The Productivity and Viability of Snake River Sockeye Salmon Hatchery Adults Released into Redfish Lake, Idaho

Abstract

AbstractIn 1991, the Snake River Sockeye Salmon Captive Broodstock Program was initiated to prevent the extinction and preserve the genetic diversity of this evolutionarily significant unit protected by the Endangered Species Act. At the time of listing, the Redfish Lake Sockeye Salmon Oncorhynchus nerka population was considered functionally extinct. One of the recovery strategies entails the release of adults for volitional spawning in Redfish Lake, Idaho, for rebuilding of the natural population. In this paper, we describe the productivity metrics from this strategy. We evaluated eight spawn years to address three primary questions: (1) What metrics for eggs to smolts, smolts per female, and smolt‐to‐adult returns (SARs) result from recent adult releases? (2) How do these metrics compare with historical estimates for Redfish Lake and estimates for other Sockeye Salmon populations throughout the range? and (3) Does the current combination of smolts per female and SARs result in population replacement? Replacement was determined as two adult recruits per female assuming an even sex ratio. We found that the reintroduced adults, despite being derived from a multigenerational captive broodstock, were able to successfully spawn and produce offspring that migrated to the ocean and returned as adults. Smolt abundance, size, and age data suggest that the population is functioning below density dependence. However, increased smolt production did not translate into greater adult returns and this is likely due to out‐of‐basin factors. Productivity metrics were similar to those of the wild population in Redfish Lake during the 1950s and 1960s. However, both current and historic productivity estimates were near the low end of the range for other Sockeye Salmon populations and have not resulted in population replacement. Until freshwater and out‐of‐basin survival can be improved, our data suggest that adult releases will continue to be an important recovery strategy to prevent cohort collapse and to rebuild naturally spawning populations.