NOTE Fulling et al : Abundance and distribution of cetaceans in the US, Gulf of Mexico 



931 



T. trunccitus from the GOM consist primarily of demersals, 

 at least close to shore, but they also prey on pelagic spe- 

 cies (Barros and Odell, 1990). The prey of S. frontalis are 

 not well characterized but descriptions include epipelagic 

 and mesopelagic fish and squid, and benthic invertebrates 

 (Perrin, 2002). Richard and Barbeau ( 1994) observed "spot- 

 ted dolphins" feeding on flyingfish (Exocoetidae) in waters 

 28-35 m deep on the West Florida Shelf This is not uncom- 

 mon because S. frontalis have been routinely observed feed- 

 ing on flyingfish at night during haulback of longline gear 

 during NMFS fisheries assessment surveys (Grace^). Fertl 

 and Wiirsig ( 1995) describe S. frontalis feeding on a school 

 of small clupeid fish at the surface south of the Florida 

 Panhandle. A S. frontalis satellite-tracked for 24 days 

 off Texas stayed in waters 12-63 m deep (mean, 32.6 m) 

 and 58.1% of its dives were <10 m (Davis et al., 1996). 

 These shallow dives observed by Davis et al. may indicate 

 feeding on epipelagic species. 



The occurrence of S. bredanensis in continental shelf 

 waters of the U.S. GOM is interesting because this spe- 

 cies is usually described as inhabiting oceanic waters (e.g. 

 Jefferson, 2002). In the northern GOM, the estimated 

 density of S. bredanensis was larger in OCS waters during 

 fall (0.50 dolphins/100 km^; Table 2) than that estimated 

 for oceanic waters during spring (0.32 dolphins/100 km'-) 

 (MuUin and Fulling-). In fact, if there is no OCS-oceanic 

 shift in distribution between spring and fall, there may be 

 similar numbers of S. bredanensis in northern GOM shelf 

 waters (1238; 0.65) as in oceanic waters (1231; 0.45). One 

 of the groups sighted in OCS waters was near the shelf- 

 edge 1 183 m) but the other two sightings were at depths of 

 31 m and 33 m off Texas (Fig. 2). The use of shelf waters 

 in the U.S. GOM by this species may not be atypical; two 

 sightings of S. bredanensis were made on the West Florida 

 Shelf in waters <55 m deep during August 1994 (Hofstetter, 

 2002). Pitman and Stinchomb (2002) provide evidence that 

 S. bredanensis may be specialized predators of dolphinfish 

 [Coryphaena hippurus) in the Pacific Ocean. Dolphinfishes 

 have a circumtropical distribution but occur in oceanic and 

 shelf waters in the northern GOM commonly associated 

 with Sargassiim and other drifting materials (Hoese and 

 Moore, 1998). Steno bredanensis in the northern GOM are 

 commonly found near flotsam, as they are in the Pacific — a 

 place where dolphinfish tend to aggregate. 



The abundance estimates presented in this study are 

 the first ship-based estimates of T. tursiops and S. fron- 

 talis from Gulf of Mexico OCS waters. Although probably 

 negatively biased, these estimates provide reliable data for 

 the management of these species. Our results suggest that 

 the diverse U.S. GOM environments provide an excellent 

 natural experiment and opportunity to further understand 



» Darnell, R. M., J. A. Kleypas, and R. E. Defenbaugh. 1987. 

 Eastern Gulf shelf bio-atlas; a study of the distribution of 

 demersal fishes and penaeid shrimp of the soft bottoms of the 

 continental shelf from the Mississippi River Delta to the Florida 

 Keys. OCS Study 86-004 1 , 548 p. Mmerals Management Ser- 

 vice, Gulf of Mexico OCS Region, New Orleans, LA 70123. 



^ Grace, M. A. 2003. Personal commun. NOAA, 3209 Frederic 

 Street, Pascagoula, MS 39567. 



the ecology of these sympatric cetacean species in OCS pe- 

 lagic waters. 



Acknowledgments 



Many people made significant contributions to the success 

 of the surveys including the officers and crews of NOAA 

 ships Gordon Gunter and Oregon II and the field party 

 chiefs. The marine mammal observers were H. Adams, N. 

 Baertlein, C. Brown, J. Brusher, C, Burks, C. Gates, J. Con- 

 tillo, L. Csuzdi, A. Debose, A. Beier-Engelhaupt, K. Maze- 

 Foley, J. Henne, W. Hoggard, K. Hough, J. Litz, T. Martinez, 

 M. Newcomer, C. Palmer, K. Rademacher, C. Roden, C. 

 Sinclair, S. Stienessen, J. Tobias, CWH, KDM, and GLF. 

 The comments of two anonymous reviewers improved the 

 manuscript significantly. This work was conducted under 

 Marine Mammal Research permit 779-1339 issued to the 

 SEFSC and supported by Interagency Agreement 15958 

 between the NMFS, SEFSC and the Minerals Management 

 Service, Gulf of Mexico Region. 



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