Estimation of Discard Mortality Rates for Pacific Halibut Bycatch in Groundfish Longline Fisheries

Mandatory release to the sea of Pacific halibut Hippoglossus stenolepis incidentally harvested in Alaskan and Canadian groundfish fisheries has the potential to close fisheries or to close fishing to individual fishermen or vessels that reach Pacific halibut bycatch mortality limits. Tagging experiments of Pacific halibut from longline gear demonstrated that Pacific halibut with similar types of injuries experienced lower mortality following release from small (13/0) circle or autoline hooks than from large (16/0) circle hooks. As a result, the current viability criteria for individual Pacific halibut overestimate discard mortality rates. Proposed, simplified four-category viability criteria based on injury codes increased accuracy of bycatch mortality calculations over the present three-category criteria. The new criteria may reduce calculated discard mortality of Pacific halibut released from longlines by 20%. Use of the new criteria would result in more accurate estimates, which in turn could lower the probability of bycatch-induced fishery closures, increase the Pacific halibut available for a directed fishery, or both.