Capture-related stress physiology and delayed mortality of sharks

The impact of fisheries capture on sharks remains understudied, despite the urgent need for effective fisheries management plans. Consequently, the fate of sharks that are incidentally caught as by-catch and subsequently discarded alive is largely unknown. Postrelease mortality may significantly contribute to overall fishing mortality, but the circumstances leading to delayed mortality are poorly understood. This study evaluated the feasibility of simulating fisheries capture of elasmobranchs in a laboratory setting, and used this method to investigate the immediate and delayed effects of capture stress on the physiology and condition of sharks. Port Jackson sharks, Heterodontus portusjacksoni, and gummy sharks, Mustelus antarcticus, were subjected to varying durations of gill-net, longline and trawl capture in captivity, and their post-capture condition was monitored via serial blood sampling during a 72-h recovery period subsequent to the capture event. Results of preliminary experiments showed that sedation with the anaesthetic AQUIS® or frequent handling of sharks can affect stress-related blood variables. There was no evidence for any habituation effects in sharks that were repeatedly exposed to experimental gill-net capture. Capture experiments revealed distinct inter-specific differences in stress tolerance and clear gear-specific differences in immediate and delayed mortality of sharks. Port Jackson sharks appear to be highly resilient to capture stress, as evidenced by a very low degree of physiological disturbance and no mortality observed during or after any experiments. However, gummy sharks reacted more sensitively to the capture treatments. Immediate and delayed mortality of gummy sharks was low during longline experiments, but was substantial during some gill-net and trawl experiments. In general, exposure to capture stress resulted in elevated plasma lactate and potassium concentrations, depressed plasma urea concentrations, and depressed blood pH caused by a combination of metabolic (increased lactate) and respiratory (increased pCCb) acidoses. Blood pH was most depressed immediately after capture, whereas maximum plasma lactate and potassium concentrations were measured 3-6 h after the capture event. In moribund gummy sharks, plasma lactate and potassium concentrations rose until death occurred, and were significantly higher than in surviving gummy sharks. Intramuscular lactate concentration of gummy sharks was substantially higher than plasma lactate concentration at all times, and was highest immediately after a capture event. High buffering capacity of myotomal n muscle of gummy sharks indicates that this species routinely accumulates large amounts of lactic acid in white muscle. Increased stress duration generally led to higher maximum plasma lactate values measured during the 72-h monitoring period, but did not result in higher mortality. Brief exposure to air (10 min) following a trawl capture event led to higher maximum plasma lactate concentrations. Simulated crowding of sharks in a trawl cod end, as performed in this study, had no additional effect on the monitored blood variables. The results of this study suggest that incidental capture and subsequent release of does not have any strong negative short-term effects on the condition of Port Jackson sharks, but gummy sharks are susceptible to the deleterious effects of fisheries capture, and severe disturbance of their homeostatic balance can lead to substantial delayed mortality. As a consequence, fisheries management efforts concerning gummy shark stocks should aim to reduce the probability of unwanted interaction with fishing gear for this species. The findings of this study are an important contribution to identifying a reliable field-based method that allows an accurate assessment of delayed mortality of discarded sharks