Identifying partners at sea from joint movement metrics of pelagic pair trawlers

Abstract

Abstract Here, we present an approach to identify partners at sea based on fishing track analysis, and describe this behaviour in several fleets: pelagic pair trawlers, large and small bottom otter trawlers, mid-water otter trawlers, all in the North-East Atlantic Ocean, anchovy purse-seiners in the South-East Pacific Ocean, and tuna purse-seiners in the western Indian Ocean. This type of behaviour is known to exist within pair trawlers, since these vessels are in pairs at least during their fishing operations. To identify partners at sea, we used a heuristic approach based on joint-movement metrics computed from vessel monitoring system data and Gaussian mixture models. The models were fitted to joint-movement metrics of the pelagic pair trawlers, and subsequently used to identify partners at sea in other fleets. We found partners at sea in all of the fleets except for the tuna purse-seiners. We then analysed the connections between vessels and identified exclusive partners. Exclusiveness was more common in pelagic pair trawlers and small bottom otter trawlers, with 82% and 74% of the vessels involved in partnerships having exclusive partners. This work shows that there are collective tactics at least at a pairwise level in diverse fisheries in the world.