Seasonal Abundance and Site Fidelity of Bottlenose Dolphins (Tursiops truncatus) in Mississippi Sound

Line-transect and photo-identification methods were used to examine bottlenose dolphin (Tursiops truncatus) abundance and site fidelity in a 446 km2 portion of the Mississippi Sound in the north-central Gulf of Mexico from May 1995 through September 1996. For estimating density and abundance, northsouth transect lines were surveyed using a 6.4-m boat. The density and abundance of dolphins in the area surveyed within Mississippi Sound varied seasonally, peaking in summer 1995 with 1.3 dolphins/km2 and 584 dolphins (CV = 0.17), and dropping to a low in fall 1995 with 0.6 dolphins/km2 and 268 dolphins (CV = 0.23). Density estimates were comparable to previous seasonal estimates for Mississippi Sound. Dolphins were widely distributed throughout the study area during all seasons. Group sizes ranged from 1 to 50 dolphins with an overall median of 4.0 (x“ = 6.5, SE = 0.45, n = 288). Group size varied by season, with the smallest groups in fall and winter (median = 3.0 for each), slightly larger groups in spring (median = 3.5), and largest groups during the summers (median = 5.0 and 5.5). Groups containing calves were significantly larger than groups without calves (median = 10.0 and 3.0, respectively) (p = 0.001). Five hundred fifteen individuals were photo-identified, but resighting rates were low, with a mean of 1.57 sightings/dolphin. The most frequently sighted dolphin was observed on six occasions. Evidence of site fidelity over various time scales was found, including possible long-term site fidelity. Two dolphins first photographed in 1991 and two dolphins freeze-branded in 1982-1983 were resighted during this study.

Carrie W. Hubard, Kathy Maze-Foley, Keith D. Mullin, and William W. Schroeder

Aquatic Mammals

Page Numbers: 299-310

DOI: 10.1578/AM.30.2.2004.299