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Ecological risk assessment (ERA), and specifically Productivity-Susceptibility Analysis (PSA), is a useful methodology for assisting the management of fisheries from an ecosystem perspective in a data poor situation. Indian Ocean tuna and tuna-like fisheries, managed by the Indian Ocean Tuna Commission (IOTC), are economically important both at local and international scales and interact with several non-target or bycatch species.
A PSA for shark caught in various longline fleets, purse seiner fleet and gillnet fleet operating in the Indian Ocean was carried out. We follow the methodology proposed by Cortés et al. (2010), which allow ranking the vulnerability of the species based on its productivity and susceptibility to the fishing gear. We estimate the species productivity parameters based on Leslie matrices analysis, in which the value of Lambda (λ), population finite growth rate, was calculated (Caswell 2001). The susceptibility analysis was carried out comparing the horizontal overlap between fisheries and stock distribution, the vertical overlap between the species and fishing gear, the gear selectivity, and post-capture mortality.
The species with the least productivity values are the pelagic thresher (Alopias pelagicus) and the crocodile shark species (Pseudocarcharias kamoharai), followed by several Lamniformes (Alopias superciliosus, Alopias vulpinus, Isurus paucus, and Lamna nasus). However, for Isurus paucus little biological information is available and most information is from the Atlantic. As had been previously observed for other Oceans, such as the Atlantic (ICCAT, 2012), the blue shark (Prionace glauca) seems to be the pelagic shark species with the higher values of biological productivity.
The species more susceptible for the longline fishing fleets are the blueshark (Prionace glauca) and shortfin mako (Isurus oxyrhinchus) followed by silky shark (Carcharhinus falciformis), porbeagle (Lamna nasus), bigeye thresher, great hammerhead and longfin mako. Then oceanic whitetip (Carcharhinus longimanus), common and pelagic threshers and smooth hammerhead (Sphyrna zygaena) are ranked in lower levels of susceptibility and the susceptibility of the rest of species is even lower. Overall, it was estimated that the most vulnerable species are the shortfin mako, silky shark, porbeagle and bigeye thresher, followed by blueshark, longfin mako, great hammerhead and oceanic whitetip. Common and pelagic threshers are rank with lower vulnerability because lower post-capture mortality after the entry in force of Resolution 12/09 on threshers.
The species more susceptible for the purse seine fishing fleets are the crocodile shark and pelagic thresher followed by longfin mako. The rest of species are ranked in much lower levels of susceptibility. The coastal shark species are less susceptible for the purse seiner fleets. Overall, and according to our analysis, for the purse seiner fleets the most vulnerable species are the crocodile shark, pelagic thresher, longfin mako, and silky shark. The most vulnerable species estimated in 2012, the oceanic white-tip and silky shark, were rank in much lower level of vulnerability in this exercise because their lower post-capture mortality after the implementation of safe release best practices in the purse seiner fleet in 2014. The rest of species are ranked in much lower levels of vulnerability.
The species more susceptible for the gillnet fishing fleets were the most coastal shark species such as smooth hammerhead, the crocodile shark and pelagic thresher followed by silky shark, scalloped hammerhead and longfin mako. Overall, for the gillnet fleets the most vulnerable species are the coastal crocodile shark, smooth hammerhead, pelagic thresher, silky shark and scalloped hammerhead. The rest of species are ranked in similar levels of vulnerability except blueshark and porbeagle with lower vulnerability.
