Using logbook data to determine the immediate mortality of blue sharks (Prionace glauca) and tiger sharks (Galeocerdo cuvier) caught in the commercial U.S. pelagic longline fishery
Commercial fisheries are recognized as one of the greatest threats to shark populations worldwide, but factors affecting the likelihood of shark mortality during fishery capture are poorly understood. We used the U.S. pelagic fishery logbook data from 1992 through 2008 to quantify the effects of several variables (fisheries regulatory periods, geographic zone, target catch, and sea surface temperature) on mortality of blue sharks (Prionace glauca) and tiger sharks (Galeocerdo cuvier). Mortality rates and trends in both species closely matched those recorded from other sources, and therefore indicated that the data on sharks discarded dead and discarded alive in the U.S. pelagic fishery logbook are accurate. The introduction of fisheries management regulations (fin weight to carcass weight ratios in 1993 [to prevent finning] and the prohibition of J-hooks in 2004) presumably decreased the immediate mortality rate of captured blue and tiger sharks (by 8.0% in blue sharks after 2004 and 4.4% in tiger sharks after 1993). Other factors that we examined had a statistically significant effect on mortality, but additional variables should be recorded or made available in logbook data to enable the determination of other causes of mortality. Our results show that the U.S. pelagic fishery logbook data can be used as a powerful tool in future studies of the immediate mortality of longline-caught animals.