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Fishery Bulletin 92(4), 1994 



sighting intervals corresponding to perpendicular 

 distances from the transect line of 40, 83, 132, 192, 

 273, 397, and 629 m. Observers searched on and near 

 the transect line and scanned periodically out to 629 m. 

 Sighting cues beyond 629 m were ignored unless the 

 observer was certain it was a cetacean. 



When cetaceans were encountered, the sighting 

 interval was noted and the herd was circled. Before 

 continuing on the transect, the herd was identified 

 and its size estimated. The identifying characteris- 

 tics of each cetacean species were noted. Data were 

 entered on a computer interfaced with a LORAN-C 

 navigation receiver. Latitude, longitude, and head- 

 ing were automatically recorded with each data record. 



Cetaceans were identified to the lowest taxonomic 

 level possible from descriptions in field guides by 

 Leatherwood et al. (1976) and Leatherwood and 

 Reeves ( 1983). Our ability to make an identification 

 was dependent on water clarity, sea state, and ani- 

 mal behavior. We were not able to distinguish spe- 

 cies of some genera or groups of species. These groups 

 included 1) the species ofMesoplodon; 2) the melon- 

 headed whale, Peponocephala elect ra, and pygmy 

 killer whale, Feresa attenuata; 3) the dwarf sperm 

 whale, Kogia simus, and pygmy sperm whale, K. 

 breviceps; and 4) the short-finned pilot whale, 

 Globicephala macrorhynchus , and long-finned pilot 

 whale, 4 G. melaena. Cuvier's beaked whale, Ziphius 

 cauirostris, and Mesoplodon spp. could not always 

 be distinguished and these sightings were classified 

 as unidentified ziphiids. While we did make positive 

 identifications of striped dolphins, S. coeruloealba, 

 spinner dolphins, S. longirostris, and clymene dol- 

 phins, S. clymene, from photographs, they were usu- 

 ally difficult to distinguish in the field and were 

 grouped together for analyses. In some cases, ani- 

 mals could only be identified as large cetaceans 

 ( greater than about 7 m ) or small cetaceans (less than 

 about 7 m). 



For species or species groups sighted 20 or more 

 times, the null hypothesis that water depth did not 

 vary among species or species groups was tested with 

 one-way analysis of variance. If the null hypothesis 

 was rejected, Duncan's multiple-range test was used 

 to determine where significant differences in mean 

 water depths occurred. 



Sighting rates of herds and individuals were used 

 as measures of overall, temporal, and spatial rela- 

 tive abundance. Seasons were defined as summer 

 (June-August), fall (September-November), winter 

 (January-February), and spring (March-May). To 

 summarize spatial relative abundance, the area sur- 



4 Only the short-finned pilot whale is known to inhabit the Gulf 

 of Mexico (Sehmidly, 1981). 



veyed was divided into an eastern zone (Blocks A4, 

 Bl, and B2), a central zone (Blocks Al, A2, B3, B4, 

 and B5), and a western zone (Blocks A3, B6, and B7). 

 All sightings from each season and zone were pooled. 

 For each season and for each zone, the sighting rate 

 of herds (herds/100 transect km) and animals (ani- 

 mals/100 transect km) of each species or species 

 group was calculated. 



We also compared the relative abundance of indi- 

 viduals of each species or species group from our sur- 

 veys to those from the Gulf stranding database 

 (Mead 3 ). The database of Gulf strandings contained 

 2,321 records identified to species. Only 516 records 

 (22%) were not those of bottlenose dolphins. To com- 

 pare our results with these data, we excluded bottle- 

 nose dolphins and unidentified cetaceans from both 

 data sets. We used our species or species-group cat- 

 egories and calculated the relative abundance of each 

 within each data set as a percentage of the total num- 

 ber of animals. 



Results 



In total, we sighted 320 herds (7,438 animals) and 

 identified 18 species of cetaceans (Table 2); 45 herds 

 (14%) could not be identified. Of the 275 identified 

 herds (6,084 animals), 93.5% of the herds and 97.7% 

 of the animals consisted of seven species or species 

 groups: Risso's dolphins (herds, 22.2%; animals, 

 12.6%); sperm whales (15.6%, 1.4%); bottlenose dol- 

 phins (14.2%, 7.4%); Atlantic spotted dolphins ( 12.8%, 

 15.0%); pygmy and dwarf sperm whales (11.6%, 

 1.0%); striped, spinner, and clymene dolphins (8.7%', 

 33.8%); and pantropical spotted dolphins (8.4%, 

 26.5%). 



Mean herd sizes of species or species groups sighted 

 more than 20 times ranged from 1.9 to 87.8 animals 

 (Table 2). The largest herd consisted of 325 striped, 

 spinner, or clymene dolphins (SSC dolphins). Dol- 

 phins of the genus Stenella had the largest mean herd 

 sizes and the largest ranges of herd sizes. However, 

 the mean herd sizes of pantropical spotted dolphins 

 and SSC dolphins were each about three times that 

 of the Atlantic spotted dolphin. The mean herd sizes 

 of sperm whales and pygmy and dwarf sperm whales 

 were close to two, and they exhibited the smallest 

 ranges of herd sizes. Bottlenose dolphins and Risso's 

 dolphins had similar means and ranges of herd sizes. 



Mean water depths of species or species groups 

 sighted 20 or more times ranged from 257 to 905 m 

 (Table 2). Differences between these means were sta- 

 tistically significant (Table 3). Mean water depths of 

 pantropical spotted dolphin, sperm whale, and SSC 

 dolphin sightings were the largest (>700 m). Each of 



