Distribution of juvenile leatherback sea turtle Dermochelys coriacea sightings

Abstract

Data on the location, date, sea temperature, and turtle size for 98 small (<145 cm) leatherback sea turtles Dermochelys coriacea demonstrate that leatherbacks less than 100 cm in carapace length occur only in waters warmer than 26degreesC.

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MARINE ECOLOGY PROGRESS SERIES
Mar Ecol Prog Ser
Vol. 230: 289–293, 2002 Published April 5
INTRODUCTION
One of the great mysteries of sea turtle life history is
where young turtles reside between the time they first
enter the sea as hatchlings and when they return to
coastal habitats as juveniles. This period can span a
few to many years. Brongersma (1968) proposed that
loggerhead sea turtle Caretta caretta hatchlings from
US Atlantic beaches move directly offshore and, as
young juveniles, reside in the pelagia passively drift-
ing to Atlantic Europe. He proposed that after some
indeterminate period they returned to benthic habitats
on the US eastern seaboard. Carr (1986) provided
additional evidence for this pattern of dispersal. Simi-
lar migrations have been described for the loggerhead
in the Pacific (Bowen et al. 1995) and are suspected for
other species, with 2 exceptions. The Australian flat-
back turtle Natator depressus may remain in coastal
waters throughout its life (Walker & Parmenter 1990),
whereas the leatherback remains pelagic.
Despite a growing understanding of the develop-
mental life stages and habitats utilized by the young of
most sea turtle species (Musick & Limpus 1997, van
Dam & Diez 1998), there is no information on where
hatchling leatherback turtles Dermochelys coriacea go
after leaving the nesting beaches (Brongersma 1970).
Deraniyagala (1936) suggested that they remain in the
open ocean, based on the sighting of a juvenile 20 km
from shore.
METHODS
There are very few records of juvenile leatherback
sightings or occurrences in the scientific literature.
Thus, for insights into the distribution of juvenile
leatherbacks, I gathered primary material from sea
turtle and marine mammal stranding coordinators,
published literature, museum records and reports, and
personal communications with qualified sea turtle bio-
logists. Because such data are often gathered in a man-
ner less rigorous than optimal, I was very careful to
evaluate each report for accuracy. If there was any
question as to location accuracy, size of turtle or other
such information, the record was rejected from my
analysis. Of particular concern was the size of the tur-
tle, as often the data records only list estimated size, or
it was unclear how size was determined. In such cases
the data were rejected. Fortunately, because the mor-
phology of the leatherback is so distinct, species
misidentification was rarely a concern as it might be
with other species of sea turtle. I restricted data to
turtles with measured curved carapace lengths
(CCL) <145 cm. For 2 turtles of 11.5 cm and 19.0 cm
length, I accepted the straight line measure, because
© Inter-Research 2002 · www.int-res.com
*E-mail: seckert@hswri.org
Distribution of juvenile leatherback sea turtle
Dermochelys coriacea sightings
Scott A. Eckert*
Hubbs-Sea World Research Institute, 2595 Ingraham Street, San Diego, California 92109, USA
ABSTRACT: Data on the location, date, sea temperature, and turtle size for 98 small (<145 cm)
leatherback sea turtles Dermochelys coriacea demonstrate that leatherbacks less than 100 cm in
carapace length occur only in waters warmer than 26°C.
KEY WORDS: Leatherback · Dermochelys · Juvenile · Temperature requirements · Developmental
habitat · Endangered species
Resale or republication not permitted without written consent of the publisher

Mar Ecol Prog Ser 230: 289–293, 2002290
Fig. 1. Dermochelys coriacea. Locations of juvenile (<145 cm curved carapace length) leatherback sightings or strandings utilized for this analysis

Eckert: Distribution of juvenile leatherbacks
such small turtles have relatively little cur-
vature to their carapaces and there should
be little difference between the 2 measure-
ment methods. Date, location, and water
temperature (if available) were noted. If
water temperature (whose accuracy could
be confirmed) was not available at the loca-
tion of the sighting, I used satellite sea sur-
face temperatures (SSTs) for the time and
location of the sighting (see http://podaac.
jpl.nasa.gov/mcsst/). Such data are readily
available on the worldwide web for the
period from 1 January 1987 to 20 November
1999. For a detailed discussion of the accu-
racy of this information see Vazquez et al.
(1998). In cases where no SST data were
available on the same date as the sighting
due to cloud cover or poor satellite coverage,
I used the nearest good date (± 4 d).
RESULTS
One hundred records of juvenile leather-
backs qualified for my analysis. Of these, 26
were from the published literature (Bronger-
sma 1969, 1970, 1972, Frair et al. 1972, Greer
& Wright 1973, McCoy 1974, Rhodin &
Schoelkopf 1982, Standora et al. 1984, Hor-
rocks 1987, Johnson 1989, Frazier 1990, Eck-
ert 1993, Sparks 1993, Acuna & Toledo 1994,
Grant 1994, Pino & Pino 1996), 66 were from
United States (State or Federal) sea turtle or
marine mammal stranding network coordi-
nators, 4 were from fishery observer records,
2 were from museum records, and 2 were
from unpublished data. (Work & Balazs un-
publ., M. Stinson pers. comm.). Their latitudi-
nal distribution extended from 56.75° N to
33.58° S, with the majority of records from
the Northern Hemisphere (Fig. 1). Of the
records, 5 were discarded from further analy-
sis due to their carapace size being listed as straight
length. A scatterplot of the size of turtles and latitude
suggested a positive and gradual increase in turtle size
with increasing latitude. To confirm that this relation-
ship was positive and statistically significant, a correla-
tion test was conducted against Northern Hemisphere
sighting data, (Pearson product-moment correlation, r =
0.693, p < 0.05). To confirm the gradual nature of the in-
crease, size data was divided into 20 cm size classes
and the most northern sighting of each size class was
used in a linear regression analysis (r = 0.85212) (Fig. 2).
In contrast, the relationship between juvenile leather-
back distribution and water temperature was not grad-
ual (Fig. 3). Rather, there appeared to be a sharp break
in the distribution at 100 cm CCL, with turtles less than
100 cm found only in waters warmer than 26°C and tur-
tles slightly larger than 100 cm found in waters as cool
as 8°C. While there is a significant (p < 0.05) negative
correlation between CCL and temperature, the statistic
is weak (r = –0.33,) (Fig. 4).
DISCUSSION
Leatherbacks have long been considered to be
facultative homeotherms capable of maintaining ele-
291
Fig. 2. Dermochelys coriacea. Scatterplot of juvenile sizes compared with
latitude of the sighting or stranding. A linear regression was plotted
against the highest latitude sighting within each (20 cm) size class
(arrows)
Fig. 3. Dermochelys coriacea. Scatterplot of juvenile sizes compared with
surface water temperature at the location of the sighting or stranding

Mar Ecol Prog Ser 230: 289–293, 2002
vated internal body temperature and thus able to
extend their range into cold northern waters (Frair et
al. 1972). Morphological and physiological characteris-
tics enhance the leatherbacks’ ability to stay warm,
including a cylindrical body form, large body mass,
thick fatty insulation and countercurrent circulation to
reduce heat loss in their extremities (Greer et al. 1973).
They are also reported to have temperature indepen-
dent cellular metabolism (Spotila & Standora 1985, Pal-
adino et al. 1990, Spotila et al. 1991, Penick et al. 1998).
The restriction of smaller leatherbacks to warmer
waters implies that size may play a role in the ability of
the species to exist in colder waters. Larger turtles
have a larger mass to surface-area ratio and greater
thermal inertia which could facilitate living in progres-
sively cooler waters as they grow. Therefore while
growth and development are poorly understood in
leatherbacks, it would be reasonble to assume that
they grow gradually over time, and that this gradual
growth would be reflected in a gradual movement of
the turtles into cooler waters with increasing size. The
positive relationship between turtle size and latitudi-
nal distribution, which is gradual, support this con-
tention if it is assumed that higher latitudes are cooler
than lower latitudes. However, the higher latitude
sightings of leatherbacks <100 cm CCL occurred only
where the water temperature was above 26°C. Further,
the relationship between temperature and distribution
of juvenile leatherbacks does not support a gradual
increase. Rather, there is a distinct transition at 100 cm
CCL, with turtles smaller than this found only in waters
of at least 26°C, and those only slightly larger (107 cm)
found in waters as cold as 12°C. One explanation for
such a rapid transition to a cold-water existence could
be developmental. If leatherbacks are able to generate
heat metabolically as proposed by Pennick
et al. (1998), then the data presented in this
paper would imply that this capacity may be
developmentally induced, and support the
theory that heat generation is physiological
rather than simply a function of morphology.
At approximately 100 cm in carapace size
there may be an onset of thermogenerating
capability which is not found in younger or
smaller leatherbacks.
The relationship between the distribution
of small leatherbacks and temperature is an
important clue to understanding the life his-
tory of this unique species. Leatherbacks
appear to spend the first portion of their
lives in tropical waters. Once they exceed
100 cm CCL they can move into the cooler
waters that have long been considered the
primary habitat for the species.
Acknowledgements. Ann Bowles, Joseph Jehl and 3 anony-
mous reviewers made helpful suggestions on the manuscript.
I am grateful to the following individuals or organizations
who provided data for this analysis: George Balazs, Joe
Cordero, Christina Fahy, Jacques Fretey, Kelly McAllister,
Sally Murphy, Donna Shaver, Margie Stinson, Craig Webster,
Brad Winn, and the Burke Museum. The sea surface temper-
ature data were obtained from the NASA Physical Oceanog-
raphy Distributed Active Archive Center at the Jet Propulsion
Laboratory, California Institute of Technology. The work was
supported under contract 43 AANF604349 from the Office of
Protected Resources, US National Marine Fisheries Service.
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293
Editorial responsibility: Otto Kinne (Editor),
Oldendorf/Luhe, Germany
Submitted: August 9, 2000; Accepted: July 26, 2001
Proofs received from author(s): March 6, 2002