Post-release survival and behavior and exposure to fisheries in juvenile tiger sharks, Galeocerdo cuvier, from the South Atlantic

Citation
Afonso AS, Hazin FHV (2014) Post-release survival and behavior and exposure to fisheries in juvenile tiger sharks, Galeocerdo cuvier, from the South Atlantic. Journal of Experimental Marine Biology and Ecology 454:55–62. doi: 10.1016/j.jembe.2014.02.008
Abstract

General declines in the abundance of sharks due to intense exploitation rates have been increasing the pressure for mitigating bycatch mortality in longline fisheries. Understanding post-release mortality and behavior of sharks is required to ensure the adequacy of management strategies focusing on the release of live animals. Also, such information is crucial for addressing shark attack mitigation strategies focusing on mechanisms other than shark culling. This study used a combination of conventional tags with acoustic and satellite transmitters to assess both post-release survival and behavior and exposure to fisheries in tiger sharks caught in a longline survey off Recife, Brazil. Although none of the sharks experienced post-release mortality, one episode of presumable natural mortality occurred after 45 days at-liberty. The exposure of tiger sharks to fishing pressure in this region seems to be high since at least 23% of the tiger sharks tagged and released off Recife were caught by coastal and oceanic fisheries, most (83%) of which after only 69 (± 42) days at-liberty. Upon release, all satellite-tracked sharks moved offshore to deeper waters, usually in the oceanic realm, within an average of 17.72 (± 10.99) h, corresponding to a minimum cross-shelf swimming speed of 0.40 (± 0.22) m · s− 1. The archival data of 3 recovered satellite tags depicted post-release behaviors with exceptional detail, with all such sharks exhibiting a yo-yo movement pattern with increasing depths until the shelf break was reached. Some sharks consistently spent a period of 12 (± 2) days in deep-diving before returning to shallower waters, presumably from the continental shelf, which could correspond to a species-specific behavioral response to hooking stress. Tagged sharks did not return to the area where they were caught and it would be unlikely that they would return before several months because most of them traveled through great distances to northward latitudes. Altogether, tiger sharks caught alive in longline gear seem to experience negligible post-release mortality if they are adequately released, thus an eventual mandatory release of live animals could be effective to improve the conservation of the species. Also, recently-released tiger sharks should not be expected to further interact with the same fishing gear that caught them for some time, or to further pose any threat to beach users in Recife, where an abnormally high shark attack rate is verified since 1992.