Interactions between cetaceans and the tuna/swordfish pelagic longline fishery in the tropical western Atlantic Ocean

摘要

Since the 1950s, interactions between cetaceans and tuna/swordfish longline fisheries have been observed and described in many seas and oceans worldwide. Depredation, in which marine mammals feed on fish caught on longlines, is one form of interaction that has led to considerable detriment to the fishing industry. Depredation can lead to losses that vary depending on the target species of the fishery and the marine mammal species involved, who run the risk of substantial injury or even death. The present study was carried out in an area in the tropical western Atlantic Ocean located along the Brazilian coast, between latitudes 7 degrees N and 24 degrees S and longitudes 16 degrees W and 42 degrees W, from February 2005 to August 2006. The aim of this investigation was to assess and characterize the occurrence of these interactions with the Brazilian longline tuna and swordfish fishery industry. Information was acquired through the collection of data by onboard observers of the foreign-operated fishing fleet operating under a lease agreement with Brazil. Observers collected data on the types of interaction (sightings, sightings with depredation, depredation and unintentional bycatch), their location, and the qualitative and quantitative characteristics of the cetaceans and depredated fish. Specific information regarding the fishery zone and longline retrieval time of each occurrence was also collected. Interactions with cetaceans were seen and recorded in 29.7 % of the total number of fishing cruises. The frequency of interaction (IF) was calculated as 10.0 %, whereas, the frequency of depredation (DF) and the frequency of sightings (SF) were 7.4 0/0 and 5.5 %, respectively. Two cetacean species were identified as interacting with the longline: false killer whales (Pseudorca crassidens), which were most frequent (92 %) with group sizes between two and hundreds of individuals, and killer whales (Orcinus orca) to a much smaller degree (8 %). Two separate regions were notable in terms of frequency of interaction: one straddling the equator, and another in the vicinity of Trindade and Martin Vaz islands. Depredation occurred more often on the target species (85 %) of the fisheries-tuna and swordfish-than on the accompanying fauna; this could indicate a preference for food that fulfills their nutritional needs. There were nine records of bycatch during this study. The investigation revealed important information on the effect of depredation caused by cetaceans in the tropical western Atlantic Ocean.