Bycatch - where a non-target species is caught by non-discriminatory fishing methods - is a common problem of the fishing industry. Owing to the fact that many species of shark are endangered and for the sake of upkeeping good fishing practice, bycatch is often required to be released alive. Whilst there have been studies on the successful handling and release of shark bycatch, there have been few studies on the post release mortality of sharks.
In their research the team, led by A. J. Gallagher, looked at whole blood lactate, pH values, pCO2 and shark size to assess the physiological disturbance that hooking has on a shark and to ascertain if any of these disturbances linked to post-release mortality. Smart Position or Temperature Transmitting (SPOT) tags were deployed on 3 of the species (bull, tiger and great hammerhead sharks) to assess potential post-release survival.
The researchers included 5 shark species in the study: blacktip shark Carcharhinus limbatus, bull shark Carcharhinus leucas, great hammerhead shark Sphyrna mokarran, lemon shark Negaprion brevirostris and tiger shark Galeocerdo cuvier. The species were caught on baited drumlines using circle hooks - which have been shown to lower mortality rates in sharks. The hooks afforded the sharks a 23m radius to swim around once caught. The hooks were checked periodically and caught sharks were brought alongside the boat in order to take samples and to measure the total length.
The results showed that of the 5 species, tiger sharks were the most likely to survive post release capture with 96.4% of the individuals caught transmitting tag data 4 weeks after the study. Blacktip sharks appeared to be the most vulnerable to hooking, with their physiological disturbances ranking the highest of the 5 species studied. The team also found that of all the measurements, blood lactate levels seemed to correlate closest with post release survival. Species which may need to use short bursts of energy to evade predation from larger sharks - such as the blacktips - displayed high levels of lactate in their blood. Lactate is generated when periods of sustained exercise cause animals to undergo a shift from aerobic to anaerobic respiration. Tiger sharks - which most likely rely on stamina and sustained swimming as oppose to bursts of energy - exhibited very little change in their lactate levels.
These stress responses - such as changes in lactate and pH - can affect the short term and long-term survival of an animal. This study found that some sharks - such as tiger and lemon sharks - are able to recover very quickly from the stress of being hooked. However, other species - such as blacktips and great hammerhead sharks - suffer greatly from post capture stresses as displayed by large changes in their blood lactate levels. This is translated to higher supposed mortality rates post capture - only 53.6% of great hammerhead sharks transmitted tag data after 4 weeks compared to 96.4% of tiger sharks.
The finding here are important as the fishing industry may currently be assuming that if a caught shark swims away from the boat that it will survive. However, the results here show that there are huge variations in the ability of different shark species to recover from the stress of being hooked and the post release survival rate is actually quite low in some species. Owing to the fact that sharks are threatened globally and live release of bycatch is currently an accepted strategy for dealing with unintentionally caught sharks, more data on actual post release survival levels will help determine whether or not hooking certain species of sharks will impact upon the long term survival of the species.
Gallagher, A.J., Serafy, J.E., Cooke, S.J. and Hammerschlag, N. (2014) Physiological stress response, reflex impairment, and survival of five sympatric shark species following experimental capture and release. Marine ecology progress series. 496: 207-218
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