Estimating Bycatch Survival in a Mark-Selective Fishery

摘要

Pacific Northwest fishery managers have recently adopted mark-selective commercial fisheries to concentrate harvest on healthy spring Chinook salmon Oncorhynchus tshawytscha stocks while conserving the weaker stocks. Mark-selective fisheries are possible where a mark, such as a clipped adipose fin, enables fishers to visually separate fish that may be retained from those that must be returned to the water. Selective fishing allows a fishery to avoid nontarget species or stocks, or when encountered, to capture and release them in a manner that minimizes mortality. Previous research comparing tangle nets, which capture fish by the snout rather than by the gills, to the conventional gill nets, revealed high postrelease survival of tangle netted fish. Consequently, beginning in 2003, Columbia River fishery managers adopted mark-selective tangle net fisheries during part of the commercial fishery. Because fishery managers must now account for the survival of the released nontargeted bycatch stocks as well as the total harvest of targeted stocks, we have developed methods to estimate the immediate, postrelease, and total survival offish captured and released from commercial fishing nets. Treatment spring Chinook salmon were captured and released from 4.25-in to 4.5-in tangle nets and control fish were captured and released from an adult fish trap just upstream. Captured fish were tagged and released for recovery in succeeding fisheries, at hatcheries, and during spawning ground surveys. The results showed that immediate survival was 98%, postrelease survival was 70.1%, and total survival was 68.6% following capture in tangle nets. Total survival combines the results of the immediate and postrelease survival. These estimation methods can be adapted toother fisheries, such as hook and line fisheries, provided an appropriate control is available.