Understanding bottlenose dolphin feeding ecology through combined analyses of stomach contents and stable isotopes
By Nélio Barros, PhD, Mote Marine Laboratory

We are studying the feeding ecology of bottlenose dolphins in Sarasota Bay by combining traditional analyses of stomach contents of stranded animals with assessments of stable isotopes in tissues of both dolphins and their preferred prey. The advantage of this approach is that it incorporates short-term evidence of prey consumption (stomach contents) with data derived from long-term assimilation of elements through the food chain (stable isotopes), thus encompassing a wide temporal spectrum of feeding data. To date, stomach content data from 30 well-known dolphins from Sarasota Bay indicate that this resident population preys on 34 species (within 21 families) of teleosts, cephalopods and elasmobranchs. Most prey are bottom-dwelling and associated with seagrass beds, suggesting that seagrass beds are important foraging areas for dolphins and the other resident marine mammal in the bay, the Florida manatee.

Analyses of carbon and stable isotopes in dolphin tissues, a long-term indicator of diet, have demonstrated that older Sarasota Bay dolphins have higher levels of carbon isotopes and lower levels of nitrogen isotopes in their tissues. This ontogenetic variability may reflect shifts in prey base and/or habitat use. To further enhance our ability to distinguish between animals living in the Bay versus Gulf waters or other areas, we’re including an additional tracer (sulfur) in our stable isotope analyses. This element should provide better discrimination of resident dolphins as it distinguishes marine versus freshwater food sources in estuarine systems. Preliminary results of sulfur isotopic analyses in teeth of dolphins from different west central Florida populations have shown a trend for lower isotopic values in estuarine to offshore waters. These encouraging results suggest that stable isotopes (and stomach content analyses) will not only provide insights into dolphin feeding ecology but may also serve as diagnostic tools in separating different dolphin populations inhabiting coastal and offshore waters of west central Florida.