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
Background:
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 predatorprey 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.
Methods:
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.
Results:
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).
Discussion:
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
Density of dolphins (based on group size of sightings)
with 5km search radius
Density of Sightings with 5km search radius
Eastern Zone
Eastern Zone
Central Zone
Central Zone
Western Zone
Western Zone
Legend
Legend
Atlantic Transition Zone
Sighting location
Atlantic Transition Zone
Sighting location
<VALUE>
<VALUE>
±
1st St. Dev.
Gulf Transition Zone
Figure 1.
2nd St. Dev.
7
3.5
0
7 Kilometers
3rd. St. Dev.
4th St. Dev.
±
Gulf Transition Zone
Figure 2.
7
3.5
0
7 Kilometers
1st St. Dev.
2nd St. Dev.
3rd St. Dev.
4th St. Dev.