Moving from measuring to predicting bycatch mortality: predicting the capture condition of a longline-caught pelagic shark

Incidental fisheries capture has been identified as having a major effect on shark populations throughout the world. However, factors that contribute to the mortality of shark bycatch during fisheries capture are not fully understood. Here, we investigated the effects of capture duration, sea surface temperature, and shark total length (snout to the tip of the upper caudal lobe) on the physiology and condition of longline-caught bronze whalers, Carcharhinus brachyurus. Plasma lactate and potassium concentration had a positive linear relationship with capture duration, indicating that this species experiences increasing physiological challenges while on fishing gear. Additionally, we used stereotype logistic regression models to determine variables that could predict the capture condition of sharks (categorized as "healthy," "sluggish," or "moribund or dead"). In these models, elevated plasma lactate concentration, plasma potassium concentration, and capture duration increased the likelihood of C. brachyurus being captured in a "sluggish" condition or in a "moribund or dead" condition. After plasma lactate concentration exceeded 27.4 mmol/L, plasma potassium concentration exceeded 8.3 mmol/L, or capture durations exceeded 293 min, the majority of captured sharks ( > 50%) were predicted to be "moribund or dead." We recommend that a reduction in the amount of time longlines are left fishing (soak time) will reduce immediate and post-release mortality in C. brachyurus bycatch and that our methods could be applied to identify causes of fisheries-induced mortality in future studies. The identification of operational, environmental, and biological variables contributing to poor condition will be necessary to implement conservation strategies that reduce mortality during capture.