Workshop on the Depredation in the Tuna Longline Fisheries in the Indian Ocean 2007

IOTC Secretariat (2007) Workshop on the Depredation in the Tuna Longline Fisheries in the Indian Ocean 2007. IOTC, Seychelles

•Depredation is the partial or complete removal of captured fish or bait from fishing gear by predators. During this workshop discussion concentrated on depredation of tuna longline fishing gear in the Indian Ocean.
•Depredation on longline gears are recorded for many marine species: fish, marine mammals, birds, crustaceans, squids, however current workshop discussed depredation by principal predators: sharks and cetaceans, The cetaceans mostly include false killer whales Pseudorca crassidens, and short-finned pilot whales Globicephala macrorhynchus but other cetacean species including killer whales Orcinus orca also contribute. The species composition estimates could be improved with better field guides/ training of fishers and observers.
•Data were available from several countries and reported in their National Reports.
•The quality of the data is variable, as are the metrics used to quantify depredation. Standard terminology should be adopted to quantify depredation.
•The identification of the species causing the depredation is largely based on the shape of the wounds. Sharp edges are usually taken to indicate sharks, ragged edges to indicate cetaceans. However, this separation may be confounded by post-depredation feeding by both cetaceans and fishes which may reshape the original wound(s). There is a need to standardize and verify the criteria used to identify the predator species.
•The magnitude of losses on sets where depredation occurs can be large, particularly where cetaceans are the major cause of the depredation.
•Catch that has suffered from depredation by sharks can often still be marketed (one country reported that damaged fish are actually in greater demand.) than catch that is landed whole. Because of these factors cetacean depredation is the more important problem.
•Those species causing depredation vary both seasonally and spatially.
•Depredation rates vary by the species being depredated.
•Species depredated vary spatially and seasonally, partly or wholly as a consequence of the availability of different species to longline gear.
•Based on the data presented, the fraction of total catch that suffers from depredation seems often to be less than 5% on average. However,
•On those sets where depredation occurs, a large part of the catch can be lost (up to 100%), with corresponding economic loss to the fishers.
•Predators appear to prefer targeted tuna to other species, although one study found swordfish to be the primary target species depredated.
•Both catch and depredation statistics need improvement for stock assessment purposes. Major fishing countries now treat the depredation data as confidential and have not provided them to the IOTC. It is essential that all fishing entities provide such data to the IOTC on a timely basis.
•Methods to mitigate depredation losses to cetaceans involve shifting patterns in gear deployment to disrupt learned behavior, passive acoustic detection to avoid setting near pods, active acoustics to harass approaching individuals, and physical barriers to minimize attacks on hooked fish. The quantitative benefits of these methods have yet to be demonstrated, but have shown positive results that should be further researched.