Moving from Measuring to Predicting Bycatch Mortality: Predicting the Capture Condition of a Longline-Caught Pelagic Shark

Abstract

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.

Meta‐analysis of post‐release fishing mortality in apex predatory pelagic sharks and white marlin

“…temperature, dissolved oxygen, salinity, temperature–depth gradients) and biological factors (e.g. species, respiratory mode, habitat class, sex, body size, skin thickness, predation, soak time or time spent hooked/fight time, hooking location, vitality or condition code, biochemical correlates of morbidity and mortality) to influence survival outcomes (Awruch et al., ; Braccini et al., ; Butcher et al., ; Campana, Joyce & Manning, ; Carruthers et al., ; Dapp et al., , , ; Davis, ; Diaz & Serafy, ; Gallagher, Orbesen et al., ; Gallagher, Serafy et al., ; Marshall et al., ; Morgan & Carlson, ; Moyes et al., ; Musyl, Brill et al., ; Musyl et al., ). Moreover, there are also data that suggest sharks may suffer from gas emboli from rapid temperature changes (Garner, ) accompanied during retrieval from deep‐set longline gear (Rodríguez‐Cabello & Sánchez, ).…”

“…(), Gallagher, Serafy, Cooke and Hammerschlag (), Dapp et al. () and Musyl and Gilman (), light touching of the nictitating membrane (found only in the Carcharhiniformes, comprising ~270 species) can be used to distinguish mortality of sharks (Bell & Satchell, ). If observers are on boats with high freeboard, a pole with a padded probe could be used to administer the test.…”

“…For fisheries management and conservation, having precise survival rates of discards and at‐vessel mortality rates are necessary to advance and justify non‐retention, catch‐and‐release policies (Ellis, Phillips & Poisson, ; Graves, Marcek & Goldsmith, ; Moyes et al., ; Pine, Martell, Jensen, Walters & Kitchell, ) and other by‐catch mitigation strategies to rebuild stocks (Carruthers, Schneider & Neilson, ; Dapp, Huveneers, Walker, Drew & Reina, ; Favaro & Côté, ; Gilman et al., ; Poisson et al., ; Uhlmann et al., ). In fisheries population biology, it is necessary to account for the two main sources of fishing mortality: at‐vessel or catch (i.e.…”

“…Accurate estimates of the two sources comprising fishing mortality are also essential in producing accurate stock and other forms of ecological risk assessments (Gallagher, Kyne & Hammerschlag, ; Hobday, Smith, Stobutzki, Bulman & Daley, ; Kelleher, ; Kitchell, Kaplan, Cox, Martell & Essington, ; Pine et al., ; Punt, Smith, Tuck & Methot, ; Viana, Graham, Wilson & Jackson, ; Viana, McNally, Graham, Reid & Jackson, ). The vast majority of discards are unaccounted for in most stock assessments (Dapp et al., ) and the uncertainty as to the true values of F c and F r are major impediments to effective management and resource conservation in many fisheries (Campana, Joyce & Manning, ; Carruthers et al., ; Dapp et al., , ; Dapp, Walker, Huveneers & Reina, ; Molina & Cooke, ; Musyl et al, ). Fishing also has the capacity to exert sublethal effects at the population level (e.g.…”

A review of capture and post‐release mortality of elasmobranchs

“…g . concentrations of lactate and potassium) may be correlated with physiological stress and trauma and likelihood of survival (Wells & Davie, ; Hoffmayer & Parsons, ; Mandelman & Farrington, 2007 a ; Brill et al , ; Mandelman & Skomal, ; Brooks et al , ; Hyatt et al , ; Marshall et al , ; Skomal & Mandelman, ; Dapp et al , 2016 a ).…”