Physiological response and immediate mortality of gill-net-caught blacktip reef sharks (Carcharhinus melanopterus)

Abstract

To investigate factors contributing to immediate mortality in gill-net-caught elasmobranchs, we caught and blood sampled a total of 64 sharks and rays. Blacktip reef sharks (Carcharhinus melanopterus) were the most commonly caught species and had significantly elevated plasma lactate (mean 23.3 mM) and potassium (mean 6.2 mM) concentrations following capture, suggesting physiological disturbance. The overall immediate mortality rate of C. melanopterus was 38% and mortality was significantly influenced by body size, but not location of entanglement or sex. Of the other species caught, nervous sharks (Carcharhinus cautus) were of particular concern, because this species is often caught as bycatch by gill-net fisheries throughout its range and experienced high immediate mortality rates when captured (66%; n = 12) in the present study. The results suggest that juvenile C. melanopterus are particularly susceptible to gill-net-related mortality and that fisheries management strategies for C. melanopterus bycatch should focus on reducing the likelihood of juvenile sharks encountering this gear. Given that this species is not currently considered to be threatened with extinction, population monitoring should be performed such that the suggested regulations can be implemented if populations decline to levels threatening the conservation status of the species.

Dead tired: evaluating the physiological status and survival of neonatal reef sharks under stress

“…Adult C. melanopterus around Moorea appear to be quite resilient to hook-and-line capture ( Mourier et al , 2017 ), and neonate and juvenile C. melanopterus and N. acutidens both exhibited near 0% mortality following gill-net and hook-and-line capture in the Mangrove Bay Sanctuary Zone on Ningaloo Reef, Australia ( Oh et al , 2017a ). In contrast, however, another study out of Western Australia reported that, when facing unspecified capture durations in gill-nets, juvenile and adult C. melanopterus were more susceptible to immediate mortality than N. acutidens ( Dapp et al , 2017 ). Local adaptation to environmental conditions at the population level may influence these contrasting trends ( Eliason et al , 2011 ; Di Santo, 2016 ); although, temperature data were not reported from the Western Australia study ( Dapp et al , 2017 ).…”

“…In contrast, however, another study out of Western Australia reported that, when facing unspecified capture durations in gill-nets, juvenile and adult C. melanopterus were more susceptible to immediate mortality than N. acutidens ( Dapp et al , 2017 ). Local adaptation to environmental conditions at the population level may influence these contrasting trends ( Eliason et al , 2011 ; Di Santo, 2016 ); although, temperature data were not reported from the Western Australia study ( Dapp et al , 2017 ). The model generated by Dapp et al (2017) to estimate immediate mortality of C. melanopterus would have predicted 100% mortality for sharks in our study using only total length as a predictor.…”

“…Local adaptation to environmental conditions at the population level may influence these contrasting trends ( Eliason et al , 2011 ; Di Santo, 2016 ); although, temperature data were not reported from the Western Australia study ( Dapp et al , 2017 ). The model generated by Dapp et al (2017) to estimate immediate mortality of C. melanopterus would have predicted 100% mortality for sharks in our study using only total length as a predictor. It is possible that differences in the duration of capture led this study to conclude that immediate mortality was 0%, whereas difficulty in identifying capture events by Dapp et al (2017) could have allowed for sufficiently long capture durations and more realistic immediate mortality estimates.…”

“…The model generated by Dapp et al (2017) to estimate immediate mortality of C. melanopterus would have predicted 100% mortality for sharks in our study using only total length as a predictor. It is possible that differences in the duration of capture led this study to conclude that immediate mortality was 0%, whereas difficulty in identifying capture events by Dapp et al (2017) could have allowed for sufficiently long capture durations and more realistic immediate mortality estimates. Alternatively, stress resulting from this study’s shorter capture durations may simply not have been fatal ( Oh et al , 2017a ).…”

“…The exhaustive challenge used throughout the entire study included approximately ~3 min of gill-net capture (3.4 ± 1.2 min SD) and 1 min of air exposure. Gill-net capture has been demonstrated to induce exhaustion in elasmobranchs ( Frick et al , 2009 , 2010 , 2012 )—including juvenile C. melanopterus and N. acutidens ( Dapp et al , 2017 )—and a standardized duration of air exposure is commonly employed along with an exhaustive challenge to maximally exhaust fish and to simulate handling of fish out of water by fishers ( Clark et al , 2013 ; Rummer et al , 2016 ). Values for physiological metrics were generated from unique individuals subjected to one of four treatments.…”