Bottlenose dolphins are the most frequently-sighted cetaceans in coastal waters of the southeastern U.S. Records also indicate that they are the most common species to strand in this area. As such, they are widely represented in marine mammal museum collections throughout the region. In the most extensive of these collections, a wealth of associated biological data may accompany these materials. Often, however, little is known about the history of most stranded animals. Without information on where the animals have been or to which population they belong, the interpretation of data collected from them may be limited. Since bottlenose dolphins occur as near shore populations resident to estuaries and bays or transient to coastal waters, as well as offshore waters beyond the continental shelf, the difference in distribution and ranging patterns may be reflected in the biological and ecological attributes these animals present.
        Off west-central Florida, cetacean strandings have been studied for the past two decades by biologists at Mote Marine Laboratory in Sarasota. The osteological collection it houses includes skulls and necropsy records of over 500 stranded bottlenose dolphins, most of which are of unknown history.
        To help understand the population membership of these animals, and therefore be in a better position to make sense of their associated biological data, we tested the hypothesis that dolphins of different distribution (and likely belonging to different populations) have different feeding ecology. To that end, we analyzed stable isotopes; a long-term indicator of feeding history, in the teeth of a subset of bottlenose dolphins stranded in west-central Florida belonging to three general putative populations: resident members of the Sarasota Bay community (SB, n= 39), dolphins occurring along the nearshore Gulf of Mexico (Gulf; n= 36) and members of the offshore ecotype (Off; n= 7). The former comprises animals from a well-studied resident community, observed across a span of five concurrent generations over nearly four decades, and the latter two include animals for which no history on origin and population membership is available. The principle behind the stable isotope approach is that the isotopic composition of a predator reflects that of its prey, as elements are assimilated through the food chain. Therefore, the isotopic signature of the predator is influenced by habitat type and distribution of its prey. In this study, we examined the carbon, nitrogen, and sulfur isotope values (δ¹³C, δ¹⁵N and δ³⁴S) of dolphin tooth collagen with the goal of gaining insight into the general habitat occupied by bottlenose dolphins off west-central Florida.
        The results obtained indicate that dolphin groups differed significantly for all three isotopes analyzed. Pair-wise comparisons showed that teeth from the Gulf and Off groups had lower carbon values relative to teeth from SB animals. For nitrogen values, pair-wise comparisons showed that teeth of the SB dolphins were lower than those of the Gulf and Off groups. Dolphin teeth from all three groups differ in sulfur values. A comparison across the three groups showed a decreasing trend in sulfur with Off > Gulf > SB. Additionally, the sulfur values of teeth from the Off group were significantly higher than Gulf animals, with non-overlapping ranges. When a combination of two elements is used, C-S and N-S clearly separate the three groups of dolphins examined, whereas C-N distinguish SB dolphins from the Gulf and Off dolphins.
        This is the first study using stable isotopes to differentiate groups of small cetaceans in the Gulf of Mexico and adjacent nearshore waters. Stable isotopes appear to be a powerful means of distinguishing among groups of dolphins along an inshore-offshore gradient. This technique will be applied to the remainder of the bottlenose dolphin osteological collection at Mote Marine Laboratory, to determine if stranded animals originated in inshore vs. coastal vs. offshore waters.