HYPOTHESIS AND THEORY
published: 08 April 2021
doi: 10.3389/fmars.2021.630590

Dynamic Thermal Corridor May
Connect Endangered Loggerhead
Sea Turtles Across the Pacific Ocean
Dana K. Briscoe 1,2,3* , Calandra N. Turner Tomaszewicz 4 , Jeffrey A. Seminoff 4 ,Denise M. Parker 5 , George H. Balazs 6 , Jeffrey J. Polovina 7 , Masanori Kurita 8 ,Hitoshi Okamoto 8 , Tomomi Saito 9 , Marc R. Rice 10 and Larry B. Crowder 1
1

Stanford University, Hopkins Marine Station, Pacific Grove, CA, United States, 2 Cawthron Institute, Nelson, New Zealand,Institute of Marine Sciences, University of California, Santa Cruz, Santa Cruz, CA, United States, 4 Southwest Fisheries
Science Center, NOAA National Marine Fisheries Service, La Jolla, CA, United States, 5 Golden Honu Services of Oceania,
Newport, OR, United States, 6 Golden Honu Services of Oceania, Honolulu, HI, United States, 7 Marine Biology Program,
The University of Hawai’i at Mānoa, Honolulu, HI, United States, 8 Port of Nagoya Public Aquarium, Nagoya, Japan, 9 Usa
Marine Biological Institute, Kochi University, Kôchi, Japan, 10 Hawaii Preparatory Academy, Kamuela, HI, United States
3

Edited by:
Alastair Martin Mitri Baylis,
South Atlantic Environmental
Research Institute, Falkland Islands
Reviewed by:
Gail Schofield,
Queen Mary University of London,
United Kingdom
Paolo Luschi,
University of Pisa, Italy*Correspondence:
Dana K. Briscoe
danakbriscoe@gmail.com
Specialty section:
This article was submitted toMarine Megafauna,a section of the journal
Frontiers in Marine Science
Received: 17 November 2020
Accepted: 12 February 2021
Published: 08 April 2021
Citation:
Briscoe DK, Turner
Tomaszewicz CN, Seminoff JA,
Parker DM, Balazs GH, Polovina JJ,
Kurita M, Okamoto H, Saito T,
Rice MR and Crowder LB (2021)
Dynamic Thermal Corridor May
Connect Endangered Loggerhead
Sea Turtles Across the Pacific Ocean.
Front. Mar. Sci. 8:630590.
doi: 10.3389/fmars.2021.630590

The North Pacific Loggerhead sea turtle (Caretta caretta) undergoes one of the greatestof all animal migrations, nesting exclusively in Japan and re-emerging several years later
along important foraging grounds in the eastern North Pacific. Yet the mechanismsthat connect these disparate habitats during what is known as the “lost years” haveremained poorly understood. Here, we develop a new hypothesis regarding a possible
physical mechanism for habitat connectivity – an intermittent “thermal corridor” – using
remotely sensed oceanography and 6 juvenile loggerhead sea turtles that formed partof a 15 year tracking dataset of 231 individuals (1997–2013). While 97% of individuals
remained in the Central North Pacific, these 6 turtles (about 3%), continued an
eastward trajectory during periods associated with anomalously warm ocean conditions.
These few individuals provided a unique opportunity to examine previously unknown
recruitment pathways. To support this hypothesis, we employed an independently
derived data set using novel stable isotope analyses of bone growth layers and assessedannual recruitment over the same time period (n = 33, 1997–2012). We suggest
evidence of a thermal corridor that may allow for pulsed recruitment of loggerheads
to the North American coast as a function of ocean conditions. Our findings offer, forthe first time, the opportunity to explore the development of a dynamic ocean corridor
for this protected species, illuminating a longstanding mystery in sea turtle ecology.
Keywords: loggerhead sea turtle, distribution, satellite remote sensing, sea surface temperature, habitat
connectivity, migration, corridors

INTRODUCTION
Conservation of long-lived pelagic organisms, like sea turtles, that navigate whole ocean basinsremains one of the most daunting challenges in marine science. Six of the seven species of sea
turtles are listed as critically endangered, endangered, or vulnerable by the IUCN and are subject
to numerous threats on land, in the Exclusive Economic Zones (EEZs) of nations, and in the open
ocean (IUCN, 2018). For sea turtles, we have long known where they nest but Archie Carr’s “lostFrontiers in Marine Science | www.frontiersin.org

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Dynamic Corridor for Migratory Mysteryreproductive outcomes achieved by coastal recruits suggests
this area represents important developmental grounds for some
portion of this population (Peckham et al., 2011; Seminoff et al.,2014; Turner Tomaszewicz et al., 2015, 2017).
How and why some loggerheads recruit to the eastern Pacificwhile others remain in the CNP is unresolved. Despite one of
the largest tracking sets of any migratory species, evidence of
only 6 out of 231 individuals moving across the EPB and into
the eastern Pacific is perplexing, but corresponds with recent
findings by Turner Tomaszewicz et al. (2017) that revealed
this ontogenetic habitat shift may be infrequent or variable.
Historically, the lack of data on recruitment and the prolonged,
slow-growing nature of loggerheads has limited our visibility
into this important ontogenetic shift for the North Pacificpopulation. But for the first time, the routes of these 6 “sentinel”
individuals illuminate this lost years puzzle and have provided us
with the first empirical insight into a recruitment pathway thiscryptic species.
Here, we extend the meta-analysis of these 231 tracks by
Briscoe et al. (2016) and follow the unique movements of the
6 individuals that progressed beyond the CNP towards theNorth American coast (Figure 1B). First, we review several
well-established connections between loggerhead movement
and their environment to develop a theory for recruitment.
We then explore the evolution of favorable oceanographic
conditions along the migratory corridor identified by these 6
individuals. Finally, we add an additional line of evidence using
an independently derived data set documenting loggerhead
recruitment to the BCP region over the same time period(1997–2012), to compare estimated annual recruitment
in years of oceanographic variability. Our findings offer a
promising hypothesis underlying habitat connectivity and a newinterpretation of this longstanding mystery in sea turtle ecology.

year” puzzle, for which he called out as the “most substantialof all obstacles to understanding the ecology of sea turtles”(Carr, 1980), is now understood to be the “lost decades”
for some species.
Even the at-sea movements of well-known sea turtle species
such as loggerheads (Caretta caretta) have until recently beena mystery. Only 25 years ago, many assumed that loggerheads
found in the eastern North Pacific Ocean originated fromundiscovered nesting sites in the region, as it was hard to
imagine a juvenile sea turtle undertaking a 12,000 km transpacificmigration from its natal beaches in Japan to the North American
coast (Nichols et al., 2000; Seminoff et al., 2012). The daunting
presence of one of Darwin’s “impassable” barriers (Darwin, 1872),
the Eastern Pacific Barrier (EPB) (Ekman, 1953), usually prevents
the dispersal of marine fauna from the central to eastern Pacific
Ocean (see Briggs, 1961; Lessios and Robertson, 2006; Baums
et al., 2012). But phylogeographic studies have provided insight
into the pathways of colonization across this biogeographic
barrier for several small (Baums et al., 2012) and large marine
species into the eastern Pacific, including sea turtles (Dutton et al.,2014). For North Pacific loggerhead sea turtles, the combinationof genetic analyses (Bowen et al., 1995), flipper tag recoveries, andsatellite tracked individuals (Uchida and Teruya, 1988; Resendiz
et al., 1998; Nichols et al., 2000) eventually confirmed theNorth Pacific population as a single, widely dispersed, geneticallydistinct stock. Nesting exclusively in Japan, hatchlings from thispopulation undertake developmental migrations that we now
know span the entire North Pacific ocean basin (Bowen et al.,
1995; Nichols et al., 2000). Foraging hotspots for juveniles havebeen identified in the Central North Pacific (CNP) (Polovina
et al., 2004, 2006; Kobayashi et al., 2008; Abecassis et al., 2013)
and in the eastern North Pacific off the Baja California Peninsula(BCP), Mexico (Peckham et al., 2011; Wingfield et al., 2011;
Seminoff et al., 2014). Upon maturity, these turtles return totheir natal beaches in Japan to reproduce, residing in the western
Pacific for the remainder of their lives (Nichols et al., 2000;
Watanabe et al., 2011).
Throughout their range, North Pacific loggerheads are subject
to high rates of fisheries-related mortality (Gilman et al., 2007;
Peckham et al., 2008). While we now have knowledge of
their movements within their core oceanic and coastal habitats,our understanding of the connectivity between these disparate
locations is fragmented, leaving critical gaps in our ability toeffectively assess and protect this cryptic species. Briscoe et al.
(2016) examined the basin-scale movement patterns of 231
satellite tracked North Pacific juveniles between 1997–2013,
revealing an extensive pelagic phase within the oceanic waters of
the CNP. Of these individuals, tracked for up to 4.9 years at atime, many displayed east-west movement patterns within theiroceanic habitat, but only 30% moved into the eastern Pacific.
Even fewer (n = 6 turtles, ∼3%) transitioned into the California
Current Large Marine Ecosystem (CCLME) (Figure 1A). And
yet over 43,000 individuals from this population are known tocongregate along the coastal waters of the BCP at any given
time (Seminoff et al., 2014). Aging analysis has shown thatthese juveniles are long-term residents (20+ years) (Turner
Tomaszewicz et al., 2015) and the differential fitness andFrontiers in Marine Science | www.frontiersin.org

THERMAL HYPOTHESIS
Temperature is a fundamental driver of sea turtle distribution(Epperly et al., 1995). As ectotherms, core body temperature isinherently tied to the surrounding environment and thus, sea
turtles rely on thermal cues to both select (or avoid) habitats andinitiate long-distance movements (Bentivegna, 2002; McMahon
and Hays, 2006; Hawkes et al., 2007; Báez et al., 2011). In
the North Pacific, loggerheads exhibit strong fidelity to the18◦ C isotherm, a widely used proxy for the highly productive
Transition Zone Chlorophyll Front (TZCF) (Polovina et al.,2000). Individuals have been shown to track the position of the
TZCF over annual and interannual cycles, which serves as botha transport and foraging mechanism for many highly mobile
species across the North Pacific Ocean (Polovina et al., 2001).
For loggerheads, defining associations with sea surface
temperature (SST) now extend beyond distribution, as
understanding and anticipating SST changes have become
pivotal to dynamic fisheries management strategies in the North
Pacific. Identification of loggerhead-SST affinities in the open
ocean motivated the development of one of the first the dynamic
bycatch avoidance tools, TurtleWatch (Howell et al., 2008), used2

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FIGURE 1 | State-space modeled distribution of (A) 231 electronically tagged juvenile loggerhead sea turtles across the North Pacific Ocean (1997 – 2013),
deployed off Japan and in the Central North Pacific (light gray). CCLME region shown in gray. While individuals released from the North American coast (light blue)
were not included in the study because they represent a larger age-size class, they are shown here to characterize the full habitat use of individuals from thispopulation. (B) The eastern North Pacific study region, utilized by the 6 individuals that migrated from the Central North Pacific towards the North American coast
study area (white box).

patterns. In the western Pacific, Ascani et al. (2016) identified
the effects of Pacific Decadal Oscillation (PDO) induced changes
in SST and ocean circulation on the recruitment of neonate
loggerheads into the open ocean. During certain PDO phases,
the strength of the Kuroshio Extension Current serves as aphysical barrier preventing (or facilitating) turtles from reachingproductive TZCF waters. In the CNP, Briscoe et al. (2016)
showed that an extended residency was in part due to eastward
moving turtles turning around upon reaching cool oceanographic
conditions in the eastern North Pacific. Included as part ofseveral intrinsic and extrinsically-motivated hypotheses, theyby the Hawaii-based longline fishery to mitigate interactions by
avoiding waters within thermally optimal habitat. In the CCLME,
increased loggerhead bycatch within the California Drift Gillnet
fishery has historically correlated with anomalously warm water
periods associated during El Niño Southern Oscillation (ENSO).
This association led to the establishment of temperature-based
operational guidelines for the fishery and since 2003, a time-areaclosure is enacted when an El Niño event has been forecast or
declared (NMFS, 2000).
For North Pacific juveniles, SST conditions may even playa role in the timing of recruitment and ontogenetic shift

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Dynamic Corridor for Migratory Mysterytheorized that interannual variability may influence east-west
movements and habitat connectivity. Indeed, between 1997 and2012, the eastern North Pacific experienced high variability inoceanographic conditions (Fleming et al., 2016). Warm andcool episodes were characterized by fluctuating ENSO indicesat intervals of approximately 2–4 years, as well as local-forcing
events that resulted in anomalous conditions (Sydeman et al.,2006; Fleming et al., 2016; Fiedler and Mantua, 2017). With
these physical variations came associated shifts in biologicalproductivity, trophic structure, and species distribution (Hughes,
2000; Mackas et al., 2006; Ainley and Hyrenbach, 2010;
Hazen et al., 2014). For turtles queued up in the eastern CNP,
we posited that exceptionally warm conditions may provide
the environmental cues and thermal requirements necessary tocreate an ecological bridge - a temporary pathway known toconnect two suitable but disparate and distinct habitat regions
for highly migratory species (Fromentin et al., 2014; Briscoe
et al., 2017) – thus facilitating movement into the CCLME.
Based on our previous findings of the behavior of the 231
juveniles, the well-established relationships between movement
and temperature, and an exploration of ocean conditions
within the region identified by our 6 sentinel loggerheads,
we explored the hypothesis that years with anomalously
warm conditions could facilitate the recruitment of juvenile
loggerheads into the eastern North Pacific and to the Baja
California foraging habitat.
Below, we detail our observations and assessments used tosupport this proposed hypothesis and discuss the importance of
future experiments to validate this work. Additional information
on materials and methods can be found in the Supplementary
Materials section.

A

OBSERVATIONS
The 6 satellite tracked sea turtles evaluated in this study were
deployed at the same date-time and location (176◦ E, 32.7◦ N,
4-May-2005, Supplementary Table 1), providing a unique
opportunity to concurrently monitor the timing and longterm movements of juveniles into the eastern North Pacific.
Impressively, these 6 individuals transmitted an average of
1,116 days (more than 3 years) ± 137 days (range: 898–1,247
days) and over an average distance of 17,115 km ± 2,884 km(range: 12,533 – 20,395 km). All loggerheads tracked the 18◦ C
isotherm across the CNP and moved into the eastern North
Pacific during the winter months (October–March), when the
TZCF migrates to its southern-most position (Bograd et al.,2004) (Supplementary Figure 1). Of these individuals, only
two successfully migrated beyond 130◦ W (Figures 2B,C); theremaining 4 eventually reversed direction and returned to the
CNP (Figures 2A,D–F).
A cursory examination of the time-matched remotelysensed thermal conditions within the eastern North Pacificstudy area (Figure 2) showed grouped SSTs to be slightly
cooler for the individuals that reversed direction (turtles A,D–F); median: 16.6◦ C, range: 14.2◦ C – 20.4◦ C), comparedto the two successful migrators (turtles B and C; median
17.1◦ C, range 15.1◦ C – 22.6◦ C, Supplementary Table 2).
Most notably, the two successful migrators tracked moreeastward moving days in water warmer than 18◦ C (46 and43 days for turtles B and C, respectively, compared tothose that reversed direction (31 days for turtle A and
0 days for turtles D-F, Supplementary Table 2). The twojuveniles that transitioned towards the coast did so between

B

A
B

C

C

D

D

E

E

F

F

FIGURE 2 | Left panel: Daily sea surface temperatures (SST) extracted under the 6 satellite-tracked individuals that moved into the eastern North Pacific study region(gray box). Turtles (A,D–F) did not complete migration to the North American coast, but instead reversed direction and moved towards the Central North Pacific.
Turtles (B,C) successfully moved through the study region in an eastward direction. Right panel: Density distribution of the corresponding daily SSTs experienced byeach turtle upon entering the study region and until reaching the location of maximum eastward longitude. Vertical line represents the median SST value.

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Dynamic Corridor for Migratory Mysterysummer months (Apr-Sep) when the turtles were crossing the
EPB for coastal recruitment. To account for both remotely
forced and localized trends in variability in the absence of
ENSO, we classified “warm” and “cool” conditions basedon the auto-correlative properties of SST within the study
area, as current conditions are influenced by previous ones,
and as previously noted, time lags often exist between an
environmental state and an animal’s response (Ainley andHyrenbach, 2010). Within the study area, we found serial
correlation of regional SST conditions to persist for up to 3
months (r = 0.51 at 3 months), with statistically significant
correlative properties diminishing thereafter (Supplementary
Figure 3). In the North Pacific, anomalous SSTs tend to peakin the winter but can persist into the subsequent summer
months (Fiedler and Mantua, 2017). For this reason, a warm (orcool) period represented a recruitment year summer (hereafterreferred to as “summer, Year 0”) and its preceding winter(“winter, Year −1”). Each period was classified as warm (orcool) if the regional or remotely forced 3-month runningmean of SST anomalies exceeded a 0.5◦ C (−0.5◦ C) thresholdfor 3 or more consecutive months. Over the entire study
period, we classified 6 warm and 5 cool periods. Warm periods
encompassed five ENSO events (1997–1998, 2002–03, 2004–
05, 2006–07, 2009–10) and 2005, a year of delayed upwelling
and anomalously warm conditions in the California Current(Mackas et al., 2006; Peterson et al., 2006), though not officiallyan El Niño. Cool periods included the 1998–99, 2000–01,
2007–08, and 2010–11 La Niña events, and 2012, which wasconsidered a cool year without the presence of a La Niña
(Fiedler and Mantua, 2017).
To explore our hypothesis through an independentline of evidence, we examined the annual recruitment ofloggerheads in relation to these thermal conditions using
the humeri of loggerheads stranded along the BCP between
2003 and 2012 (Turner Tomaszewicz et al., 2017). The
recruitment year, and then an annual recruitment count,
was determined by extending the work of Turner Tomaszewicz
et al. (2017). In this study, the humeri were examined using
skeletochronology and stable isotope analyses to identify
the timing of ontogenetic habitat shifts. Specifically, Turner
Tomaszewicz et al. (2017) reported a distinct differentiationin isotope values for 33 loggerheads that revealed the size
and age when each turtle transitioned from oceanic waters
to the North American coast. Here we continued thisanalysis to determine the year this habitat shift occurredfor each of these individuals. This consisted of assigning
the year of stranding to the outermost bone layer and backassigning the corresponding year to each sequential bone
growth layer until the recruitment was dated for each ofthese 33 individuals, thereby providing an annual turtle
recruitment count (Supplementary Materials). Twenty ofthese individuals had recruitment years associated with warm
conditions while a total of 13 individuals recruited during years
associated with cool conditions. Annual recruitment during
warm ocean conditions was significantly greater than annual
recruitment during cool conditions (one-tailed Mann-Whitney,
P = 0.0201 Figure 3).

April-July, suggesting an arrival that would align with peakcoastal productivity (Jacox et al., 2015a). Of those that turned
around, turtles D, E, and F spent less than 2.5 months
in the region before returning to the CNP by April-May.
Turtle A arrived within the study area in early Marchand continued an eastward trajectory until early August.
A close inspection of the bimodal distribution in SSTs
for this individual (Figure 2A, right panel) indicated that
this turtle encountered the warmest SSTs during an arearestricted search just prior to turning around (Figure 2A,
left panel). Further investigation of the mesoscale bio-physicalprocesses may shed light on the behavioral movements forthis individual.
While these 6 juveniles provided a descriptive understandingof SSTs experienced within the eastern North Pacific, we
caveat that the small sample size of satellite tracked individuals
limited our ability to draw robust conclusions between
real-time environmental conditions and behavioral responses,
especially given unknown biophysical lags. Despite this, these
individuals provided us with the first-ever glimpse into theseasonal timing of recruitment and the specified locationof a migratory pathway, greatly enhancing our previous
understanding of these unknown routes.
With this information, we explored the thermal conditioningof this eastern North Pacific pathway with respect tointerannual variability. ENSO is a well-documented phenomena(Emery and Hamilton, 1985; Liu and Alexander, 2007) andconsiderable attention has been paid to changing ocean
conditions in the eastern North Pacific driven by this broad
scale climatic forcing mechanism. However, it should be
noted that oceanographically warm or cool conditions may
evolve in the absence of an ENSO event. Recent studies
using regional ocean circulation models have begun toprovide insight into the importance of regional forcing asa driver of interannual SST variability (Frischknecht et al.,2015; Jacox et al., 2015b), as evidenced by recent marine
heatwaves within the California Current Large Marine
Ecosystem (CCLME, e.g., the Blob), (Bond et al., 2015; Di
Lorenzo and Mantua, 2016; Fiedler and Mantua, 2017). To
investigate correlative relationships between remote, broadscale climate forcing and localized conditions, we compared
the monthly SST anomalies in the study region with thosein the equatorial Pacific (Niño 3.4, a widely used index of
ENSO events) (Trenberth, 1997) over the 15 year study
period (Supplementary Figure 2). Our analyses showed onlya moderate correlation in phase and amplitude (r = 0.42)
These results were similar to those found in a study byFiedler and Mantua (2017), highlighting the occurrence ofmore short-term trends in anomalous conditions in the
CCLME in the absence of an El Niño or La Niña event(e.g., 2005 and 2012).
Using the timing and location of our sentinel tracks, wecharacterized the oceanographic conditions that turtles would
experience during the two previously identified time periods:(1) the winter months (Oct-Mar) preceding each year of
recruitment when the turtles were approaching the eastern
CNP (the typical turn-around location); and (2) the following

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Dynamic Corridor for Migratory Mysterycoastal recruitment. These conditions possibly prompt eastwardmoving turtles to turn around and remain in the CNP. The one
exception was 2007, which met the criteria for cool conditions
but experienced relatively high recruitment (n = 5, Figure 3).
In this instance, individuals may have exploited the warm but
receding summer SST anomalies (0.1◦ C–0.3◦ C) that persisted as
the California Current transitioned from a moderate El Niño to astrong La Niña event.
The idea that fluctuations in ocean-atmosphere conditions canmodulate recruitment dynamics is not new but is less frequently
demonstrated. For North Pacific loggerheads, the theory thateastern boundary recruitment may be tied to changing ocean
conditions is in agreement with several independently establishedobservations. First, the aforementioned role of SST as a significant
predictor in sea turtle movement and distribution (Spotila andStandora, 1985), and thus their well-documented coupling andsensitivity to thermal conditions (e.g., McMahon and Hays,
2006; Hawkes et al., 2007). Second, interannual variability
and anomalous SSTs have been shown to influence migration
strategies of several other highly mobile species in the North
Pacific, including albacore (Thunnus alalonga) (Kimura et al.,1997), eels (Anguila japonica) (Lin et al., 2017), and northernelephant seals (Mirounga angustirostris) (Abrahms et al., 2018).
Finally, we drew upon the highly relevant studies that have linked
interannual and decadal oscillations to indices of loggerhead sea
turtle recruitment. As previously mentioned, Ascani et al. (2016)provided evidence of recruitment and survival of neonate North
Pacific loggerheads into the TZCF as modulated by synchronousbasin-wide changes in SST associated with the PDO. In a negative
PDO phase, the Kuroshio Extension Current is strong, acting as
a barrier that prevents northward transport into the productive
TZCF that is otherwise favored during a positive phase. In the
North Atlantic, Báez et al. (2011) have linked environmental
variability to east-west migrations for juvenile loggerheads. The
western Mediterranean serves as a staging area for late stage
juveniles preparing to return to the western Atlantic (Eckert et al.,2008). During positive phases of the North Atlantic Oscillation,strong winds and cool SSTs impede migration back to thewestern Atlantic, forcing an accumulation of loggerheads aroundthe Strait of Gibraltar and a significant increase in strandings(Báez et al., 2011).
These studies, combined with advancements in biologging andsatellite telemetry have greatly enhanced our ability to understandhow migratory animals move through their environment.
However, tracking individuals through multiple life history stages
remains inherently difficult (Hazen et al., 2012a), especially fora long-lived, slow growing species that remains in the juvenilestage for up to several decades (Turner Tomaszewicz et al., 2015).
Despite the unique opportunity to observe recruitment during
an otherwise cryptic life history stage, our multi-year dataset
represents only a snapshot of this important developmental
period. At present, the small number of individuals that moved
into the eastern North Pacific limits the power associated with
our ability to fully test our hypothesis. However, exploring the
behavior of a unique subset of the population should not be
underestimated. McMahon and Hays (2006) and Caughley andGunn (1996) aptly noted that such limitations in sample size
should not preclude crucial life-history information from being

5

Number of Recruits per Year

4

3

2

1

0
C ool

W a rmSST Condition

FIGURE 3 | Relative number of annual juvenile loggerhead sea turtle recruits
during years of cool (blue) and warm (red) regional ocean conditions in theeastern North Pacific Ocean study area (Figure 1B) between 1997–2012,
n = 33. Raw data values are shown with color dots. Error bars and dark circle
represent the grouped median and standard error.

DISCUSSION
Out of our previously published 15-year study tracking the multiyear movements of 231 satellite tagged turtles, the movements ofjust 6 sentinel individuals examined here offer the first empiricalinsight into a previously unknown migratory pathway and thecapability to offer a new interpretation to explain a long-standing
mystery of one of the ocean’s greatest migrants. The tracking ofthese individuals, coupled with remotely sensed oceanographic
conditions and novel methods in sequential sampling of annualbone growth layers, provided us with the ability to elucidate
juvenile recruitment in relation to favorable ocean conditions.
From these data, we propose a new hypothesis regarding a
possible physical mechanism for habitat connectivity and provide
the first evidence of a “thermal corridor” that would allow forpulsed recruitment of loggerheads from the Central North Pacificto the North American coast.
The spatial composites of SST anomalies across the study
region illustrate the year-to-year variability in oceanographic
conditions that were associated with annual recruitment to theeastern Pacific (Figure 4). During warm periods, it appears
a transport pathway opened intermittently as a functionof the warm sea surface temperatures, thereby creating athermal corridor that may facilitate successful recruitment
across the Eastern Pacific Barrier to coastal foraging grounds(Figure 4D). The corridor was not only present during thesummer but showed a preconditioning of warmer temperaturesin the preceding winter months (Figure 4B). Such anomalous
conditions, especially if sustained for several months at a time,
may provide key environmental cues to animals concentrated
in the eastern edge of the CNP that the thermal corridor is
opening. By contrast, during cool years the thermal corridor
appears to close (Figures 4A,C), leading to relatively weak or no
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A

Cool

B

°C

Warm

1.0

Winter (Year -1)

35°N

30°N

0.5

25°N

C

0.0

D

Summer (Year 0)

35°N

-0.5

30°N

25°N

-1.0

135°W

125°W

115°W

125°W

115°W

FIGURE 4 | Six-month mean spatial composites of monthly SST anomalies for cool (A,C) and warm (B,D) ocean conditions during recruitment year summers(Apr-Sep, Year 0) and their preceding winters (Oct-Mar, Year -1). The region utilized by the six easternmost transiting juvenile loggerhead sea turtles is outlined in grayand represents the location of a thermal corridor that may facilitate the successful recruitment to coastal foraging grounds.

provided evidence to dismiss the theory that these individuals
were already BCP coastal recruits following warmer water
northwards. These studies, combining data from loggerhead
aerial surveys, at-sea-sightings, stranding records, and tissue
samples support our hypothesis of a thermal corridor facilitating
coastal recruitment during anomalously warm ocean conditions.
Oceanographic conditions are expected to become more
extreme with climate change, leading to shifts in the distributions
and migratory pathways of sea turtles, sea birds, marine
mammals, and pelagic fishes (Hazen et al., 2012b). If athermal corridor across the Eastern Pacific Barrier were toopen more frequently in a warming ocean, it may resultin an increase in the relative abundance of loggerheads andother highly migratory marine megafauna shifting from theCentral North Pacific to foraging grounds along the eastern
Pacific coast (e.g., Pacific bluefin tuna (Kitagawa et al.,2007), white sharks (Tanaka et al., 2021), and northernelephant seals (Abrahms et al., 2018). Finally, a change in the
distribution of protected species can pose increased conservation
challenges. For the North Pacific loggerhead, it could mean
higher exposure to significant regional bycatch not only for
the Baja California coast (Peckham et al., 2008), but forgathered or studied; rather it is imperative that endangered
animals be studied while they can, so that the most informed
conservation and management decisions can be made (McMahon
and Hays, 2006). This is especially true given that anotherdataset of this magnitude (i.e., several hundred individuals
from a single population over a 15-year period) may be quitedifficult to reassemble. Our preliminary evaluation provides
novel insights into the existence of a dynamic ocean corridor, but
future satellite tagging and bone stable isotope studies focused
within the eastern North Pacific are necessary to experimentally
test our hypothesis under varying ocean conditions. In theinterim, other lines of evidence by Allen et al. (2013), Eguchi
et al. (2018), and Welch et al. (2019) provide strong support tothe proposed thermal corridor. Recently, the ephemeral presenceof large numbers of juveniles off the United States West Coast
coincided with the 2014–2015 El Niño and 2015–2016 marine
heatwave (Eguchi et al., 2018; Welch et al., 2019) and triggered theoperational closure of the California Drift Gillnet fishery. A study
by Allen et al. (2013) confirmed that the influx of these small
juveniles into the Southern California Bight during anomalously
warm water events were in fact open ocean recruits from the CNP
and not displaced coastal residents. Distinct stable isotope values

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Briscoe et al.

Dynamic Corridor for Migratory Mysteryauthors contributed to the draft revisions and gave final approval
for publication.

other potentially important North American foraging grounds,
including the Southern California Bight (Welch et al., 2019).
As the planet undergoes unprecedented changes in theanthropogenic era, barriers to species dispersal that wereonce considered “impassable,” like the Eastern Pacific Barrier,are being redefined and so are the associated ecologicalimplications. Understanding mechanisms facilitating migration
between discrete habitats and behavioral responses of species,
like the North Pacific loggerhead, is crucial to interpreting thelife history, ontogenetic habitats, and threats these species faceat sea. For sea turtles, Archie Carr’s “lost years” puzzle is still oneof most substantial of all obstacles to understanding the ecology
and conservation challenges of sea turtles, but multiple emerging
technologies and analyses can now be integrated to elucidate thischallenging problem. The puzzle is far from complete, but the
picture is coming into view.

FUNDING
Funding for DB was provided by the Crowder Lab at Hopkins
Marine Station, Stanford University. Funding for CT wasprovided by NMFS Southwest Fisheries Science Center and the
National Research Council Research Associateship. Funding for
LC was provided by the Department of Biology and by the Woods
Institute for the Environment, Stanford University.

ACKNOWLEDGMENTS
The authors would like to gratefully acknowledge the personnel
at Port of Nagoya Public Aquarium, S. Hoyt Peckham andthe community and members of Grupo Tortuguero de lasCalifornias for collection of bone samples from BCP, Mexico.
The authors wish to thank Steven Bograd and Elliott Hazen
for their constructive feedback in the early stages of this work,and the reviewers for their helpful contributions that improved
this manuscript.

DATA AVAILABILITY STATEMENT
Publicly available datasets were analyzed in this study.

AUTHOR CONTRIBUTIONS
DB and LC conceived the hypothesis and its evaluation. TS, MK,
and HO provided animal care. GB, MR, TS, and MK conducted
fieldwork including tag deployment. DB, CT, and LC conceived
and designed methodology. DP, GB, JP, DB, CT, and JS acquired
and analyzed data. DB and CT performed formal analyses. DB
led and CT, LC helped with the drafting of the manuscript. All

SUPPLEMENTARY MATERIALThe Supplementary Material for this article can be foundonline at: https://www.frontiersin.org/articles/10.3389/fmars.
2021.630590/full#supplementary-material

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Conflict of Interest: The authors declare that the research was conducted in theabsence of any commercial or financial relationships that could be construed as apotential conflict of interest.
Copyright © 2021 Briscoe, Turner Tomaszewicz, Seminoff, Parker, Balazs, Polovina,
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