Indicators of Depredation by Sharks and Toothed Whales Impacting the Domestic Pelagic Longline Fishery in Seychelles

Depredation on pelagic longlines targeting swordfish and tuna involves short-finned pilot whales, false killer whales and some pelagic sharks. It has adverse impacts affecting conservation, fisheries economy and fishery statistics. Thereby, there is a real need for robust and accurate indicators in order to assess the extent of depredation, which still remains a misunderstood phenomenon with poorly studied impacts. In the Seychelles archipelago, where a semi-industrial pelagic longline fishery was launched in the mid-90s, a database was built for the 2004-2006 period to assess the extent of this phenomenon through analyses of the trend of five different depredation indices: depredation occurrence, gross depredation impact, depredation rate, depredation per unit effort and economic loss per unit effort. The depredation occurrence is higher for sharks than toothed whales. The gross depredation impact reached 19.5% of the catch, while the depredation rate was significantly greater for toothed whales than for sharks. Toothed whales significantly damaged more swordfish than tuna in proportion, whereas sharks equally damaged both species. Finally, the economic loss per unit effort due to depredation averaged USD120/1000 hooks. These values are among the highest known in the world, suggesting that Seychelles region might be a depredation “hotspot” in pelagic longlining.