My Master’s research focused on shark bite scar and wound frequencies in the Sarasota Bay resident bottlenose dolphins as an indirect measure of the threat sharks pose to this dolphin community. Using photographs taken during Sarasota Dolphin Research Program (SDRP) capture-release projects from 1975 to 2013, I found that of the 246 resident dolphins that have been handled, 37% had shark inflicted scars or wounds. I found no difference in shark bite frequency between males and females, and the frequency of shark bites on the resident dolphin community has remained relatively unchanged during 1984 through 2013.
Previous studies of the Sarasota Bay dolphin community – along with common predator-prey theories – suggest animals may modify habitat use and social behaviors to adjust for increased risk from potential predators. I tested to see if differential habitat use existed among dolphins with and without shark bite scarring, and if so, in which habitats would we predict to see higher and lower shark bite scar and wound occurrence. In 2013, Boyd Carnal (Duke University) and I classified coastal habitat from Apollo Beach to Venice Inlet into eight categories – seagrass, sandflat, pass, Gulf, open bay, mangrove, channel, and river. Using updated GIS techniques, I calculated each individual dolphin’s lifetime home range, so long as they had a minimum of 100 sightings from the age 6 years or greater. Because other factors may influence a dolphin’s risk to negative shark encounters, I also tested for the influence of water depth, average group size, sex and home range size on the probability of an individual dolphin having a shark bite wound or scar. Using GIS and statistical modeling, I concluded that there was a difference in habitat use between individual dolphins with shark bites and those without shark bites.
I assessed whether habitat type or social- or ranging behavior influenced the probability of a dolphin having a shark bite scar over two spatial scales – the core range, or area of intense use, and the overall home range. In the core area, I found that as an individual’s average group size increased, the probability of a dolphin having a shark bite decreased. I also found that as the proportion of channel habitat making up the core area increased, the probability of a dolphin having a shark bite decreased. The opposite was found with open bay habitats –as the proportion of open bay habitat increased, the probability of a dolphin with a shark bite increased. Other tested variables were not significant indicators for shark bite presence in the core range model. For the overall home range, I found that as the proportion of open bay and mangrove habitats increased in the home range area, the probability of a dolphin having a shark bite increased; where-as increasing the proportion of seagrass habitat in the home range decreased the probability of shark bite occurrence. Additionally, an increase in average depth increased the probability of a dolphin having a shark bite. Other variables were not significant indicators for shark bite presence in the home range model.
This project has been made possible by an anonymous donor to the Chicago Zoological Society and by the University of Florida, Department of Wildlife Ecology and Conservation.
This article was published on page in the November 2014 issue of Nicks n Notches