Leigh G. Torres and Andrew J. Read

Leigh G. Torres and Andrew J. Read

Dolphin Ecology Project, Key Largo, FL

Our research is focused on the effects of habitat restoration on bottlenose dolphins in Florida Bay. Bottlenose dolphins range throughout Florida Bay and beyond, and their patterns of distribution reflect variation in various environmental parameters. We are particularly interested in the dynamics of dolphin predator-prey interactions in the context of habitat and water quality. As the first step in this research, we tested the hypothesis that dolphins occur and feed preferentially in habitats where prey densities are high. We have also examined whether or not variation in the density of potential prey is related to spatial variation in water quality and habitat type.

We assessed dolphin distribution and habitat use in dedicated boat-based surveys throughout each basin of Florida Bay between June 15^th and August 15^th, 2002. The bay was divided up in five zones (Eastern, Central, Western, Gulf Transition and Atlantic Transition) based on similar environmental conditions (e.g. Thayer and Chester 1989, Sogard et al. 1989) and 6 bottom habitat types (seagrass, hardbottom, open mud, open sand, mud bank, mixed bottom type) based on the “Florida Bay Bottom Types” map created by the USGS in 1997 (Haley 1997). GPS waypoints were collected every five minutes and water quality parameters (salinity, temperature and clarity) were recorded every 30 minutes. Each area of the bay was surveyed twice throughout the summer to ensure equal survey effort in all habitats. At all sightings of dolphins, GPS location was recorded and water quality parameters measured. Photo-identification was conducted to determine group composition and site fidelity of individuals. Group size, the presence or absence of calves, and behavioral state were also noted.

Bottlenose dolphins were distributed non-randomly among the various habitats of Florida Bay (p < 0.001, χ² = 235, df = 5). Only 12% of Florida Bay is covered by mud habitats, but 33% of all sightings and 42% of all animals sighted in the Bay occurred in these habitats. In addition, the number of sightings (p = 0.0002, χ ² = 22, df = 4) and the number of dolphins (p < 0.001, χ ² = 142, df = 4) were distributed non-randomly in the five zones of the Bay. Very few dolphins were observed in the eastern and Atlantic transition zones (Figures 1 and 2). Only 1% of the eastern zone and less than 1% of the Atlantic transition zone is covered by mud. Spatial analysis revealed that dolphin sightings occurred most frequently in the northern part of the Central zone (Figure 1), but the actual density of dolphins (corrected for group size of each sighting) was greatest in the western part of the bay, particularly in the Gulf Transition zone and Western zone (Figure 2).

Florida Bay is characterized by strong environmental gradients, particularly with regard to salinity, and a diversity of habitat types. In this environment, fish are patchily distributed, occurring in environments that are within their physiological tolerances and provide suitable habitat. Piscivorous predators, such as bottlenose dolphins, respond to the patchy distribution of prey by concentrating their foraging efforts in areas where prey are abundant. Due to their visibility, bottlenose dolphins are a good indicator of distribution or prey and, in turn, of the quality of the habitat and environment which support these fishes. By monitoring the habitat use and distribution ecology of bottlenose dolphins with synoptic sampling of the fish community and water quality, we quantified the response of bottlenose dolphins to environmental variation in Florida Bay to obtain a better understanding of the effects of environmental variability on upper trophic level predators throughout the Bay.

Laura Engleby, Dolphin Ecology Project, P.O. Box 1142, Key Largo, FL 33037, Phone: 305-852-0649, lengleby@aol.com

Question 5 – Higher Trophic Level

Figure 1.