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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).