ENDANGERED SPECIES RESEARCH
Endang Species Res
Vol. 32: 89–101, 2017
doi: 10.3354/esr00793
Published February 7
INTRODUCTION
Delineating high-use areas for marine species is
critical for identifying essential habitat and times of
the year when vulnerable species may warrant
increased levels of protection. However, it is chal-
lenging to delineate such high-use areas for highly
mobile marine species. Satellite telemetry affords the
opportunity to reveal location-based habitat-use pat-
terns for these species (Godley et al. 2008, Hart &
Hyrenbach 2009), and a growing body of literature
indicates that this tool continues to evolve, allowing
researchers to address increasingly complex ques-
tions for many cryptic vertebrates of conservation
concern. To date, the global use of satellite tracking
in marine turtle studies has revealed much about
their spatial use of breeding areas, migration zones,
and foraging sites (Schofield et al. 2009, Fossette et
© The authors and (outside the USA) the US Government 2017.
Open Access under Creative Commons by Attribution Licence.
Use, distribution and reproduction are un restricted. Authors and
original publication must be credited.
Publisher: Inter-Research · www.int-res.com
*Corresponding author: kristen_hart@usgs.gov
Resident areas and migrations of female green
turtles nesting at Buck Island Reef National
Monument, St. Croix, US Virgin Islands
Kristen M. Hart1,*, Autumn R. Iverson2, Allison M. Benscoter1, Ikuko Fujisaki3,
Michael S. Cherkiss1, Clayton Pollock4, Ian Lundgren4, Zandy Hillis-Starr4
1US Geological Survey, Wetland and Aquatic Research Center, Davie, FL 33314, USA
2CNT, contracted to US Geological Survey, Wetland and Aquatic Research Center, Davie, FL 33314, USA
3University of Florida, Ft. Lauderdale Research and Education Center, Davie, FL 33314, USA
4National Park Service, Buck Island Reef National Monument, Christiansted, US Virgin Islands, USA
ABSTRACT: Satellite tracking studies can reveal much about turtles’ spatial use of breeding
areas, migration zones, and foraging sites. We assessed spatial habitat-use patterns of 10 adult
female green turtles Chelonia mydas nesting at Buck Island Reef National Monument (BIRNM),
US Virgin Islands (USVI; 17° 47.4’ N, 64° 37.2’ W) from 2011 to 2014. Turtles ranged in size from
89.0 to 115.9 cm curved carapace length (CCL) (x− ± SD: 106.8 ± 7.7 cm). The inter-nesting period
for all turtles ranged from 31 July to 4 November, and sizes of the 50% core-use areas ranged from
4.2 to 19.0 km2. We observed consistency of inter-nesting habitat-use patterns, with all turtles
using near-shore (<1.5 km), shallow waters (<–20 m depth) within approximately 10 km of Buck
Island. Seven of the 10 turtles remained locally resident after the nesting season; 5 turtles (50%)
established resident foraging areas around Buck Island, 2 established resident foraging areas
around the island of St. Croix, and the other 3 (30%) made longer-distance migrations to Antigua,
St. Kitts & Nevis, and Venezuela. This is the first empirical dataset showing limited migration and
use of ‘local’ resources after the nesting season in the USVI by this unique management unit of
green turtles. Five of the turtles had resident foraging area centroids within protected areas; thus,
inter-nesting and foraging areas at BIRNM that overlap with human use zones present an impor-
tant management concern. Delineating spatial areas and identifying temporal periods of near-
shore habitat use can be useful for natural resource managers with responsibility for overseeing
vulnerable habitats and protecting marine turtle populations.
KEY WORDS: Chelonia mydas · Green sea turtle · Inter-nesting · Foraging · Switching state-space
model · Kernel density estimation · Migration · Minimum convex polygon
OPEN
ACCESS
Endang Species Res 32: 89–101, 2017
al. 2010, Shillinger et al. 2010, Hart et al. 2014). Cou-
pled with recent advances in analytical modeling
techniques such as switching state-space modeling
(SSM; see Jonsen et al. 2003, 2005, 2006, 2007, Pat-
terson et al. 2008, Hoenner et al. 2012), tracking data
can provide an unprecedented window into behav-
ioral modes of marine turtles (i.e. directed movement
or migration, area restricted search or foraging; see
Bailey et al. 2008, 2009, Hart et al. 2015).
Green turtles Chelonia mydas (Linnaeus, 1758) use
nesting and foraging sites throughout tropical and
subtropical zones (Hirth 1997, Seminoff 2004). In the
Caribbean, at approximately 2 yr intervals, mature
female green turtles breed and lay eggs at natal
beaches roughly every 2 wk from July through No-
vember (Carr et al. 1974, Allard et al. 1994, Broderick
et al. 2002, 2007, Plotkin 2003), then return to distinct
foraging areas (Limpus et al. 1992, Broderick et al.
2007). Although green turtles can migrate large dis-
tances (i.e. 1000s of km) between nesting beaches and
foraging sites (Mortimer & Portier 1989, Plotkin 2003),
recent evidence of non-migratory behavior has emerged
through satellite-tracking studies (Whiting et al. 2007,
Hart et al. 2013, Esteban et al. 2015). In their role as
herbivores, green turtles consume seagrasses and al-
gae, which consequently maintains the structure and
productivity of seagrass pastures (Thayer et al. 1982,
1984, Zieman et al. 1984, Moran & Bjorndal 2005) and
coral reefs (Goatley et al. 2012).
Listed as Endangered by the International Union
for the Conservation of Nature (Groombridge 1982,
Groombridge & Luxmoore 1989, Seminoff 2004), and
threatened under the US Endangered Species Act,
Caribbean green turtles are part of the North At -
lantic distinct population segment (DPS), yet little is
known about the species in the US Virgin Islands
(USVI). Analysis of mitogenomic haplotype frequen-
cies from rookeries within the southern Greater Car-
ibbean region indicates that green turtles at Buck
Island Reef National Monument (BIRNM) in St. Croix
warrant recognition as a distinct management unit
that is part of the larger USVI stock (Shamblin et al.
2012).
Much reduced from historic numbers (Jackson
1997, McClenachan et al. 2006), green turtle popula-
tion numbers in Caribbean rookeries are low, but re -
covering. Three separate long-term saturation nest-
ing programs in St. Croix (The Nature Conservancy
[TNC] at East End Marine Park [EEMP], National
Park Service at BIRNM, and US Fish and Wildlife
Service [USFWS] at Sandy Point) have documented
increases in nesting green turtle numbers since the
1990s, but annual numbers of nesting individuals at
each of these 3 main sites are only in the 20s (e.g. 26
in 2015 at BIRNM; C. Pollock pers. obs.). An increase
in seagrass distribution at BIRNM (Ken dall et al.
2004a,b), federal protection for green turtles in the
1970s (NMFS & USFWS 1991), a decrease in poach-
ing, and the removal of mongoose from BIRNM nest-
ing beaches in the 1980s all provided in creased habi-
tat security for green turtles at BIRNM. By 1995, the
number of nesting green turtles at BIRNM had in -
creased, followed by increased numbers at East End
beaches (K. Lewis [TNC] pers. comm.).
Currently, critical habitat for green turtles is delin-
eated in waters surrounding Culebra Island, Puerto
Rico (NOAA 1998), and there is a pending proposal
to list 11 DPSs of green turtles as endangered or
threatened, with a revision of current listings (NMFS
& USFWS 2015, March Federal Register). Under-
standing the specific movement ecology for different
DPSs can help inform conservation efforts targeted
to wards those population segments. Further, under-
standing green turtle movement patterns in protec -
ted areas is considered a priority for ongoing conser-
vation efforts and Federal recovery plans (NMFS &
USFWS 1991, Hart et al. 2013). However, green turtle
spatial habitat use within BIRNM waters has not
been assessed.
To assess spatial habitat-use patterns of green tur-
tles nesting at BIRNM, our goals were to delineate
zones used during inter-nesting periods, define any
migration paths after the nesting season, and identify
foraging sites where turtles remain resident. Where
possible, we quantified spatial overlap of habitat use
for individuals at inter-nesting areas and foraging
sites. To glean possible reasons behind spatial selec-
tion of habitats and whether resident areas were
selected by turtles of a specific size or experience
level, we quantified ecological and spatial correlates
such as the relationship between turtle size and
bathymetry values at inter-nesting areas, character-
ized the habitats associated with foraging areas, and
determined whether habitat overlap varied depend-
ing on nesting ‘experience’ (i.e. ‘neophyte’ for first
time nester vs. ‘re-migrant’ status). Finally, we dis-
cuss green turtle habitat use in relation to nearby
marine protected areas (MPAs).
MATERIALS AND METHODS
Study sites
Sampling and tagging of green turtles occurred
from 15 July to 30 September annually from 2011 to
90
Hart et al.: Buck Island green turtle habitat
2014 in the USVI at BIRNM (17° 47.4’ N, 64° 37.2’ W),
which includes a 0.71 km2 uninhabited island (Buck
Island) located on the shallow St. Croix shelf (depth
range approx. –15 to –20 m), 2.4 km northeast of the
island of St. Croix (see Fig. 1). Buck Island is 1.82 km
long and 0.8 km wide, and rises 103 m above sea
level at the highest peak. With the expansion of the
monument boundaries in 2001, the amount of sub-
merged lands around Buck Island now covers
76.3 km2. BIRNM is a nesting and foraging area for
loggerhead Caretta caretta, leatherback Dermochelys
coriacea, hawksbill Eretmochelys imbricata, and
green sea turtles.
Turtle capture and transmitter deployment
Nightly surveys were conducted from 19:00 to
05:00 h local time. Turtle interception and tagging
followed methods similar to those of Hart et al.
(2013); turtles were documented and fitted with trans-
mitters using established protocols (NMFS-SEFSC
2008). Briefly, female green turtles were intercepted
on the beach after nesting. Immediately after mark-
ing each turtle with Inconel and PIT tags, standard
carapace measurements were recorded, in cluding
curved carapace length (CCL). We adhered platform
transmitter terminals (PTTs; Wildlife Computers
SPOT5s; length × width × height: 7.2 × 5.4 × 2.4 cm,
mass: 119 g in air) using slow-curing epoxy. Attach-
ment materials were streamlined to minimize the
epoxy footprint and potential buoyancy and/or drag
effects on turtle swimming ability. Each tag was pro-
grammed to transmit 24 h d−1.
Sea turtle tracking and switching
state-space modeling
Location data were retrieved using Satellite Track-
ing and Analysis Tool (STAT; Coyne & Godley 2005)
available at www.seaturtle.org. Location classes (LCs)
3, 2, 1, 0, A, and B were used to assess the position of
the turtles during the tracking period, and served as
the location data to characterize inter-nesting, for -
aging, and migration behavior of each turtle. We
used switching state-space modeling (SSM; Jonsen
et al. 2003, Patterson et al. 2008) as described in Jon-
sen et al. (2005) to assess the fine-scale behavioral
modes of individual green sea turtle tracks that
originally nested in BIRNM. Switching SSM tech-
niques followed our previous studies (see Hart et al.
2013, 2014, 2015, Shaver et al. 2013, 2016 and the
Supplement at www. int-res. com/ articles/ suppl/ n032
p089 _ supp. pdf for general information on this tech-
nique). Earlier applications defined a binary behav-
ioral mode, categorized as either ‘foraging’ or ‘migra-
tion’ (Jonsen et al. 2005, 2007, Breed et al. 2009);
however, since we tagged animals during the nesting
season, behavioral modes predicted by the SSM
algorithm were defined as either ‘inter-nesting or for-
aging’ or ‘migration’.
We summarized data acquired until the transmit-
ters stopped sending information or until the time of
data synthesis: 13 November 2014. We used the SSM
approach to determine the beginning and end date of
the inter-nesting, migration, and foraging behavioral
periods for each turtle. After assigning the beginning
and end dates of each behavioral mode for each
turtle track, we used the original satellite locations
within those time periods for all further analyses.
Inter-nesting and foraging periods
We verified SSM behavioral modes both spatially
and temporally using the satellite data. Inter-nesting
periods occurred before migration to foraging
grounds. Some turtles exhibited local foraging be -
havior around Buck Island, and also did not show a
distinct migration period. Nesting activity data for
this species on Buck Island indicates that the end of
October is the end of the inter-nesting season,
based on the last beach encounters for 9 nesting
green sea turtles from 1995 to 2013. Therefore, a
temporal cutoff of 31 October was applied to differ-
entiate between inter-nesting and foraging behav-
ior for the turtles that did not migrate. From the
satellite data during the inter-nesting and foraging
periods, we filtered out locations that represented
movement speeds >5 km h−1, locations on land, and
very distant spatial locations (>120 km from the
nearest valid point). We also filtered out points asso-
ciated with ocean water depths deeper than –200 m
(neritic zone delineation). Green turtles have been
found to stay in shallow waters during residency
times (see Meylan 1995), remaining in mean depths
shallower than –10 m in the Mediterranean (Hays et
al. 2002) and the Gulf of Mexico (Hart et al. 2013),
and shallower than –25 m in the Caribbean (Esteban
et al. 2015). For all tracks in this study, the removed
locations deeper than –200 m comprised less than
7.0% of available speed-filtered loca tions. For
bathymetry, we used the ETOPO1 global relief mo -
del (bedrock, cell-registered, 1 arc-minute; Amante
& Eakins 2009).
91
Endang Species Res 32: 89–101, 2017
Characterization of inter-nesting
and foraging areas
After assigning the tracking data to inter-nesting
and foraging behavioral modes, we quantified inter-
nesting and foraging home ranges using kernel
density estimation (KDE) and minimum convex poly-
gon (MCP) analyses. To minimize autocorrelation of
points, we generated mean daily locations (MDL)
within each inter-nesting and foraging period in the
software program R version 3.1.2 (R Development
Core Team 2014) from the filtered satellite locations.
We used MDLs (when n ≥ 20) for KDE analysis and
filtered satellite locations (when MDLs < 20) for 95%
MCPs.
Kernel density is a non-parametric method used
to identify one or more areas of disproportionately
heavy use (i.e. core areas) within a home-range
boundary (Worton 1987, 1989, White & Garrott 1990),
with appropriate weighting of outlying observations.
We used the software program R and the package
‘adehabitatHR’ (Calenge 2006) to calculate home
range analyses. We applied the fixed-kernel least-
squares cross-validation smoothing factor (hcv) for
each KDE (Worton 1995, Seaman & Powell 1996).
Following Walcott et al. (2012), we used 95% of
points to create MCP polygons, as it is possible for a
proportion of distant filtered locations to represent
infrequent movements or explorations external to the
home range (sensu Burt 1943, Rodgers & Kie 2011).
When the standard deviations of the x and y coordi-
nates were unequal (<0.5 or >1.5), data were re-
scaled prior to home range calculations by dividing
the coordinates by their standard deviation (follow-
ing Seaman & Powell 1996). We used ArcGIS 10.2
(ESRI 2013) to plot the data and to calculate the area
(km2) within each kernel density contour (50 and
95%) and each MCP. The 95% KDEs were used to
represent the overall home range, and the 50% KDEs
represented the core area of activity (Hooge et al.
2001).
We tested location data for, and quantified site
fidelity to, the in-water inter-nesting and foraging
locations using the Animal Movement Analysis
Extension for ArcView GIS 3.3 (ESRI 2002). Using
Monte Carlo random walk simulations (100 repli-
cates), we tested tracks during each inter-nesting
and foraging period for spatial randomness against
randomly generated walks (Hooge et al. 2001). We
bounded the range for random walks from −200 to
0 m bathymetry to encompass all filtered locations,
and smoothed the bounding polygon with a 250 m
inland and 5 km seaward buffer to account for
numerous small bays, and allow for the generation of
random walks with points in close proximity to land
(the 5 km seaward border was extended an addi-
tional 5 km for one turtle track: Turtle ID 7 inter-nest-
ing). Tracks exhibiting site fidelity indicate move-
ments that are more spatially constrained rather than
randomly dispersed (Hooge et al. 2001); any tracks
that failed site fidelity were not analyzed in the home
range analyses.
We also calculated the centroid of each 50% KDE
and MCP; if a 50% KDE included multiple activity
centers, we calculated the centroid for the largest
activity center. We extracted bathymetry depths for
all centroids and the distance from each centroid to
the nearest land.
Turtle inter-nesting and foraging days per grid cell
To depict the inter-nesting and foraging locations
used by the turtles over time, we calculated the num-
ber of turtle inter-nesting days and foraging days in
grid cells (2 × 2 km). For each turtle track containing
days in either inter-nesting or foraging mode (re -
gardless of whether a KDE or MCP was calculated),
we determined the number of days that each turtle
was recorded in each grid cell (turtle days) using all
filtered satellite locations, and summed the number
of turtle days across all turtles for each grid cell. We
derived mean bathymetry in each 2 km grid cell and
examined associations between the turtle days and
bathymetry for each inter-nesting and foraging
period.
Core area space-use sharing
We calculated the amount of home range overlap
for the core-use areas (50% KDEs) during inter-nest-
ing and foraging using the ‘adehabitatHR’ package
in R (Calenge 2006, R Development Core Team 2014)
to quantify the extent to which green turtles share
their home ranges. The utilization distribution over-
lap index (UDOI) is a non-directional joint measure
that is a function of the product of the 2 utilization
distributions (UDs), a modification of Hurlbert’s
(1978) E/Euniform statistic to allow for continuous spa-
tial UDs, and is the most appropriate statistic for
measuring space-use sharing between animals. The
UDOI is zero for 2 UDs without overlap, and 1 for uni-
formly distributed UDs with complete overlap; how-
ever, UDOI can be >1 for non-uniformly distributed
UDs with a high degree of overlap, indicative of
92
Hart et al.: Buck Island green turtle habitat
higher than normal overlap relative to uniform space
use (Fieberg & Kochanny 2005).
We calculated UDOI space-use sharing for all
turtles during inter-nesting (n = 10 turtles, 45 pairs),
because KDEs were calculated for each turtle. Of the
5 turtles for which foraging KDEs were determined,
we calculated UDOI space-use sharing for the 3 tur-
tles that foraged in the same area around Buck Island
(n = 3 pairs). Four turtles had foraging MCPs; one
was located near Antigua, but for the others we cal-
culated the distance between same-region centroids.
This included 2 turtles foraging SW of St. Croix and 1
turtle foraging in the Buck Island area (we deter-
mined the mean distance from this MCP centroid to
the 3 KDE centroids). We also determined the level of
temporal overlap (days) that same-region turtle pairs
had during foraging. For inter-nesting, we conducted
a non-parametric Mann-Whitney rank sum test to
determine whether there was a difference in space-
use sharing between turtle pairs with and without
neophytes in order to examine whether newly nest-
ing turtles shared inter-nesting habitat with experi-
enced nesters any differently than experienced pairs
shared habitat. We classified any turtles that were 1st
time nesters at BIRNM as neophytes.
Ecological and spatial correlates
For the inter-nesting periods, we conducted linear
regression analyses to determine whether there was
a relationship between turtle size (i.e. CCL) and the
size of the 50% KDE, the bathymetry values at the
50% KDE, and the bathymetry values at the MDLs;
additionally, we conducted linear regression analy-
ses to examine whether there was a relationship
between the number of tracking days and the size of
the 50% KDE using SigmaPlot (Systat Software
2012). For turtles foraging around Buck Island and St.
Croix (n = 6), we determined the benthic habitat type
located at the foraging centroids using NOAA ben-
thic habitat maps of Puerto Rico and the USVI
(Kendall et al. 2001).
Migration periods
We summarized the primary migration periods that
represented movement away from the inter-nesting
area to the foraging grounds; these periods occurred
after inter-nesting and directly before the foraging
period. After the migration periods were assigned,
they were verified both spatially and temporally
using the satellite location data. We filtered out satel-
lite locations during the primary migration periods
representing movement speeds >5 km h−1, locations
on land, and very distant spatial locations (>120 km
from the nearest valid point). We quantified the num-
ber of days in the primary migration period, the
straight-line migration distance, the migration path
distance (when applicable), and the depth along
migration paths. Some turtles in our study did not
exhibit a migration path (e.g. turtles that foraged
locally near Buck Island), and therefore only the
straight-line migration distance was quantified, cal-
culated as the distance between inter-nesting and
foraging centroids. For Turtle ID 8, the mean center
of the filtered foraging satellite locations was quanti-
fied and used for the straight-line distance calcula-
tion, because this turtle did not exhibit site fidelity
and therefore no centroid was calculated.
RESULTS
Turtle size and tracking duration
We tagged 10 adult female green turtles over a 4 yr
period between 2011 and 2014 (Table 1). Turtles
ranged in size from 89.0 to 115.9 cm CCL (Table 1). In
a total of 1681 tracking days across all turtles, in -
dividual turtle tracking durations ranged from 100
to 372 d (Table 1). Turtles were a mix of neophytes
(n = 2) and re-migrants (n = 8), with varied nesting
histories since 1995 (see Table S1 in the Supplement
at www. int-res.com/ articles/ suppl/ n032 p089 _ supp.
pdf).
In-water inter-nesting areas
We obtained SSM results for all 10 turtles (see Fig.
S1, Table S2 for example SSM prediction paths and
model parameters). We tagged turtles early enough
in each nesting season to track them through inter-
nesting periods, and all showed site fidelity (propor-
tion of tracks that were more constrained than ran-
dom movement paths >99.0099 for all 10 turtles;
Table 1) and comprised enough MDLs (≥20) to con-
duct KDE analyses (Table 1). One individual (Turtle
ID 8) was also observed on 13 September 2013 nest-
ing on property that TNC monitors at the East End of
St. Croix (i.e. EEMP). The 10 inter-nesting periods
totaled 760 d across all turtles, and ranged from 21 to
92 d (Table 1). The inter-nesting period across all tur-
tles (regardless of year) ranged from 31 July (x− ± SD
93
Endang Species Res 32: 89–101, 2017
of first inter-nesting date: 7 August ± 8.2 d, n =
10) to 4 November (x− ± SD of last inter-nesting
date: 21 October ± 16.0 d, n = 10). In total, we
obtained 624 MDLs for inter-nesting KDE ana -
lyses (Table 1). Mean size of the 50% core-use
areas during inter-nesting was 7.3 km2 (range:
4.2 to 19.0 km2); mean size of the 95% KDE
areas during inter-nesting was 35.8 km2 (range:
20.5 to 91.3 km2); and distance to the nearest
land from the 50% KDE centroids (Fig. 1)
ranged from 0.0 to 1.4 km (Table 1). Bathyme-
try values (i.e. a proxy for water depth) at the
50% KDE centroid locations ranged from
−17.0 to –1.0 m (Table 1).
The grids of the turtle days showed that
high-use areas during inter-nesting periods
were concentrated around Buck Island (Fig. 2).
There was a weak but significant association
between turtle days and bathymetry in the
grid cells (Spearman’s rank correlation, rS =
0.34, n = 253, p < 0.0001).
Core area space-use sharing
During inter-nesting, across all pairs of inter-
nesting females, UDOI ranged from 0 to 0.18
(x− ± SD: 0.04 ± 0.05, n = 45; Table S3 in the
Supplement), where greater UDOI indicates
greater space-use sharing between turtle pairs.
Temporal overlap across all pairs of inter-
nesting females ranged from 0 to 91 d (x− ± SD:
19.2 ± 32.5, n = 45). Results of the non-para-
metric Mann-Whitney rank sum test indicated
there was no difference in UDOI between tur-
tle pairs with and without neophytes (Mann-
Whitney U-test, U = 182.5, n1 = 22, n2 = 23, p =
0.169, where n1 = pairs with neophytes and
n2 = pairs without neophytes).
Ecological and spatial correlates
We did not find any significant relationships
between turtle CCL and either the size of the
50% KDE (linear regression, r2 = 0.01, F1,4 =
0.003, p = 0.87) or the bathymetry values at the
50% KDE centroid (linear regression, r2 = 0.01,
F1,8 = 0.11, p = 0.75), or between the number of
tracking days and the size of the 50% KDE
(linear regression, r2 = 0.04, F1,8 = 0.30, p =
0.60). Thus, turtle size was not predictive of
inter-nesting habitat selection.
94
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Hart et al.: Buck Island green turtle habitat
Migration
Four of the 10 turtles exhibited a migration pe -
riod, as determined by SSM (Fig. 3). Three of these
4 turtles had migration points along their tracks
after filtering (Turtle ID 5 had a single unfiltered
point during her short 3 d migration, and zero
points after filtering, therefore the migration path
could not be delineated; Table 1). Migration
periods lasted 3 to 14 d, and the distance along the
migration path ranged from 84.4 to 906.0 km
(Table 1). The straight-line migration distance for
all 10 turtles ranged from 0.2 to 694.0 km (Table 1).
Mean water depths associated with migration
points across the 3 migration periods (Table 1)
ranged from −3613.7 to −387.3 m.
In-water foraging areas
All 10 turtles were tracked past inter-nesting and
migration to their respective foraging grounds.
Five of these turtles (50%) settled around Buck
Island, 2 settled around the island of St. Croix, and
the other 3 (30%) made longer-distance migrations
to Antigua, St. Kitts & Nevis, and Venezuela,
where they took up residence (Fig. 3). Nine of the
10 turtle tracks passed site fidelity
tests during the foraging period
(proportion of tracks that were more
constrained than random move-
ment paths >96.0396; Table 2). Of
the 9 foragers that passed site
fidelity tests, 5 had enough MDLs
to conduct KDE analyses; we con-
ducted MCP analyses for the other
4 turtles. The 5 KDE foraging peri-
ods (Fig. 3) totaled 772 d across all
turtles, and ranged from 35 to
372 d (Table 2). In total, we ob -
tained 265 MDLs for KDE analyses
(Table 2). Mean size of the 50%
core-use areas during foraging was
5.6 km2 (range: 2.0 to 11.9 km2;
Table 2). Mean size of the 95%
KDE areas was 26.6 km2 (range: 9.5
to 59.4 km2; Table 2). The 4 MCP
foraging periods (Fig. 3) totaled
172 d across all 4 turtles with 44
MDLs (Table 2). Mean size of the
MCPs across the 4 turtles was
20.6 km2 (range: 9.0 to 30.7 km2;
Table 2).
95
Fig. 2. Grids of inter-nesting days for 10 adult female green sea turtles Chelonia
mydas (top panel) and foraging-days for 7 adult female green sea turtles (bottom
panel). All turtles were satellite-tagged on Buck Island, US Virgin Islands
Fig. 1. Buck Island Reef National Monument (BIRNM; dark
line represents Monument boundary) study site (small black
square in the inset) where adult female green sea turtles
Che lonia mydas were satellite-tagged from 2011 to 2014.
Kernel density estimation (KDE) for 10 inter-nesting adult
female green sea turtles are shown with hatched blue lines
(95% KDE contours) and blue areas (50% KDE core-use
contours; contour areas for all 10 turtles were merged for vi-
sualization purposes). Centroids of core-use area kernel
density estimation (50% KDE) are shown as open circles.
Orange shading indicates marine protected area at East End
Marine Park and green shading indicates marine protected
area at Sandy Point National Wildlife Refuge
Endang Species Res 32: 89–101, 2017
Distances to the nearest land from the 50% KDE
foraging centroids (Fig. 3) ranged from 0.7 to
1.6 km, and bathymetry values at the 50% KDE
centroid locations ranged from −23.0 to −1.0 m
(Table 2). Distances to the nearest land from the
MCP foraging centroids ranged from 0.5 to 1.6 km,
and the bathymetry values that corresponded to
the foraging MCP centroids ranged from −13.0
to –6.0 m (Table 2).
Turtle foraging days per grid cell
In addition to the area around Buck Island, where
we observed high use by turtles during the inter-
nesting period, turtles were frequently located off the
SW coast of St. Croix (Fig. 2). The number of turtle
days per grid cell was weakly but significantly as -
sociated with bathymetry (Spearman’s ρ = 0.31, p <
0.0001).
96
Fig. 3. Foraging and migration for adult female green sea turtles Chelonia mydas satellite-tagged on Buck Island, US Virgin Is-
lands. Kernel density estimations (KDEs) for 5 turtles are shown with hatched blue lines (95% KDE) and blue areas (50%
KDE); open circles represent 50% KDE centroids. Minimum convex polygons (95% MCPs) are shown by red polygons for 4
turtles; red diamonds represent 95% MCP foraging centroids. The rectangles labelled A, B, C, in the upper left panel show the
figure extents for the other 3 panels: (A) 4 turtles foraging near Buck Island (Turtle IDs 2, 4, 6, and 9) and 2 turtles (Turtle IDs 3
and 10) foraging southwest of St. Croix; the boundary for Buck Island Reef National Monument is represented by the black line;
(B) 1 turtle foraging near St. Kitts & Nevis Islands (Turtle ID 1) and 1 turtle (Turtle ID 5) foraging southwest of Antigua; and (C)
1 turtle foraging near Los Roques National Park (LRNP), Venezuela (Turtle ID 7). Both KDEs and MCPs were merged for visu-
alization purposes. Migration paths from inter-nesting grounds to foraging grounds for 3 turtles are shown in the top left panel.
Turtle ID 1 had a migration path to St. Kitts & Nevis (orange triangles), Turtle ID 7 had a migration path to LRNP (green
circles), and Turtle ID 9 had a brief migration path to and from Buck Island, thereafter foraging locally (purple square)
Hart et al.: Buck Island green turtle habitat
Core area space-use sharing
There were 5 turtles for which foraging KDEs
were calculated, 3 of which foraged in the same
region (near Buck Island). These 3 turtle pairs were
all re-migrants, and the amount of UDOI space-use
sharing for these turtle pairs (n = 3 turtles, n = 3
pairs) ranged from 0.09 to 0.14 (x− ± SD: 0.12 ± 0.02,
n = 3; Table S3), where greater UDOI indicates
greater space-use sharing between turtle pairs.
Temporal overlap for these 3 pairs of foraging
females near Buck Island ranged from 0 to 138 d
97
Turtle
Foraging
FFLs MDLs 50%
95% Centroid Depth at
Foraging
Centroid
Centroid
ID
period
KDE
KDE
distance centroid
location
in
habitat type
(d)
area
area
to shore
(m)
protected
for local
(km2)
(km2)
(km)
area?
foragers
KDEs
1
09/19/11−03/03/12
481
82
11.9
59.4
1.6
−23.0 St. Kitts & Nevis
No
na
(167)
2
11/01/12−04/09/13
(160)
361
66
6.5
31.7
1.1
−1.0
Buck Island
Yes, EEMP
SVS
4
11/23/12−08/06/13
(372)
387
55
4.2
19.9
0.7
−1.0
Buck Island
Yes, EEMP
CRCH
6
11/01/13−12/08/13
(38)
325
33
2.0
9.5
0.8
−1.0
Buck Island
Yes, EEMP
SVS
7
11/05/13−12/09/13
(35)
338
29
3.2
12.3
1.2
−19.0
Venezuela
Yes, LRNP
na
Mean
154.0
378.4
53.0
5.6
26.6
1.1
−9.0
SD
137.2
62.0
22.3
3.9
20.3
0.4
11.0
Min.
35.0
325.0
29.0
2.0
9.5
0.7
−23.0
Max.
372.0
481.0
82.0 11.9
59.4
1.6
−1.0
Tutle
Foraging
FFLs MDLs
95% Centroid Depth at
Foraging
Centroid
Centroid
ID
period
MCP
distance centroid
location
in
habitat type
(d)
(km2)
to shore
(m)
protected
for local
(km)
area?
foragers
MCPs
5
10/12/12−12/21/12
36
13
30.7
1.6
−13.0
Antigua
No
na
(71)
3
11/01/12−12/28/12
49
13
9.0
0.7
NE
St. Croix
No
SVS
(58)
9
11/04/13−12/07/13
33
9
23.2
0.5
NE
Buck Island
Yes, EEMP
USS
(34)
10
11/05/14−11/13/14
114
9
19.6
1.4
−6.0
St. Croix
No
SVS
(9)
8
11/01/13−11/15/13
7
4
na
na
na
Buck Island
na
na
(15)
Mean
37.4
47.8
9.6
20.6
1.1
–9.5
SD
26.8
40.0
3.7
9.0
0.5
4.9
Min.
9.0
7.0
4.0
9.0
0.5
−13.0
Max.
71.0
114.0
13.0
30.7
1.6
–6.0
Table 2. Foraging area characteristics for 10 satellite-tracked female green sea turtles Chelonia mydas that originally nested
on Buck Island, US Virgin Islands. Foraging kernel density estimation (KDE) areas presented here were comprised of at least
20 mean daily locations (MDLs), and minimum convex polygons (MCPs) were comprised of less than 20 MDLs; variables in-
clude information regarding the timing, duration, area, water depth, distance to shoreline, and foraging area characteristics
for each foraging period. Centroids were derived from 50% KDEs or 95% MCPs. Foraging periods are given as mm/dd/yy.
FFL: filtered foraging locations; EEMP: East End Marine Park; LRNP: Los Roques National Park; SVS: submerged vegetation
(seagrass); CRCH: coral reef and colonized hardbottom; USS: unconsolidated sediments (sand); na: not applicable. Turtle ID 8
did not pass site fidelity, therefore no MCP was calculated. All proportions for KDE site fidelity were >99.0099; turtle tracks
with values >95.00 indicate movement paths that are more constrained than random movement paths. Proportions for MCPs
were >95.00 except for Turtle ID 8, which had value of 56.4356 and did not pass site fidelity. NE = not estimated because
centroid depth had positive value
Endang Species Res 32: 89–101, 2017
(x− ± SD: 46.00 ± 79.67, n = 3). Of the 4 turtles with
foraging MCPs, 2 (1 turtle pair) foraged in the same
area, southwest of St. Croix; the distance between
centroids for these 2 turtles was 0.69 km. For the
turtle with a foraging MCP in the Buck Island area,
the mean distance from the MCP centroid to the
3 KDE centroids was 1.47 km.
Ecological and spatial correlates
The benthic habitats associated with foraging cen-
troids for turtles foraging near Buck Island and St.
Croix consisted of submerged seagrass vegetation,
coral reef and colonized hardbottom, and unconsoli-
dated sand sediments (Table 2). Of the 9 turtles that
had a foraging KDE or MCP (n = 5 KDEs, n = 4
MCPs), 5 turtles had foraging centroids located in
protected or managed areas, including EEMP, USVI
and Los Roques National Park (LRNP), Venezuela.
The other 4 turtles had foraging centroids that were
not located in protected areas (Table 2).
DISCUSSION
Through 4 yr of satellite tracking, we gained
insight into the movement and habitat-use patterns
of adult female green turtles nesting at BIRNM,
which represents a unique management unit (see
Shamblin et al. 2012), during breeding, migration,
and foraging time periods. Our study presents key
information on the BIRNM green turtle rookery, sup-
ports previous findings on spatial use from 3 other
Caribbean studies (Blumenthal et al. 2006, Esteban
et al. 2015, Becking et al. 2016), and our robust SSM
approach accurately quantifies time periods of
migration and residency at foraging and inter-nest-
ing sites.
We observed consistency of inter-nesting habitat-
use patterns over the 4 study years, with all
turtles using near-shore, shallow waters (shallower
than –20 m depth) within approximately 10 km of
Buck Island. Green turtle inter-nesting habitat in Dry
Tortugas National Park (Hart et al. 2013) and other
Caribbean locations was also in near-shore, shallow
waters (Lesser Antilles, Esteban et al. 2015; Cayman
Islands, Blumenthal et al. 2006), although some
exceptions have been found for turtles traveling
between nesting sites and swimming through deep
waters (e.g. mean depth −2940 m for 1 Cayman
Islands turtle; Blumenthal et al. 2006, see also Beck-
ing et al. 2016).
Neophyte or first-time nesters may show different
patterns during inter-nesting than established remi-
grants. For example, experienced re-migrant hawks-
bills were the only turtles to select distant resident
areas during inter-nesting in Barbados (Walcott et al.
2012). However, we did not find differences in inter-
nesting resident areas between green turtle neo-
phytes and re-migrants; 2 of our tagged females (Tur-
tle IDs 7 and 8) were neophytes, and their habitat-use
patterns mirrored those of the other 8 turtles. One
exception was that Turtle ID 7 showed variability in
nesting beach selection and nested at a nearby TNC
study site at EEMP (6.5 km straight-line distance
away), whereas all other nesting events for tagged
turtles occurred at BIRNM. This indicates that nest-
ing site fidelity within a single nesting season may
vary for some BIRNM green turtles. Flexibility in
nesting-site use was also observed in the Cayman
Islands, where 1 of 7 green turtles shifted her nesting
site within a season (Blumenthal et al. 2006). Contin-
ued satellite-tracking and nest-monitoring at BIRNM
and neighboring beaches could help determine if
both neophytes and re-migrants share the same level
of flexibility in site-use within a nesting season.
Ours is the first empirical dataset showing limited
migration and use of ‘local’ resources in the USVI by
green turtles. We found that 7 of 10 turtles did not
migrate, supporting recent findings of plasticity in
migration and selection of local foraging sites in Dry
Tortugas National Park and the Florida Keys Nat -
ional Marine Sanctuary (Hart et al. 2013) where 9 of
11 turtles showed year-round residency, in the Indian
Ocean (Whiting et al. 2007), where all 6 turtles
migrated to shallow foraging grounds within 40 km
of the nesting beach, and elsewhere in the Caribbean
(Esteban et al. 2015) where 2 of 3 green turtles for -
aged within 50 km of their original nesting grounds.
Not all turtles in our study remained near nesting
grounds to forage, however; some migrated long dis-
tances (e.g. up to 694 km straight-line distance to
Venezuela waters; Turtle ID 7). This was also the
case for turtles studied in the Lesser Antilles, where 1
green turtle that did not forage locally migrated
607 km straight-line distance (Esteban et al. 2015)
and 4 turtles studied in Bonaire migrated 198 to
3135 km away after nesting (Becking et al. 2016). In
a study of green turtles in the Cayman Islands, all 7
tagged green turtles traveled 520 to 856 km straight-
line distance to foraging locations (Blumenthal et al.
2006). Of all these studies, neither Bonaire nor the
Cayman Islands had locally foraging post-nesting
green turtles despite both areas supporting resident
juvenile greens (Blumenthal et al. 2006, Stapleton et
98
Hart et al.: Buck Island green turtle habitat
al. 2014, Becking et al. 2016). Local habitat may
not be particularly suitable for adults in these loca-
tions. In the Cayman Islands, legal and illegal sea
turtle fisheries that mainly target larger turtles (Bell
et al. 2006) could also play a role, potentially captur-
ing resident turtles more often than those that
migrate away. Fishing for turtles is not legal in US
Gulf of Mexico waters, St. Eustatius and St. Maarten
(Lesser Antilles) or the USVI (Bräutigam & Eckert
2006), where turtles were found to forage locally
after nesting.
Similar to the inter-nesting period, foraging cen-
troids for BIRNM nesters were also close to shore (at
most 1.6 km away) and in relatively shallow waters
(up to −23 m depth). This finding is similar to that of
Esteban et al. (2015), where the 2 green turtles that
migrated to the Lesser Antilles remained resident
there in shallow nearshore foraging areas (within
approximately 5 km of land).
The grid analysis revealed high-use zones for
BIRNM green turtles to the northeast of St. Croix
during inter-nesting, especially surrounding Buck
Island. During foraging periods, high-use grid cells
were to the northeast and southwest of St. Croix.
Grid cells to the northeast of St. Croix lay directly in
the path of commercial and recreational boats travel-
ing to BIRNM. Inter-nesting and foraging areas at
BIRNM that overlap with human use zones present
an important management concern, especially be -
cause some individual adult female turtles are resi-
dent year-round in this relatively small area. The
area from Green Key Marina (on the mainland of St.
Croix) to Buck Island is a major human thoroughfare
for boating, so boat strikes of turtles are a potential
concern. Given that we found some overlap of satel-
lite-tagged green turtle home ranges with human
use areas at BIRNM, the density of turtles at these
sites may be higher than we can demonstrate from
satellite tracking alone. Investigation of these forag-
ing sites to determine resource condition and to fur-
ther quantify turtle numbers at these sites would be
valuable for informing future management actions.
While some turtles remained resident, 3 of our tur-
tles migrated to other countries, supporting the need
for international cooperation in conservation of these
turtles (e.g. Blumenthal et al. 2006, Becking et al.
2016). We tracked BIRNM nesting green turtles on
their migrations to Antigua, St. Kitts & Nevis, and
Venezuela, and only one of these ended up in a pro-
tected area (Venezuela’s LRNP). Exploitation of sea
turtles in Antigua and St. Kitts & Nevis is not com-
pletely prohibited, although it is restricted (Bräutigam
& Eckert 2006). Given the genetic distinctness of
BIRNM green turtles, continued long-term protection
of the nesting beach and protection of the in-water
inter-nesting and foraging sites delineated here may
improve the likelihood of population recovery.
Acknowledgements. We thank NPS interns and USGS
employees J. Beauchamp, M. Denton, A. Daniels, B. Smith,
H. Crowell, and A. Crowder for assistance deploying satel-
lite tags in the field, and E. Connolly-Randazzo for help with
an earlier draft of the manuscript. Permission to tag and
sample turtles was given under BUIS permit BUIS-2012-
SCI-0002 and USGS-SESC-IACUC 2011-05, and USFWS
permits issued to K.M.H. Funding was provided by the
USGS Natural Resources Protection Program, National Park
Service and USGS Ecosystems Program. Any use of trade,
product, or firm names is for descriptive purposes only and
does not imply endorsement by the US Government.
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Editorial responsibility: Brendan Godley,
University of Exeter, Cornwall Campus, UK
Submitted: June 29, 2016; Accepted: Nov 21, 2016
Proofs received from author(s): January 20, 2017