Bottlenose dolphins of San Luis Pass, Texas: occurrencs patterns, site fidelity, and habitat use

Abstract

Boat-based photo-identification surveys were conducted during a 12-month (1995-96) study of bottlenose dolphins, Tursiops truncatus, in San Luis Pass/Chocolate Bay, Texas, a relatively undisturbed area located at the southwestern end of Galveston Island. Seventy-one individuals were identified, including 37 residents (Bay) and 34 transients (Gulf). Group sizes ranged from I to 29 with a median of 10.5 and mean of I 1.0 ( ︢SD 8.37). A significant difference in group size by season was documented, with largest and smallest groups present during spring and fall, respectively. Groups containing calves (mean = 14.9 ︢SD 7.29) were significantly larger than non-calf groups (mean = 3.3 ︢SD 3.74). A seasonal calving peak occurred between late March and mid-July. Distribution shifted from Chocolate Bay in the summer to the Gulf of Mexico during winter. Number of affiliates for Bay animals ranged from I 46, with an overall mean of 33.27 ( ︢SD 9.904). Coefficients of association (COAS) were calculated for all individuals identified more than five times, resulting in relatively high association indices between residents (median = 0.455). COAs for known male pairs were among the highest recorded. During all seasons except spring, feeding was the most frequently observed behavior. Individuals identified during this study were compared with 63 individuals identified in the study area in 1990. Fourteen of 71 (19.7%) animals identified in 1995-96 were present in 1990, suggesting that some dolphins exhibit longterm site fidelity to this area. To investigate movement patterns, dolphins identified in San Luis Pass were compared to photographs taken during weekly surveys of Galveston Bay (approximately 48 km away) in 1995. Three animals were resighted in Galveston Bay, an indication of coastal movements between study sites. Results were compared to previous findings from Galveston Bay, an industrial area located at the northeastern end of Galveston Island, to assess the potential impact of humans and industry on this species' distribution and behavior.