608 



Fishery Bulletin 101(3) 



Figure 2 



Locations of on-effort sightings of bottlenose dolphins (n=38), 

 "coastal" (n=27) and "offshore" (n = l) Atlantic spotted dol- 

 phins, pantropical spotted dolphins (n=6), striped dolphins 

 (n=5), and Clymene dolphins (n=2). The 200-, 500-, 1000-, 

 2000-, and 3000-m isobaths are shown. 



Abundance 



The following were the most abundant species (abundance; 

 coefficient of variation) observed in our study: Atlan- 

 tic spotted dolphins (14,438; 0.63); bottlenose dolphins 

 (13,085; 0.40); pantropical spotted dolphins (12,747; 0.56); 

 and striped dolphins (S. coeruleoalba) (10,225; 0.91); and 

 Risso's dolphins (9533; 0.50). Sperm whales were the most 

 abundant large whale (1181; 0.51). Abundances for other 

 species or taxonomic categories ranged from 20 to 6086. 

 There were an estimated 77,139 (0.23) cetaceans in the 

 study area. 



Group sizes 



Mean group sizes for balaenopterids, physeterids, and 

 ziphiids were less than three animals per group. Bottle- 

 nose dolphins, pilot whales, Risso's dolphins, and "coastal" 

 Atlantic spotted dolphins were in groups that averaged 

 12-18 animals. The average group sizes of pantropical 

 spotted, Clymene, and striped dolphins ranged from 75 to 

 110 individuals (Table 3). 



Distribution 



Cetaceans were distributed throughout the study area, 

 but few sightings occurred on the eastern Blake Plateau 



Figure 3 



Locations of on-effort sightings of Risso's dolphins (7i=22), 

 pilot whales (n=10), rough-toothed dolphins (n = l). Mesoplo- 

 don spp.(n=4), and unidentified beaked whales (n=3). The 

 200-, 500-, 1000-, 2000-, and 3000-m isobaths are shown. 



(Fig. 1). The distribution of species varied regionally and 

 by water depth (Fig. 2-4, Table 4). Bottlenose dolphins and 

 "coastal" Atlantic spotted dolphins were sighted throughout 

 the study area but primarily in or near continental shelf 

 waters. Pilot whales and Risso's dolphins were widely dis- 

 tributed seaward of the continental shelf Sperm whales, 

 unidentified large whales, "offshore" Atlantic spotted dol- 

 phins, striped dolphins, and Clymene dolphins occurred 

 almost exclusively in oceanic waters (>200 m) from Cape 

 Hatteras northward. Most pantropical spotted dolphin 

 sightings were in the southern part of the study area. 



Discussion 



Abundance 



Cetacean abundances for the entire study area have not 

 been estimated previously. Based on stranding records 

 and previous surveys within or near the study area, all 

 the species encountered were expected to be sighted. Pre- 

 vious abundance estimates for the western North Atlantic 

 stocks of short-finned pilot whales and of dwarf and pygmy 

 sperm whales, 749 (0.64) and 420 (0.60), respectively, were 

 based on a winter 1992 ship survey in U.S. oceanic waters 

 (>200 m) south of Cape Hatteras (Waring et al., 1997). The 

 1992 dwarf and pygmy sperm whale estimate is similar to 

 our estimate (580; 0.57); although the 1998 study area was 



