Tracking post-nesting movements of loggerhead turtles (Caretta caretta) with sonic and radio telemetry on the southwest coast of Florida, USA

Abstract

Nine post-nesting loggerhead turtles (Caretta
caretta) were tracked using sonic and radio telemetry. Tracking began immediately after the turtles left the beach and continued until contact was either lost or terminated. As sonic tags transmit continuously underwater, they were much more effective than the radio tags in determining the paths of the turtles. Radio tags transmit only at the surface and were useful in ascertaining submergence durations. For nine of the ten turtles tracked with sonic signals, the gross movement was away from the beach in a westerly direction. The tracking periods ranged from 3.35 to 8.25 h, while the straight-line movements ranged from 3.05 to 12.88 km, respectively. Sixty-seven percent of the submergence durations recorded were <3 min. This respiratory behavior suggests continuous swimming, and the paths of the turtles suggested directed movement offshore immediately after nesting. The gradual littoral slope and lack of nearshore structure in this part of the Gulf of Mexico could be contributing factors to the patterns of dispersal observed, as benthic structures provide resting and foraging habitat for loggerheads.

Full text

D.S. Addison ÆJ.A. Gore ÆJ. Ryder ÆK. Worley
Tracking post-nesting movements of loggerhead turtles
(
Caretta caretta
) with sonic and radio telemetry
on the southwest coast of Florida, USA
Received: 8 November 2000 / Accepted: 16 August 2001 / Published online: 15 March 2002
ÓSpringer-Verlag 2002
Abstract Nine post-nesting loggerhead turtles (Caretta
caretta) were tracked using sonic and radio telemetry.
Tracking began immediately after the turtles left the
beach and continued until contact was either lost or ter-
minated. As sonic tags transmit continuously underwater,
they were much more effective than the radio tags in
determining the paths of the turtles. Radio tags transmit
only at the surface and were useful in ascertaining sub-
mergence durations. For nine of the ten turtles tracked
with sonic signals, the gross movement was away from the
beach in a westerly direction. The tracking periods ranged
from 3.35 to 8.25 h, while the straight-line movements
ranged from 3.05 to 12.88 km, respectively. Sixty-seven
percent of the submergence durations recorded were
<3 min. This respiratory behavior suggests continuous
swimming, and the paths of the turtles suggested directed
movement offshore immediately after nesting. The grad-
ual littoral slope and lack of nearshore structure in this
part of the Gulf of Mexico could be contributing factors
to the patterns of dispersal observed, as benthic structures
provide resting and foraging habitat for loggerheads.
Introduction
Sea turtles regularly migrate long distances between
foraging grounds and specific nesting beaches (Mortimer
and Carr 1987; Bowen 1995; Papi et al. 1995; Bowen
et al. 1996; Renaud et al. 1996). This homing ability and
fidelity to a specific beach is also evident during the
nesting season, as gravid females may nest up to seven
times on the same beach (Addison 1996). When gravid
loggerhead turtles (Caretta caretta) were relocated con-
siderable distances from their nesting beaches, nearly all
returned to the same area (Murphy and Hopkins-Mur-
phy 1990; Papi et al. 1997). While these studies focused
on long-range navigation after displacement, the in-
ternesting movements of females within a season are less
well known. In studies of internesting movements of
green turtles (Chelonia mydas) and hawksbill turtles
(Eretmochelys imbricata), neither species appeared to
range far from their nesting beaches (Mortimer and
Portier 1989; Starbird and Hillis 1992). Stoneburner
(1982) found that the internesting movements of log-
gerheads varied, with five individuals remaining inshore,
while two moved offshore, one in excess of 35 km. Ex-
tensive internesting movements have also been recorded
for leatherback (Dermochelys coriacea; Eckert 1993) and
Kemp’s ridley turtles (Lepidochelys kempi; Mendonca
and Pritchard 1986). While internesting movements are
likely influenced by the habitat needs of each species, the
proximity of suitable habitat to nesting beaches may
also affect the extent of such movements.
During the reproductive season, gravid female turtles
frequent coastal waters. On these occasions they can be
more vulnerable to boat strikes, harassment, entangle-
ment in commercial fishing gear, and chemical contam-
ination such as oil spills (Lutcavage et al. 1997). Data on
the extent of internesting movements, habitat dimen-
sions, and the ecological needs of turtles during the re-
productive season are essential to develop effective
conservation and management programs (Meylan 1995).
This need is particularly evident in South Florida, where
rapid urbanization places increasing stress on the marine
environment. Data on the direction and extent of post-
nesting movements would be useful to natural resource
managers and to permitting agencies in making deci-
sions that could affect nearshore areas frequented by sea
Marine Biology (2002) 141: 201–205
DOI 10.1007/s00227-002-0811-z
Communicated by W. Sammarco, Chauvin
D.S. Addison (&)ÆK. Worley
The Conservancy of Southwest Florida,
1450 Merrihue Drive, Naples, FL 34102, USA
E-mail: DaveA@conservancy.org
J.A. Gore
Dept. of Environmental Science,
Columbus State Univ., 4225 Univ. Ave.,
Columbus, GA 34907, USA
J. Ryder
Rookery Bay Nat. Estuarine Research Reserve,
300 Tower Road, Naples, FL 34116, USA

turtles during the reproductive season. The objective of
this study was to track the early post-nesting movements
of loggerhead turtles between nesting events.
Materials and methods
This study was conducted from 1996 to 1998 during the loggerhead
turtle nesting seasons on Keewaydin Island, Florida (Fig. 1).
Nesting begins in May and continues through August in this re-
gion. No telemetry tags were applied to nesting females prior to
mid-June, thereby minimizing the probability of male turtles
damaging tags during copulation. As reproductively mature fe-
males have high fidelity to specific nesting beaches, only those
turtles that had been flipper-tagged earlier in the nesting season
were selected. This methodology increased the probability of
tracking a single turtle more than once and also improved the
chances of recovering telemetry tags.
Sonic tags (Sonotronics CHP-87-L) transmitted continuously
underwater using a series of pings unique to each tag. As specified
by the manufacturer, the range of the sonic transmitters was 1 km.
Radio tags (Telonics MOD-555 HP) were tuned to different
frequencies and were manufactured to have a range of 8 km. Radio
tags transmitted only when the turtle was at the surface. The sonic
tags were used for most of the tracking, while the radio tags served
to maintain long-range contact if the sonic signal was lost.
Floats for the radio tags were made from blocks of balsa wood,
each sanded into a rounded oblong shape to reduce drag. The floats
were 17 cm long with a maximum diameter of 27 cm. The radio
transmitter was inserted in a hole drilled in the base of the float.
Seven coats of fiberglass resin were then applied to each float fol-
lowed by three coats of antifouling paint. Floats were attached to a
75 cm long tether made by connecting two rubber shock absorbers
(‘‘snubbers’’; Atlantic and Gulf Fishing Supply, no. 2021-1). Radio
tags were attached to the carapace by drilling a hole in the poste-
rior-most post-marginal scute. A nylon tie-wrap was used as a
breakaway link between the tether and the carapace. Sonic tags
were attached with aluminum wire threaded through small holes at
each end of the tag and wrapped around the tag lengthwise.
Electrician’s tape was wrapped around the middle of the tag, thus
forming a loop at each end of the tag. Sonic tags were attached to
the opposite side of the carapace beneath the posterior-most post-
marginal scute. Two holes were drilled, and the tags attached with
nylon tie-wraps.
Before attaching any tags to the turtles, the carapace was
scrubbed with a brush and washed with betadyine. All the nylon
Fig. 1. Location of Keewaydin
Island, Florida
202

tie-wraps were threaded through pieces of surgical tubing and
wiped with Neosporin prior to insertion in the holes to prevent
infection. During the entire tagging process, the turtles were re-
strained in a portable plywood box held in place with 1.25 m
lengths of steel reinforcing rod pushed into the sand.
Prior to release, the location of the turtle was recorded with a
hand-held global positioning system (GPS; Garmin GPS 75).
Tracking began as soon as the turtle entered the water. An open
6.4 m boat with a tiller-mounted, four-stoke 35 horse power out-
board motor was used for tracking. The hand-held GPS was used
to determine the location of the boat relative to the turtle.
Two radio receivers (AVM Instruments, LA12-Q) were used for
radio telemetry. One was equipped with a hand-held, three-element
yagi antenna and used as a backup if contact was lost with the sonic
tag. The other radio receiver was deployed on the beach with a
seven-element yagi antenna. This receiver was monitored periodi-
cally at night beginning 9 days after a turtle was first tracked.
Compass headings were obtained from a compass mounted on the
Table 1. Caretta caretta.
Telemetry data for post-nesting
females from Keewaydin
Island, Florida (n.d. no data,
turtles tracked principally with
radio telemetry)
Turtle
tag no.
Date tracked Hours
tracked
Linear distance
traveled (km)
Total distance
traveled (km)
Extent of
meander (km)
Rate
(km h
–1
)
PPQ707 26 Jun 1996 4.85 n.d.n.d n.d n.d
KYT076 11 Jun 1997 5.38 n.d n.d n.d n.d
KYT003 20 Jun 1997 6.13 9.10 12.00 2.90 2.0
1 Jul 1997 3.05 3.35 3.95 0.60 1.3
12 Jul 1997 6.87 7.09 10.36 3.27 1.5
KYT071 22 Jun 1997 5.00 7.35 11.12 3.77 2.2
PPQ704 24 Jun 1997 6.40 9.18 10.40 1.22 1.6
KYT081 26 Jun 1997 6.65 8.18 11.93 3.75 1.8
6 Jul 1997 6.37 7.05 7.82 0.77 1.2
QQN220 27 Jun 1997 6.27 12.88 13.77 0.89 2.2
QQE502 18 Jun 1998 8.25 10.77 13.06 2.29 1.6
QQE415 25 Jun 1998 8.17 11.30 12.78 1.49 1.6
Mean 6.12 8.62 10.72 2.09 1.7
SD 1.43 2.62 2.92 1.25 0.34
Fig. 2. Caretta caretta. Routes of post-nesting loggerheads tracked
with sonic telemetry, Keewaydin Island, Florida
203

antenna pole. A Sonotronics directional hydrophone and receiver
(USR5 W) were used for sonic tracking. The hydrophone was
mounted on one end of a 2 m length of 5 cm PVC pipe. To obtain
headings, a magnetic compass was mounted on the opposite end of
the PVC pipe. Per the manufacturers’ specifications, the radio
transmitters’ possessive range was about 8 km, while sonic pings
could be detected for up to 1 km. These specifications were verified
in field tests prior to deployment. After release, contact with the
turtles was maintained continuously with the hydrophone. The boat
was allowed to drift until the sonic signal faded noticeably. A
compass heading was recorded, and the boat operated at idle speed
until the signal began to strengthen. The motor was then shut down,
the boat allowed to drift or, at times, was anchored, and the process
repeated. This protocol was used to reduce disturbance by main-
taining maximum distance from the turtle. Periodically, the boat
was operated until it was well seaward of the turtle to determine if its
presence had any noticeable influence on the direction of movement.
Invariably, the turtle passed the boat and continued moving out to
sea. When conditions permitted, the boat either drifted or was an-
chored as long as the sonic signal remained audible. The radio re-
ceiver operated continuously to detect radio pulses. When signals
were received, the antenna was swung through an arc to attempt to
obtain a compass heading. Radio and sonic tags were removed from
turtles found nesting after 25 July of each year.
Results
Nine female loggerhead turtles were tracked during this
study (Table 1). Of these, seven were tracked once, while
two others were tracked twice and three times, respec-
tively. The paths of the turtles were determined princi-
pally with sonic telemetry (Fig. 2). Sonic contact with
turtles PPQ707 and KYT076 was lost early in each
tracking episode. The routes of these turtles could not be
accurately determined using radio telemetry alone, so
only the points of last contact are shown in Fig. 2. Only
one turtle was detected with the land-based receiver. On
1 July 1997, a storm forced termination of tracking
KYT003 after 3 h. At that time, she was 4 km offshore.
Later that afternoon, a very weak radio signal was de-
tected on the Keewaydin Island beach on a bearing of
270°. This suggests that she was still within 8 km of land
and, like the other turtles that were tracked for longer
periods, had continued moving offshore.
When tracking newly tagged turtles, the radio re-
ceiver was occasionally set to different frequencies to
detect radio signals from previously tagged turtles. None
were detected, nor were any sonic signals detected from
turtles other than the turtle being tracked. The only
evidence that turtles were in the area after the initial
tracking terminated came from a fisher who returned a
radio transmitter found adrift on 17 July 1997. It was
encountered ca. 24 km offshore of the northern end of
Keewaydin Island. The turtle (KYT084) was tracked on
27 June until she had traveled approximately 13 km
offshore. She nested again on Keewaydin Island on 18
July 1997.
The tracking data indicated that the initial post-
nesting movements were generally offshore (Fig. 2). The
extent of meandering of individual turtles varied and is
reflected in the difference between the straight-line dis-
tance and total distance traveled (Table 1). In nine of the
ten turtles tracked with sonic signals, the rates of gross
movement ranged from 1.2 to 2.2 km h
–1
(Table 1).
Discussion
The periods during which radio signals were received
were typically too brief to obtain bearings for the turtles,
thus sonic telemetry was clearly the more useful means
of tracking. Had the turtles remained in shallow, inshore
waters, where transmissions might have been for longer
durations, the radio transmitters would have been more
useful in determining compass headings. Offshore their
utility was limited to monitoring submergence durations.
The rates of movement recorded (1.2–2.2 km h
–1
)
were within the 1–3 km h
–1
documented by Hopkins and
Murphy (1980) for post-nesting loggerheads in South
Carolina. Of the 439 submergence durations recorded,
67% were for <3 min, which suggests that swimming
was continuous (Schmid 2000). The direction of the
swimming was typically away from the beach. These
data suggest that loggerhead turtles in Southwest Flor-
ida disperse offshore immediately after nesting. The lit-
toral shelf in this region of the Gulf of Mexico slopes
gradually. The bottom is sandy with scattered structure,
which is of low relief. Typically, depth does not exceed
12 m until a distance of about 24–32 km offshore. Hard-
bottom features such as limestone rock outcroppings,
which provide suitable resting and foraging habitat for
loggerheads, are present in deeper waters offshore
(Continental Shelf Associates 1986). These features
could be contributing factors to their dispersal offshore.
Stoneburner (1982) indicated that the loggerheads
tracked in the Georgia Bight did not move randomly,
but moved directly to areas with natural or artificial
substrate.
The determination of the full extent to which logger-
head turtles disperse offshore was beyond the scope of
this project. Flipper tagging studies (Dodd 1988; Limpus
et al. 1992) have demonstrated fidelity to nesting and
foraging areas. This behavior suggests that loggerheads
may use the same areas between nesting events within a
season and between nesting seasons. Future refinements
in the accuracy of satellite telemetry will allow re-
searchers to gain insight into the movements and habitat
preferences of sea turtles during internesting periods.
Acknowledgements The authors sincerely thank the following in-
dividuals for their assistance with transmitter attachment and
tracking: K. Addison, A. Andrews, A. Berg, E. Bugbee, K. Croak,
M. Ebenroth, C. Granger, T. Grazia, M. Henig, K. Nelson, J.
Opdyke, M. Polzar, H. Ross, N. Ryan, D. Soles, and S. Steele. The
authors also thank S. Bortone and J. Schmid for their reviews of
the manuscript. We also wish to thank three anonymous reviewers
for their contributions.
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