Predicting Postrelease Survival in Large Pelagic Fish

—Sharks, turtles, billfish, and marine mammals are frequently caught accidentally in commercial fisheries. Although conservationists and fisheries managers encourage the release of these nontarget species, the long-term outcome of released animals is uncertain. Using blue sharks Prionace glauca, we developed a model to predict the long-term survival of released animals based on analysis of small blood samples. About 5% of the sharks were landed in obviously poor condition (lethargic and unresponsive to handling); these moribund sharks were sampled and euthanized. A subset of the remaining sharks was sampled and tagged with pop-up satellite archival tags (PSATs). Each of the PSATs that reported data (11 tags) showed that the sharks roamed at sea for at least 3 weeks postrelease. Five variables differentiated moribund sharks from survivors: plasma Mg2þ (moribund, 1.57 6 0.08 mM; survivor, 0.98 6 0.05 mM; P , 0.00001), plasma lactate (moribund, 27.7 6 4.1 mM; survivor, 5.80 6 2.96 mM; P , 0.001), erythrocyte heat shock protein 70 (Hsp70) mRNA (relative levels: moribund, 3.96 6 0.53; survivor, 1.00 6 0.29; P , 0.005), plasma Ca2þ (moribund, 3.70 6 0.14 mM; survivor, 3.13 6 0.11; P , 0.005), and plasma Kþ (moribund, 7.01 6 0.66 mM; survivor, 5.12 6 0.44 mM; P , 0.05). These analyses were used to develop logistic regression models that could ‘‘predict’’ the long-term survival of captured sharks, including a larger group of sharks that we sampled but did not tag. The best logistic model, which incorporated Mg2þ and lactate, successfully categorized 95% of fish of known outcome (19 of 20). These analyses suggest that sharks landed in an apparently healthy condition are likely to survive long term if released (95% survival based on biochemical analyses; 100% based on PSATs). Commercial fishing activity has the potential to adversely affect the populations of large pelagic species, whether they are target species or bycatch. Though there is considerable disagreement about the current state of large pelagic fish populations (Hampton et al. 2005; Burgess et al. 2005), the animals at greatest risk are the large apex predators, particularly sharks (Baum et al. 2003; Baum and Myers 2004), scombrids (tuna and billfish) (Myers and Worm 2003), and turtles (Spotila et al. 2000). Substantial reductions in reproductive biomass of these long-lived, latematuring predators could cause prolonged effects on the population; effects that are seen locally in predator hotspots (Worm et al. 2003) or ecosystem-wide (Jackson et al. 2001; Frank et al. 2005), and in extreme cases might threaten a species with extinction (Spotila et al. 2000). Sharks dominate the bycatch of many marine pelagic fisheries (Bonfil 1994; Rose 1996; Hurley 1998; Francis et al. 2001). Some fisheries have a very high mortality in shark bycatch (e.g., 75% in the Atlantic menhaden Brevoortia tyrannus purse seine fishery; de Silva et al. 2001). However, in the pelagic longline fisheries for tuna and billfishes, sharks are captured in good condition. For example, in a survey of the 1997– 1998 New Zealand tuna longline fishery, about 87% of blue sharks Prionace glauca were alive at the time of gear recovery (Francis et al. 2001). Though unwanted bycatch is typically released from the fishing gear, it is unknown if these animals would survive long term. This uncertainty about postrelease survival is a management challenge in many fisheries for large pelagic species, from commercial longline to sports fisheries. Long-term postrelease survival is typically estimated by means of tagging programs. Historically, large-scale conventional tagging programs were employed but these yielded low return rates. For example, in a 30year study of Atlantic blue sharks, only about 5% of the tags were recovered (Kohler et al. 1998). Such * Corresponding author: [email protected] Received September 11, 2005; accepted March 31, 2006 Published online October 2, 2006 1389 Transactions of the American Fisheries Society 135:1389–1397, 2006 Copyright by the American Fisheries Society 2006 DOI: 10.1577/T05-224.1 [Article]