Bottlenose dolphin population differentiation and trophic studies using Carbon, Nitrogen, and Sulfur stable isotopes and stomach content analyses
By Nélio Barros, PhD, Mote Marine Laboratory, Peggy Ostrom, PhD, Michigan State University, Craig Stricker, PhD, U.S. Geological Survey, and Randall S. Wells, PhD

We used a combined approach of stable isotope and conventional stomach content analyses in dietary and population differentiation studies of bottlenose dolphins from central west Florida. Teeth obtained from dolphins stranded in Sarasota Bay (resident animals of known feeding history), the adjacent Gulf of Mexico, Charlotte Harbor and offshore waters were analyzed for their carbon (ä¹³C), nitrogen (ä¹⁵N) and sulfur (ä³⁴S) isotopic signatures. Selected fish prey species collected in Sarasota Bay were also analyzed for C and N isotopes (ranges of ä13C: -16.2 to -10.2‰ and ä¹⁵N: 2.9 to 8.6‰). Sarasota Bay dolphins (n= 48) had significantly higher ?ä 13C values (-10.7‰ ± 1.2 SD) than animals from other populations (adjacent Gulf: -11.9‰ ± 0.9 SD, n= 30; Charlotte Harbor: -12.0‰ ± 1.5 SD, n= 27; offshore: –11.6‰ ± 1.0 SD, n 10), whereas ä15N values did not differ (Sarasota Bay: 12.4‰ ± 1.5 SD, Adjacent Gulf: 12.7‰ ± 0.8 SD, Charlotte Harbor: 12.3‰ ± 0.9 SD, offshore: 12.9‰ ± 0.9 SD). Isotope data for Sarasota Bay are consistent with preference for seagrass-associated fish prey, particularly pinfish, as determined from stomach content analyses and behavioral observations. Preliminary analysis of sulfur isotope data suggests nearly non–overlapping ranges in the few specimens analyzed, where estuarine (Sarasota Bay and Charlotte Harbor) were less than Gulf of Mexico which were less than offshore dolphins (Fig.1). Thus, animals inhabiting the Charlotte Harbor estuary, an area influenced by runoff from three major rivers, had lower Sulfur values (ä³⁴S: 7.2 ± 2.1 SD, n= 4) compared to other populations (inshore animals stranded in Gulf beaches: 10.8 ‰ ± 3.1 SD, n= 6; animals from the offshore ecotype: 17.3 ‰ ± 1.1 SD, n= 4), likely reflecting the freshwater signature acquired in these brackish waters. Sarasota Bay dolphins had Sulfur values similar to those from Charlotte Harbor (Fig. 1). With additional samples being analyzed, further isotopic discrimination among parapatric populations is expected. Stable isotopes hold great promise in tracing energy flow in pelagic, neritic and estuarine bottlenose dolphin populations and, together with stomach content analyses, can be used as tools in understanding trophic ecology, assessing ontogenetic variability at the individual and population levels, and assigning population identification of stranded animals of unknown history.

Figure 1. Sulfure values of dolphin tooth samples collected from different populations off central west Florida; results are expressed in delta relative to the V–CDT scale. Samples sizes are shown above the histograms; error bars represent ± standard deviation.