Zollet and Read Depredation of the king mackerel troll fishery by Tursiops truncatus 



345 



Eating discarded 



bait 

 Milling near the 



boat but not 



interacting with 



the boat 

 Following the boat 



Passing the boat 

 or being passed 

 by the boat 



a dolphin consumed bait thrown 

 from a fishing vessel 

 a dolphin was in the same area 

 as the fishing vessel but did not 

 seek food or become entangled in 

 gear. 



a dolphin was actively following or 

 pursuing the boat 

 a dolphin was observed travel- 

 ing or was passed by the vessel, 

 but the dolphin neither followed 

 nor interacted with the vessel 



Impact on the king mackerel fishery 



To assess the extent and impact of depredation on the 

 king mackerel fishery, we recorded the type of fishing 

 gear, the number and species of fish caught, and the 

 number and species of fish lost or damaged by dep- 

 redation during each 30-minute fishing interval. We 

 attributed lost fish to bottlenose dolphin depredation 

 if we observed dolphins following the boat or chasing 

 the fish. We recorded the species of the fish if a defini- 

 tive part of the fish was retrieved or if we observed the 

 fish before depredation occurred. We also recorded the 

 fishermen's response to depredation. The categories of 

 response included leaving fishing grounds, ignoring dol- 

 phin depredation, throwing objects or shooting, cutting 

 fishing line, or increasing boat speed. 



Outrigger clip 



Bait line 



/ Metal wire 



Figure 2 



A device designed to reduce bottlenose dolphin iTursiops 

 truncatus) depredation in the commercial king mackerel 

 iScomberoinorus cavalla) troll fishery. The device is 

 created with an outrigger clip secured to the back of a 

 planer. The wire, attached at one end to the outrigger clip 

 and clipped at the other end to the bait line, is released 

 from the outrigger clip when a fish bites the line, and it 

 travels to the end of the bait line. At the end of the bait 

 line, the metal wire flaps about and deters a dolphin from 

 taking a captured fish. (Diagram by Eric Blankfieldi. 



Testing a deterrence device 



We also worked with fishermen to devise and test a 

 deterrence device to be used on outriggers of commercial 

 fishing vessels. The device was equipped with a planer 

 that is used on outrigger lines. An outrigger release clip 

 was secured to the back of the planer (Fig. 2). The bait 

 line passed through the outrigger clip, which released 

 when a fish bit on the bait. The clip also released a metal 

 wire that was attached to the bait line. The wire trav- 

 eled towards the fish and flapped around the fish at the 

 end of the bait line, making it difficult for a dolphin to 

 approach the fish. 



We tested the device on commercial fishing vessels in 

 Fort Pierce during December 2003 and January 2004. 

 We randomly placed the device on one of two outrig- 

 gers, noting on which of the two outriggers the device 

 was placed and the time and GPS coordinates for each 

 event. An event occurred when the fishing line was 

 placed in or taken out of the water and when a fish was 

 caught or depredated. We recorded the number and spe- 

 cies of each fish that was caught, taken, or damaged. 



Results 



Interviews with fishermen 



We conducted interviews with 26 king mackerel boat 

 operators in Islamorada and 23 operators along Florida's 



Table 1 



Vessel type and location of respondents in interview 

 surveys of fishermen for a study of bottlenose dolphin 

 depredation in the Florida king mackerel fishery. Nine 

 individuals selected more than one vessel type. 



Commercial Charter Recreational 



Islamorada 

 East Coast 



1 



17 



23 



12 



east coast. Several individuals indicated that their 

 boats served multiple purposes or that they operated 

 different types of boats during different times of the 

 year (Table 1). 



All fishermen responded that they saw or interacted 

 with bottlenose dolphins while fishing. Forty-seven fish- 

 ermen provided useful responses to questions regarding 

 depredation; all of these respondents indicated they 

 had observed dolphins taking bait or catch. Other re- 

 ported interactions included entanglement in fishing 

 gear (10.6%), begging (4%), and eating discarded bait 

 (10.6%). Ninety-seven percent of participating fishermen 

 reported that king mackerel were taken by bottlenose 

 dolphins. King mackerel was the species most often 

 identified as being taken by bottlenose dolphins, but 

 other fish reportedly taken included amberjack {Seriola 



