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Fishery Bulletin 101(4) 



dance estimates were negatively biased because the central 

 assumption that all cetacean groups on the transect line are 

 detected (i.e.^(G)=l), certainly was not met, and data were 

 not collected to correct estimates for perception and avail- 

 abihty bias (Marsh and Sinclair, 1989). Barlow (1995) esti- 

 mated perception bias in a ship survey in the Pacific Ocean, 

 and although the group-sizes were not estimated at close 

 range, the majority of groups missed by the primary team 

 were apparently small groups. From this. Barlow (1995) 

 estimated g(0) to range from 0.73 and 0.79 for small groups 

 of delphinids (<21 animals). Delphinids have relatively short 

 dive-cycles but diving synchrony among members of a group 

 can affect availability bias; if dives are asynchronous, the 

 probability that at least one animal will be at the surface 

 increases with group-size. Because availability bias varies 

 by species due to differences in individual dive cycles, group 

 diving behavior, and group-sizes, we were not able to address 

 this potential bias based on Barlow's ( 1995) results. 



The use of the effective strip half-width [l//",(0)] from 

 the 25x binocular sightings for the strip width for the 

 strip-transect estimates (Table 2) was assumed to be con- 

 servative and somewhat negatively biased. The distance 

 from which animals will come to the ship to ride the bow 

 is unknown and variable, depending on factors such as 

 the animals' previous behavior, number of bowriding op- 

 portunities, and the type of ship. If the strip width was too 

 narrow, the strip-transect estimates of abundance would 

 be positively biased. 



Our abundance estimates were for the entire U.S. OCS, 

 but the surveys did not extend south of 26.0°N in the 

 eastern Gulf Sightings from a 1994 survey of the eastern 

 Gulf (Hoffstetter, 2002) indicated that the distribution of 

 T. truncatus and S. frontalis does not change dramatically 

 between 26.0°N and Key West; therefore we believe this 

 potential bias is minimal. 



Because our estimates are from four combined years, an- 

 other source of bias would occur if there were annual shifts 

 in cetacean distribution, that is, if the majority of animals 

 of any species occurred in a different part of the OCS in 

 one year during fall compared to others years. However, 

 there was no indication that this variation in distribution 

 occurred and therefore potential bias is probably minimal. 

 Potential bias due to the seasonality of the survey is also 

 possible but cannot currently be addressed. 



Additionally, survey effort from the 2001 cruise was the 

 most complete effort of all years and may have carried more 

 weight than all the other cruises. However, the 2001 survey 

 provided adequate eastern GOM coverage. Variable survey 

 effort in the fall is common because tropical weather can 

 create rough sea conditions. Additionally, fall surveys al- 

 ways began in the west and terminated in the east. Because 

 the same cruise track was always followed, we rarely had 

 the opportunity to survey those areas not surveyed previ- 

 ously during nighttime transit, and thus may have created 

 both a spatial and temporal bias. 



Distribution 



The observed distributions of both T. truncatus and 

 S. frontalis were not surprising given previous descriptions 



of their distributions. The greater number of S. frontalis in 

 the U.S. GOM off Florida compared to the western GOM 

 was suggested by Schmidley and Melcher (1974), and the 

 distribution of sightings reported by Mills and Rademacher 

 (1996) supported this finding. The density of S. fi-ontalis 

 was much greater in the eastern GOM OCS than the west- 

 ern GOM OCS but the density of T! truncatus was similar 

 in the two regions (Table 2). 



The West Florida Shelf and Texas-Louisiana Shelf are 

 very different marine environments, but how habitat dif- 

 ferences specifically affect cetacean density patterns is not 

 clear. The oceanography of the U.S. GOM continental shelf 

 is complex, variable both spatially and temporally, and dif- 

 ficult to characterize briefly. Nevertheless, there are some 

 clear distinctions between eastern and western OCS. First, 

 there are 3415 active oil and gas platforms in the U.S. GOM 

 OCS (0-200 m); the vast majority of these platforms (with 

 their attendant boat and helicopter traffic) occur in waters 

 west of Mobile Bay (MMS^). Also, -95% of the U.S. GOM 

 fisheries landings by weight occur west of Mobile Bay (10 

 years of NMFS^ data). Additionally, sediment- and nutri- 

 ent-laden fresh water from the Mississippi River and its 

 distributary, the Atchafalaya River, usually moves west and 

 predominately affects the Texas-Louisiana and Mississippi- 

 Alabama shelves. The bottom of the Texas-Louisiana Shelf 

 is primarily clay-slit mud and sand, and that of the West 

 Florida Shelf is a mosaic of sand, gravel, shell, and coral 

 (Rabalais et al., 1999). Primary production associated with 

 the Mississippi River outfiow is the highest measured in the 

 GOM (Lohrenz et al., 1999). However, productivity on the 

 West Florida Shelf can be enhanced by a variety processes 

 (e.g. Gilbes et al., 1996). The deep eastern GOM is subject 

 to the quasi-annual incursion of the Loop Current, which 

 can extend to the Mississippi-Alabama Shelf (Wiseman 

 and Sturges, 1999). This incursion can lead to upwelling 

 episodes along the Loop Current front that may increase 

 productivity along the shelf edge and on the West Florida 

 Shelf (Paluszkiewicz et al., 1983; Gilbes et al., 1996). 

 Baumgartner et al. (2001) reported greater sighting rates 

 of cetaceans in the eastern GOM shelf-edge and oceanic wa- 

 ters and suggested that greater feeding opportunities may 

 occur because of the influence of the Loop Current. Griffin 

 and Griffin (2003), whose study included coastal waters 

 (<20 m), reported that S. frontalis on the West Florida Shelf 

 was found in deeper, more saline, and less turbid water than 

 those where T. truncatus was found. 



Demersal fish (e.g. sciaenids) are abundant and diverse 

 on the western GOM OCS, but less abundant on the east- 

 ern OCS (Darnell et al"; Darnell et al.^). The known prey of 



^ Mineral Management Service, Gulf of Mexico Region website: 

 http://www.gomr.mms.gove/hompg/fastfactsAVaterDepthAVater 

 Depth.html. [Accessed on 7/8/2003.] 



^ National Marine Fisheries Service web site: http://www. 

 st.nmfs.gov/stl/commercial/. [Accessed on 8 July 2003.] 



 Darnell, R. M., R. E. Defenbaugh, and D. Moore. 1983. North- 

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

 demersal fishes and penaeid shrimp of the soft bottoms of the con- 

 tinental shelf from the Rio Grande to the Mississippi River Delta. 

 Open File Report No. 82-04, 438 p. Minerals Management Ser- 

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



