Coastal Distribution and Consequent Factors Influencing Production of Endangered Snake River Sockeye Salmon

Abstract

Snake River Sockeye Salmon Oncorhynchus nerka were declared endangered in 1991 after several years of decreasing abundance. Several factors, including poor marine survival, likely contributed to the decline of Snake River Sockeye Salmon. Little is known about their migration and ocean distribution and the factors influencing their production. We sampled (1) coastal waters from southern British Columbia (BC) to southeast Alaska during June–July, October–November, and February–March 1998–2011; and (2) Oregon and Washington coastal waters during May–June and September 2007–2010. In total, 8,227 juvenile Sockeye Salmon were captured. Despite their extremely low abundance relative to other stocks, 15 coded-wire-tagged juveniles from Redfish Lake were recovered since 2007, primarily in spring and summer surveys off the BC coast. Genetic analyses revealed that an additional eight Redfish Lake juveniles were also present in this area during summer. Snake River smolts undertook a rapid northward migration that brought them well beyond the Columbia River estuary and plume, exposing them to ocean conditions prevailing off BC. Through a multimodel inference approach, we characterized associations between the number of returning adults and a suite of ocean and river variables. Seven ocean variables and five river variables were chosen for the model selection analysis (e.g., copepod biomass anomalies, coastal upwelling indices, date of the spring transition, river discharge, river temperature, and the proportion of smolts transported through the hydropower system). Although adult returns were highly correlated with smolt abundance, our analyses suggest that ocean conditions encountered during the first growing season (as indexed by copepod anomalies) contribute to the variability in total adult returns. There was also evidence for a negative effect of transporting smolts through the hydropower system, with the caveat that we used transportation data for steelhead O. mykiss as a proxy.

Received April 4, 2014; accepted September 15, 2014

ACKNOWLEDGMENTS

We thank the crews of the CCGS W.E. Ricker, F/V Viking Storm, F/V Ocean Selector, and F/V Frosti and the numerous scientists and technicians for their assistance with the field work and laboratory analysis. We acknowledge M. Bradford, B. Burke, C. Kozfkay, D. Schill, and three anonymous reviewers who provided helpful comments on previous drafts of the manuscript. We are grateful to C. Petrosky for generously providing river and hydropower system variables. We also thank DFO, the National Marine Fisheries Service, the Bonneville Power Administration, the Canadian Space Agency, and the World Wildlife Fund for their financial support.