Zollet and Read Depredation of the king mackerel troll fishery by Tui stops tiuncatus 



347 



gun shooting blanks, in response to 

 depredation. Anecdotal accounts of 

 the use of seal bombs, guns, and bird 

 bangers were also reported by com- 

 mercial and charter fishermen. 



Experimental testing of 

 deterrence device 



Charter fishermen 



Commercial fishermen 



30% 



29% 



In three cases when the device 

 was in use, bottlenose dolphins ap- 

 proached a king mackerel on the 

 fishing line, but left the fish appar- 

 ently after they detected the device. 

 The number of fish caught per hour 

 for each outrigger was 1.48. A f-test 

 demonstrated no significant differ- 

 ence exists between the rate offish caught by each 

 outrigger when the device was not used (P=0.99). 

 We also found no difference in the number of fish 

 caught per hour by outriggers equipped with the 

 device (1.40) with those without the device. The 

 device did not cause a reduction in catch of tar- 

 geted species {P=0.83). 



Discussion 



We documented frequent depredation in both the 

 king mackerel charter and commercial troll fish- 

 eries in southeastern Florida. All commercial and 

 charter fishermen indicated that they experienced 

 bottlenose dolphins taking their bait or catch. 

 During our observations, approximately one in every 

 five fish caught by commercial fishermen was lost to 

 bottlenose dolphins, but only 6% of catch was lost by 

 charter fishermen. This difference in depredation rates 

 may be attributed to seasonal variation in the distribu- 

 tion of king mackerel or differences in the gear used 

 by the two fishery sectors. Fishermen in Islamorada 

 reported that most conflicts with dolphins occur in 

 the winter. Because of the highly migratory nature of 

 king mackerel and the mixing of the South Atlantic 

 and Gulf stocks in the winter in the Florida Keys, we 

 anticipated that a higher depredation rate would be 

 experienced by the charter fishery in winter (Gold et 

 al., 2002). We observed commercial fishing operations 

 along Florida's east coast during the season with the 

 most reported conflicts. Depredation rates along the east 

 coast, however, may be even higher in the spring than 

 at the end of spring and summer when our observations 

 were made. 



In addition, commercial fishermen target king mack- 

 erel, whereas charter boats use fishing gear that targets 

 a variety of fish species, including amberjack, barra- 

 cuda (Sphyraena obtusata), and bonito (Sarda sarda). 

 The higher depredation rates encountered by commer- 

 cial fishermen may result from the regular capture of 

 king mackerel compared to the various species caught 

 by charter boats during a fishing trip. In addition to 



S strongly disagree 



□ Disagree 



□ Neutral 

 B Agree 



□ Strongly agree 



50% 



Figure S 



Response of surveyed fishermen when asked if they agree or disagree with 

 the following statement: "bottlenose dolphins [Tursiops truncatus) are caus- 

 ing economic loss to my business by stealing bait and/or catch." 



30-, 



18,1 



13.9 



2.5 



9^ 7.4 



Depredation 



Following 

 boat 



Near boat, 

 no interactions 



Passing 

 boat 



Observed bottlenose dolphin behaviors 



Figure 6 



Bottlenose dolphin (Ti/r.s/oyy.v truncatus) behaviors observed 

 from king mackerel iScomberoiitorus cavalla) fishing vessels. 



king mackerel, species that were reported taken by 

 both commercial and charter boat fishermen included 

 Spanish mackerel, amberjack, and blackfin tuna. We 

 did not observe depredation of these species, most likely 

 because they were rarely caught during our study. How- 

 ever, depredation of these species has been reported 

 from Spanish mackerel fisheries elsewhere (Read et 

 al., 2003). 



We observed bottlenose dolphins engaging in dep- 

 redation only while the vessels were fishing for king 

 mackerel. Bottlenose dolphins do not generally prey on 

 king mackerel; Barros (1993) did not find king mackerel 

 in the stomach contents of any stranded dolphins in 

 Florida. Exploitation of fisheries by marine mammals 

 may introduce a new food resource that was either not 

 previously available or used, as seen in the case of pilot 

 whales (Globicephala spp.) that feed on Atlantic mack- 

 erel (Scomber scombrus) in trawl fisheries off the north- 

 eastern United States (Waring et al., 1990; Gannon 

 et al., 1997) and in the case of killer whales iOrcinus 

 orca) that prey on swordfish (Xiphias gladius) hooked on 

 longlines in southern Brazil (Secchi and Vaske, 1998). 

 It is likely that fishing affects not only the diet but 

 also the behavior and spatial distribution of bottlenose 

 dolphins (Leatherwood, 1975; Chilvers et al., 2003). The 

 bottlenose dolphins in this study may spend less time 

 foraging, but as indicated by the observed depredation 



