Depredation, defined as the damage or removal of fish from fishing gear by predators, is an issue leading to negative impacts on animals and fisheries. Toothed whale depredation in pelagic longline fisheries targeting swordfish (Xiphias gladius) and tuna (Tuna spp) involves short-finned pilot whales (Globicephala macrorhynchus) and false killer whales (Pseudorca crassidens). In the Seychelles archipelago, a database was built to assess the extent of this phenomena and to analyse how fishing practices could be involved in. Data analyzed came from fishermen logbooks and scientific cruises and covered the 2002-2006 period, representing a total of 705 fishing operations. The proportion of sets impacted by toothed whale depredation was 16% while the proportion of damaged fish reached 58% when toothed whales interacted with longlines. Toothed whale global depredation rate reached 10.7%. Logistic regression analysis and generalized additive models showed that both depredation occurrence and rate were positively related to the abundance of target fish, suggesting the co-occurrence of toothed whales in areas of high concentration of pelagic fish. Obviously, alternative fishing practices cannot be considered as an efficient way to mitigate depredation. Consequently we investigated fishing gear improvement by deploying a technology designed to physically protect the hooked fish by hiding them to predators. The efficiency of “spiders” was tested during a fishing trial of 26 longline fishing operations when 12480 hooks and 1970 devices were set. 117 fish were caught on branchlines equipped with spiders and among those devices, 87.3% were correctly triggered and 80% of capture were correctly protected. While more trials should be carried out to deeply verify the efficiency of “spider” devices, we remain convinced that the consideration of the fishing gear technology might be more actively investigated to propose innovative measures to mitigate toothed whale depredation in pelagic longlining.
Thursday, 07 February 2019 / Published in Posters
Toothed whale depredation on pelagic longlining in Seychelles waters: identification of risky fishing practices and assessment of the efficiency of the physical protection of fish as mitigation measure
Rabearisoa, N., Lucas, V., Bach, P., 2011. Toothed whale depredation on pelagic longlining in Seychelles waters: identification of risky fishing practices and assessment of the efficiency of the physical protection of fish as mitigation measure, in: 19th Biennial Conference on the Biology of Marine Mammals. Presented at the 19th Biennial Conference on the Biology of Marine Mammals, Society for Marine Mammalogy, Tampa, Florida. (pdf)