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Sharks are of considerable interest from at least three important scientific or practical perspectives. First, many aspects of the biology of these fishes have not yet been studied in detail (Carrier et al. 2004; Grogan and Lund 2004). Second, they are ecologically important as predators in pelagic food webs (Kitchell et al. 2002; Schindler et al. 2002; Compagno 2008). Third, sharks comprise much of the nontarget catch in many commercial fisheries (Beerkircher et al. 2004; Gilman 2007a; Erickson and Berkeley 2008; Pikitch et al. 2008) and are vulnerable to overfishing because their typical life history traits include slow growth, relatively late maturation, and low fecundity (Smith et al. 1998; Cortés 2004). Moreover, because most sharks are of low value, they may be under‐reported or not reported at all in logbooks (Walsh et al. 2002; Nakano and Clarke 2006), which can increase the difficulty of discerning population trends and can introduce uncertainty into stock assessment models.
The Hawaii‐based pelagic longline fishery, which targets bigeye tuna Thunnus obesus in the deep‐set sector and swordfish Xiphias gladius in the shallow‐set sector, takes sharks, especially blue sharks Prionace glauca, in substantial numbers (Kleiber et al. 2001; Walsh et al. 2002; Dalzell et al. 2008). Unlike most other fisheries that take sharks, the Hawaii‐based longline fishery is very well suited to analyses of shark catches because detailed operational and catch data are gathered by the Pacific Islands Regional Observer Program (PIROP). This program was established as the Hawaii Longline Observer Program in March 1994 for the purpose of monitoring interactions between fishing vessels and protected or endangered sea turtles (leatherback sea turtles Dermochelys coriacea and loggerhead sea turtles Caretta caretta) (DiNardo 1993). At the present time, PIROP observers monitor interactions with protected or endangered species, record a large suite of operational details (e.g., position, number of hooks, set and haul times, target species, bait type), obtain species‐specific tallies of the catch, including the animal's condition on retrieval (i.e., live or dead) and subsequent disposition (i.e., retained or discarded), and measure fish (Pacific Islands Regional Office 2006). The PIROP is now the largest...