348 



Fishery Bulletin 104(3) 



of king mackerel, the diet and possibly the distribution 

 of these animals is impacted by their interaction with 

 the fishery. 



In this study, fishermen reported observing female 

 bottlenose dolphins "teaching" their calves how to engage 

 in depredation, indicating a behavioral transmission of 

 knowledge. Depredation of king mackerel by bottlenose 

 dolphins may have resulted from a learned behavior that 

 results in a low-cost foraging specialization. 



Over three quarters of interviewed fishermen reported 

 increasing conflicts with bottlenose dolphins. The fre- 

 quency of these interactions most likely result from a 

 combination of factors, including behavioral learning, in- 

 creasing fishing effort, and spatial overlap and resource 

 competition between cetaceans and fisheries (Donoghue 

 et al.'-). In addition, an upsurge in depredation may be 

 correlated with a rise in troll fishing effort that resulted 

 from the July 1995 statewide ban of gill nets in Florida 

 (Wells et al., 1998). The increase in depredation places 

 bottlenose dolphins in close proximity to fishing vessels 

 and gear, increasing the risk of injury or death to the 

 dolphins. It is unknown if the dolphins in this study 

 were injured by hooks. Although we did not observe 

 entanglement in this study, entanglement in and inges- 

 tion of fishing gear by bottlenose dolphins could result 

 from depredation. Hucke-Gaete et al. (2004) observed a 

 fatal entanglement of a sperm whale (Physeter tnacro- 

 cephahis). likely engaging in depredation, in a longline 

 fishery off southern Chile. Monofilament fishing line 

 does not degrade rapidly, and injury or death can result 

 from the entanglement in or ingestion of fishing gear 

 (Mann et al., 1995). Previous research has documented 

 the deaths of bottlenose dolphins from entanglement 

 (Wells et al., 1998) and from ingestion of monofilament 

 line (Gorzelany, 1998). 



As a result of lost or damaged gear and catch, fisher- 

 men experience economic loss from these interactions. 

 Commercial fishermen reported significantly higher 

 economic losses than charter boat fishermen. Our obser- 

 vations confirm the potential for high economic loss by 

 commercial fishermen due to lost fishing gear and dep- 

 redation. Because of the cost to commercial fishermen 

 and risk to marine mammal safety, we investigated 

 gear modification as a potential solution to reduce these 

 conflicts (FR, 1996). Gear modification has proven suc- 

 cessful in decreasing depredation and bottlenose dolphin 

 mortality caused by entanglement in the blue crab fish- 

 ery in Florida, in reducing seabird bycatch in coastal 

 gillnet fisheries, and in reducing sea turtle entangle- 

 ment in shrimp trawlers (Crowder et al., 1994; Melvin 

 et al., 1999; Noke and Odell, 2002;). Our preliminary 

 tests demonstrated that a modification to the outrigger 

 planer will successfully deter bottlenose dolphins from 

 engaging in depredation, without causing a reduction 

 in catch. The deterrence device is made of fishing gear 

 already owned by most fishermen and therefore the cost 

 of the gear modification is minimal. 



The deterrence device could prove beneficial in reduc- 

 ing fishery-related injury or mortality of bottlenose dol- 

 phins in the king mackerel troll fishery, including harm 



that may result from retaliatory measures of some fish- 

 ermen. The 1994 amendments to the Marine Mammal 

 Protection Act of 1972 allow the operator of a fishing 

 vessel to deter a marine mammal from damaging his 

 gear or catch. However, potentially harmful methods, 

 such as guns and seal bombs observed and reported in 

 the Florida king mackerel fishery, are strictly prohibited 

 (FR, 1995). The deterrence device offers an alternative 

 to such illegal measures and their associated harmful 

 consequences. We recommend that the deterrence device 

 be fully tested and, if successful, employed as a strategy 

 to reduce depredation and its adverse effects on both 

 fishermen and bottlenose dolphins. 



Acknowledgments 



We thank two anonymous reviewers who provided sug- 

 gestions that improved this manuscript. Laura Engleby 

 and the Dolphin Ecology Project provided housing and 

 support that made this study possible. We acknowledge 

 the Edna B. Sussman Fund and the Environmental 

 Internship Fund at Duke University's Nicholas School of 

 the Environment and Earth Science for providing funds 

 to support this research and Steve McCulloch at Harbor 

 Branch Oceanographic Institute for supplying housing on 

 the east coast of Florida. Leigh Torres, Dave Johnston, 

 Danielle Waples, and Kim Urian provided invaluable 

 assistance with this study. We also thank Eric Blank- 

 field who diagramed the fishing gear device. Finally, our 

 appreciation goes to all participating fishermen. 



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