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Fishery Bulletin 101(4) 



surveyed (see Fig.l in Baumgartner [1997]). Complete 

 coverage would have certainly led to more S. frontalis 

 sightings and it is possible the lines that were surveyed 

 were in areas with more T. triuicatus. Blaylock and Hog- 

 gard (1994) estimated from aerial surveys that about 31% 

 of the T. truncatus in OCS waters west of Mobile Bay were 

 in rather a small area from the Mississippi River Delta 

 west to about 90.5°W. Our ship survey effort in this area 

 was small and resulted in only one sighting of T! truncatus 

 (Fig. 2). Therefore, our ship-based estimates may have un- 

 derestimated the abundance ofT. truncatus in the western 

 OCS. Aerial abundances were based on survey lines that 

 extended from 9.3 km past the 18 m (10 fm) curve to 9.3 

 km past 183 m ( 100 fm) curve; therefore the area surveyed 

 was somewhat different than our 20-200 m OCS study 

 area for ship surveys. Aerial survey effort in waters >200 m 



may have resulted in more sightings ofT. truncatus than 

 S. frontalis because the deeper waters are not the common 

 habitat of S. frontalis (Mullin and Fulling'^) and sightings 

 in waters <20 m would have also been biased toward T. 

 truncatus. 



Stenella frontalis and T. truncatus are similar in length 

 and shape. Stenella frontalis are born without spots and 

 become progressively more spotted with age, but young ani- 

 mals look very similar to T. truncatus (see Herzing, 1997). 

 Therefore, depending on the composition of the group, from 

 a distance S. frontalis are not always easily distinguished 

 from T. truncatus: therefore it is possible that some groups 

 were misidentified as T. truncatus during aerial sui-veys, 

 leading to bias in the relative abundance of each species. 



The annual PBR for the OCS stock of T. truncatus was 

 432 dolphins, and for the U.S. GOM stock of S. frontalis, 



