Estimation of Postrelease Longline Mortality in Pacific Halibut Using Acceleration-Logging Tags

Pacific Halibut Hippoglossus stenolepis captured in directed commercial longline fisheries in Canada and the USA that are below the legal minimum size for retention must be returned to the sea without incurring additional injury. Estimates of mortality caused by discarding sublegal-sized fish are included in annual estimates of total mortality from all sources and affect the results of stock assessment and the yield available to fisheries. Currently, an average discard mortality rate (DMR) of 16% is applied to all sublegal-sized longline discards. These discards consist of fish that suffer injuries ranging from minor to severe. The 16% DMR that is currently applied was derived by averaging injury-specific DMRs that in turn assume 3.5% mortality of Pacific Halibut that are released to the sea with only minor injuries. The latter has been derived experimentally but only in captivity. Here, we used acceleration-logging pop-up archival transmitting tags to infer individual survival outcomes for Pacific Halibut that were released in situ following capture on longline gear. Postrelease behavioral data were evaluated for 75 fish that were at liberty for 2-96 d. Three fish were confidently inferred to have died after periods at liberty of 41-80 d, and another three fish may have died 96 d after release, resulting in minimum and maximum estimated 96-d postrelease DMRs of 4.2% (range = 0.0-8.7%) and 8.4% (range = 1.7-14.6%), respectively. These ranges are consistent with the currently applied value of 3.5%. However, the observation that no mortalities occurred until after 40 d postrelease departs from the findings of captive studies, in which the majority of capture-induced mortality occurred within 20 d of release.