Identifying Foraging Hotspots of Bottlenose Dolphins in a Highly Dynamic System: A Method to Enhance Conservation in Estuaries

Estuaries are biologically productive systems that support many cetacean populations and serve as important nursery grounds for their prey but face continued habitat degradation from increasing coastal development. Because estuaries are highly dynamic systems with fine-scale environmental gradients and microhabitats, it is challenging to identify foraging hotspots. To investigate whether bottlenose dolphin (Tursiops truncatus) foraging hotspots occur at fine spatial (500 m) and temporal (time of day and season) scales in a large estuary in the northern Gulf of Mexico, we conducted boat surveys from January to December 2001 in five subareas of Galveston Bay, totaling 3,815 km. Using geospatial techniques, we analyzed the number of dolphins, group behavior, and environmental variables (e.g., water temperature, salinity, turbidity, number of boats and seabirds, and distance to the Gulf) on a 500-m resolution grid. We observed 1,802 dolphins in 262 groups, 57% of which were foraging. Two subareas, Bolivar Roads and the Galveston Ship Channel, comprising only one-fifth of the total surveyed area, accounted for 91% of foraging groups. We identified six foraging hotspots in these two areas that were used throughout the day and in every season. Hotspots were located in deeper channels where dolphins often foraged with bottom trawl shrimp vessels, near ferry landings, and along the jetties where prey are likely exposed or aggregated by currents and tidal fronts. In addition, a greater number of seabirds and vessels were recorded in hotspots relative to where dolphins were not observed. We suggest that this fine spatiotemporal scale approach is a valuable tool for the conservation of vulnerable estuarine cetacean populations, particularly if paired with population and site-fidelity studies. Specifically, determining prime foraging habitat and identifying baseline hotspot density (number of foraging dolphins per unit area) provides useful metrics for detecting changes in habitat usage resulting from habitat degradation or restoration efforts.