Estimating a release mortality rate for Coho Salmon in a commercial tangle-net fishery in the Columbia River

ABSTRACT Objective To protect imperiled wild stocks of Pacific salmon Oncorhynchus spp. while still providing opportunities for harvest of hatchery stocks, fishery managers in the Pacific Northwest have implemented live-capture fisheries that allow for the selective harvest of hatchery fish (marked by excision of the adipose fin) and the release of wild (unmarked) fish. Managing these fisheries requires the application of a mortality rate to the released nontarget catch to account for incidental mortalities. Our objectives were to estimate a release mortality rate for wild Coho Salmon O. kisutch in a mark-selective commercial tangle-net fishery in the Columbia River and to develop a framework that could be used to estimate mortality rates for other species and gears. Methods We held adult Coho Salmon in net-pens to assess postrelease survival after their capture in tangle nets by research vessels in the Columbia River. We analyzed data for short-term (0–48-h) and long-term (48–192-h) holding periods to evaluate survival over time, and we calculated postrelease survival using the product of the short- and long-term survival estimates. Results The mean short-term survival was 0.921 (95% CI = 0.895–0.941), and long-term survival was 0.947 (95% CI = 0.895–0.974), with a postrelease survival estimate of 0.873 (95% CI = 0.832–0.915). We combined the postrelease survival estimate from the study with the observed immediate (onboard) survival of released adult Coho Salmon from monitoring of the commercial tangle-net fishery (0.875), resulting in a cumulative survival estimate of 0.764 (95% CI = 0.734–0.795) or a release mortality rate of 23.6% (95% CI = 21.9–25.5%). Conclusions Managers currently use the 23.6% mortality rate for the Coho Salmon tangle-net fishery in the Columbia River. This study provides a method of estimating release mortality rates for use in the management of mark-selective fisheries that are meant to protect imperiled stocks.