Comparison of Group Size, Abundance Estimates and Movement Patterns of Common Bottlenose Dolphins (Tursiops truncatus) in Mississippi Sound, Mississippi

Abstract

Common bottlenose dolphins (Tursiops truncatus; dolphins) inhabiting Mississippi Sound (MSS) in the north-central Gulf of Mexico (GMx) are considered a part of a single stock of dolphins that includes Bay Boudreau and Lake Borgne by the National Marine Fisheries Service (NMFS). MSS is bounded by the mainland (north) and several barrier islands (south). Dolphins inhabiting coastal waters directly south of the barrier islands constitute part of the Northern Coastal Stock. Abundance in MSS has been reported to fluctuate seasonally, with higher abundances of dolphins estimated in summer versus winter. Analysis of covariance was used to compare previous abundance estimates. Results indicated significantly more dolphins in the summer and when boats were used as survey platforms. To explore the possibility of finer scale distributions of dolphins within MSS based on movement patterns of individuals, from 2002 to 2005, 78 photo-identification surveys were conducted that sampled four zones in MSS: one Inner-Sound zone near Round Island, two Outer-Sound zones on the northern sides of Horn Island and Petit Bois Island and one Coastal zone outside MSS south of Petit Bois Island. Analysis of variance was used to test for main effects of zone, season (summer and winter) and presence of calves on mean group size. There were no interactions of main effects. Mean group sizes were significantly larger in Outer-Sound zones, in summer and when a calf was present in the group. Limited movement was observed between the Inner-Sound zone and the other zones. Seventy-seven individual dolphins (40%) were sighted both within and outside MSS, therefore spanning two NMFS stock units. Larger summer group sizes at Outer-Sound zones could reflect a seasonal concentration of dolphins, possibly due to zone differences that may increase prey resources or protection. The finding that some individual dolphins routinely use north and south sides of the barrier islands suggested stock boundary modification could be warranted.