Florida imposed a state-wide ban on the use of large gillnets in inshore waters in 1995.
This set the stage for a natural experiment to examine the indirect effects of fisheries on two well-studied bottlenose dolphin populations, one in Sarasota Bay on the west coast of Florida and the other in the Indian River Lagoon on the east coast.
The major focus of the study was to look for changes in feeding habits of these two resident dolphin populations. To do this we looked for shifts in diet and feeding habits from animals stranded in Pre-Net Ban and Post-Net Ban years by analyzing stomach contents allied with analysis of diet based on stable isotopes (carbon, nitrogen and sulfur) from muscle tissue.
In the Sarasota Bay population, the number of prey species found in the dolphin stomachs tripled from Pre-Net Ban to Post-Net Ban and the average number of different types of prey per stomach doubled. Along with this change in diversity of diet, a shift in 13C and 34S indicated that the dolphins moved from a primarily seagrass foraging habitat Pre-Net Ban to including more open bay foraging habitat Post-Net Ban as prey became more available. We concluded that the result of the gillnet ban for the dolphins in Sarasota Bay was associated with a reduction in by-catch, indicating that there had been recovery of at least some fish species that were incidentally caught by the nets. This stressed the impact that gillnets have on fish species not specifically targeted by these gillnets.
For the Indian River Lagoon dolphin population, the results were different. We saw a smaller change in the major prey items seen in the stomachs of the stranded dolphins Post-Net Ban, and different from Sarasota Bay, the frequency with which mullet (the gillnet target fish) were present increased. While there was also an increase in the overall fish species taken, the change in diet based on stomach contents was not as striking as in Sarasota Bay. Stable isotope analysis suggested that, contrary to Sarasota Bay, there was an increased use of seagrass habitat, suggesting that more prey was available in the seagrasses. This finding, allied with the stomach content data, suggested that this was from an increase in mullet availability as well as from reduced by-catch.
In the years Post-Net Ban, seagrass beds in the Indian River Lagoon substantially increased due to changes in ocean conditions while in Sarasota Bay this increase was not as marked. This factor is thought to have played a major role in the differences in response to the gillnet ban noted in the two areas around Florida and is an excellent example of the complexity of the factors that contribute to the health of inshore habitats.
This study was possible because of the years of data that have been collected for the Sarasota Bay and Indian River Lagoon bottlenose dolphin populations and because of the availability of samples made possible by intensive stranding response in each of these areas. It sets the stage for on-going studies of the effects of increased fishing activity and habitat change in the coastal marine ecosystems in Florida.
This project was funded by the U.S. Marine Mammal Commission to examine indirect effects of fisheries on marine mammals and represents a novel synthetic approach to detecting the impact of gillnet fishing on local populations of dolphins. The results reported here come from a final report submitted December 2012.
This article was published on pages 23-24 in the January 2014 Nicks n Notches.