NOTE Fulling et al : Abundance and distribution of cetaceans in the U.S. Gulf of Mexico 



925 



distinguished at long distances and were therefore some- 

 times recorded as "T. truncatus + S. frontalis." 



Analytical techniques 



Survey effort that was parallel to the bathymetry gradi- 

 ents, occurred in waters outside the OCS study area, or 

 occurred in a Beaufort sea state >4 was excluded from 

 analyses (Fig. 1). Survey effort used in analyses is sum- 

 marized in Table 1. Survey effort was not uniformly dis- 

 tributed throughout the study area due to poor survey 

 conditions, particularly in the eastern GOM, during two of 

 the four years. Because S. frontalis sightings were clearly 

 more numerous in the east, we delineated the study area 

 into west (106,186 km^) and east (139,614 km^) regions at 

 88°1.5.0'W (ca. Mobile Bay, Alabama) and estimated abun- 

 dances separately for each region. A combination of line- 

 transect and strip-transect methods were used to make 

 abundance estimates. Line-transect methods were used for 

 sightings detected with 25( binoculars, which constituted 

 the majority of sightings (129/140). Strip-transect methods 

 were used for the 11 sightings that were made without 

 the 25x binoculars (naked-eye sightings) and that were 

 observed by the primary team. 



Line-transect estimates 



For each species or species group (i) [i.e. T. truncatus, S. 

 frontalis, rough-toothed dolphins iSteno bredanensis) and 

 T. truncatus+S. frontalis] detected by 25x binoculars, and 

 for each region (J) (east and west), abundance estimates 

 were made with line-transect methods (Af^, j) by using the 

 software program DISTANCE (Colorado Coop. Fish and 

 Wildlife Research Unit, Colorado State Univ., Fort Collins, 

 CO) (Laake et al., 1993; Buckland et al., 2001 ) and by mcor- 

 porating data into the following equation: 



A',, 



2L-g{0) 



(1) 



where A^ = area of region j; 



n^ = number of group sightings of species; in region 



7; 

 ^Li I ~ niean group size of species i in region j; 

 f (0) = sighting probability density function at per- 

 pendicular distance zero for species i; 

 L = total length of transect line in region j; and 

 g(0) = probability of seeing a group on the transect 

 line. 



The parameter g(0) was not estimated; g(0) = 1 was used 

 for each abundance estimate. Abundances were negatively 

 biased because observers usually miss some groups at the 

 surface on the transect line, and some groups were under 

 the surface while in the observation area, therefore g(0) <1 

 (see "Discussion" section). The log-normal 95% confidence 

 interval was computed (Buckland et al., 2001) for each 

 abundance estimate because it was a product of estimates 

 and tended to have a skewed distribution. The variance of 

 ^Li ^^^ estimated by using 



var(A',,^) = A'-,^ 



var(/i,, ) var(S,, 



^l. 



) van 



- + 



[/(O)] 



f,(or 



(2) 



The sampling unit was the length of the transect completed 

 on-effort each day with Beaufort sea state <3 in a region. 

 The formula used to estimate each component of the vari- 

 ance is given in Buckland et al. (2001). Varin^ , ^) was 

 length-weighted and based on the variation in the number 

 of on-effort group sightings between sampling units that 

 ranged up to 191 km/d. 



Estimation of f(0) 



The perpendicular distance (y ) was estimated by using bear- 

 ing and reticle measurements. The reticle readings were 

 converted to radial sighting distances {R) by the method of 

 Lerczak and Hobbs ( 1998;y=R sin(6), where 6=angle between 

 the sighting and the transect line). Because of the difference 

 in observer height (5.3 m) between the Oregon II and Gunter, 

 each ship could potentially yield a different sighting function, 

 g(x). However, only seven sightings were made in sea states 

 <3 from the Oregon II during the one year it was used; there- 

 fore data from both ships were pooled. Estimates of/'/O) were 

 made by using a hazard-rate, uniform, or half-normal model 

 with exact perpendicular sighting distances and no adjust- 

 ments. Model selection was determined by using Akaike's 

 Information Criterion (AIC; Buckland et al., 2001). 



The number of S. bredanensis groups and the number of 

 T. truncatus+S. frontalis groups sighted was insufficient 

 to estimate /ID) for each. Because the S. bredanensis group 

 and T. truncatus+S. frontalis group had similar sighting 

 characteristics (e.g. body size, group-size, surface behavior), 

 we pooled them with sightings of T. truncatus to estimate 

 f'iO). Total number of sightings for both T. truncatus and 

 S. frontalis was sufficient to estimate fiO) for each without 

 pooling with other species. Truncation for T. truncatus, S. 

 bredanensis, and T. truncatus + S. frontalis was 3300 m, 

 and was 5000 m for S. frontalis. Each estimate of /"/O) was 

 based on pooled sightings from the east and west regions. 



Estimation of mean group-size 



Group-sizes tend to be related toy, because in many cases 

 larger groups are easier to see than small groups with 



