Use of generalized linear models to investigate the fishing strategies of tuna purse seiners, in presence of floating objects in the western Atlantic Ocean.

The fishing strategies used by Venezuelan purse-seine fishermen in the Caribbean Sea are based on sighting indices, which mark the presence of tuna schools at the surface of the ocean. Once detection has been made, the purse seiner can seek the help of a baitboat to increase the percentage of successful sets. In order to consider the variability of factors, such as season, floating objects associated with schools, help of a baitboat, tuna species and mean weight of fish in the school, the probabilities of some events have been estimated by logit modeling, drawn from log-linear models. The probabilities of capturing a school of large tunas are high when yellowfin tuna is the main species in the school and when the set is made on non-associated school or on whale-sharks; this effect is greater during the dry season. In contrast, if skipjack tuna is the dominant species in the school, the probability of catching large tunas is low, especially for schools associated with whales. The results of these models have been included in a tree decision analysis. If we use the criterion of the mathematical expectation of outcomes, the tactic which consists of using a baitboat does not lead to the best choice. However, the effect of this tactic is significant, since the rate of positive sets increases from 58%, without the baitboat, to 85% with the baitboat. The efficiency of this help is linked with the nature of the sighting index. This efficiency is greater for sets associated with whales and especially with non-associated schools than for sets associated with whale-sharks. A Monte Carlo simulation shows that some important outcomes can be expected, without the baitboat, for sets associated with whales. Hence, the captains of the small and median purse seiners, who use baitboats more frequently than the captains of the large ones, could be characterized as risk-averse, without neglecting some limiting characteristics of their boats such as the speed. Compared to other tuna fishing grounds, the Caribbean Sea is characterized by a high proportion of sets made in presence of whale sharks (37% of the sets) and whales (24%). In contrast, the frequency of log sets is very low. This fact could be explained by the nature of ocean currents which carry away these logs out off the Venezuelan purse-seiner fishing grounds. To conclude, the similarities of adaptive attitudes developed against risk in different fields, such as agriculture and fisheries, are briefly discussed.