The authors have declared that no competing interests exist.
Conceived and designed the experiments: KD ML. Performed the experiments: KD. Analyzed the data: KD BG TM. Wrote the paper: KD BG TM ML.
Leatherback sea turtles,
Highly migratory marine predators such as leatherback sea turtles encounter a diversity of habitats during their long-distance movements. Oceanographic processes create regional ecosystems with distinct rates of primary productivity and community structure
Leatherback sea turtles (
Broad-scale tracking studies over the past decade have given new insight into the relationship between leatherback behavior and their environment
In the present study, we deployed satellite tags on adult male, female and sub-adult leatherbacks turtles captured off Massachusetts, USA. This is the first in-water tagging study conducted in leatherback foraging grounds in the US Atlantic, and one of three direct-capture studies of leatherback turtles worldwide. We collected geolocation and dive data to: 1) determine leatherback occupancy of distinct oceanographic regions in the North Atlantic; 2) characterize leatherback regional movements, dive behavior, and environmental associations; 3) identify seasonal high-use habitat and 4) determine key environmental features associated with leatherback search behavior in the NW Atlantic.
This work was conducted under the authority of the National Marine Fisheries Service Endangered Species Act Section 10 Permit # 1557-03 and the University of New Hampshire IACUC # 060501 and #090402. Turtle disentanglement was conducted under the authority of NOAA 50 CFR Part 222.310.
Leatherbacks were located off the coast of Massachusetts, USA (∼41°N, 70°W) from August 2007 to September 2009, and captured with a breakaway hoopnet (n = 11)
Letters represent satellite tag deployments as listed in
Turtle ID | PTT |
CCL |
Age |
Sex |
Capture method | Tag Model |
Tagging location |
Tagging date | Days at liberty | Distance (km) | No. ARGOS locations | No. GPS locations |
A | 68366 | 140.7 | S | M | Entangled | MK10-AF | N. Sound | 19-Aug-07 | 34 | 938 | 137 | 263 |
B | 68364 | 143.2 | S | M | Entangled | MK10-AF | CC Bay | 29-Aug-07 | 18 | 461 | 82 | 90 |
C | 68369 | 123.0 | S | U | Entangled | MK10-AF | N. Sound | 29-Aug-07 | 16 | 277 | 64 | 89 |
D | 68370 | 137.5 | S | U | Entangled | MK10-AF | CC Bay | 22-Sep-07 | 183 | 6444 | 572 | 777 |
E | 68365 | 136.0 | S | F | Entangled | MK10-AF | CC Bay | 1-Oct-07 | 35 | 991 | 109 | 252 |
F | 68365a | 149.5 | A | M | Hoopnet | MK10-AF | N. Sound | 17-Jul-08 | 174 | 8004 | 1520 | 1067 |
G | 68364a | 146.0 | A | F | Hoopnet | MK10-AF | V. Sound | 26-Jul-08 | 199 | 7920 | 1407 | 1498 |
H | 82052 | 161.5 | A | F | Hoopnet | MK10-A | V. Sound | 29-Jul-08 | 272 | 8435 | 2114 | na |
I | 76988 | 152.2 | A | M | Hoopnet | MK10-AF | Nantucket | 10-Aug-08 | 214 | 8878 | 1932 | 1469 |
J | 76990 | 140.4 | S | U | Hoopnet | MK10-AF | Nantucket | 10-Aug-08 | 150 | 5967 | 1147 | 593 |
K | 82055 | 133.8 | S | U | Hoopnet | MK10-A | Nantucket | 10-Aug-08 | 152 | 5792 | 1306 | na |
L | 82051 | 153.3 | A | M | Hoopnet | MK10-A | Nantucket | 10-Aug-08 | 242 | 9466 | 1846 | na |
M | 76989 | 144.8 | A | F | Hoopnet | MK10-AF | Nantucket | 21-Aug-08 | 180 | 9191 | 1427 | 1563 |
N | 85538 | 154.0 | A | M | Hoopnet | MK10-AF | Nantucket | 22-Aug-08 | 183 | 6528 | 1707 | 796 |
O | 85537 | 138.5 | S | M | Hoopnet | MK10-AF | Nantucket | 22-Aug-08 | 181 | 5883 | 1722 | 307 |
P | 82053 | 146.4 | A | M | Entangled | MK10-A | N. Sound | 23-Aug-08 | 234 | 9765 | 1095 | na |
Q | 82054 | 140.0 | S | U | Entangled | MK10-A | N. Sound | 28-Aug-08 | 191 | 5980 | 1306 | na |
R | 82056 | 126.5 | S | U | Weir | MK10-A | N. Sound | 10-Jul-09 | 414 | 14168 | 3570 | na |
S | 82057 | 127.7 | S | U | Hoopnet | MK10-A | N. Sound | 27-Aug-09 | 278 | 11541 | 1616 | na |
T | 27579 | 155.0 | A | M | Entangled | MK10-A | N. Sound | 3-Sep-09 | 203 | 7096 | 1458 | na |
PTT: platform transmitter terminal.
CCL: curved carapace length.
S: sub-adult (<145 cm CCL); A: adult (≥145 cm CCL).
M: male; F: female, U: unknown sex.
MK10-A: Argos-only locations; MK10-AF: Argos and Fastloc GPS locations.
N. Sound: Nantucket Sound; CC Bay: Cape Cod Bay; V. Sound: Vineyard Sound; Nantucket: waters south of Nantucket.
Twenty leatherbacks were fitted with Wildlife Computers, Inc. (Redmond, WA, USA) model MK10-A (n = 8) and MK10-AF (n = 12) ARGOS-linked satellite time depth recorders. The tags deployed in 2007 had flexible, plastic-coated metal baseplates that conformed to the leatherbacks' medial ridge. We worked with Wildlife Computers to improve this design in future seasons (2008–2009), resulting in the “ridge-mount” tag model specifically developed for leatherback turtles. We attached satellite tags directly to the leatherbacks' medial ridge following methods first developed by Lutcavage et al.
Leatherbacks were measured to the nearest 0.1 cm (curved carapace length: CCL and curved carapace width: CCW) with a flexible fiberglass measuring tape, and ranged from 123.0 to 161.5 cm CCL (
The satellite tags transmitted Fastloc GPS locations (MK10-AF model only), ARGOS-derived locations (all tags) and dive information (depth resolution ±0.5m and temperature resolution ±0.05°C) via Service ARGOS (Toulouse, France) (
We defined a dive as vertical movement below two meters for at least one minute. The number of dives within specified depth and duration ranges and the time spent within depth and temperature ranges were collected as frequency histograms based on preprogrammed bins (
Years | Number of tags | Depth bin (m) | Duration bin (min) | Years | Number of tags | Time-at-Depth bin (m) | Time-at-Temp bin (°C) |
2007–2009 | 20 | 2–5 | 1–4 | 2008–2009 | 15 | 0–2 | 0–4 |
2007–2009 | 20 | 5–10 | 4–8 | 2008–2009 | 15 | 2–10 | 4–6 |
2007–2009 | 20 | 10–15 | 8–12 | 2008–2009 | 15 | 10–15 | 6–8 |
2007–2009 | 20 | 15–20 | 12–16 | 2008–2009 | 15 | 15–20 | 8–10 |
2007–2009 | 20 | 20–25 | 16–20 | 2008–2009 | 15 | 20–25 | 10–12 |
2007–2009 | 20 | 25–30 | 20–24 | 2008–2009 | 15 | 25–30 | 12–14 |
2007–2009 | 20 | 30–50 | 24–28 | 2008–2009 | 15 | 30–40 | 14–16 |
2007–2009 | 20 | 50–75 | 28–32 | 2008–2009 | 15 | 40–50 | 16–18 |
2007–2009 | 20 | 75–100 | 32–36 | 2008–2009 | 15 | 50–75 | 18–20 |
2007–2009 | 20 | 100–200 | 36–40 | 2008–2009 | 15 | 75–100 | 20–22 |
2007–2009 | 20 | 200–300 | 40–44 | 2008–2009 | 15 | 100–125 | 22–24 |
2007–2009 | 20 | 300–400 | 44–48 | 2008–2009 | 15 | 125–150 | 24–26 |
2007–2009 | 20 | 400–500 | 48–52 | 2008–2009 | 15 | 150–200 | 26–28 |
2007–2009 | 20 | >500 | >52 | 2008–2009 | 15 | >200 | >28 |
We selected environmental data likely to influence production and distribution of gelatinous prey
We filtered 30,173 raw ARGOS and GPS locations using Kalman filter methods outlined in Royer & Lutcavage
To investigate variation in seasonal habitat use, we created density utilization maps of filtered leatherback positions for pooled data across all turtles by season. Seasons were defined as: July – September (summer), October – December (autumn), January – March (winter), and April – June (spring). Daily locations were summed into hexagonal area bins, with the area of each hexagon approximately 669 km2 (or 4 hexagons per degree). These bins are larger than the error associated with our filtered ARGOS and GPS location data, but small enough to identify regional high-use areas. Density utilization maps were produced using R
We applied generalized linear mixed-effects models to investigate the influence of ecoregion, SST, chl
Model | |
null | |
ecoregion | |
ecoregion + SST |
|
ecoregion × SST |
|
ecoregion + chla |
|
ecoregion × chla |
|
ecoregion + SSTg |
|
ecoregion × SSTg |
|
ecoregion + chlag |
|
ecoregion × chlag |
|
ecoregion + bathymetry |
|
ecoregion × bathymetry |
|
ecoregion + SST |
|
ecoregion × SST |
|
ecoregion + chla |
|
ecoregion × chla |
|
ecoregion + SSTg |
|
ecoregion × SSTg |
|
ecoregion + chlag |
|
ecoregion × chlag |
Turtle
Slopes held constant in regions of interest.
Slopes and intercepts allowed to vary in regions of interest.
We fit the models by maximum marginal likelihood in R
We received data from all tagged leatherbacks: four tags transmitted for less time than expected and 16 tags met or exceeded predicted battery life. Tags reported between 16 and 414 days, with a median tracking duration of 184 (152 to 219; Q1–Q3) days (
Tracks show turtle movements from point of release (Cape Cod) to point of last Argos transmission (red triangles). Tags were deployed on adult males (F, I, L, N, P, T), adult females (G, H, M), and sub-adults (A, B, C, D, E, J, K, O, Q, R, S).
a) Dive-depth (n = 19) and b) dive-duration (n = 17) from turtles tagged 2007–2009. c) Time-at-depth (n = 15) and d) time-at-temperature (n = 15) from turtles tagged from 2008–2009.
Of the 210,556 dives reported for the dive-depth parameter, over 28% were to depths less than 5 m and 90% were shallower than 75 m (
Leatherbacks ranged widely between 39°W and 83°W, and between 9°N and 47°N (
Summer, July – September (n = 19 turtles), autumn, October – December (n = 17 turtles), winter, January – March (n = 16 turtles), and spring, April – June (n = 5 turtles). There are four hexagons per degree; each hexagon represents approximately 669 km2. Color scale shows the number of track days per hexagon. Ecoregions from Longhurst
Turtles modified their movements and dive behavior while occupying different ecoregions. Leatherbacks in the Northwest Atlantic Shelves had the lowest travel rates and path straightness of all regions (
a) Rate of travel, b) straightness, c) SST, d) SST gradient magnitude, e) chl
Depth and duration bins (top panels) and leatherback hours in depth and temperature bins (bottom panels). NWCS, Northwest Atlantic Shelves; GFST, Gulf Stream; NASW, North Atlantic Subtropical Gyral West; NATR, North Atlantic Tropical Gyral; CARB, Caribbean; GUIA, Guianas Coastal. The second y-axis (red) corresponds to NWCS (red line) while all other regions reference the first y-axis (black). This highlights the increased dive activity of tagged turtles in the NWCS region.
Turtles experienced highly variable environmental conditions across ecoregions, where bathymetry ranged from shallow bays and sounds on the continental shelf to deep oceanic waters, SST from 9.6°C to 28.9°C and chl
Based on AIC values, the most well supported model showed that differences in leatherback search behavior (represented by logit-transformed path straightness) were best explained by ecoregion and effects of bathymetry and SST, with effects of SST depending on the ecoregion (
Leatherback path straightness is shown in relation to: a) observed log-transformed bathymetry (m) in each of two distinct ecoregions of the northwest Atlantic and b) observed sea surface temperature (SST) in each of two distinct ecoregions of the northwest Atlantic; Northwest Atlantic Shelves (NWCS); Gulf Stream (GFST); Guianas Coastal (GUIA). Mean SST value in each region was used to create the bathymetry plot and mean bathymetry value in each region was used to create the SST plot.
Model | p |
AIC | ΔAIC |
ω |
null | 4 | 7258.06 | 390.51 | 0 |
ecoregion | 9 | 6927.63 | 60.08 | 0 |
ecoregion + SST |
10 | 6927.26 | 59.71 | 0 |
ecoregion × SST |
11 | 6921.62 | 54.07 | 0 |
ecoregion + chla |
10 | 6908.31 | 40.76 | 0 |
ecoregion × chla |
15 | 6908.97 | 41.42 | 0 |
ecoregion + SSTg |
10 | 6928.73 | 61.18 | 0 |
ecoregion × SSTg |
11 | 6929.90 | 62.35 | 0 |
ecoregion + chlag |
10 | 6923.94 | 56.39 | 0 |
ecoregion × chlag |
15 | 6922.50 | 54.95 | 0 |
ecoregion + bathymetry |
10 | 6871.17 | 3.62 | 0.07 |
ecoregion × bathymetry |
11 | 6873.15 | 5.60 | 0.03 |
ecoregion + SST |
11 | 6871.52 | 3.97 | 0.06 |
ecoregion × SST |
12 | 6867.55 | 0.00 | 0.42 |
ecoregion + chla |
11 | 6871.76 | 4.21 | 0.05 |
ecoregion × chla |
16 | 6868.89 | 1.34 | 0.21 |
ecoregion + SSTg |
11 | 6873.13 | 5.58 | 0.03 |
ecoregion × SSTg |
12 | 6874.93 | 7.38 | 0.01 |
ecoregion + chlag |
11 | 6871.61 | 4.06 | 0.05 |
ecoregion × chlag |
16 | 6870.84 | 3.29 | 0.08 |
p: number of parameters in the model.
ΔAIC: difference in AIC value between best fitting model and other model.
ω: Akaike weight.
Slopes held constant in regions of interest.
Slopes and intercepts allowed to vary in regions of interest.
*Best fitting model.
Effect | Estimate | Standard Error |
Intercept | 1.563 | 0.119 |
GFST |
−0.723 | 0.522 |
GUIA |
−1.312 | 0.235 |
NASW |
0.303 | 0.125 |
NATR |
0.337 | 0.121 |
NWCS |
−1.467 | 0.321 |
log(bathy) |
0.231 | 0.030 |
sst_NWCS |
−0.032 | 0.014 |
sst_GFST |
0.034 | 0.024 |
Random effect parameter estimate intercept was 0.196 and residual was 0.960, and estimated autocorrelation was 0.306.
GFST: Gulf Stream ecoregion.
GUIA: Guianas coastal ecoregion.
NASW: North Atlantic Subtropical Gyral West ecoregion.
NATR: North Atlantic Tropical Gryral ecoregion.
NWCS: Northwest Atlantic Shelves ecoregion.
log(bathy): logarithm of bathymetry in NWCS and GUIA ecoregions.
sst_NWCS: sea surface temperature in NWCS ecoregion.
sst_GFST: sea surface temperature in GFST ecoregion.
Seasonal density utilization maps showed leatherback movements were the least extensive during summer, with turtles tagged off Massachusetts showing a strong preference for the Northeast US continental shelf, concentrating movements off southern New England and Virginia/North Carolina (
Overall, dispersal patterns differed between adult and sub-adult leatherbacks. Most adults followed widely spaced but highly oriented south/southeast headings during their southward migration until they reached latitudes between 10°N and 13°N (
We deployed GPS-linked and conventional ARGOS STDRs to simultaneously collect data on movements and dive behavior of adult and sub-adult leatherbacks in the North Atlantic. This study is one of the first to obtain highly accurate GPS locations from leatherback turtles, allowing us to identify high use habitat, movement patterns and environmental associations with less observation error. We also used novel design (e.g. “ridge-mount tag”) and direct attachment techniques to deploy low profile, hydrodynamic tags. In an analysis of transmitter drag and tag attachment procedures, Jones et al.
Leatherbacks tagged off Massachusetts showed a strong affinity to the Northeast US continental shelf before dispersing widely throughout the northwest Atlantic. One individual tracked for >1 year exhibited site fidelity to the US shelf, returning in late spring and remaining through late summer. Surprisingly, only one Massachusetts-tagged leatherback moved onto the eastern Canada shelf, an important and well-documented leatherback foraging ground
There was a strong seasonal component to habitat selection, with most leatherbacks remaining in temperate latitudes in the summer and early autumn and moving into subtropical and tropical habitat in the late autumn, winter and spring. This latitudinal shift is consistent with previous studies of leatherbacks tracked from foraging grounds in the North Atlantic
During the over-wintering period, sub-adults, small adult males, and a single inter-nesting-year female primarily remained in oceanic habitat, while large adult males and two reproductive females moved into coastal breeding areas. Little is known about the demographics of male leatherbacks, but there may be a size constraint whereby smaller males are unable to compete for females, and are effectively displaced from breeding areas by larger, more dominant individuals
Our density utilization maps demonstrate that the Northeast US shelf, particularly southern New England, provides important seasonal habitat for leatherback turtles tagged of Massachusetts. The Northeast US shelf is one of the most well-studied and productive large marine ecosystems in the world
Previous studies have identified the Northwest Atlantic Shelves as important foraging habitat for leatherbacks
The average mixed layer depth on the Northeast US shelf is also shallowest (10–20 m) during the summer and early autumn
We observed marked behavioral changes as leatherbacks left continental shelf habitat and began their southward migrations through subtropical oceanic habitat. As turtles moved through the Gulf Stream and Subtropical Atlantic, they showed rapid, directed travel and began spending more time at depths >50 m. Most turtles spent minimal time in these ecoregions, suggesting that these are less important feeding areas for Massachusetts-tagged leatherbacks and are primarily used for transiting between temperate (i.e., foraging) and tropical (i.e., breeding) habitat. However, two individuals did make more extensive use of the Gulf Stream region during summer, fall and early winter. The Gulf Stream's strong horizontal SST gradient, particularly in fall and winter, is evident in the strong SST fronts encountered by leatherbacks there. Leatherback movements in the Gulf Stream became slightly more sinuous at lower SST, possibly associated with upwelling along the Gulf Stream front, but this relationship was weak in our model. The Gulf Stream has been previously identified as probable foraging habitat for leatherback turtles
Leatherback movement patterns and dive behavior in the Subtropical Atlantic were consistent with other studies of this species in the North Atlantic
The percentage of time that leatherbacks spent at the surface in the Gulf Stream and the Subtropical Atlantic was similar to the percent surface times recorded by James et al.
Leatherbacks overwintered in tropical ecoregions, with reproductively active adults primarily occupying the Guianas Coastal and Caribbean regions while non-reproductively active adults and sub-adults mainly used oceanic habitat in the Tropical Atlantic. Turtles slowed down in the tropics compared to the subtropical gyre but travel was still directed compared to the sinuous movements we observed in the Northwest Atlantic Shelves, suggesting a mix of behaviors that may include foraging, transiting and breeding. The Guianas Coastal and Caribbean regions encompass important breeding and nesting habitat for leatherbacks
The average mixed layer depth varies throughout the tropics, with deepest depths occurring in winter when leatherbacks are present
The convergence of the North Equatorial Current, North Equatorial Counter-Current and the North Brazil Current appears to play an important role for overwintering leatherbacks in the southern part of their range. From June to January, the upper North Brazil Current joins the meandering North Equatorial Counter-Current at a retroflection zone near 5–10°N, where large, anti-cyclonic eddies are formed
Both adult and sub-adult leatherbacks in our study adjusted their movements and dive behavior in response to regional differences in environmental features. Leatherbacks increased their path sinuosity with decreasing water depth in temperate and tropical shelf habitats. This relationship is consistent with increases in gelatinous zooplankton biomass with decreasing water depth
We thank A. Myers, C. Merigo, C. Innis, M. Dodge, G. Purmont, M. Leach, B. Sharp, S. Landry, M. Murphy, G. Tomasian, N. Fragoso, K. Sampson, R. Smolowitz, K. Hirokawa, J. Casey, S. Leach, J. Wilson, and E. Eldredge for invaluable assistance in the field. We thank A. Rhodin for surgical expertise and guidance in development of humane and effective direct tag attachment procedures for leatherbacks. S. Benson, P. Dutton and M. James gave helpful suggestions and advice regarding leatherback captures at sea. We thank F. Royer for insightful discussions and expert assistance with track reconstruction. M. Coyne provided valuable assistance with oceanographic data extraction. We are grateful to B. Prescott and K. Dourdeville for timely leatherback sightings information in southern New England (