Sarasota Dolphin Research Program

Understanding the feeding ecology of free-ranging dolphins is crucial for the interpretation of data obtained on distribution and movements, behavioral and social structure, human interactions and competition with fisheries, ecophysiology, and bioaccumulation of contaminants and pollutants through the food chain. To assess geographic variability in bottlenose dolphin diet along the west central coast of Florida, analyses of stomach contents from beach-cast animals and stable isotopes in tissues of predator (dolphin) and prey (fish) are being conducted. Earlier studies with RandyWells in Sarasota Bay have shown that resident dolphins preyed exclusively on fish. Analyses of additional samples collected since then extended the known list of prey in Sarasota Bay to 30 species of fish and one squid. In contrast, dolphins of unknown history that stranded along the Gulf coast of the barrier islands have a different prey composition, and prey more often on squid (Fig. 1). These findings are similar to those obtained for estuarine and exposed-beach populations of dolphins in east Florida (research by Barros) and North Carolina (work by Damon Gannon and colleagues). In those areas, bottlenose dolphins are thought to employ different foraging strategies, preying upon soniferous fish in estuaries and relying more heavily on squid and schooling fish in more open habitats.

As many dolphins strand with empty stomachs, or do not contain diagnostic remains of prey (fish earbones or squid “beaks”), analyses of stable isotopic ratios provide an alternative method of studying diet, as particular elements (e.g., carbon and nitrogen) assimilated by the predator are thought to reflect those of their prey. Previous analyses of carbon and nitrogen isotopic compositions in teeth of dolphins from different central west Florida populations have shown distinct signatures between populations (estuarine and coastal marine). In addition, isotopic ratios suggest ontogenetic differences in diet for known Sarasota Bay dolphins, with older animals showing enriched carbon and depleted nitrogen values. Funds are being sought to investigate the isotopic variability (spatial and seasonal) among three adjacent estuarine systems (Tampa Bay, Sarasota Bay, Charlotte Harbor) in tissues of dolphins and their prey. As multiple element isotope ratio studies often provide much better insights into ecosystem processes than single element studies, sulfur will be added to the list of elements being currently analyzed. Whereas there appears to be little or no enrichment in sulfur isotopes per trophic level, the sulfur is useful in distinguishing benthic versus pelagic producers, and marsh plants versus estuarine phytoplankton. In tropical estuarine systems, variables such as terrestrial versus marine inputs to the system, seasonality, different primary producers in the area are of particular interest as they may influence the isotopic signatures of dolphins and their main prey. The results obtained in this study will shed light onto how applicable these data are to other Florida estuarine systems. This approach highlights the importance of developing and refining trophic models in smaller geographic scales as applied to bottlenose dolphins, so as to allow for meaningful comparative studies. Funding for this project was provided by NOAA Fisheries, through the Chicago Zoological Society, and Harbor Branch Oceanographic Institution Protect Wild Dolphins Program.

Figure 1. Different types of prey consumed by resident and unknown bottlenose dolphins in central west Florida.