Validating stranding data in feeding ecology studies of bottlenose dolphins from Sarasota Bay, using long-term observations
By Nélio Barros, PhD, Janet Gannon, MS, and Randall Wells, PhD

     We have been using the ratios of stable isotopes in dolphin tissues to examine feeding patterns of bottlenose dolphins along Florida’s west coast.  Results from the stable isotope analyses suggest that differences exist at the population level, which allow us to distinguish offshore and various inshore dolphin populations, and we have started to examine potential individual variability in feeding preferences. We hope to accomplish this by examining inter-annual variability in isotopic composition of dolphin teeth (in progress) and by collecting tissues from known free-ranging animals during occasional capture-release health assessments in Sarasota Bay. As described above, it has been recently shown that individual dolphins strongly select the habitat they occupy, which in turn could reflect dietary preferences. Our previous stomach content analyses have also shown individual differences in dolphin food habits, with some individuals consuming seagrass-associated fish, whereas others preferred prey fish found around rocky structures and pilings. 
     Considering that feeding plays an important role in where the animals are found, and the habitats they select, particular interest has been paid to the affects of human-induced and natural disturbances in the environment that might negatively impact the dolphins. Those include perturbations in estuarine ecosystems caused by prolonged and intense red tide events, which may cause depletion of their food sources, a shift in the animals’ distribution, and increased morbidity and mortality.  As a possible result of decline of their preferred prey, known resident dolphins were impacted by increased mortality due to ingestion of recreational fishing gear (as described above).  The magnitude of this mortality has raised concerns about the long-term survival of the resident dolphin community if this trend continues.
     Our goal is to investigate the feeding ecology of Sarasota Bay dolphins at the individual and population levels, using data obtained from stranded and free-ranging animals. We plan to address seasonal as well as inter-annual variability in feeding and assess any dietary shifts that might take place as a result of environmental disturbances (for example, red tide). As isotopic ratios in dolphin teeth represent the feeding history of the animal over its entire life, we will use tissues that reflect a briefer history (weeks to a few months), such as skin and muscle. To be able to use archived samples collected from stranded animals in addressing these ecological questions, we tested the hypothesis that dolphin stranding location reflected the area occupied by the animals when they were alive.  We analyzed historical sighting data from 42 resident bottlenose dolphins and created three measures of home range. With only a few exceptions, dolphins stranded where they had been historically sighted, and the three home range measures largely reflected those findings – dolphins typically strand well within the confines of their home range (Figure 1). Those who deviate from this pattern don’t seem to stray too far, being typically found within 13 km of their home range boundaries. With these results we feel confident that data derived from strandings can be applied to the population at large for these resident dolphins.  

Figure 1.  Sighting history of long-time resident female "FB29."  She died in 1998 at 35 years of age, and stranded in Palma Sola Bay.  "FB29" died from ingestion of a fishing lure, and her newborn calf disappeared upon her death.