Movements, site fidelity, and surfacing patterns of Bottlenose Dolphins on the central Texas coast

Abstract

Radio-tracking of 10 bottlenose dolphins (Tursiops truncatus), from 9 July 1992 to 13 September 1992, and photographic surveys of 35 freeze-branded dolphins, from May 1992 to June 1993, were conducted in the Matagorda Bay area of Texas, in response to a mass mortality event which occurred between Matagorda and Aransas Bays, Texas, during spring 1992. The primary goals of the study were to assess range size and site fidelity, as well as to initiate a long-term ecological study by collecting data on social and behavioral patterns. The Matagorda Bay dolphin population was found to be numerically robust, occupying all regions of the bay surveyed. Mean range size, based on radio telemetry, was 140 km2 (SD = 90.7, n = 10 dolphins). Males and females had similar range sizes though males visited the extremities of their ranges more frequently or for longer periods. Most, if not all 35 freeze-branded dolphins, appeared to be resident to the Matagorda-Espiritu Santo Bay area with much fluidity of group membership. Overall mark/recapture population size estimates from photo-identification suggested that 218 + 71.4 (95% CI) dolphins utilized an area of 312 km2 in Matagorda and Espiritu Santo Bays, similar to an estimate made in 1981. Dolphins spent longer times at the surface and dove less often at night, indicating lower activity levels at night. Observations of long-distance movement between Texas bays, and an autumn increase in dolphin numbers in the study area, suggested that the study animals were not an isolated population.