The paucity of research into the environmental requirements, stock membership, abundance and residency patterns of bottlenose dolphins (Tursiops truncatus) in coastal Louisiana creates difficulty in understanding how local ecosystems and threats (such as fishery interactions, habitat degradation and pollution) affect populations. This study combined fine-scale environmental measurements and photo-identification techniques to describe patterns of habitat usage and abundance of bottlenose dolphins in lower Barataria Basin from June 1999 to May 2002. In addition I investigated the validity and limitations of using mark-recapture models to estimate abundance from cetacean photo-identification data. Bottlenose dolphins were present year-round in a wide range of water temperatures (10.9 33.9 ēC), dissolved oxygen levels (3.7 16.6 mg/L), salinities (11.7 31.5 psu), turbidity levels (1.4 34.0 NTU), distances from shore (3 800 m), and water depths (0.4 - 12.5 m). However, feeding activity was concentrated in a narrower range of conditions, 20 24 ēC water temperature, 6 9 mg/L of dissolved oxygen, turbidity values between 20 28 NTU, 200 500 m from shore, and depths of 4 6 m. Spatial mapping showed differences in the seasonal distribution of individuals and a tendency for feeding activity and larger group sizes to be concentrated in passes. Using distinctive natural markings present on dorsal fins, I identified 133 individual dolphins. Closed-population models were improved by inclusion of temporal and individual heterogeneity as sources of sighting variability and produced estimates of between 138 and 238 (95% CL range = 128 297) bottlenose dolphins for the study area. Analysis of Jolly-Seber model assumptions demonstrated the importance of ensuring cetacean surveys accurately represent temporal, geographic and demographic properties of a study population. In addition such factors as non-preferential image acquisition, group size, gender, behavior, stability and distinctiveness of natural markings, weather conditions and boat traffic must be considered. Evidence of a relatively closed Barataria Basin population agrees with current assumptions that bay bottlenose dolphin stocks are distinct from those found in deeper, offshore waters. Furthermore, the characterization of environmental usage patterns for this bay population strengthens adequate description and management of this relatively discrete Gulf of Mexico bottlenose dolphin stock.