fish. Although studies have used telemetry to monitor Chinook salmon movement and survival through the LRE, comparisons between outmigration years are confounded by differences in tag technologies, array locations, and experimental designs. In the present study, multiple releases of barged and in-river Snake River spring/summer Chinook salmon were implanted with acoustic tags and monitored at multiple locations between Lower Granite Dam on the Snake River (695km from the mouth of the Columbia River) to within 3km of the Pacific Ocean. LRE survival estimates and transit rates of barged fish significantly varied throughout the outmigration season. The transit rates of in-river fish also varied, but without a corresponding seasonal difference in LRE survival estimates. Early release groups of barged salmon were slower and had lower survival in the LRE than in-river salmon. Estuary arrival timing and the magnitude of transit rates may contribute to significant differences in LRE mortality between in-river and barged juvenile salmon. Survival in the Lower River reaches was stable and exceeded 0.90 for both barged and in-river fish, while survival decreased markedly in the Estuary. Differential distributions of arrival to the LRE, transit rates, and survival suggest that the outmigration experience is not homogenous for barged and in-river yearling Snake River Chinook salmon, and that previous outmigration experience of threatened and endangered salmon should be considered in future management decisions and recovery plans.