Assessing the Trophic Ecology of Sarasota Bay Dolphins using Conventional Analyses and Stable Isotopes
By Nélio Barros, PhD, Peggy Ostrom, PhD, and Randall Wells, PhD

The movements and distribution of a predator are generally thought to reflect those of its prey, both in space and time. To better understand the trophic ecology of bottlenose dolphins in the Sarasota Bay area we used conventional (stomach content analyses) and novel technology (stable isotopic analyses), in addition to long-term behavioral observations of feeding. Analyses of stomach contents of stranded dolphins of known feeding history revealed a diet composed primarily of fish, most of which were associated with seagrass beds. We found a close agreement between the species represented in the diet and the observed feeding behaviors of resident dolphins. Dolphins in this area usually feed alone or in small groups in shallow, vegetated habitats. We applied stable isotopic techniques to reconstruct the dietary history of dolphins in Sarasota Bay and adjacent areas. Thus, isotopic signatures of carbon and nitrogen were obtained from available tissues of predators (dolphin teeth from the Mote Marine Lab specimen collection) and their main prey fish (pinfish, pigfish, spot, mullet). Comparisons of carbon isotopic ratios in tissues of dolphins and fish prey indicate that seagrasses and associated vegetation provide important feeding habitats for dolphins in Sarasota Bay. Trophic comparisons using nitrogen isotopic ratios in dolphins and fish also confirm the importance of particular species in the diet of dolphins. In addition, we found that dolphins of older ages had enriched carbon isotopic values and depleted nitrogen values, possibly an indication of ontogenetic shifts in diet or an exploration of different sites during foraging.

The isotopic composition of dolphin teeth from Sarasota Bay differed from that of dolphins from the adjacent Gulf of Mexico and the Charlotte Harbor estuary. The former had significantly higher carbon isotopic ratios than the latter two groups (Figure), confirming that seagrass beds provide important feeding habitats for dolphins in Sarasota Bay. We are pursuing these dietary studies to further investigate the trophic dynamics of dolphins along the central west coast of Florida. Funding for this study was provided by Chicago Zoological Society, Mote Marine Laboratory, NOAA Fisheries, Harbor Branch Oceanographic Institution and the Mote Scientific Foundation.