Is it all about the haul? Pelagic false killer whale interactions with longline fisheries in the central North Pacific

摘要

Pelagic false killer whales (Pseudorca crassidens) are killed or seriously injured in the Hawaii-based deep-set longline fishery more than any other cetacean, with bycatch regularly exceeding allowable levels. Telemetry data from five satellite-tagged whales (from three groups) and longline logbook entries (4182 sets) from the Hawaii-based longline fisheries are used to assess the range of the population and potential interactions with longline gear. A switching state-space model with a 4 -h time step was used to assess the behavior of the tagged whales. Two of the groups remained within the U.S. EEZ surrounding the Northwestern Hawaiian Islands, while one group spent 87.5 % of its time in international waters to the east of the Hawaiian archipelago. Tagged whales came within 100 km of only 26 sets over the 184 days of tag data, with only two of the three groups coming within 50 km of a set. Only twice were whales (from only one group) known to approach closely enough to interact with gear, during two series of three deep-sets, with only one of the six sets recording no catch (indicating probable catch depredation). Movement towards the sets was most dramatic during the haul phase, in one case the tagged whales moved almost 100 km towards the gear in 7 h. During one set in each of the two interactions, whale behavior changed to 'area restricted search' (indicative of foraging) during periods that overlapped with hauling of the gear. Overall, our results show that pelagic false killer whales spend a relatively small proportion of their time interacting with U.S. longline gear, and suggest that hauling gear may be an important cue initiating interactions.