Garrison: Interactions between marine mammals and pelagic longline fishing gear 



415 



study, there were relatively few circle hooks ob- 

 served in the fishery and thus there was limited 

 power to test this potential gear effect. Marine 

 mammal interactions with the fishery will con- 

 tinue to be closely monitored to verify that ac- 

 tions to reduce turtle interactions do not inadver- 

 tently increase marine mammal interactions. 



The logistic regression results demonstrate 

 that longline fishery interactions with both pilot 

 whales and Risso's dolphins are largely driven 

 by environmental factors and spatial overlap of 

 marine mammals and fishery operations. Pilot 

 whales are concentrated along the shelf break 

 between the 200- and 1000-m isobaths, and the 

 highest densities are found between Cape Hat- 

 teras, NC, and New Jersey and along the south- 

 ern flank of Georges Bank (Payne and Heine- 

 mann, 1993; Mullin and Fulling, 2003; Waring 

 et al., 2006). Risso's dolphins are similarly con- 

 centrated along the shelf break, but they have 

 a more northern distribution from New Jersey 

 to southern New England (Waring et al., 2006). 

 Both species are found along the southeast At- 

 lantic U.S. coast from North Carolina to Florida; 

 however, they have much lower densities and are 

 not as strongly associated with the shelf break 

 (Mullin and Fulling, 2003; Waring et al., 2006) 

 as in the former region. The fishery effort is 

 similarly concentrated along the shelf break in 

 both the MAB and NEC (Fig. 1; Abercrombie 

 et al., 2005). Fishery effort is generally concen- 

 trated just north of Cape Hatteras, NC, during 

 the winter months from January through April. 

 The fishery then expands northward and over- 

 all effort increases through the MAB and NEC 

 fishing areas, reaching peak effort levels dur- 

 ing September and October and remaining high 

 through December. 



The strong overlap between fishery effort and 

 high marine mammal presence is related to prey 

 distribution. The pelagic longline fishery targets 

 tunas and swordfishes that feed on small fish 

 and squids. Both pilot whales and Risso's dol- 

 phins are also pelagic predators, and squids are 

 a major component of their diets (Overholtz and 

 Waring, 1991; Gannon et al., 1997; Kruse et al.. 

 1999). Both longfin (Loligo pealei) and shortfin 

 (Illex illecebrosus) squids are primary prey items, and 

 both concentrate near the convergence zone of shelf- 

 slope waters during summer and autumn (Brodziak 

 and Hendrickson, 1999). The high interaction rate in 

 the MAB area during summer and fall, and in warmer 

 water temperatures, is therefore likely a function of 

 environmental features driving availability of squid and 

 small pelagic fishes that are prey for both the target 

 fish species and marine mammals. 



For both pilot whales and Risso's dolphins, there was 

 a correlation between observed damage to swordfish 

 catch and the likelihood of an interaction with a marine 

 mammal. It should be noted that damage to catch may 



.a 1 



100 120 140 160 



0,06 - 



0.04 



20 40 60 80 100 120 



Distance from 200-m isobath (km) 



Figure 4 



Predicted (lines) and observed (symbols) interaction rates 

 (number of positive sets/total number of observed sets) for 

 pilot whale (Globicephala spp.) in the Mid-Atlantic Bight fish- 

 ing area (A) and other areas (B), with increasing distance from 

 the 200-m isobath and by mainline-length category. Error bars 

 indicate 95% confidence intervals of predicted values. Note the 

 difference in the v-axis scale. 



also be related to feeding by sharks, as well as by ma- 

 rine mammals. The strong correlation with damage to 

 catch indicates that marine mammals are likely feeding 

 upon the catch or bait. Similarly, the negative correla- 

 tion between fish bait and Risso's dolphin interactions 

 indicates that the animals were attracted to gear with 

 squid bait as a ready food source. 



The consistent effect of mainline length may also be 

 related to depredation of catch on the longline gear. 

 Longer mainlines represent a larger, more attractive 

 food source or may be more easily detectable. Marine 

 mammals therefore likely spend a longer time around 

 the gear, creating greater opportunities for hooking 



