Mullin et al.: Cetaceans of the north-central Gulf of Mexico 



783 



phins, and pilot whales in the Atlantic exceeded 350 

 animals (CeTAP 6 ). Herds of striped dolphins, 

 pantropical spotted dolphins, and spinner dolphins 

 exceeding 1,000 animals in the Pacific are not un- 

 common (Leatherwood and Reeves, 1983). In 

 Monterey Bay, California, Risso's dolphin herds as 

 large as 500 animals have been reported (Kruse, 

 1989). These differences in maximum herd sizes may 

 be related to how prey or predators, or both, are dis- 

 tributed in these areas (Norris and Dohl, 1980; Wells 

 et al., 1980). 



We sighted two mixed species herds (Risso's dol- 

 phins and pilot whales; Atlantic spotted and bottle- 

 nose dolphins). Fritts et al. ( 1983) reported only three 

 mixed species herds in the Gulf: two herds of pilot 

 whales and Stenella sp., and one herd of Risso's dol- 

 phins with an unidentified whale. Risso's dolphins 

 are often associated with other oceanic cetaceans 

 (Leatherwood and Reeves, 1983; Kruse, 1989). In the 

 eastern tropical Pacific, spinner and pantropical spot- 



ted dolphins are commonly found together (Au and 

 Perryman, 1985). Bottlenose dolphins were associ- 

 ated with other species (primarily pilot whales) in 

 20% of the sightings in the Pacific (Scott and Chivers, 

 1990). While we sighted only five herds of pilot 

 whales, other species that are commonly in mixed 

 species herds elsewhere (Risso's dolphins, bottlenose 

 dolphins, and pantropical spotted dolphins), ac- 

 counted for 44% of our identified herd sightings. The 

 abundance and distribution of prey or predators, or 

 both, may be factors involved in the formation of 

 mixed species herds (see Scott and Chivers, 1990). 

 There may be differences in these factors in the Gulf 

 of Mexico compared with those in the northwestern 

 Atlantic and Pacific oceans. 



The abundance of prey species has been demon- 

 strated to be positively correlated with the abundance 

 of several species of cetaceans (e.g. Kenney and Winn, 

 1986; Payne et al., 1986; Selzer and Payne, 1988). 

 Fish and squid are the primary prey of most odon- 



