First satellite tracks of neonate sea turtles redefine the ‘lost years’ oceanic niche

Abstract

Few at-sea behavioural data exist for oceanic-stage neonate sea turtles, a life-stage commonly referred to as the sea turtle 'lost years'. Historically, the long-term tracking of small, fast-growing organisms in the open ocean was logistically or technologically impossible. Here, we provide the first long-term satellite tracks of neonate sea turtles. Loggerheads (Caretta caretta) were remotely tracked in the Atlantic Ocean using small solar-powered satellite transmitters. We show that oceanic-stage turtles (i) rarely travel in Continental Shelf waters, (ii) frequently depart the currents associated with the North Atlantic Subtropical Gyre, (iii) travel quickly when in Gyre currents, and (iv) select sea surface habitats that are likely to provide a thermal benefit or refuge to young sea turtles, supporting growth, foraging and survival. Our satellite tracks help define Atlantic loggerhead nursery grounds and early loggerhead habitat use, allowing us to re-examine sea turtle 'lost years' paradigms.

Full text

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Research
Cite this article: Mansfield KL, Wyneken J,
Porter WP, Luo J. 2014 First satellite tracks of
neonate sea turtles redefine the ‘lost years’
oceanic niche. Proc. R. Soc. B 281: 20133039.
http://dx.doi.org/10.1098/rspb.2013.3039
Received: 20 November 2013
Accepted: 4 February 2014
Subject Areas:
behaviour, ecology
Keywords:
sea turtle ‘lost years’, ocean migration,
satellite telemetry, oceanic stage sea turtles,
Caretta caretta, thermal niche
Author for correspondence:
Katherine L. Mansfield
e-mail: kate.mansfield@ucf.edu
First satellite tracks of neonate sea turtles
redefine the ‘lost years’ oceanic niche
Katherine L. Mansfield1,2, Jeanette Wyneken3, Warren P. Porter4
and Jiangang Luo5
1
Department of Biology, University of Central Florida, Orlando, FL 32816, USA
2
Southeast Fisheries Science Center, National Marine Fisheries Service, 75 Virginia Beach Drive, Miami,
FL 33149, USA
3
Department of Biological Sciences, Florida Atlantic University, Boca Raton, FL 33431, USA
4
Department of Zoology, University of Wisconsin Madison, Madison, WI 53706, USA
5
Division of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science,
University of Miami, Miami, FL 33149, USA
Few at-sea behavioural data exist for oceanic-stage neonate sea turtles, a
life-stage commonly referred to as the sea turtle ‘lost years’. Historically, the
long-term tracking of small, fast-growing organisms in the open ocean was
logistically or technologically impossible. Here, we provide the first long-term
satellite tracks of neonate sea turtles. Loggerheads (Caretta caretta) were remotely
tracked in the Atlantic Ocean using small solar-powered satellite transmitters.
We show that oceanic-stage turtles (i) rarely travel in Continental Shelf waters,
(ii) frequently depart the currents associated with the North Atlantic Subtropical
Gyre, (iii) travel quickly when in Gyre currents, and (iv) select sea surface
habitats that are likely to provide a thermal benefit or refuge to young sea turtles,
supporting growth, foraging and survival. Our satellite tracks help define
Atlantic loggerhead nursery grounds and early loggerhead habitat use, allowing
us to re-examine sea turtle ‘lost years’ paradigms.
1. Introduction
Classic sea turtle life-history models assume discrete shifts in habitat use during
different life stages [1– 3]. Sea turtles hatch from nests on coastal beaches, enter
near-shore waters and swim offshore, transitioning to oceanic habitats where
they remain for a minimum of 1– 2 years [1–6]. Known as the sea turtle ‘lost
years’, few data exist on the in-water behaviour of young, oceanic-stage sea turtles
[7]. These knowledge gaps reflect the logistical and technological limitations of
observing small, fast-growing, migratory species in the open ocean. Rare sight-
ings, at-sea collections [1,4,8], genetic sampling [9] and spatially discrete size
distributions of loggerhead turtles [10] resulted in long-standing hypotheses
regarding oceanic-stage sea turtle dispersal and behaviour. These include the
hypotheses that neonate Atlantic loggerhead turtles:
(1) transition to and remain offshore in oceanic waters, away from predator-rich
Continental Shelf waters [2– 4];
(2) are passive drifters that entrain within currents associated with the North
Atlantic Subtropical Gyre [1,4]; and
(3) occupy sea surface habitats [1,4,8,11] and associate with floating Sargassum
communities [1,11,12].
Carr [1] hypothesized that loggerhead hatchlings from eastern Florida (USA)
nesting beaches swim offshore and enter the North Atlantic Subtropical Gyre
(NASG) via the southern Gulf Stream. Theoretically, turtles are passively trans-
ported across the North Atlantic to eastern Atlantic waters and are known to
associate with floating Sargassum communities for predator refuge and food avail-
ability [1,4,11,12]. Size distributions of oceanic loggerheads from the eastern
Atlantic (Azores, Cape Verde, Madeira), when compared with those along the
&2014 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution
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author and source are credited.

Atlantic seaboard, support the hypothesis of a long-term,
unidirectional, gyre-based, developmental migration [1,4,5,
9,10,13]. Loggerheads found in the eastern Atlantic are geneti-
cally linked to nesting assemblages along the western Atlantic
coast [9,14]. Laboratory studies demonstrate that hatchlings
orient appropriately to remain within the NASG when exposed
to magnetic fields replicating those fields found at discrete
regions along this system [15]. Travel time within the NASG
is generally assumed to be correlated to surface current
speeds—similar to drift bottles; Carr [1,4] noted that passively
drifting loggerheads within the NASG would require 235
days to traverse from Florida natal beaches to the Azores.
However, no actual long-term turtle movements or travel
times had been directly observed.
Temperature influences poikilothermic turtle growth, move-
ment, feeding behaviour, physiology and immune competence
[16,17]. Experimental tests of hatchling loggerheads during
their post-hatching frenzy found that swimming activity
decreased in 308C water and locomotor coordination was lost
in waters above 338C [18]. At cool temperatures (less than
108C), smaller sea turtles are vuln erable to hypothermic stunning
[19,20]. Despite its importance, the thermal environment
encountered by oceanic-stage loggerheads has not been directly
measured. The significance of Sargassum communities as refuge,
foraging and early developmental habitat for oceanic juvenile
loggerheads is well documented [4,14]. Yet an important func-
tion of this surface-based habitat has been overlooked: the
thermal benefit of associating with these communities.
Here, we provide the first long-term, at-sea movement data
for small, oceanic-stage loggerhead sea turtles. Using novel
satellite telemetry methods [7], we remotely tracked neonate
loggerheads in the AtlanticOcean. By characterizing the turtles’
offshore dispersal, thermal niche and habitat characteristics, we
re-examine the long-standing hatchling dispersal paradigms
and hypotheses associated with the sea turtle ‘lost years’.
Specifically, we test whether oceanic-stage loggerhead sea
turtles (i) remain exclusively offshore within oceanic (non-
neritic) waters, (ii) entrain within the currents associated with
the North Atlantic Subtropical Gyre as part of a unidirectional
developmental migration and (iii) occupy sea surface habitats.
Finally, (iv) we test whether occupying sea surface habitats or
association with Sargassum communities would confer thermal
benefits to small, oceanic-stage sea turtles.
2. Material and methods
(a) Turtle data and movement analyses
Microwave Telemetry’s PTT-100 9.5 g solar-powered satellite
transmitters were used to track the at-sea movements of 17 neonate
loggerhead sea turtles collected from nests along the south-
east coast of Florida, and laboratory-reared to release size
(300– 720 g, 11 – 18 cm straight carapace length) and age (3.5–9
months old). Tag coloration matched that of a typical loggerhead
carapace (brown). Tags were adhered to turtles’ carapaces using
a flexible acrylic–silicone– neoprene attachment described by
Mansfield et al. [7]. Tag duty cycle was programmed to 10 h on,
48 h off per manufacturer requirements—tags required 48 h of
solar charging. All turtles were released in the Gulf Stream
within floating Sargassum mats approximately 18.5 km offshore
of their natal beaches (near 26.98N latitude, 79.58W longitude).
Using the Argos satellite data processing system and Kalman filter-
ing algorithm, transmitter data were filtered based on accuracy of
transmission using Argos location codes (LC) 3–0, A and B [21].
Location data were tested for spatial randomness and orientation
using circular point and Raleigh’s Z statistics (ARCVIEW v. 3.2,
AMAE ext.;
a
,0.05). Mean orientation was determined for
locations occurring below and above 358N latitude, roughly corre-
sponding to the replicated magnetic field locations tested on
captive hatchlings by Lohmann & Lohmann [22].
(b) Thermal ecology and habitat use
Mean daily ambient temperature (8C; +0.338C accuracy per
manufacturer specifications) and solar cell charge (volt;
+0.02 V accuracy) were collected from transmitter sensor data.
Bathymetry data and MODIS 9 km resolution daily sea surface
temperatures (SSTs) were extracted using the Satellite Tracking
and Analysis Tool (STAT) [23]. Additional SST and bathymetry
data were derived using the Global Hybrid Coordinate Ocean
Model (HYCOM þNCODA Global 1/128Analysis; 7 km
resolution) and 2 min Gridded Global Relief Data (ETOPO2 v. 2).
To characterize time spent in association with oceanographic
features such as the Gulf Stream and other currents or meso-scale
eddies, the Kalman-filtered tracks were regularized to a frequency
of 6 h intervals, using piecewise Be
´zier interpolation methods simi-
lar to Tremblay et al. [24], but modified with the algorithm by Lars
Jensen (http://ljensen.com/bezier/). To determine turtle associ-
ation with eddy features, we compared the 6 h interpolated turtle
tracks with daily current vector maps from HYCOM model
output for all individuals and tabulated the number of track days
associated with each of the three features: (i) main Gulf Stream
(main part of Gulf Stream not including eddy features), (ii) eddy
feature (defined as a current vector group forming a circular
pattern) and (iii) other areas not included in (i) and (ii).
To determine whether turtles remained at the sea surface,
and to characterize the thermal environment encountered by
the turtles, we used four complementary approaches. (i) We
tested for differences among ambient (transmitter-derived)
versus satellite- (MODIS) and model-derived (HYCOM) temp-
eratures encountered by the turtles using the Mann– Whitney
U-test (
a
,0.05). (ii) We characterized the Argos location accu-
racy and tags’ solar charge rates in order to determine relative
exposure of the tags to air and direct sunlight. The satellite
tags do not transmit unless exposed to air. Longer periods of
air exposure allow longer periods of communication between
tags and overhead satellites, thereby increasing the accuracy of
the transmitted location data [7,25,26]. Mansfield et al.[7]
showed that the power output from comparable solar cells
declined with depths as shallow as 30 cm; power output was
one-seventh that of solar cells left to charge at 5 cm depths.
Thus, we infer that higher Argos location accuracies and greater
charge rates relate to longer periods of transmitter exposure to
air and to sunlight, enabling the tags to effectively communi-
cate with overhead satellites or exposing tags to the solar
energy required to successfully recharge. We use these data as
a proxy to determine whether the turtles occupied sea surface
habitats. (iii) To determine whether exposure to direct solar energy
could influence temperatures encountered by neonate sea turtles
(e.g. ambient temperatures recorded by the satellite tags), we mea-
sured the solar reflectivity of the Microwave Telemetry 9.5 g PTTs
solar-powered satellite tags, loggerhead carapaces and mats of
fresh Sargassum spp.collected from waters offshore of southeast
Florida. Solar reflectivity measurements represent the fraction of
incident radiation reflected rather than absorbed by a surface
or substrate. Measurements were collected in a dark room using
a portable ASD spectroradiometer set to a spectral range of
350– 2500 nm (10 nm resolution; http://www.asdi.com/products/
fieldspec-spectroradiometers). Measurements were made with a
1 cm diameter sensor window that delivered a white light source
to illuminate the test object. A white reference standard was used
to determine 100% reflectivity across the full spectrum; the sensor
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2

head was transferred from the white reference standard to the object
of interest and a full spectrum scan was performed. Seawater reflec-
tivity (albedo) was obtained from literature and satellite-based
measures [27,28]. These data were applied to a heat balance
equation: Q
in
þQ
abs
¼Q
out
þQ
st
,whereheatin(Q
in
)isbylong
wavelength infrared thermal radiation (IR) from the sky and
clouds, Q
abs
is by solar radiation absorption, Q
out
is by conduction,
convection, evaporation and emitted IR, and Q
st
is stored heat.
We used these calculations to determine whether any observed
differences between ambient (transmitter-derived) and satellite- or
model-derived SST measures could be explained by reflectivity
differences between ambient seawater and turtles, tags or Sargassum.
Finally, (iv) we measured and compared thermal profiles
of seawater with and without Sargassum to verify that the
Sargassum thermal environment is warmer than that of open
seawater. Fresh Sargassum was collected offshore of southeast
Florida placed in one of two identical buckets (28 cm inside
diameter 36.8 cm high) filled with filtered seawater. The buckets
were each placed in circular plastic tubs (82 cm diameter 15 cm
high) filled with freshwater. Temperature was recorded every
30 min using Hobo U22 temperature data loggers (with +0.28C
accuracy; http://www.onsetcomp.com/products/data-loggers/
u22-001). Data loggers were placed in air adjacent to the buckets,
and each bucket had data loggers placed centrally, 2.5 cm
below the water surface or water–Sargassum surface, and
suspended at half-depth. We estimated percentage cloud cover
visually; sunrise and sunset, wind speed and humidity were
inferred from data at a nearby airport (less than 1 km away), and
corrected for bucket height above the ground. Temperature data
from the data loggers were overlaid to compare thermal profiles
among treatments.
3. Results
(a) Turtle movements
Turtles were remotely tracked for between 27 and 220 days
(mean ¼86.6 days +55.2 s.d.; table 1) and travelled distances
that ranged from 200 to more than 4300 km (figure 1a). All tur-
tles initially travelled north, remaining within or in close
proximity to the Gulf Stream immediately post-release. Ten tur-
tles continued within the Gulf Stream, past Cape Hatteras
(North Carolina, USA), then moved eastward into the north-
western Atlantic. Initially, turtles remained along the outer
edge of Continental Shelf (defined by the 200 m isobath;
figure 1a). Complete departure from near-shelf waters occurred
past Cape Hatteras (approx. 358N). One turtle spent approxi-
mately 21 days within Continental Shelf waters; however,
98.6% of all locations across all turtles (n¼1472 track days)
were off the Shelf.
All tracks showed significant directional movement
(Raleigh’sZ;p,0.05) throughout their tags’ transmission
period. Turtles’ net paths were directed to the NNE–NE
(38.98+16.6 s.d.; n¼17) between release points to approxi-
mately 358N latitude. Turtles travelled to the ENE (63.48+
21.4 s.d.; n¼11) within north Atlantic waters, north of 358N
latitude. With one exception, none of the turtles moved west-
ward of the Gulf Stream boundary; however, turtles did
move east beyond the eastern Gulf Stream boundary. Turtles
spent 24.3% (+9.3% s.d.) of their track time (n¼1472 days)
within the main Gulf Stream current (table 1). Seven turtles
travelled out of the Gulf Stream, moving into the Sargasso
Sea (figure 1b). Some movements out of the Gulf Stream
were associated with meso-scale eddies (table 1). Our current
feature utilization analysis showed dominant utilization of
eddy features (table 1 and figure 2a–c). Turtles spent between
13 and 81% of their time in association with eddies (mean:
66.7% +39.6 s.d.); typically along the edges of the meso-scale
features (an example is shown in figure 2a–c).
Nine turtles reached 358N latitude (n¼9) in 11– 19 days
from their release off southeast Florida while travelling in the
Gulf Stream. Turtles continuing in the Gyre reached waters
south of Georges Banks in 20–30 days, and waters off the
Grand Banks (approx. 458N, approx. 538W) in 50–70 days.
One turtle travelled to a point west of the Azores in 219 days
prior to tag transmission cessation (figure 1a).
(b) Thermal ecology and vertical habitat use
Argos location code accuracy, transmitter solar charging rates
and temperature sensor data combine to allow for niche
characterization. The majority of Argos location codes
received were high quality: 77.2% of messages had high
LCs of 0–3 (figure 3a). All tags maintained adequate oper-
ational charges (greater than 3.2 V) and optimal mean
charges (greater than or equal to 4.0 V) throughout their
transmission periods (figure 3b). These data combine to
suggest that turtles were remaining at the sea surface. The
tags’ high Argos location accuracy confirms that tag antennae
were exposed to air and in communication with overhead
satellites. Satellite-derived average daily SST encountered
by the turtles was 20.8 +3.48C and was similar to model-
derived HYCOM average daily temperature of 21.4+3.48C
(figure 3c). Internal tag temperature sensors recorded ambient
temperatures that ranged from 178Cto358C (mean: 25.6+
3.78C; figure 3c)—consistently averaging 4– 68C higher than
temperatures derived from remote satellite data or HYCOM.
This difference was statistically significant (Mann – Whitney
U;p,0.05). Optimal battery charging and the higher tag-
recorded ambient temperatures compared with satellite- and
HYCOM-derived SST data suggest that the tags’ solar cells
were exposed to the sun’s rays [7].
Reflectivity of the Sargassum (10–14%), tags (6–11%) and
loggerhead turtle shells (7%) did not differ substantially.
Empirical tests of seawater surface temperatures within buck-
ets containing Sargassum mats versus plain seawater confirm
the assumption that water temperature near the surface
within Sargassum was consistently warmer when exposed to
sunlight than in seawater without Sargassum (figure 4a,b).
The Sargassum intercepted more solar radiation just below
the surface where the water was consistently warmer than in
the seawater bucket without Sargassum (figure 4a). The water
column halfway to the bottom in the bucket without the Sargas-
sum was warmer than the water temperature in the middle of
the Sargassum bucket because more sunlight reaches deeper
when there is no Sargassum to intercept it (e.g. figure 4b). As
part of the Sargassum was above the water surface, night-time
evaporation cooled it below the temperature of the seawater
bucket during non-daylight hours.
4. Discussion
Our study provides the first successful satellite tracks for any
neonate sea turtle. We also provide the first long-term empiri-
cal and in situ tracking data to characterize neonate loggerhead
oceanic movements and surface habitat use. Tracked turtles
rarely occupied Continental Shelf waters, supporting the log-
gerhead oceanic nursery paradigm. The turtles’ tracks were
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3

Table 1. Metadata for tracked turtles including turtle ID, straight carapace length (SCL) and total weight at time of release, sex, age of turtle, hatch date, release date, release location and track duration, and the number of days
associated with each current feature: Gulf Stream, eddy feature and other. The Gulf Stream is defined as the main part of Gulf Stream not including eddy features. Eddy feature is defined as any current vector group forming a circular
pattern. ‘Other’ includes the remaining locations (and days) not included in previous Gulf Stream or eddy groups.
turtle ID SCL (mm) weight (g) sex age (days) hatch date release date release location
track duration
(days)
Gulf
Stream (days) eddy (days) other (days) % eddy
92590a 150 577 F 247 04 Sep 2008 09 May 2009 26.883 N, 79.883 W 80 26 54 0 68
92584a 149.5 537 F 214 07 Oct 2008 09 May 2009 26.883 N, 79.883 W 39 19 20 0 51
92585a 182.8 615 F 251 04 Oct 2008 12 June 2009 26.829 N, 79.823 W 58 22 36 0 62
92587a 163 720.5 F 251 04 Oct 2008 12 June 2009 26.829 N, 79.823 W 56 15 41 0 73
92588a 169 692 F 281 04 Sep 2008 12 June 2009 26.829 N, 79.823 W 49 17 32 0 65
92586a 133.5 364 M 127 10 Aug 2009 15 Dec 2009 26.761 N, 79.823 W 39 17 22 0 56
92589a 146 475 F 127 10 Aug 2009 15 Dec 2009 26.761 N, 79.823 W 171 12 129 30 75
85512 117 314.8 F 109 30 June 2010 18 Oct 2010 26.292 N, 79.683 W 32 28 4 0 13
92585 121 309.5 F 109 30 June 2010 18 Oct 2010 26.292 N, 79.683 W 65 23 42 0 65
92586 129.9 315.4 F 114 11 July 2010 02 Nov 2010 26.761 N, 79.856 W 71 18 53 0 75
92587 132.8 338.3 F 127 28 June 2010 02 Nov 2010 26.761 N, 79.856 W 220 45 128 47 58
92590 124 329.6 F 117 08 July 2010 02 Nov 2010 26.761 N, 79.856 W 93 36 57 0 61
85511 114.8 297.3 F 141 14 July 2010 02 Dec 2010 26.738 N, 79.510 W 27 9 18 0 67
92588 124.9 339.5 F 168 20 July 2010 04 Jan 2011 26.750 N, 79.783 W 74 8 60 6 81
85513 119.5 328.3 F 173 14 July 2010 04 Jan 2011 26.750 N, 79.783 W 169 20 123 26 73
85514 122.6 337.2 F 155 02 Aug 2010 04 Jan 2011 26.750 N, 79.783 W 127 26 101 0 80
92584 133.3 390.6 F 168 20 July 2010 04 Jan 2011 26.750 N, 79.783 W 102 17 62 23 61
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distinctly constrained east of the 200 m isobath, along the outer
edge of the Continental Shelf. This off-shelf demarcation differs
from the mostly on-shelf distributions and habitats used by
larger, older, juvenile or sub-adult and adult loggerheads in
western Atlantic waters [29].
Turtles travelled more variable routes than implied by the
classic Gyre dispersal hypotheses [1,4]; rather, the turtles’
routes are more consistent with, and help validate, theoretical
migratory trajectories derived by Putman et al. [30] using
oceanographic models and experimentally derived navigation
behaviour. While the GulfStream provided initial transport, tur-
tles did not select the fastest or most direct routes to known
oceanic developmental habitats (e.g. the Azores, Madeira or
Cape Verde). Turtles instead travelled along net clockwise trajec-
tories using a variety of paths. These paths indicate that dispersal
is not uniformly unidirectional; deviations from outer Gyre cur-
rents and boundaries are common, invalidating previous
hypotheses assuming a unidirectional developmental migration
route following or entrained within the currents of the NASG.
The paths of our tracked neonate loggerheads were
environmentally constrained. No turtles moved into lethally
cold waters. Among the fraction of turtle tracks associated
with the Gulf Stream or NASG current regime, net directio-
nal movements were consistent with the use of regional
guideposts (e.g. magnetic cues) to orient the turtles along
hypothesized routes [22]. Consistent with Lohmann &
Lohmann [22], turtles did not travel beyond the constraints
of the outer Gyre boundaries; orientation on a macro scale
to remain within the Gyre boundaries is likely. However,
deviations from the Gyre currents were oriented towards
the interior of the Gyre. Turtles’ paths in and out of the
Gulf Stream and Gyre currents as well as turtles’ association
with meso-scale eddies imply that localized (‘micro-scale’)
orientation and the duration of the turtles’ travels or regions
they encounter can vary.
Some of our tracked turtles left the Gulf Stream and travelled
into the Sargasso Sea, a behaviour that might be explained by the
seasonal distribution of Sargassum in the northwestern Atlantic.
Sargassum travels from the Gulf of Mexico along the eastern USA
coast to the northwestern Atlantic before settling to the south as
epipelagic mats in the Sargasso Sea [31]. Oceanic-stage turtles
may opportunistically remain with those Sargassum habitats,
leaving the Gyre currents and instead exploiting favourable
foraging and thermal niches within the Sargasso Sea.
Neonate loggerheads can travel from southeast Florida to
Azorean waters in less than a year, somewhat faster (e.g.
approx. 220 days in the case of one turtle) than Carr’s [4]
drift bottle hypothesis suggests (235 days) despite the poten-
tial of slowed movement owing to tag effects (such as
hydrodynamic drag). Mansfield et al. [7] demonstrate that
biologically significant costs to the turtles from tag effects
are minimal. This work compared neonate turtles with and
without tags under controlled laboratory conditions over a
period of several months. Mansfield et al. [7] found no signifi-
cant differences in growth, condition, swimming behaviour
and feeding among the test groups, suggesting that there
are minimal energetic costs to the turtles due to the hydro-
dynamic effects of the tags [7]. Potential drag effects were
75° W
Cape
Hatteras, NC Azores
depth (m)
temperature (°C)
10 15 20 25 30
–6000 –5000 –4000 –3000 –2000 –1000 0
60° W 45° W 30° W 15° W
75° W 60° W 45° W 30° W 15° W
75° W 60° W 45° W 30° W 15° W
20° N
30° N
40° N
50° N
20° N
30° N
40° N
50° N
20° N
30° N
40° N
50° N
(a)
(b)
20° N
30° N
40° N
50° N
Figure 1. (a,b) Satellite tracks of neonate loggerhead sea turtles (109–281 days old) overlaid with bathymetric Gridded Global Relief Data, ETOPO2v2 (figure 1a;
turtle tracks in white) and composite SST data (figure 1b; turtle tracks in black). Photo credit for figure 1 (b): J. Abernethy (2009).
rspb.royalsocietypublishing.org Proc. R. Soc. B 281: 20133039
5

mitigated by creating a teardrop attachment shape and pla-
cing the tags behind and between vertebral ‘spikes’. Using
the techniques developed by Jones et al. [32], estimated
drag may range as low as 4% or as high as 10% depending
on turtle size (T. Jones 2014, personal communication). How-
ever, it is important to note that these estimates assume
laminar flow conditions. The surface-based habitat occupied
by the turtles is one in which the tags are likely to be out
of the water, exposed to air, so that drag may be further mini-
mized by two orders of magnitude [33,34]. There also
remains the hypothesis that oceanic-stage sea turtles are
passive drifters [1,4]. Working on the assumption that
oceanic-stage turtles are, at a minimum, part-time passive
drifters, then the net energetic cost due to drag would be
further reduced. Finally, the in-water behaviour of the turtles
tracked in this study was similar to that of larger, wild-caught
turtles, suggesting some degree of natural behaviour. Specifi-
cally, the neonates tracked in this study showed a similar
association with meso-scale eddies as larger, wild-caught
subadult or neritic juveniles satellite tracked in the western
Atlantic ocean [35].
By occupying oceanic surface layers, young turtles
probably receive thermal benefits from solar absorption—
either directly via their carapace at the air–sea interface, or
indirectly through association with Sargassum or other flotsam.
Tag sensor data, coupled with solar reflectivity tests, suggest
that turtles are indeed occupying this air– sea interface, thus
bolstering the thermal niche hypothesis. The tags’ high Argos
location accuracy confirms that tag antennae were exposed to
air and in regular communication with overhead satellites.
Optimal tag battery charges indicate that the tags’ solar cells
were exposed to the sun’s rays [7]. The difference in the tags’
recorded ambient temperatures compared with satellite-
and HYCOM-derived SST data (4– 68C) could be due to
(i) biases owing to location, model and data resolution or
error, and/or (ii) the thermal effects sun exposure. Surface sea-
water where neonate turtles are typically found [36] is highly
transparent. Solar energy may be dissipated over a substantial
vertical depth. However, Sargassum, neonate sea turtles and
satellite transmitters have zero transparency and low reflectiv-
ity; thus, the absorption of solar radiation is concentrated near
their respective surfaces. Furthermore, floating Sargassum mat
structure impedes lateral water flow, thereby inhibiting con-
vective transport of absorbed solar energy (heat) into the
surrounding water. Under these conditions, energy retention
can raise local water temperatures up to 68C above that of
surrounding water, as observed by ambient tag sensor data
and as our bucket experiments demonstrated (e.g. figure 4a).
The thermal benefits that small sea turtles gain from remain-
ing at the seasurface or associating with Sargassumcommunities
probably differs somewhat from basking—a common thermo-
regulatory behaviour used by reptiles. Atmospheric basking
(out of water) is common among reptiles, including turtles,
for thermal regulation, as well as enhanced digestive efficiency,
80° W 75° W 70° W 65° W
80° W
35° N
30° N
35° N
28/12/2009
28/12/2009
Bermuda
10 15 20
temperature (°C)
25 30
28/5/2010
Cape Hatteras
30° N
75° W 70° W 65° W
(a)
(b) (c)
Figure 2. (a) Track of turtle (ID 92589_2009, red dots) overlaying temperature and current vectors from HYCOM model output on 28 December 2009. (b) A close-up
view of track positions around 28 December showing two eddy features: one clockwise at lower left, the other counter-clockwise on the right. (c) A close-up view of
track positions around 28 May 2010 showing the turtle between three eddy features: one anti-clockwise on the left, one on the bottom also anti-clockwise, and the
other clockwise on the top.
rspb.royalsocietypublishing.org Proc. R. Soc. B 281: 20133039
6

epibiont control and enhanced vitamin D synthesis [37,38].
A common feature of turtle basking, generally, is that it is
episodic (not chronic as in the case of neonate turtles at the
sea surface), being initiated when temperatures approach
operative environmental temperatures [37,38]. Atmospheric
basking is known in some populations of Chelonia mydas and
results in increased body temperatures [39,40]. A single study
of Caretta caretta found that loggerhead internal temperatures
were higher when basking at the water’s surface during sunny
periods [41]. The authors attributed this increase in body temp-
erature to increased absorption of solar radiation [42].
If exposed to sunlight, turtles’ shells will be likely to gain
some degree of warmth. A surface-based, thermally driven
developmental niche makes sense in a broader evolutionary
context. Sea turtles exhibit a number of traits that natural
selection probably acted on for a surface-based thermal
developmental niche to have evolved. Sargassum habitats pro-
vide young, cold-blooded turtles with a thermal environment
that promotes growth, eventually reducing the assemblage of
predators capable of consuming them. Sea turtles are
ectotherms; exposure to cooler habitats tends to reduce
rates of food consumption and individual growth compared
to exposure to warmer environments. Thermal differences
and chronic or acute exposure to unfavourable temperatures
can influence age and size at maturity in turtles [42,43].
Thermal habitat availability and early exposure to thermally
beneficial developmental habitats probably has broad impli-
cations for age (or size) at neritic recruitment, particularly
within different ocean basins or relative to different sea
turtle rookeries. Exposure to UV radiation enhances reptilian
25
20
15
frequency (%)
10
5
321
ARGOS location codes
0ABZ
mean daily charge:
avg. charge per day (V)
track day
0
3
4
5
50 100 150 200
4.1 ± 0.1 V s.d.
range: 3.7 –4.2 V
daily temperature (C) versus hour of day by data source
daily temperature (°C)
hour of da
y
data source
HYCOM
STAT
tag sensor
0
27
26
25
24
23
22
21
20
19
5 101520
(a)
(b)
(c)
Figure 3. (a) Frequency (%) of Argos location codes reported with satellite
track locations from neonate loggerhead sea turtles released in the western
Atlantic. (b) Average daily solar cell charge (volt) reported from satellite tags
(n¼17) deployed on neonate loggerhead sea turtles. Operational charge
level (per manufacturer specifications) is represented by a black line
(3.0 V) at the bottom of the graph; manufacturer-specified optimal charge
(4.0V)isrepresentedbyaredline,andmeandailyobservedcharge(4.1+
0.1 V s.d.) is represented by a blue line. (c) Daily temperatures (8C) derived from
satellite tag sensors (red line), HYCOM model (dotted line) and satellite-derived
SSTs (STAT; dot-dashed line).
TSargassum –Tseawater2.5 cm below the surface
start time = 1 pm, 29 Dec 2012
temperature difference (°C)
hour of da
y
0
8
(a)
(b)
6
4
2
0
–2
temperature difference (°C)
1
0
–1
–2
–4
–6
–8
20 40 60 10080
cumulative hours
middle of water column
TSargassum–Tseawater
0 20 40 60 10080
Figure 4. (a,b) Seawater temperature differences within paired containers of
Sargassum mat versus plain seawater.
rspb.royalsocietypublishing.org Proc. R. Soc. B 281: 20133039
7

calcium-dependent functions, including vitamin D-associated
skeletal mineralization and growth [44]. Life at the surface
also exposes turtles to airborne cues that can lead to the
next patch of productive ocean [45]. Localized warming
by only a few degrees can have significant impacts on
temperature-dependent processes in reptiles, including diges-
tion, growth and activity time. Availability of thermally
beneficial habitat early in life can have important long-term
impacts on the survival and fitness of sea turtles.
By combining persistent sea-surface-based behaviour
with oceanic-stage turtles’ known association with Sargassum,
we propose a new thermal niche hypothesis and possible
mechanistic framework for why the sea surface and Sargas-
sum habitats are important for the development and likely
survival of oceanic-stage sea turtles.
All research was conducted in compliance with the protected species
laws of the United States and under Florida Atlantic University
IACUC approval (A08–40), Florida Marine Turtle Permit (MTP-
073) and US Fish and Wildlife Service Permits (USFWC-TE05217-2).
Acknowledgements. We especially thank M. Marrero, A. Stiles, S. Epperly,
N. Thompson, C. Gonzales, L. Bachler, C. Mott, R. Matyisin,
E. Stenersen, E. Wood, J. Abernethy, Jim Abernethy’s Scuba-Adventures,
K. Phillips, K. Rusenko, The Gumbo Limbo Nature Center, Michael
Coyne and seaturtle.org. We also thank K. Lohmann, B. Wallace,
S. Maxwell and S. Vogel for constructive discussion.
Data accessibility. Datafrom this study are archived online using seaturtle.
org’s STAT (http://www.seaturtle.org/tracking/?project_id=717).
These data are directly linked to the OBIS-SEAMAP data repository
(http://seamap.env.duke.edu/provider/STAT).
Funding statement. Funding for this research was provided by the Large
Pelagics Research Center Extramural Grants Program, NOAA Fish-
eries, Southeast Fisheries Science Center, Florida Sea Turtle Grants
Program, Save Our Seas Foundation, Disney Wildlife Conservation
Fund, the National Academies Research Associateship Program, the
Ashwanden Family Fund, the Nelligan Sea Turtle Research Support
Fund and personal funds. Funding for J. Luo was provided by the
Robertson Foundation and the Bonefish & Tarpon Trust.
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