Overlooked bycatch mitigation opportunities in pelagic longline fisheries: Soak time and temperature effects on swordfish ( Xiphias gladius) and blue shark ( Prionace glauca) catch

Abstract

Bycatch mitigation approaches aim to either reduce the incidence of unwanted catch or reduce bycatch mortalities. In pelagic longline fisheries incidence of unwanted catch can be reduced by limiting the availability of baited hooks (e.g., within bycatch species’ preferred depths and water temperatures), whereas bycatch mortalities can be decreased by gear modifications and changes to fishing practices, e.g., by limiting soak time. To evaluate the effects of temperature, depth, and soak time on catch of target and bycatch species, temperature recorders were set along the length of the longline to characterize the environment at which hooks were fishing. Although few instrumented sets were fished, observations at the within set scale – specifically, that swordfish ( Xiphias gladius) catch did not increase with longer soak times – led us to reexamine assumptions made in fleet-wide catch models. Swordfish catch did not increase with soak time in generalized linear models based on fisheries observer data collected from swordfish-targeted sets fished by the Canadian pelagic longline fleet in 2008 and 2009 ( n = 42 and n = 78, respectively). Minimum soak time, from end of setting to start of hauling, was used in swordfish catch models. Total soak time is inappropriate for catch models because it includes haulback time, which increases as a function of catch. If landed catch does not increase as a function of soak time, then limiting longline soak time to reduce bycatch mortalities would not cause decreased swordfish catch nor result in economic losses for fishers. While minimum soak time limits would likely decrease bycatch mortality rates in swordfish longline fisheries, impacts on other aspects of the fishing process would need to be considered, such as negative impacts on fisher safety.