Factors determining the interaction between common bottlenose dolphins and bottom trawlers off the Balearic Archipelago (western Mediterranean Sea)

Factors determining bottlenose dolphin association with bottom trawlers were studied off the Balearic Islands, western Mediterranean, by studying dolphin distribution around the islands and their interaction with fishing operations. Results showed that bottlenose dolphins avoided the upper shelf (shallower than 50 m) in the warm season, but not in the cold season and that the slope was avoided all year round. Bottlenose dolphins approached most of the trawlers surveyed in the continental shelf, but seldom interacted with those operating in the slope in the warm season. As a consequence, the average depth of trawling operations that attracted dolphins was shallower than those not attracting dolphins. No statistical difference was observed in the average catch of hauls conducted in the presence or in the absence of dolphins. However, discriminant analysis showed differences in catch composition between hauls with and without dolphin presence, but this difference was not attributed to the palatability of the catch, but to influence of trawling depth on the catch. It is concluded that depth is the main factor ruling occurrence of interaction.