780 



Fishery Bulletin 92(4), 1994 



The ecological implications of Gulf stranding 

 records are not clear since there are only a small 

 number of strandings of most species. It is not known 

 whether the stranded animals strayed into the Gulf 

 from their primary ranges or whether they inhab- 

 ited Gulf waters on a regular basis. The number and 

 broad seasonality of sightings during this study of 

 Risso's dolphins, sperm whales, pygmy and dwarf 

 sperm whales, SSC dolphins, and pantropical spot- 

 ted dolphins indicate that they are probably perma- 

 nent residents of the Gulf. 



How accurately our results reflected the abundance 

 of each species relative to other species is uncertain. 

 Factors that vary among species, such as surface 

 behavior, herd size, and time spent at or near the 

 surface, can affect sighting rates. In our study, wa- 

 ter depth or area, or both (e.g. Mississippi Canyon), 



affected the distribution of some species. However, 

 our survey effort was not equal seasonally over wa- 

 ter depths or by area and this probably affected at 

 least some of our relative abundance results (Tables 

 4-5). Forty-nine percent (19/39) of the bottlenose 

 dolphin herds we sighted were encountered during 

 summer and fall at the head of the Mississippi Can- 

 yon and on the Upper Mississippi Fan at depths less 

 than 100 m. (Survey effort at <100 m made up <5% 

 of the total effort, 8.5% of both the summer and fall 

 effort, and 0% of the winter and spring effort.) With- 

 out these sightings, the seasonal sighting rates of 

 bottlenose dolphins were less variable. Also, 28% (9/ 

 32) of the pygmy and dwarf sperm whale sightings 

 and 26% (9/35) of the Atlantic spotted dolphin 

 sightings occurred in the Mississippi Canyon (Block 

 A2, surveyed during summer and fall) where only 



