Fate of the fish caught on longline gears and potential mitigation measures

Poisson F (2009) Fate of the fish caught on longline gears and potential mitigation measures. IOTC, Mombasa, Kenya

This document summarises some major results obtained during experiments conducted in collaboration with the Reunion Island (France) (20-22 degrees N and 53-57 degrees N) fishing industry. These studies may aid fishermen in modifying fishing operations and selecting a fishing strategy to increase economic benefits and also to reduce the impact on bycatch mortality. Firstly, we investigated the behaviour of the fishes when caught on the longline gear and the survivorship of fish hooked, using longline gears instrumented with hook time recorders (HT) and temperature depth recorders (TDR). We showed that the percentages of fish recovered alive at hauling varied among species. The percentages of fish recovered alive up to 8 h after capture provides a rough idea of the resistance of each species to the capture process; these rates were recorded for the blue shark (Prionace glauca), the oceanic whitetip shark (Carcharhinus longimanus) and for the bigeye tuna (Thunnus obesus) and were respectively 29%, 23 % and 27% while this rate was lower for the swordfish (8%). Moreover, we demonstrated that shortening the soaking time during the fishing operation could be beneficial in many ways for fishermen. A second study on the reproduction dynamic of the swordfish in the vicinity of Reunion Island showed that the Big Old Fat Fecund Female Fish (BOFFFF) hypothesis could effectively apply to this species. Consequently, the removal of the larger, older individuals could be detrimental for the stock and the current results may be used, in the future, to support new policies preserving population age structure. One management method available to conserve older fish would be to institute slot size limits for retention (minimum and maximum size) but this potential measure to be successful need the individuals to survive their release back to the water. The last study aimed at investigating the possibility of developing a method to reduce the stress of the fishes caught with hooks. Prototypes of "sleeping hook" were developed and tested, using rod and reel, around moored fish aggregating devices (FAD's). During the fishing experiments a total of 162 fish comprising 3 main species were caught including: yellowfin tuna (Thunnus albacares), skipjack tuna (Katsuwonus pelamis) and dolphinfish (Coryphaena hippurus). Analyses of blood chemistry stress indicators revealed the "sleeping hook" method to be successful in reducing the fish stress. Additional research should be conducted to evaluate the feasibility of reducing the soaking period in the current fishing strategy. However, the "sleeping hook" could contribute to the development of alternative fishing technology enabling also to reduce the side effect of protracted soaking times e.g. by reducing post hooking mortality and increasing the post release survivorship of species of conservation concern and unwanted sized target species.