Leigh G. Torres and Andrew J. Read

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Using bottlenose dolphins as an indicator species in Florida Bay: Analyzing habitat use and distribution relative to water quality, habitat and fish community

Leigh G. Torres and Andrew J. Read

Duke University Marine Lab, Beaufort, NC
Laura Engleby

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 15th and August 15th, 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.

To assess the fish community throughout Florida Bay, we used two fishing techniques: bottom trawls and gillnets. Demersal prey was sampled using a 3-m research otter trawl, towed at a speed of 4-5 knots for 3 minutes. The gillnet was comprised of 3 ¼ inch mesh and was set for 30 minutes. The locations of bottom trawls and gillnet sets were randomly generated, using GIS, to adequately sample the different habitat types and zones within Florida Bay. Fish were identified and measured before being released alive. Water quality was measured at each trawl and gillnet site. A total of 121 trawls and 26 gillnet sets were conducted during the summer of 2002. Trawling and gillnetting caught different species and size classes of fish, regardless of location.
All information was analyzed using Geographic Information Systems (GIS) software (Arc) to determine spatial relationships between (1) effort within habitat types and zones, (2) dolphin distribution and density, (3) water quality, (4) fish diversity and abundance and (5) dolphin behavior.

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).

Catch per unit effort (CPUE) of fish caught in trawls was related to the zone in which the trawl was conducted (p < 0.001, χ 2 = 34.5, df = 4). The relationship between CPUE and bottom habitat type was not significant (p = 0.103, χ 2 = 7.7, df= 4). CPUE of fish decreased significantly with depth (p = 0.002). (Analysis of CPUE compared to salinity and temperature has not been completed yet.) Total fish caught in gillnet sets was significantly related to both zone (p < 0.01, χ 2 = 13.3, df = 4) and bottom habitat type (p < 0.03, χ 2 = 12.2, df = 5).

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.

It is apparent from our preliminary examination of our data that the number of sightings, the number of dolphins and fish caught in both trawls and gillnets are related to zone and, less strongly, to bottom habitat type. We plan to conduct further analysis of dolphin distribution, abundance and behavior, as related to habitat type and the distribution of fish communities, to shed light on the spatial and temporal dynamics of dolphins and their prey in Florida Bay.
Leigh Torres and Andy Read, Duke University, 135 Duke Marine Lab Rd., Beaufort, NC 28516, Phone: 252-504-7629, lgt3@duke.edu, aread@duke.edu

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.

Figure 2.

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