Decreases in Shark Catches and Mortality in the Hawaii-Based Longline Fishery as Documented by Fishery Observers

Walsh WA, Bigelow KA, Sender KL (2009) Decreases in Shark Catches and Mortality in the Hawaii-Based Longline Fishery as Documented by Fishery Observers. Marine and Coastal Fisheries 1:270–282.

This article summarizes catch data for sharks collected by fishery observers during two periods (1995-2000 and 2004-2006) in the Hawaii-based pelagic longline fishery, which targets swordfish Xiphias gladius in the shallow-set sector and bigeye tuna Thunnus obesus in the deep-set sector. The blue shark Prionace glauca was the predominant shark species caught throughout the study period (84.5% of all sharks). Five other species (bigeye thresher Alopias superciliosus, oceanic whitetip shark Carcharhinus longimanus, shortfin mako Isurus oxyrinchus, silky shark C. falciformis, and crocodile shark Pseudocarcharias kamoharai) were relatively common (1.0-4.1%). Two major developments affected shark catches in this fishery during the study period. The first was the prohibition in 2000 of shark finning under most circumstances. The second development was that management measures were taken in 2000 and 2001 to protect sea turtles (leatherback sea turtles Dermochelys coriacea and loggerhead sea turtles Caretta caretta) and these measures included a closure of the shallow-set (swordfish-targeting) sector for more than 3 years. The closure caused decreases in shark catches because the shallow-set sector was typically characterized by high catch rates. The shallow-set sector was reopened in 2004. Comparisons of nominal catch per unit effort (number of sharks/1,000 hooks) revealed significant differences in catch rates between the two fishery sectors and the two periods. Blue shark and shortfin mako catch rates were significantly greater in the shallow-set sector than in the deep-set sector of the fishery, whereas the opposite was true for the deeper-dwelling bigeye threshers and crocodile sharks. Catch rates for the blue shark, oceanic whitetip shark, bigeye thresher, and crocodile shark were significantly lower in 2004-2006 than in 1995-2000. For the blue shark in particular, the combination of reduced catch rates, the finning ban, and an apparent capacity to resist the stress of capture on longline gear resulted in low (4%-5.7%) minimum mortality estimates. Therefore, we conclude that the Hawaii-based pelagic longline fishery has made substantial progress in reducing shark mortality.