At-haulback mortality of elasmobranchs caught on the Portuguese longline swordfish fishery in the Indian Ocean
Whilst the use of techniques for the safe handling and release of bycatch species has been shown to development of safe handling and release techniques for bycatch species shown to increase want to know what is happening after release to evaluate how fishing practices affect bycatch survival and to identify opportunities to reduce bycatch mortality, we estimated the odds of hooking survival for common bycatch species in the Canadian longline fishery for swordfish (Xiphias gladius) and tunas (Thunnus spp.)
In this study we analyze at-haulback fishing mortality of elasmobranchs caught by Portuguese longliners that target swordfish in the Indian Ocean. Information was collected by an IPIMAR on-board fishery observer that monitored 103 longline sets between May and September 2011, and recorded information on 2910 elasmobranch specimens from 11 different species. At-haulback mortality is species-specific, with some species having high percentages of alive specimens at time of haulback (e.g. manta rays, pelagic stingray and blue shark), while others have higher percentages of dead specimens (e.g. smooth hammerhead, silky shark and bigeye thresher). The most captured elasmobranch species was the blue shark and the odds-ratios of mortality at different sizes and for each sex were estimated with GLM logistic models. Blue shark specimens tended to have decreasing odds of mortality with increasing sizes, and those results are in accordance to what has been previously reported for the Atlantic Ocean. The results presented in this paper can now be integrated in future ecological risk assessment analysis for pelagic elasmobranchs, and can be used to estimate the survival of sharks after being captured and discarded by longline commercial fisheries.