Briscoe et al. Movement Ecology (2016) 4:23
DOI 10.1186/s40462-016-0087-4

RESEARCH

Open AccessMulti-year tracking reveals extensive
pelagic phase of juvenile loggerhead sea
turtles in the North Pacific
D. K. Briscoe1*, D. M. Parker2, S. Bograd3, E. Hazen3, K. Scales3, G. H. Balazs4, M. Kurita5, T. Saito6, H. Okamoto5,
M. Rice7, J. J. Polovina4 and L. B. Crowder1,8

Abstract
Background: The juvenile stage of loggerhead sea turtles (Caretta caretta) can last for decades. In the North Pacific
Ocean, much is known about their seasonal movements in relation to pelagic habitat, yet understanding their multi-year,
basin-scale movements has proven more difficult. Here, we categorize the large-scale movements of 231 turtles satellite
tracked from 1997 to 2013 and explore the influence of biological and environmental drivers on basin-scale movement.
Results: Results show high residency of juvenile loggerheads within the Central North Pacific and a moderate influenceof the Earth’s magnetic field, but no real-time environmental driver to explain migratory behavior.
Conclusions: We suggest the Central North Pacific acts as important developmental foraging grounds for young
juvenile loggerhead sea turtles, rather than just a migratory corridor. We propose several hypotheses that may influence
the connectivity between western and eastern juvenile loggerhead foraging grounds in the North Pacific Ocean.
Keywords: Loggerhead sea turtle, Migration, Foraging, Movement, Distribution, Pelagic

Background
Highly mobile marine species utilize dynamic oceanographic habitats as they move between breeding andnon-breeding habitats [1, 2]. While advancements intracking have greatly enhanced our ability to understand how migratory animals move through their environment (seabirds, marine mammals, and sea turtles), it
still remains a challenge to understand the degree ofconnectivity between the reproductive and foraging
grounds [3]. Environmental flows such as wind andcurrents are known to influence early animal movement into the open ocean, and can potentially impact
the ontogeny of foraging and migratory patterns asanimals develop [4–6]. However, limitations still exist
in our ability to track individuals throughout life history
stages [7]. For many species, the pelagic stage is inferred rather than empirically observed [8]. This isespecially true for sea turtles, as the oceanic period ofearly life history has been termed the ‘lost years’ [9].
* Correspondence: dbriscoe@stanford.edu
1
Hopkins Marine Station, Stanford University, Pacific Grove, CA, USA
Full list of author information is available at the end of the article

The long-term tracking data of juvenile 231 loggerhead
sea turtles (Caretta caretta) provides an unprecedented
opportunity to examine the large-scale movements anddistribution of individuals during a poorly understood life
history stage (Fig. 1). In the North Pacific, hatchlings leave
their natal beaches of Japan and undergo a multi-year
migration within the North Pacific Gyre [10, 11]. Juveniles
are known to forage throughout the Central North Pacific(CNP) [12, 13], migrating to eastern developmental
grounds, along the Baja California Peninsula, Mexico(BCP) [14, 15]. Upon reaching maturity, turtles migrateback to their natal beaches of Japan and remain in thewestern Pacific as adults [11, 16, 17]. Studies of this population have provided exceptional insight into seasonal
foraging movements, diet, and active dispersal of juveniles
throughout the North Pacific [12, 13, 18–25].
Because the juvenile life history stage can last up to
three decades [26–28], the timing, duration, and longterm patterns of movement across the ocean basin still
remain relatively unknown [29]. As a result, there
remain important gaps in our understanding of thebasin-scale movements and residence time within© 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Briscoe et al. Movement Ecology (2016) 4:23

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Fig. 1 Map of the North Pacific Ocean and 231 satellite tagged juvenile loggerhead sea turtle locations from 1997 to 2014. Gray tracks represent
197 captive-reared juveniles; red tracks represent wild-caught juveniles. Black dots indicate the deploy locations for individual tracks

foraging habitats, in particular the connectivity between
juvenile foraging grounds, which impedes conservation efforts along migratory routes [30]. Here, we sought to: (1)explore movements and connectivity between nesting andforaging grounds of juvenile North Pacific loggerhead sea
turtles, and (2) explore the influence of biological and environmental drivers on basin-scale movements. Results
show a lengthy residence time of juveniles within the Central North Pacific. These results challenge the longstanding belief that juvenile loggerheads use the Central
North Pacific as a migratory corridor to eastern basin
developmental grounds. The extended time within the
open ocean reveals many individuals move back andforth within the pelagic environment for several years.

Methods
Animal tracking

Our study involves the synthesis of 186 previously published tracks [12, 13, 18–21, 24, 25, 31] and includes 45
new tracks from 2011 to 2013 (Table 1). This is the firstpublication of all tracks from 1997 to 2013. Briefly, we
review the outfitting procedure. One hundred andninety-seven sea turtles were hatched and raised in the
Port of Nagoya Public Aquarium in Minoto-ku, Japan.
Thirty-four were wild-caught juveniles. Individuals werereleased in the ocean in several locations off of Japan or in
the Central North Pacific (CNP) (Fig. 1a). Argos-linked
satellite transmitters were attached to the carapace of all
juveniles, following the procedures recommended in [32].
Argos-derived surface locations were collected by the
NOAA PIFSC, Marine Turtle Research Program, Honolulu, Hawaii.
All raw surface locations were filtered and regularizedusing a Bayesian State Space Switching Model (SSSM). Developed by [33], the SSSMs account for observation error,
in order to regularize animal location estimates in time, as
well as interpolate over small gaps resulting from missing
observations of animals locations [1]. Final position estimates along each track were generated at 24-h intervals.

Characterization of basin-scale movements

In order to characterize long-term, large-scale movementsof juvenile loggerheads in the North Pacific, we applied
several filters to account for tagging bias and deploy location, and the presence of short or incomplete tracks[34, 35]. Tracks were first categorized by deploy region(Japan or CNP). Individuals that transmitted for lessthan 60 days were removed from the data set. Fromthese data, spatial density maps were used to calculate
areas of high residency, based on deploy location. Using
hexagonal polygon binning, we calculated the numberof days spent in a 1° longitudinal area, similar to [36](Fig. 2). The movements of each individual track wereclassified into one of three dominant migratory routes,
based on direction of travel. Throughout all tracks, theseasonal north-south migration with the Transition
Zone Chlorophyll Front (TZCF) is evident, as describedby [31]. Because we have long-term, multi-year data forthese animals, we can go beyond seasonal movements
to look at the larger, basin scale movements of their migrations. For this reason, movements were categorized
by their east-west dynamics: (1) moving eastward
within the CNP foraging grounds (e.g. Fig. 3a), (2) moving westward within the CNP (e.g. Fig. 3b), (3) moving
eastward but underwent a considerable change in direction (e.g. moving east then west, or ‘turning around’,
thereby staying within the CNP) (e.g. Fig. 3c), and (4)moving eastward to Baja California, Mexico foraging
grounds (e.g. Fig. 3d). Since almost all tracks displayed
portions of eastward migration (Fig. 2), tracks thatmoved east then west were identified by a subsequent
longitudinal displacement of at least 3° in the westward
direction (Fig. 2c).
Exploration of environmental and biological drivers ofbasin-scale movements
Remotely-sensed oceanographic data

Remotely sensed environmental data were obtained foreach loggerhead track location using Xtractomatic (http://

Briscoe et al. Movement Ecology (2016) 4:23

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Table 1 Summary of 231 satellite tracked juvenile loggerhead sea turtles in the North Pacific Ocean
Deploy Date

Deploy E Lon

Deploy N Lat# Deployed

Captive/Wild Caught

SCL (cm)

Age at Deploy(months)

Days
Transmitted

Distance
Traveled (km)24-Apr-03

140.2

34.6

7

Captive

38.9–59.4

20–44

67–565

1939–12,357

28-Nov-03

140.2

34.9

17

Captive

26.2–56

15–40

48–1270

2517–22,415

19-Nov-04

140.6

34.9

26

Captive

27.7–35.3

15

85–4643263–11,832

19-Apr-05

141.1

35.4

12

Captive

25.6–64.8

24–48

27–617873–16,338

4-May-05

176.6

32.7

40

Captive

29.6–38.4

20

229–1434

3852–25,900

27-Oct-06

176.8

32.9

34

Captive

24.4–29.5

NA

47–4931258–8075

31-Mar-09

141.4

35.4

16

Captive

30.9–37.2

19

97–6963044–15,196

9-Apr-10

130.5

29.7

17

Captive

32.8–40.7

20

268–558

5937–12,226

12-Jul-11

141.3

35.7

15

Captive

34.5–71.1

24–60

11–480

275–11,112

12-Jul-11

153.7–180.0

31.7–42.4

13

Captive

34.9–39.1

24

571–865

11,425–17,863

23-Jan-97

169.8

28.7

1

Wild

44.5

NA

55

1013

2-Feb-97

163.3

29.5

1

Wild

52

NA

115

2592

15-Feb-97

161.0

29.8

1

Wild

41

NA

90

1311

17-Mar-97

154.4

30.9

1

Wild

62

NA

136

3480

30-Mar-97

160.6

26.2

1

Wild

73

NA

42

989

10-Apr-97

169.0

26.7

1

Wild

73.6

NA

13

372

20-Apr-97

154.7

29.2

1

Wild

53.7

NA

12

295

22-Apr-97

156.6

28.8

1

Wild

81

NA

178

5199

11-Sep-97

131.1

37.7

1

Wild

45

NA

67

1703

6-Jan-98

143.0

33.6

1

Wild

45.5

NA

206

3136

7-Jan-98

142.5

34.4

1

Wild

48

NA

191

3518

7-Feb-98

154.7

30.6

1

Wild

58

NA

103

1876

7-Feb-98

155.1

30.5

1

Wild

61

NA

71

1442

26-Aug-98

162.6

36.3

1

Wild

58

NA

167

2301

26-Aug-98

163.3

36.4

1

Wild

57.7

NA

106

2001

18-Oct-98

164.8

37.7

1

Wild

52.5

NA

41

935

20-Oct-98

139.6

38.5

1

Wild

59.1

NA

161

2448

2-Nov-98

161.9

36.7

1

Wild

62.5

NA

51

737

10-Dec-98

136.1

34.2

1

Wild

56.5

NA

6

13

23-Dec-98

210.0

33.6

1

Wild

57.5

NA

211

4426

31-Jan-99

156.3

24.8

1

Wild

83

NA

51

1024

3-Feb-99

153.8

32.0

1

Wild

52.5

NA

131

1727

14-Dec-99

209.1

32.9

1

Wild

51.5

NA

271

5180

17-Jan-00

143.3

32.8

1

Wild

62

NA

72

1766

3-Feb-00

169.3

32.7

1

Wild

67

NA

157

4221

12-Feb-00

138.6

32.3

1

Wild

55

NA

49

1177

5-Mar-00

146.6

31.1

1

Wild

60

NA

597

13,864

7-Mar-00

146.7

31.1

1

Wild

56

NA

246

3757

30-May-00

154.8

25.0

1

Wild

83

NA

138

3543

19-Aug-00

226.4

35.8

1

Wild

61

NA

177

3237

14-Oct-02

140.1

41.3

1

Wild

55.5

NA

358

9591

Briscoe et al. Movement Ecology (2016) 4:23

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Table 1 Summary of 231 satellite tracked juvenile loggerhead sea turtles in the North Pacific Ocean (Continued)
25-Dec-02

219.1

34.7

1

Wild

45.5

NA

245

4464

7-Jan-03

216.5

32.4

1

Wild

43.5

NA

226

3800

1-Aug-03

140.6

34.3

1

Wild

68.1

NA

336

14,579Sea turtles were deployed within two regions: the Western North Pacific (Japan) and the Central North Pacific (CNP)

coastwatch.pfel.noaa.gov/xtracto/). The data sets included
time-series of SST, surface chlorophyll-a concentrations, geostrophic current components (u and v), andSST variability (i.e. SST root mean square, SST RMS)
(Additional file 1: Table S1). For each oceanographic
parameter, a mean value was calculated based on themean latitude and longitude error (0.1° longitude ×
0.1° latitude × (1 to 8 day intervals) and centered at
the position of each daily SSSM-interpolated turtle

position (sensu [37]). Transformations of the parameters
were explored to ensure data were normally distributed. A
logarithmic transformation was required for chlorophyll-a.
A square root transformation was applied to SST RMS.
IGRF magnetic field data

Geomagnetic data were available from NOAA’s National
Geophysical Data Center (NGDC; http://www.ngdc.noaa.
gov/geomag). Magnetic field observations were calculated

Fig. 2 Spatial use of juvenile loggerhead sea turtles, color-coded by deploy location: a Japan (purple) and (b) the Central North Pacific (red). The
number of turtle days are represented by each 1° hexagonal bin. c Frequency use by degree longitude, color-coded by deploy region: Japan(purple) and the Central North Pacific (red)Briscoe et al. Movement Ecology (2016) 4:23

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Fig. 3 Example tracks showing the dominant migratory routes from these data: a moving eastward within the CNP, b moving westward within
the CNP, c moving east then west, staying within the CNP, and d moving eastward to Baja California, Mexico. Track deploy locations are in green
and end locations are in red

using the International Geomagnetic Reference Field(IGRF) coefficients. As the Earth’s magnetic fieldchanges over time, these coefficients were updated by
the International Association of Geomagnetism andAeronomy to accurately reflect the magnetic field atpresent (NGDC 2015, link above). The most recent,11th generation coefficients for total magnetic intensity,
inclination, and declination were used in this study, as

each have been shown to be detectable by sea turtles[25, 38–42]. A 1° by 1° monthly grid of each field component was calculated for the North Pacific Ocean basinusing the GeoMag 7.0 software available from the
NGDC.
Generalized Additive Models (GAMs) were used to
explore the environmental and biological factors associated with sea turtles that moved eastward across theBriscoe et al. Movement Ecology (2016) 4:23

North Pacific but then reversed direction, staying within
the CNP (Fig. 3c, Additional file 1: Table S2).

Results
Track movements

Two hundred and thirty-one juvenile loggerhead sea turtles (23.3–83 SCL cm) were tracked from January 1997
– November 2013. One-hundred and ten aquariumreared and 24 wild-caught individuals were released off
of Japan. Seventy-four aquarium-reared and 23 individuals were released within the CNP. Deployments ranged
from 41 to 1434 days (mean 351 days ± 256 days SD). Of
these, 95 individuals transmitted for greater than 1 year;
14 for more than 2 years and 8 for more than 3 years.
Individuals deployed off Japan utilized the Kuroshio Extension Current (KEC) to disperse into pelagic areas(Fig. 2a). Individuals deployed within the CNP showed
high use between 180° and 160°W (Fig. 2b). Mapping ofloggerhead movements showed that irrespective of deploy
location, turtles displayed an extended residence time
(greater than 100 turtle days per grid cell), between 165°
E–158°W longitudes (Fig. 2c). This long-term residency
was demonstrated by several individuals that traversedback and forth within the CNP for several years (Fig. 4).
One hundred and forty-seven individuals (63.6 % of
the total) displayed an eastward only migratory pattern
before end of transmission (376.2 days ± 252.3 days),moving an average distance of 2028 km (±2454.3 km).
Of these, ten were wild-caught turtles.
Nineteen sea turtles (8.2 % of the total), all wild-caught,
moved westward from their CNP deploy locations, towards Japan (mean 199.6 days ± 192.9 days, 3105 km ±2096.3 km). Sixty-five individuals (28.1 % of the total)
moved eastward and then reversed direction along theirmigration routes, staying within the CNP (582 days ±304.7 days, mean 11,128 km ± 4939.3 km). Five were wildcaught individuals. Thirty-four of these 65 turtles were
deployed off of Japan and reached an average maximum
eastward longitude of 180° before reversing direction(Additional file 1: Figure S1). The remaining 31 turtles
were deployed throughout the Central North Pacificand reached an average maximum longitude of 160°W(Additional file 1: Figure S1). Only one out of all 231
turtles migrated to Baja California (Fig. 3d).
Environmental data

Median sea surface temperatures experienced by turtles
was 17.6 °C (±2.2 °C, range of 10.0–28.74 °C) and 0.2 °C(±0.2 °C, range of 0–1.9 °C) for SST RMS. Median concentration of chlorophyll-a was 0.3 mg m-3 (±1.8 mg m3, range of 0.0–72.8 log mg m-3). Median magnetic fieldvalues were 51.3° (±5.1°, range of 26.3–62.1°) for inclination, -0.1° (±5.0°, range of -16.1–6.3°) for declination,and 43,820 nT (±1749.2 nT, range of 35,640–49,410 nT)

Page 6 of 12

for total intensity. Sea turtles deployed off of Japan experienced the greatest change in the Earth’s magnetic field
declination, corresponding to 160° E - 180° (Fig. 5b-c).
Individuals that were deployed within the CNP were
deployed eastward of this gradient, and thus did notexperience the same regional change in the Earth’s Magnetic declination as the individuals deployed off of Japan.
GAM results

No real-time environmental variables were significant inunderstanding a change in direction for the 65 individuals that moved from east to west (Additional file 1:
Table S3). The two variables that were primarily attributed to a change in direction for the western Pacificdeployed turtles were magnetic field declination (Fig. 5
and the number of days traveled (Additional file 1:
Figure S4a-b)). For the central North Pacific, the twovariables most correlated to a change in direction weremagnetic field inclination and the month. Thesemodels revealed that western Pacific deployed turtles
are more likely to continue traveling eastward underlower magnetic field declination values and for the
first 250–300 days of travel, whereas the CNP turtles
are more likely to continue traveling eastward underhigher values of magnetic field inclination and towards
the latter half of the year (Fig. 5 and Additional file 1:
Figure S4c-d).

Discussion
This study combined information from two decades ofsatellite tracking of juvenile loggerhead sea turtles in the
North Pacific to describe the movements of individuals
during a poorly understood life history stage. Results
show a long-term residence of sea turtles within theCentral North Pacific and an exceptional level of variability in their individual migration strategies across theocean basin. Contrary to expectations, we found no realtime environmental influences to explain migratory behavior. However, a moderate influence of the Earth’s
magnetic field was detected, suggesting that movements
may be driven by the navigational markers that help
guide them towards thermally optimal and biologicallyfavorable habitats within the open ocean, similar to
Atlantic loggerhead sea turtles [40].
Traditional loggerhead sea turtle life history assumed
that young, at-sea turtles were passive migrants, transported cross-basin along migratory corridors to easternboundary foraging grounds [9]. Upon reaching these developmental grounds, juveniles were thought to undergo
an ontogenetic shift from oceanic to neritic habitat [10].
Currents may carry individuals to suitable places thatthey revisit as juveniles, thereby shaping the ontogenetic
development of migration routes [43]. Indeed, recent
studies in both the Atlantic and Pacific Ocean basins

Briscoe et al. Movement Ecology (2016) 4:23

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Fig. 4 Example tracks showing the high residence time within the Central North Pacific (CNP). Tracks are color-coded by year: a Track ‘23045’,
number of days transmitted: 1270; b Track ‘50136’, number of days transmitted: 1247; c Track ‘57148’, number of days transmitted: 1434; and d
Track ‘23002’, number of days transmitted: 865. Track deploy locations are in green and end locations are in red

have suggested that active dispersal and orientation playlarger than expected roles in the at-sea movements ofyoung loggerheads [22, 44]. In this study, both wildcaught and captive-reared individuals moved in eastward
and westward directions across the ocean basin, suggestivethat it may be part of the natural behavior of individuals
from this population to move back and forth within the
CNP. However, of the 231 juvenile loggerheads tracked up
to 4.9 years, only one underwent a successful migration to
Baja California (Fig. 1a). This is perplexing, as the coastal
waters of the BCP in the eastern North Pacific are believed

to be an important developmental foraging ground for theentire population [15]. Conditions off of BCP in the east
are more energetically efficient including faster growth
rates and eventually higher fecundity, albeit with thepotential trade-off of higher predation risk [28]. Recent
estimates suggest that up to 43,000 juveniles utilize thisforaging hotspot each year [15]. Based on these results, we
propose several hypotheses that may explain why juveniles
may stay within the CNP for extended periods of time,
instead of migrating directly to the Baja California
Peninsula.

Briscoe et al. Movement Ecology (2016) 4:23

Page 8 of 12b)

Latitude

50˚N

80
60
40

40˚N

20
0

30˚N

20

50˚N

10
Latitude

a)

40˚N

0
-10

30˚N

-20

-20

20˚N

20˚N
140˚E

160˚E

180˚

160˚W

140˚W

120˚W

100˚W

140˚E

160˚E

Longitude

180˚

160˚W

140˚W

120˚W

100˚W

Longitude

8

0.4
0.2

6

0.0

4

-0.2
-0.4

2

Declination Gradient

Number of Turtles

c)

-0.6
0
160E

180

160W

140W

120W

Longitude

Fig. 5 IGRF-10 Earth magnetic field values for (a) inclination and (b) declination, across the North Pacific Ocean basin; (c) Number of turtles thatreversed direction (staying within the CNP) by longitude (green). Overlain in orange is the average change in declination by longitude (gradient).
The sharpest change in declination occurs between 160°E - 180° longitude (dashed lines). This region is known for its biological productivity [13]Hypothesis 1: CNP juveniles are not mature enough torecruit to BCP

To date, most studies of foraging loggerheads off theBaja California coast have focused on large juveniles(55–85 SCL cm) [28, 45]. For this reason, it could besuggested that turtles from this data set were too young
to undergo an ontogenetic shift to neritic waters. However, recent skeletochronology has aged BCP turtles as
young at 3 years old, which overlaps with ages of turtles
tracked in this study [46]. This matches the age and size
range of the one turtle from this data set that did
migrate to the BCP. It should be noted that this turtle
was originally deployed in the Central North Pacific, essentially giving it a head start towards its eastern foraging
grounds. However, [28] found there to be no significant
difference in the SCL sizes of CNP and BCP juveniles.
Hypothesis 2: Some utilize pelagic waters throughout
their entire juvenile life history stage

Because loggerheads have been shown to take advantage
of both oceanic and neritic habitats as both juveniles
and subadults [47, 48], it is entirely possible that not all
loggerhead turtles in the North Pacific undergo transPacific dispersals, but instead use pelagic habitat for
their entire juvenile phase, as suggested by [13]. A recent
study of a subset of individuals from this population
showed active orientation of juveniles within the CNP[22]. For this reason, it may be that the CNP is not just a

migratory corridor that juveniles pass through on the way
to foraging grounds off Baja California, but representativeof juvenile foraging grounds altogether. The extended residence time within the CNP and lack of a migration to the
BCP could be indicative of an alternative life history strategy for juveniles of this population [28].
Hypothesis 3: Turtles returning to previously experienced
preferable habitat

For many migratory species (birds, tuna, sharks, andother species of sea turtles), complete migrations are notsimple or direct [34, 49–51]. In fact, indirect routes are‘followed not to only avoid unfavorable areas but as apragmatic solution to completing a long journey successfully’ [50]. This may be especially true for early stage
sea turtles, as they are free from the constraints ofbreeding, and are able to seek out the most productive
areas to optimize growth while avoiding thermal
stressors [28, 52]. Long distance migration across the
CNP is likely energetically costly. Upon leaving the seasonally productive waters of the Kuroshio Extension
Bifurcation Region (KEBR) and the Transition Zone
Chlorophyll Front (TZCF) (see [13, 31], foraging opportunities in the eastern CNP may be increasingly difficult.
That turtles frequently appear to turn around as they
move further east across the North Pacific and after amean of 334.5 (±227.8 SD) days at sea, may be due toreaching a maximum energetic threshold (see [50]). Such

Briscoe et al. Movement Ecology (2016) 4:23

a detour, or re-entry, of juveniles into the biologically
productive waters of the KEBR and TZCF would allow
turtles to minimize their energetic costs of travel, thatprevents fasting and allows turtles to refuel for a potentially long-distance migration to the eastern Pacific developmental grounds. Similar behavior has been shownfor green turtles (Chelonia mydas) in the South Atlantic,
as they undergo transoceanic migrations between breeding sites (Ascension Island) and coastal foraging sites
along Brazil [50].
Just how loggerheads may be able to successfully navigate a return to foraging hotspots has been better understood in other populations. In the North Atlantic Ocean,
extensive research has shown that early stage loggerheads display a versatile navigational system. Namely,
their open ocean migration is guided in part by passive
drift associated with North Atlantic Subtropical Gyre
(NASG) and Gulf Stream circulation, and active orientation due to swimming [9, 47, 53, 54]. There is a strong
selective pressure for a juvenile turtle to remain within
geographic areas that provide suitable conditions [55].
Poor navigation outside the gyre could lead to lethal
temperatures [41]. This is done by active orientation in
relation to a regional magnetic map [39–42, 54, 56].
These studies have effectively shown that the transoceanic migration of loggerheads in the North Atlantic,
and the geographic regions they utilize along their migratory path, including ontogenetic shifts in habitat—are
in large part bounded by the navigational markers associated with changes in the magnetic field [40–42]. In
the North Pacific, magnetic influences have not beenstudied as in depth, however [25] noted that changes in
the magnetic field could influence habitat choice on abasin-scale level.
Results from this study show that, similar to North
Atlantic studies, North Pacific juveniles may use theEarth’s magnetic field [41] to reorient themselves backto the favorable habitat of the KEBR, while staying
within thermally suitable latitudes. The isoclines ofmagnetic field inclination are similar to the latitudinalchanges in SST (Fig. 5a and Additional file 1: Figure S3a).
As individuals move with the north-south trend of the
TZCF [20, 31], isoclines of inclination may help prevent
animals from being swept by currents into inhospitabletemperatures, also similar to the North Atlantic [41].
Turtles deployed off Japan travel through the Kuroshio
Extension Current and Bifurcation Region (KEC and
KEBR, respectively), and thus, a sharp gradient in declination as they make their way through the CNP, from 160°
E - 180° longitude (Fig. 5b-c). This is the same region
known for its biological productivity [13] (Additional
file 1: Figure S3b). Several studies have examined thepotential for magnetic maps to be ‘imprinted’ uponturtles throughout their transoceanic migrations [43,Page 9 of 12

57, 58]. Therefore, one explanation for a reversal in
migration is the use of indirect movements based on
the number of days traveled and the increase in energetic costs. As turtles move eastward throughout the
CNP, foraging opportunities may be less prevalent,
triggering them to turn around and return to morefavorable habitat using a combination of regional navigational markers.
Hypothesis 4: Migration routes may be tied to genetics

Until the 1990s, the origins of Baja California loggerheads
were entirely unknown. Work by [10] connected the haplotypes of Baja juveniles to observed haplotypes of loggerheads found off Japan. It is now known that nesting forthe entire North Pacific loggerhead population is restricted to the Japanese Archipelago [10, 17, 59].
More recent work has begun to highlight significant
differentiation among Japanese rookeries [11, 60] andtheir influences on the spatial distribution of feeding
aggregations, suggesting that genetic composition ofloggerheads may be tied to post-nesting migration
patterns [61]. We hypothesize that these genetic differences may also express themselves during developmental migrations, such that there may be a genetic
component from some nesting beaches that contribute to Baja California migrations. Despite the large
sample size, it is possible that satellite tagged animals
from this study representative of nesting beaches that
do not display this genetic contribution. However,until there is better resolution of the genetics of juvenile North Pacific loggerheads, this hypothesis willremain difficult to elucidate.
Hypothesis 5: Environmental conditions influence
recruitment to BCP

Like all ectotherms, sea turtles are inherently tied to thetemperature of their surrounding environment. Studies
have shown that higher water temperatures are energetically more favorable for sea turtles in terms of growth,digestion, and maintenance of core body temperature,
up to a 30 °C thermal maxima [14, 62, 63]. Moreover,
several management strategies currently use SST and
ENSO events as a metric of bycatch avoidance for the
North Pacific loggerhead population (see [24, 64]). During El Nino years, i.e. when SSTs are anomalously
warmer, the California Drift Gillnet Fishery is closed due
to increased interaction with loggerhead sea turtles [64];an event that is not experienced during other oceanographic regimes. It is possible that interannual variability
and/or anomalous SSTs play a larger role in east-west
movements, promoting or prohibiting movement eastwards, towards Baja California, Mexico. However, moredata is needed to fully explore this hypothesis.

Briscoe et al. Movement Ecology (2016) 4:23

Caveats

It should be noted that there are some caveats to these
data, which may hinder our ability to fully understandthe relationship between loggerhead movement and theirenvironment. Daily SST measurements may capture
mesoscale features, but may not be representative of relationships to anomalous events or larger-scale oceanographic and atmospheric variability (e.g. ENSO). The
absence of a prey field and intermittent satellite data due
to cloud cover (i.e. chlorophyll concentration) may prevent the model from being more robust. Ultimately,models that incorporate an index of forage and that
address turtle energetics may be needed to further
advance our understanding of movement. Further, themajority of the tracks represent captive-reared turtles(n = 204 out of 231 turtles). While some caution must
be taken in comparing captive with wild caught turtle
behavior, no study to date has observed significant
differences in migration and swimming behavior between the two [13, 21]. However, further studies areneeded to compare the habitat and behaviors of
captive-reared and wild-caught sea turtles under similar environmental conditions [21].

Conclusions
North Pacific loggerhead sea turtles travel long distances
and across entire ocean basins to reach developmental foraging grounds, yet connectivity between east-west movements has remained difficult. While individuals from thisendangered population display a range of movement strategies, results from multi-year tracking reveals extensive
use of the Central North Pacific. Here, we have shown that
east-west movements may be due to environmental cuesin the Earth’s magnetic field, which may aid in navigationback to preferable habitat. We suggest the Central North
Pacific acts as important developmental foraging grounds
for young juvenile loggerhead sea turtles, rather than justa migratory corridor, as results of this study show thatmany potentially important areas utilized by oceanic loggerheads may fall within unprotected areas of the highseas, offering critical geographic information that may be
used for spatially-explicit conservation approaches within
the pelagic environment. Further understanding the
movement ecology of juvenile North Pacific loggerheads
is therefore crucial for more efficient conservation strategies of this population.
Additional file
Additional file 1: Table S1. Satellite product and spatio-temporal
resolution of environmental variables sampled underneath of loggerhead
tracks. Table S2. Summary of the 65 satellite tracked juvenile loggerhead
sea turtles in the North Pacific Ocean that displayed turn around behavior.
Turtles were deployed within two regions: the Western North Pacific (Japan)

Page 10 of 12

and the Central North Pacific (CNP). Table S3. Model selection results fromgeneralized additive modeling of the environmental conditions at thelocation of reversal in migration (‘turnaround’) versus the conditions
experienced by a turtle as it continued to move eastward across the North
Pacific Ocean. Presented are the p-values, r-squared, estimated degrees of
freedom, and AIC for each of the eleven environmental parameters for (a)Japan deployed turtles and (b) turtles deployed within the Central North
Pacific. Figure S1. Density histogram of maximum eastward longitude for(a) all turtles, n = 231 (gray), long-term tracks deployed off Japan, n = 34
(blue line), and long-term deployments in the Central North Pacific, n = 31
(green line). Figure S2. Examples of east-west movement in individual
long-term tracks. Example of 2 turtles deployed in the western (a) and (b)central North Pacific that reached a maximum eastward trajectory andchanged dominant direction. Panels c and d show each track moving in an
east-west-east direction, with the start location designated by a blue triangle
and final location designated with a red square. Both tracks initially moved
eastward (gray line) and then reversed direction, moving westward (bluesegment of track). Both changed dominant direction for second time, once
again moving eastward (green segment of track). Track ‘68330’ transmitted
for 614 days (15,289 km). Track ‘22534’ transmitted for 1047 days
(18,238 km). Figure S3. (a) Average sea-surface temperature (SST °C) and (b)Chlorophyll-a concentrations (mg m-3) for the North Pacific Ocean basin,from 1997 to 2014). Figure S4. GAM response curves of juvenile
loggerhead east-west movements. Positive values show the likelihoodof an animal reversing direction under a range of environmental
values. Results indicate that turtles deployed off Japan are more likely
to reverse direction with an increase in declination (a) and the longer
it travels since deployment (b). For the turtles deployed in theCentral North Pacific, turtles are more likely to reverse direction underlower values of magnetic field inclination (c) and during the first half of theyear (d). Gray shading represents the 95 % confidence intervals for the fitted
relationships. (DOCX 1198 kb)
Acknowledgements
The authors would like to acknowledge the personnel at Port of Nagoya
Public Aquarium and the three anonymous reviewers, whose comments
improved this work.
Funding
Funding for DKB was provided by the Crowder Lab at Hopkins Marine
Station, Stanford University.
Authors’ contributions
MK, TS, HO conducted fieldwork including animal care and tag deployment.
DKB, SB, EH, KS,GHB, JJP, MK, TS, HO and MR conceived and designed
experiments. DMP, GHB, DKB acquired and analyzed data. DKB performedmodeling analysis. DMP, SB, EH, KS, GHB, JJP, LBC, MK, TS, HO and MR
contributed to theoretical concepts and research materials. DKB drafted
paper. DMP, SB, EH, KS, GHB, JJP, LBC contributed to paper revisions forimportant intellectual content. All authors approve of the final version to be
published.
Competing interests
The authors declare that they have no competing interests.
Ethics approval and consent to participate
Animal care and permitting needs for this study were fulfilled by the Port ofNagoya Public Aquarium. All treatments were humane and in fullcompliance with the requirements and approval of the government of Japanof which the Aquarium is an entity. Permission to tag and release turtles
were completed by Port of Nagoya Public Aquarium of Japan.
Author details
1
Hopkins Marine Station, Stanford University, Pacific Grove, CA, USA. 2Joint
Institute for Marine and Atmospheric Research, National Oceanic andAtmospheric Administration, Newport, OR, USA. 3Environmental Research
Division, National Marine Fisheries Service, National Oceanic andAtmospheric Administration, Southwest Fisheries Science Center, Monterey,
CA, USA. 4National Marine Fisheries Service, National Oceanic andAtmospheric Administration, Pacific Islands Fisheries Science Center,

Briscoe et al. Movement Ecology (2016) 4:23

Honolulu, HI, USA. 5Port of Nagoya Public Aquarium, Minato-ku, Nagoya
455-0033, Japan. 6Usa Marine Biological Institute, Kochi University, Usa Tosa,
Kochi 781-1164, Japan. 7Hawaii Preparatory Academy, 65-1692 Kohala Mt. Rd.
Kamuela, Hawaii 96743, USA. 8Center for Ocean Solutions, Stanford
University, Monterey, CA, USA.

Page 11 of 12

21.

22.
Received: 22 April 2016 Accepted: 13 September 201623.
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