Stable isotope variation in the northern Gulf of Mexico constrains bottlenose dolphin (Tursiops truncatus) foraging ranges

Site-specific differences were found in consumer isotope values among ten sites examined in the northern Gulf of Mexico. Average delta C-13 values among sites ranged -21.7 to -15.7 aEuro degrees, delta N-15 ranged < 3 aEuro degrees: from 9.8 to 11.5 aEuro degrees, and delta S-34 ranged from 5.9 to 18.3 aEuro degrees. Isotope variation among these sites provided insight into the ranging habits of bottlenose dolphins (Tursiops truncatus) sampled in St. George Sound (29.8N, 84.6W), a nearshore seagrass habitat, during May of 2005. Isotope comparisons suggest that the majority of dolphins sampled (average +/- A one standard deviation: delta C-13 = -17.5 +/- A 0.8, delta N-15 = 14.5 +/- A 0.9, and delta S-34 = 10.6 +/- A 1.5) did not forage significantly on offshore species which are significantly S-34-enriched, nor is it likely that they ranged either eastward or westward along the coast within the sampling region. Despite their capability for ranging, these dolphins occupied a restricted home range, during the spring before our sampling efforts. These results demonstrate significant fine-scale isotope variation among coastal habitats explained by differences in freshwater inputs, organic matter loading, and modes of primary production that may be used to constrain the foraging ranges of a highly mobile apex predator.