Preliminary Observation of Depredation by Bottlenose Dolphins on the King Mackerel Troll Fishery in Florida

Abstract

Conflicts between marine mammals and fisheries can be attributed to spatial overlap and resource competition between cetaceans and fisheries and/or behavioral learning among marine mammals. In the present study, I documented depredation by bottlenose dolphins (Tursiops truncatus) in Florida’s king mackerel troll fishery. Depredation refers to the behavior in which predator species remove or damage fish on fishing lines or in nets. I conducted twenty-six surveys of charter and commercial fishermen in Islamorada, Florida and twenty-three along Florida’s east coast from Fort Pierce south to Lake Worth Inlet, and I observed dolphin behaviors from working vessels in these two study sites. All survey respondents indicated they had observed bottlenose dolphins depredating bait or catch. Fishermen reported that king mackerel was the species most often taken by bottlenose dolphins. From behavioral observations, I found that dolphins took 6% of king mackerel caught by charter fishermen and 20% of fish caught by commercial fishermen. I attributed these differences to seasonal variation in fish distribution or gear differences between vessel types. Preliminary tests demonstrated that a modification to the outrigger planar will successfully deter bottlenose dolphins from engaging in depredation, without causing a reduction in catch.