ORIGINAL RESEARCH
published: 06 December 2018
doi: 10.3389/fmars.2018.00452

Loggerhead Turtles (Caretta caretta)
in the California Current: Abundance,
Distribution, and Anomalous
Warming of the North PacificTomoharu Eguchi 1* , Sam McClatchie 2 , Cara Wilson 3 , Scott R. Benson 4,5 ,Robin A. LeRoux 1 and Jeffrey A. Seminoff 1
1

Edited by:Jeremy Kiszka,
Florida International University,
United States
Reviewed by:
Gail Schofield,
Queen Mary University of London,
United Kingdom
Jonathan D. R. Houghton,
Queen’s University Belfast,
United Kingdom*Correspondence:
Tomoharu Eguchi
tomo.eguchi@noaa.gov
Specialty section:
This article was submitted toMarine Megafauna,a section of the journal
Frontiers in Marine Science
Received: 19 June 2018
Accepted: 12 November 2018
Published: 06 December 2018
Citation:
Eguchi T, McClatchie S, Wilson C,
Benson SR, LeRoux RA andSeminoff JA (2018) Loggerhead
Turtles (Caretta caretta) in the
California Current: Abundance,
Distribution, and Anomalous Warmingof the North Pacific.
Front. Mar. Sci. 5:452.
doi: 10.3389/fmars.2018.00452

Marine Mammal & Turtle Division, Southwest Fisheries Science Center, National Marine Fisheries Service, La Jolla, CA,
United States, 2 Fisheries Resources Division, Southwest Fisheries Science Center, National Marine Fisheries Service, La
Jolla, CA, United States, 3 Environmental Research Division, Southwest Fisheries Science Center, National Marine FisheriesService, Monterey, CA, United States, 4 Marine Mammal & Turtle Division, Southwest Fisheries Science Center, National
Marine Fisheries Service, Moss Landing, CA, United States, 5 Moss Landing Marine Laboratories, Moss Landing, CA,
United States

Environmental variability affects distributions of marine predators in time and space. With
expected changes in the ocean climate, understanding the relationship between species
distributions and the environment is essential for developing successful management
regulations. Here we provide information on an ephemeral but important habitat for
North Pacific loggerhead turtles (Caretta caretta) at the northeastern edge of theirrange. North Pacific loggerhead turtles nest on Japanese beaches and juveniles
disperse throughout the North Pacific; some remain in the high seas of the central
North Pacific whereas others transition to the eastern Pacific and forage near Baja
California, Mexico. Loggerheads have also been reported along the United States west
coast, with the majority of sightings off southern California. Here we describe theirdemography and distribution in the area, based on two aerial surveys (2011, 2015),
at-sea sightings, and stranding records. Our aerial survey during fall 2015 determined
density, abundance, and distribution of loggerheads in the area, when anomalous
warming of the North Pacific and El Niño conditions co-occurred. Using line-transect
analysis, we estimated ca. 15,000 loggerheads at the sea surface (CV = 21%) andmore than 70,000 loggerheads when accounting for those that were submerged and notavailable for detection. Our survey during fall 2011 resulted in no loggerhead sightings,
demonstrating a high variability of loggerhead density in the region. We encourage
further research on loggerheads in the area to determine the mechanisms that promote
their occurrence. These studies should include regular surveys throughout their foraging
areas along the west coast of the North America as well as assessments of preyavailability and local oceanographic conditions.
Keywords: distance sampling, Eastern Pacific, El Niño Southern Oscillation, line transect, stranding

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Loggerheads in the California Currentof loggerheads in the California-based drift gillnet fishery(CADGN) and El Niño conditions led the U.S. National
Marine Fisheries Service to develop a time-limited spatial
closure of the CADGN fishery off southern California (Federal
Register, 2004, 2007) (Figure 1). The closure is triggered
when the National Oceanic and Atmospheric Administration(NOAA) officially declares or predicts El Niño conditionsin any given year, and since its designation it has been
implemented three times so far (Federal Register, 2014, 2015,2016).
In the eastern Pacific, loggerhead presence has been reported
from Alaska to Sinaloa, Mexico with a major foraging hotspot
identified along the BCP, within the California Current Large
Marine Ecosystem (CCLME; Hodge and Wing, 2000; Peckham
et al., 2008; Carretta and Barlow, 2011; Carretta et al., 2017;
Zavala-Norzagaray et al., 2017; Halpin et al., 2018). Occasionalpresence of loggerheads off southern California has beenreported, with the recorded bycatch largely occurring during
El Niño conditions (National Marine Fisheries Service [NMFS],
2000; Carretta et al., 2003). When the aforementioned CADGN
fishery closure off southern California was established, theimmediate origin of loggerheads in the region was thought to be
Mexico. However, a study of body sizes and stable isotope values
indicated that loggerheads caught in the CADGN fishery most
likely originated from the CNP (Allen et al., 2013).
The exact path by which loggerheads arrive in the CCLME
region is unknown, as are their movement patterns and residence
times in the region. Environmental factors that promote thetransition of loggerheads to the CCLME are likely to be associatedwith the North Pacific Central Water mass that has its origin
in the Central Pacific Gyre. Warm, oligotrophic North PacificCentral Water lies to the west of the California Current, andthere is no clear boundary between the western margin of the
California Current and the edge of the Central Gyre (Lynn andSimpson, 1990). Waters of the California Current are cooler andmore nutrient rich than further offshore; therefore, it is possiblethat when waters warm throughout the region a thermal corridor
may develop that allows loggerheads to access waters off southern
California, and upon arrival, encounter sufficiently warm andproductive waters that enable their sustained presence for several
months or more.
In this paper, we describe the presence of loggerheads off thesouthern United States west coast using various data sources.
To determine the distribution and density of loggerheads in theregion, aerial surveys were conducted during September andOctober of 2011 and 2015. We examined the oceanographic
conditions associated with extreme inter-annual variability inturtle abundance between these 2 years. We also analyzed
a variety of other datasets to enhance our understanding ofloggerhead presence in relation to SST patterns in the region.
To describe the demography of loggerheads in the region, size
distributions were obtained from stranding and fishery observer
data. These data provided valuable insight into the spatial andtemporal distribution of juvenile loggerheads in the eastern
North Pacific, and would serve as a baseline to compare with
future distribution of loggerheads in the region as the ocean
climate changes.

INTRODUCTION
Spatial distributions of marine predators are strongly affectedby the environmental conditions they are exposed to via
physiological constraints and shifts in prey distribution (Pörtner
and Knust, 2007; Hazen et al., 2012). As ocean temperatures
are expected to increase over the next decades (IPCC, 2013),
distributions of marine animals will undoubtedly continue tochange. To predict the expected changes of distributions, it
is essential to understand the environmental conditions that
influence a species’ whereabouts. Such understanding, however,requires dedicated study over large spatial and temporal scales.
Here, we describe how shifting oceanographic conditions affectthe distribution of a threatened marine species.
The loggerhead turtle (Caretta caretta) is found throughout
temperate and tropical regions of all oceans (Dodd, 1988). In
the North Pacific, loggerheads nest exclusively in Japan, whereannual nesting populations are a fraction of their former levels(Kamezaki et al., 2003). Upon hatching, small loggerheads depart
Japanese nesting beaches and disperse eastward following theKuroshio and its extensions, and eventually disperse throughout
the central North Pacific (CNP; Polovina et al., 2006; Kobayashi
et al., 2008). Some juveniles transition to the eastern Pacific,
particularly along the west coast of the Baja California Peninsula(BCP) (Bowen et al., 1995; Peckham et al., 2007), where they
reside for decades until they mature (Turner Tomaszewicz et al.,2015). Other juveniles remain in oceanic habitats of the North
Pacific where their movements have been studied extensively(Polovina et al., 2000; Kobayashi et al., 2008; Abecassis et al., 2013;
Briscoe et al., 2016a). At the onset of maturity, loggerheads departthese areas and return to their natal nesting beaches in Japan forreproduction and remain in the western Pacific for the remainderof their lives (Resendiz et al., 1998; Nichols et al., 2000; Hatase
et al., 2002).
Throughout their range, North Pacific loggerheads are
impacted by incidental capture in fishing gear (i.e., bycatch;
Gilman et al., 2007; Finkbeiner et al., 2011), which in somecases has reached catastrophic levels (Senko et al., 2017).
Indeed, artisanal fisheries bycatch resulted in substantial juvenile
mortality along the BCP throughout the 1990s and 2000s (Kochet al., 2006, 2013; Peckham et al., 2007, 2008), and entrapment
in pound nets has been reported as a major source of deathnear the loggerhead nesting beaches in Japan (Gilman et al.,2010). In the CNP, interactions of loggerheads with the Hawaiibased longline fishery stimulated development and applicationof various fishing gear modifications (Gilman and Huang, 2016)
and fishing regulations (Bartram et al., 2010). Further, a dynamic,
realtime bycatch avoidance tool for longline fishers targetingswordfish (Xiphias gladius) was developed based on the affinityof loggerheads to a specific sea surface temperature (SST) range(TurtleWatch; Howell et al., 2008). This was among the first-evertools to help fishers avoid areas of suspected high concentrationsof protected species based on environmental conditions (e.g.,
Howell et al., 2015; Hazen et al., 2018).
Fisheries management to reduce loggerhead bycatchwithin United States waters has also been developed in theeastern Pacific. For example, a correlation between bycatch

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MATERIALS AND METHODSTABLE 1 | Datasets used in this study with corresponding numbers of turtle
sighting locations.

Six independent datasets were used in this study: oceanographic
surveys, aerial surveys, shipboard marine mammal surveys,
opportunistic sighting reports from the public and oceanographic
cruises, CADGN fishery observer data, and stranding reports(Table 1). Observations from the oceanographic surveys wereused to characterize the oceanographic conditions during aerial
surveys in 2011 and 2015. Aerial surveys were limited to thesouthern-most portion of the United States west coast andfollowed basic line-transect survey protocols (Buckland et al.,2001). Data from the aerial surveys were used to estimate density
and abundance within the respective study areas (see below for
details). Sightings from ship-board marine mammal surveys in2006 and 2014 also were used to estimate densities of loggerheads
when there were sufficient sightings. Loggerhead stranding datafrom the United States west coast since 1980 were obtained
from the NMFS West Coast Region Stranding Network. Fisheryobserver data since 1990 and at-sea sightings data since 1992
were obtained from NOAA Southwest Fisheries Science Center(SWFSC). These data sources were used to identify timing andlocation of loggerhead occurrence as well as size distributions.

Datasets

Loggerheads

Unidentified

Aerial surveys

2010, 2015

212

Marine mammal ship surveys

1991–2014

84

79

Opportunistic sightings

2015–2017

41

102

DGN fishery observers

1992–2006

17

Stranding

1980–2017

75

These numbers differ from the number of turtles sighted from marine mammal
ship survey sightings (Table 3) because multiple turtles often were sighted at one
location.

time from airports and to shorten the longest track lines. In 2015,
the southern boundary was extended south of the United States –Mexico border. Not all transect lines were completely surveyed
due to airspace limitations, such as military activity and flightpaths of airplanes using commercial and military airports. Detailsof the aerial survey protocol can be found in Supplementary
Material.

Ship-Board Marine Mammal Surveys
We compiled sightings of loggerheads and unidentified turtles
from ship-board marine-mammal surveys off the United Stateswest coast (28–36◦ N, 127.5–115 ◦ W) since 1991 (Figure 1). There
were sufficient numbers of sightings in this dataset to conductline-transect analyses for 2006 and 2014. Details of ship-board
surveys can be found in Supplementary Material.

Environmental Conditions
To characterize environmental conditions that might affectdistributions of loggerheads in the area, we examined
oceanographic conditions when aerial surveys were conducted.
Data were obtained from two oceanographic surveys by
California Cooperative Oceanic Fisheries Investigation(CalCOFI, Anon, 2012, 2016). The CalCOFI program survey
methods and results are described in the cruise data reports(Anon, 2012, 2016; McClatchie, 2014). Based on these datareports, we examined flow regimes and water properties in theupper 50 – 100 m of the water column, comparing the flows
and properties during the fall of 2011 and 2015 when the aerial
surveys were conducted.

Distance Analysis
Line-transect analysis was conducted on distance data fromaerial and ship-board surveys using the mrds package (v.
2.1.17; Laake et al., 2016) for R statistical software (v. 3.3.2; R
Development Core Team, 2016). For each platform (airplaneand ship), all sightings were pooled to estimate the detection
function. For the aerial survey data, the most distant 1% ofsightings were truncated (n = 3) because the observers were
instructed not to look for turtles beyond 60 degrees from thetrack line. For the ship-board survey data, the most distant 15%
of sightings were truncated (n = 18, >2269 m), as recommended
by Buckland et al. (2001). Half-normal and hazard-rate key
functions with cosine adjustments or Beaufort sea state asa covariate were used to model the detection function. We
determined the best fitting detection function using AIC (aninformation criteria; Akaike, 1973). The best model, then, wasused to estimate loggerhead density. The shipboard survey data
were only used to estimate densities along portions of thesurveyed track lines, not total abundance, because there wasno predefined study area for loggerheads. For the aerial survey
data, post-survey stratification was attempted to reduce thecoefficient of variation (CV) of estimated density. There wasno estimate of the probability of detecting a loggerhead on thetransect line, g(0), because no studies had been conducted todetermine the diving behavior of juvenile loggerheads in thisarea. Previous studies elsewhere in the North Pacific indicatedloggerheads may spend 20–40% of their time at the ocean
surface (Polovina et al., 2003; Howell et al., 2010), although

Aerial Surveys
The aerial survey study areas in 2011 and 2015 encompassed
approximately 114,000 km2 and 129,000 km2 , respectively, from
31.0 to 34.6◦ N and 123.0◦ W to the southern California coast(Figure 1). Given the warm water affinity of juvenile loggerheads
in the CNP (mean = 16.3◦ C, SE = 1.6◦ C; Howell et al., 2008),
we expected to find most loggerheads during late summer andfall when the water off southern California was likely to bewarmest (Lynn and Simpson, 1987). We divided the study area
into inshore and offshore strata by the approximate inshoreedge of the California Current and previous studies of marine
mammals in the area (Forney et al., 1995). The offshore stratumis dominated by the south-flowing California Current, whereas
the inshore stratum is dominated by the summer-intensified,northward surface expression of the California Undercurrent(also known as the Southern California Countercurrent; Dailey
et al., 1993; Hickey, 1993). Because we lacked information on thespatial distribution of loggerheads in the study area, we arranged
survey track lines evenly within the study area. The angle of tracklines from the shoreline was changed in 2015 to decrease transit

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FIGURE 1 | Survey effort and loggerhead sightings from dedicated aerial survey in 2011 (A) and 2015 (B), indicating inshore/offshore strata and truncated study
area in 2015, and from marine mammal ship-board survey (C). The study area for the aerial survey is indicated by solid black lines, whereas completed track lines
are indicated by dark cyan. Colored dots indicate sightings of loggerheads. The current time-area CADGN closure area (i.e., Loggerhead Turtle Management Area) is
indicated with dashed lines; during closure years fisheries are prohibited in this area from 1 June to 31 August.

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surface time could vary significantly with water temperature,
location, season, and age/size (Mansfield and Putman, 2013).
To account for the uncertainty associated with a lack ofg(0) for this survey, we computed two separate abundance
estimates encompassing the range of likely values: a raw
estimate without any g(0) correction (minimum abundance)
and an estimate that assumes loggerheads in the region spend
only 20% of their time at the ocean surface (maximum
abundance).

Size Distribution of Loggerheads in
CCLME
Size distribution of loggerheads in the CCLME was characterizedfrom stranding and CADGN fishery observer datasets. Mean sizeand size range were compared to those for loggerheads in BCP,
Mexico based on published reports by Peckham et al. (1995–
2007, 2008, n = 1918) and Turner Tomaszewicz et al. (2003–2011,2015, n = 146). All sizes were recorded as curved carapace length(CCL). Gaussian kernel density estimates with Silverman’s ruleof-thumb bandwidth [0.9 × A × n−1/5 , where A = min(SD,
IQR/1.34) and n = sample size; Silverman, 1986, p. 48] were used
to smooth CCL of the four datasets. Means were compared todetect differences in CCLs among the datasets.

Stranding Records, Fishery Observer
Data, and Opportunistic Sightings
Spatial and temporal distribution of loggerheads in the eastern
North Pacific was examined by compiling sighting records ofloggerheads from various sources, including scientific cruises,
opportunistic reports from the public, official stranding reports,
and CADGN fishery observer data. Scientific cruises included
marine mammal surveys (Table 2) and CalCOFI cruises.
A marine turtle sightings hotline email address (the SWFSC
Marine Turtle Sightings Hotline) was created on April 1, 2015
for the public to report sightings of marine turtles along the west
coast of the United States.
We used all sightings to determine the thermal habitat forloggerheads off southern California. We obtained sea surface
temperature data (AVHRR Pathfinder version 5.3 before 2002
and Multi-scale ultra-high resolution MUR SST analysis after2001) from NOAA1 , available at the resolution of 0.0417
or 0.01 degrees, respectively, at the time and location ofsightings. To take into account the spatial and temporalautocorrelation of SST within the aerial survey data, we showthese distributions for different data sources, i.e., aerial survey
data, CADGN observer data, opportunistic sightings fromshipboard marine mammal surveys, oceanographic surveys,
and the general public. For the aerial survey data, SST along
the track lines and SST at loggerhead sighting locations werecompared.
1

RESULTS
Environmental Conditions
In fall 2014, ocean conditions along the United States west
coast were significantly different from the long-term average,a trend that lasted throughout 2015 (Leising et al., 2015) due
to a combination of anomalous North Pacific warming andinteraction with developing El Niño conditions. These conditions
coincided with the 2014 marine mammal ship survey and the2015 aerial survey.
CalCOFI survey data indicated strong inter-annual
oceanographic differences between fall 2011 and 2015, theyears of the aerial surveys. The main inter-annual difference
was the contrast between the cool-core eddy south of Pt.
Conception and an anomalously warm and saline surface poolthat appeared to be related to an intrusion of waters from thesouth. Near surface (10-m depth) temperatures in fall 2011 were
dominated by a cool-core eddy with a core of 13–15◦ C water
contrasting with warmer temperatures of 17.5–19.5◦ C further
offshore (Supplementary Figure S1). Dynamic height anomalies
were dominated by this eddy and by strong meandering of thecurrents (Supplementary Figure S2). In contrast, the dominantsurface temperature feature in fall 2015 was a warm intrusion

http://coastwatch.pfeg.noaa.gov

TABLE 2 | Aerial survey effort during 2011 and 2015.
Survey date

Effort (km)Survey date

Effort (km)2011-09-07

118.94

2015-09-23

448.17

2011-09-08

458.42

2015-09-24

462.36

2011-09-11

618.46

2015-09-27

530.56

2011-09-12

622.74

2015-09-28

507.03

2011-09-13

259.802015-10-02

384.56

2011-09-18

568.78

2015-10-05

542.45

2011-09-30

751.38

2015-10-06

466.78

2011-10-02

250.41

2015-10-07

498.202015-10-20

571.232015-10-23

167.15

2015-10-24
Total

3648.9417.114995.6

Effort during each day was computed as distance between beginning and end of each transect line. Infrequent off-effort time due to flying over islands and otherunobservable locations was not subtracted from the total length.

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Loggerheads in the California Currentof 21.0–22.5◦ C water coming from south of the United States –Mexico border, clearly shown in dynamic height anomalies(Supplementary Figure S3). The intrusion warmed the coastal
waters of the Southern California Bight (SCB) (Supplementary
Figure S1) and mean mixed layer temperature was ∼2.5◦ C
warmer during fall 2015 than during fall 2011. Surface water was
as much as 4–5◦ C warmer than adjacent water and was confinedto the upper 20–30 m of the water column.

the polygon (Table 3). During those 2 years, most of the turtles
were identified as loggerheads (n = 110), and the remainder were
unidentified to species (n = 56).

Distance Analysis
For the 2015 aerial survey data, a hazard rate (HR) detection
function without any adjustment terms or Beaufort sea statecovariate was found to be most parsimonious (Table 4). The
estimated effective strip half-width was 156.2 m (SE = 7.2;
Figure 2). Encounter rate was more than three times greaterin the offshore stratum than inshore stratum (Table 5).
Post-stratification reduced the CV of the estimated densities(Table 5). The overall density of loggerheads in the study
area was 0.15 km−2 (0.08 vs. 0.24 km−2 in the inshore
and offshore strata, respectively) and the estimated total
minimum abundance was 15,286 (95% CI: 9945–23,495,
CV = 0.21; Table 5) during the 2015 survey period. Removingsightings that occurred when the Beaufort sea state wasgreater than 3 (n = 21) did not affect the selection of
the best detection function or parameter estimates (results
not shown), which permitted maximal spatial coverage ofthe study area. Applying the g(0) correction, i.e., assuming
loggerheads in this area spend 20% of their time at the ocean
surface, the maximum abundance was 76,430 (95%CI: 49,725–
117,475).
For the ship-board survey data, the half-normal key function
with cosine adjustments without Beaufort sea state covariate wasfound to be most parsimonious based on AIC values (Table 4).
Estimated effective strip half-width was 540.0 m (SE = 75.1 m,
Figure 2). Estimated densities for 2006 and 2014 were 0.08
(SE = 0.04, n = 70) and 0.04 (SE = 0.01, n = 67) turtles km−2 ,respectively.

Survey Effort
In 2011, approximately 3,650 km of aerial track lines weresurveyed during 8 days from 7 September to 2 October and nologgerheads were observed. In 2015, approximately 5,000 kmof track line were surveyed over 11 days from 23 September
to 24 October and 215 loggerheads were observed (Table 2
and Figure 1). Strong wind, cloud cover, and military exercises
limited our survey effort during both years, especially in theoffshore strata. Consequently, the northwest portion of the study
area (∼25,000 km2 ) was excluded from abundance estimation(Figure 1). The spatial distribution of loggerheads off southern
California was more concentrated in the southern portion of
the offshore area than the northern area; however, this finding
may reflect the low search effort in the northern offshore area
(Figure 1).
During the 2006 and 2014 ship-board marine mammal
surveys, approximately 960 and 1,926 km of track line weresurveyed, respectively, during adequate sea state conditions(Figure 1 and Table 3). Search effort within the polygon occurredfrom 7 April to 6 December 2006 and from 5 August and 9
December 2014. In 2006, turtles were seen from 8 April to 30 July,whereas in 2014 they were seen from 14 October to 7 December.
Sightings occurred on less than 50% of the track lines within

TABLE 3 | Marine mammal survey effort (total and when Beaufort sea state < 4) and number of turtle sightings (Beaufort < 4 in parentheses when different) between
1991 and 2014 within the polygon defined by latitude 28–36◦ N and longitude 127.5–115◦ W.
Year

Effort (km)Effort when Beaufort < 4

1991

5789

4980

0

0

0

1992

478

320

0

0

1

1993

5620

2574

3

0

3(2)

1995

3795

1325

0

0

1

1996

8516

4033

0

0

3(2)

1997

256

68

0

0

0

1998

826

402

0

0

0

1999

910

94

0

0

0

2000

675

221

0

0

0

2001

3516

1200

0

0

1

2003

822

521

1

4

4

2005

3889

1907

1

0

0

2006

2058

960

64

19

0

2008

4955

1752

0

0

0

2009

4920

2685

0

1

0

2014

3919

1925

46 (45)

36

1

Total

50944

24967

115 (45)60

14 (4)CCUH

UT

A sighting may include more than one turtle. CC, loggerhead turtle; UH, hard shell turtle without species identification; UT, turtle without species identification. Bold letters
indicate the years for which data were used in the current study.

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state of California. Stranding records indicated loggerheads havebeen reported as far north as Alaska and British Columbia,
Canada. The majority of strandings, however, occurred within
California (n = 62), specifically south of Pt. Conception (34.45◦ N;
n = 37). For stranded loggerheads with size information, curved
carapace lengths (CCL) ranged from 24.1 to 86.3 cm (54.5 ± 2.4,
mean ± SE, n = 40). Among 75 loggerhead strandings, 40
occurred during summer months (July, August, September, andOctober), whereas 35 occurred from November through June.
Fishery observers aboard CADGN fishing vessels reported 17
bycaught loggerheads from 1992 to 2006, whose CCLs ranged
from 22 to 59 cm (42.1 ± 0.6, mean ± SE). All bycatch eventsoccurred south of Pt. Conception, bounded by latitudes 34.1 and31.7◦ N (Figure 3). Although bycatch occurred throughout theyear, these events were most frequent in August (n = 7 out of 17).
A total of 306 hard shell turtles were reported from Marine
Mammal survey cruises and opportunistic reporting, which
included 125 loggerheads and 181 unidentified species (Table 1).
Since its inception in April 2015 to the end of July 2017, 39

TABLE 4 | Delta AIC values among six detection-function models that were fitted
to aerial and ship-board sightings data.
1AIC (Aerial)

1AIC (Ship-board)

Hazard rate (HR)0–

HR + Cosine0–

HR + Beaufort

1.94

–

Half-normal (HN)

6.23

4.84

HN + Cosine

1.53

0.0

HN + Beaufort

8.22

7.94

HN + Year–

6.68

HN + Beaufort + Year

–

7.81

Model (key + adjustment/covariates)

Stranding Data, Observer Data, andOpportunistic Sightings
From 1980 to August 2017, 75 loggerheads were reported to
the NMFS West Coast Region Stranding Network, of which
22 did not have exact coordinates but were all found in the

FIGURE 2 | Perpendicular sighting distances (m) and fitted detection functions for aerial (A) and ship-board marine mammal survey (B) when detecting turtles. For
the aerial survey, hazard-rate detection function without adjustment terms was the best fit, whereas half-normal with cosine adjustment terms (order 2) was the best
fit for the ship-board survey based on AIC values (Table 4). The estimated effective strip half-widths were 156.1 m (SE = 7.2 m, n = 212) for aerial survey and 540.0m (SE = 75.1 m, n = 99) for ship-board survey.

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TABLE 5 | Estimates and uncertainties of encounter rates, densities, and abundance of loggerheads in the study area from the line-transect survey conducted during fall
2015.
Stratumn

Estimate

0.14

0.03

0.24

0.08

0.22

212

13963.5

3401.30.24

8594.622686.3

SE

CV

LCL

UCL

No stratification
Density (km−2 )Abundance
With stratification
Encounter rate (km−1 )Inshore

82

0.025

0.009

0.35

Offshore

130

0.076

0.020

0.26

Total

212

0.042

0.010

0.24

Inshore

0.08

0.03

0.35

0.04

0.16

Offshore

0.24

0.06

0.26

0.14

0.42

Total

0.15

0.03

0.21

0.10

0.23

Density (km−2 )Abundance
Inshore4762.11687.70.35

2321.99766.8

Offshore

10524.2

2751.20.26

6133.2

18058.7

Total

15286.3

3260.30.21

9945.423495.3

The estimates are based on the most parsimonious model (HR; Table 4). SE, standard error; CV, coefficient of variation; LCL, lower 95% confidence limit; UCL, upper
95% confidence limit.

locations. Sea surface temperatures at sighting locations ranged
from 14.4 to 27.5◦ C (Figure 4). During the aerial survey in 2015,
loggerheads were found in water with SST between 20 and 21◦ C
in a disproportionately higher proportion (49.3%) than it wasavailable (20.9%). Another disproportionate peak of SST at turtle
sightings was found at approximately 23.5◦ C (Figure 4). CADGN
bycatch occurred in waters at the lower half of the temperature
range (14.4–21.1◦ C). Sea surface temperature at opportunistic
sighting locations indicated a bimodal distribution with peaks at
approximately 19 and 21◦ C.

DISCUSSION
Our study is the first to characterize loggerhead presence off
the United States west coast using multiple lines of evidence.
Clearly, the most influential data for our analyses come fromthe aerial surveys in 2011 and 2015, and the disparity in survey
results (0 vs. 215 loggerheads) underscores the ephemeral natureof loggerheads in the area. Our analyses of ship-board survey data
as well as sighting and stranding records complement our aerial
surveys and further support that loggerheads are present along
the southern California coast most commonly during warm waterconditions.

FIGURE 3 | Location and timing of loggerheads and unidentified turtles
opportunistically sighted by the public and from oceanographic cruises andbycaught in the CADGN fishery. Bycatch occurred from 1992 to 2006,
whereas reported observations are in 2015 and 2016. The current time-area
CADGN closure area (Loggerhead Turtle Management Area) is indicated withdashed lines.

loggerheads have been reported to the SWFSC Marine Turtle
Sightings Hotline by recreational boaters, surfers, sport fishingboat captains, and naturalists onboard whale watching boats.
All but two sightings included latitude and longitude with theprecision of at least 0.1 min.

Environmental Conditions
The Oceanic Niño Index (ONI), which is NOAA’s climate
indicator for categorizing El Niño Southern Oscillation
conditions as El Niño or La Niña, characterized fall 2011 as La
Niña (anomalously cool) conditions and fall 2015 as El Niño
(anomalously warm) conditions (Supplementary Figure S4).
The Pacific Decadal Oscillation index (PDO; Mantua et al., 1997)
also exhibited contrasting oceanographic conditions between
2011 and 2015; the PDO was negative in 2011, indicating lower

SST at Sighting Locations
Among the 306 sightings, SST was obtained for 293 locations.
Although these included unidentified hard shell turtle species,
we think the majority of them were loggerheads based on their

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FIGURE 4 | Distributions of SST at loggerhead sightings from various data sources. The (top) “Aerial” shows the SST at aerial survey sighting locations (n = 215)
and along the track lines (split into 2 km chunks; n = 2643) during the 2015 aerial survey. The (middle) “CADGN” shows the SST at the 17 bycatch locations. The(bottom) “Sightings” shows the SST for other sighting locations including marine mammal shipboard surveys, oceanographic cruises, and from the general public
for loggerheads (n = 123) and unidentified hard shell turtles (n = 170).

than average North Pacific SST pattern, but changed to positive
beginning in 2014 and extending to 2015 (Supplementary
Figure S4). During 2014, the CCLME was affected by the North
Pacific warm anomaly in the Gulf of Alaska (Bond et al., 2015)
and by August 2014, most of the CCLME was affected by amoderate El Niño (Leising et al., 2014, 2015). In December
2014, the North Pacific and the southern California warm
anomalies started (Leising et al., 2015) and the PDO reached its
highest value since June 1997, indicating the abnormally highNorth Pacific SST condition (Supplementary Figure S4; Leising
et al., 2015). Combined with other factors, such as atmosphericforcing, advection, and responses to El Niño, waters off the west
coast of United States were experiencing a marine heat wave
throughout 2014 and 2015 (Leising et al., 2015). The anomalously
warm conditions were associated with changes in planktonic
communities (Sakuma et al., 2016; Santora et al., 2017) as wellas the arrival of several exotic species to the region, including
pelagic red crabs, sea snakes (Pelamis platura), hammerhead
sharks (Sphyrna sp.), dolphin fish (Coryphaena hippurus), wahoo(Acanthocybium solandri), manta rays (Manta hamiltoni), andpygmy killer whales (Feresa attenuata) (Barlow, 2016, Captain
Paul Fischer, pers. comm., various news reports2 ), as well as

loggerheads. During our aerial survey in 2015, we observed someof these species (hammerhead sharks, manta rays, and dolphinfish). With the predicted warming of oceans in the future (Cavole
et al., 2016), the presence of these exotic species in the southern
California waters may become a more common occurrence.

Abundance and Density
This study provides the first estimate of spatial and temporalvariability of loggerhead density at the northeastern limit of theirrange in the Pacific. The extreme warm condition in 2014–2015
appeared to be a large factor in promoting their presence during
this period, and the observed temporal variability in density
across years indicated that this is an ephemeral habitat for the
species. In contrast, the Baja California habitat in Mexico is
known to be a permanent habitat for the species, apparently
regardless of the presence or absence of El Niño. The estimateddensity of loggerheads in 2015 (0.24 km−2 ) was comparable tothose from the Baja habitat (0.205–0.265 km−2 : Seminoff et al.,2014), suggesting that when available, this northern habitat may
be as important for the species as the Baja habitat.
The high productivity of the CCLME likely promotes
sustained high quality habitat for juvenile North Pacificloggerheads, especially during anomalously warm periods. Basedon the difference in size distributions between the central west
coast of the BCP, Mexico, and southern California (Figure 5),these two locations appear to contain distinct life-stage groups.
The predominant age group in the BCP habitat is large
juveniles, whereas a large variability of age groups, including verysmall individuals, were found in southern California. Seminoff
et al. (2014) reported the mean annual abundance estimateof approximately 43,000 loggerheads (approximate 95% CI –2

Pelagic red crabs: https://www.washingtonpost.com/news/capital-weather-gang/
wp/2015/06/17/red-crabs-swarm-southern-california-linked-to-warm-blob-inpacific/?utm_term=.aeed5bd10a50, sea snake: http://www.livescience.com/53182venomous-sea-snake-california.html, hammerhead sharks: http://losangeles.
cbslocal.com/2015/09/07/why-more-hammerhead-sharks-are-popping-up-offsouthern-california-coast/, dolphin fish: http://www.sandiegouniontribune.com/
sports/outdoors/sdut-dorado-encinitas-oceanside-fishing-california-2015aug30story.html, wahoo: http://www.reuters.com/article/us-california-elnino-fishidUSKCN0T214X20151113, manta ray: http://www.ocregister.com/articles/rays64054-manta-sighting.html.

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Loggerheads in the California Current

FIGURE 5 | Size distributions of loggerheads from four data sources. Data for Peckham et al. (2008) were simulated from the means, standard deviations, andsample sizes provided in Table 4 in Peckham et al. (2008) using normal distributions.

found in United States west coast waters originate from Baja,
Mexico.

15,000 to 100,000) in the Mexican foraging area. Simply adding
the abundance estimates from these two regions results in>100,000 loggerheads within the CCLME, albeit the estimates
were separated by 10 years.
We are unsure what proportion of all North Pacific juvenile
loggerheads this number represents. The most recent estimate
of annual nesting female abundance from nesting beaches inJapan is approximately 2300 (Matsuzawa, 2011). If this number isstill valid, the CCLME provides foraging habitat for a significant
proportion of loggerheads in this population. Based on thesenumbers, it is clear that the California Current foraging areas areimportant for North Pacific loggerheads. To further clarify thevalue of these areas, regular aerial surveys should be conducted
to estimate loggerhead abundance and distribution.
It is also possible that some or all of the turtles found off
southern California are part of a central Pacific foraging groupthat moves between the eastern and central Pacific as thermal
corridors open (e.g., Abecassis et al., 2013). Allen et al. (2013)
found that loggerheads bycaught in California-based fisheries
originated from the CNP based on stable isotope analysis.
However, the destination for loggerheads departing offshore
areas of California are less clear and deserving of future study.
Satellite telemetry efforts should be implemented to characterize
the movements of individual turtles tagged within the southern
California region. Further sampling and stable isotope analyses
should also be conducted to help clarify if the CaliforniaCNP linkage is a consistent pattern or if perhaps some turtles

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Distributions of Loggerheads
Although less common north of southern California, loggerheads
are found widely along the United States west coast. In thestranding records, a loggerhead was reported as far north as
58.5◦ N. Hodge and Wing (2000) described two loggerheads (onedead and one alive) along the coast of Alaska north of 57◦ N in
the early 1990s, and more recently Halpin et al. (2018) reported
a loggerhead in waters of British Columbia, Canada. Strandingsof loggerheads in northern/central California, Oregon, andWashington have become more frequent during the last
decade, coinciding with warmer water conditions throughout
the CCLME. Eight strandings were reported since 1997 northof Pt. Conception (34.45◦ N). In 2017, two strandings werereported in Oregon in the first 3 months. It is well known
that only a small fraction of carcasses at sea reach the shore
and is reported (Epperly et al., 1996; Eguchi, 2002; Koch et al.,2013). Postmortem drifting adds another source of difficulty ininterpreting stranding locations as a proxy for the distributionof live animals without considering oceanographic information,such as wind and current (Hart et al., 2006). In this study,
we provided the observed latitudinal distribution of stranded
loggerheads, which indicated their occurrence off southern
California, but we hesitate in drawing firm conclusions about
the origins of these turtles. Nevertheless, this information can

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Eguchi et al.

Loggerheads in the California Currentof telemetered juvenile loggerheads were significantly different
from those of simulated drifting particles. This suggests thatthe movements of these three turtles was not necessarily tied toprevailing ocean currents, perhaps to stay within a preferred SST
range. The ability of small turtles to stay within a desirable rangeof SSTs in oceanic regions has been reported elsewhere in the
Pacific (Abecassis et al., 2013).
During the 2015 aerial survey, loggerheads off southern
California were found in warmer waters than in the CNP (middle
50th percentiles: 18.6–22.9◦ C vs. 15.6–17.1◦ C, Howell et al.,2008). Using several data sources together, however, loggerheads
off southern California were found in SST between 14.4 and27.5◦ C (Figure 4), which encompassed many reported SST
ranges around the globe, where means ranged from 16.3◦ C in the
North Pacific to 28.8◦ C in the Gulf of Mexico (Table 6). Although
SST may be a proxy for loggerhead habitat, it may not be a goodpredictor of loggerhead presence in all systems. Instead, affinity
to a certain range of SST may be ecosystem-specific, as has been

be treated as the baseline to which future stranding patterns arecompared as the large-scale oceanographic conditions and totalabundance of North Pacific loggerheads change in the future.
The distribution of loggerheads in our study area appeared tobe heterogeneous (Figure 1). Presence of loggerheads appeared
to be ephemeral, where they were not present in the same
location on consecutive days (Paul Fischer, the captain of F/VOuter Limits, a San Diego based sport fishing vessel, personalcommunication). Although the apparent absence of the turtles
could have been caused by the change in sighting conditions
rather than movements of turtles, this disparity is likely due, atleast in part, to their high motility. Satellite telemetry of juvenile
loggerheads within the SCB supports this possibility, as threesmall juveniles (29.0, 31.5, and 61.9 cm straight carapace length:
SCL) tracked in the area had high daily vagility (7.6 ± 1.6 km
d−1 , mean ± SE) and moved large distances in the study
area (399–730 km) during their tracking durations (SWFSC,
Unpublished data). Briscoe et al. (2016b) showed that movements
TABLE 6 | Published information about SST and loggerhead turtle relationships.
Ocean BasinnSize range (cm)Temp. range (C)Atlantic

27

49–83

10.2–28.4

Mean/median ± SD (C)SourceWinter: 18.0 ± 1.8

Barceló et al., 2013

Spring: 19.0 ± 2.0
Summer: 22.0 ± 2.0
Fall: 20.0 ± 2.0
Atlantic

Year round: 13

Adult females

Seasonal: 42
Atlantic

Gulf of Mexico

MediterraneanYear round: 25.7

Summer: 20.5–27.21

Summer: 23.4 (med)Winter: 17.8–23.51

Winter: 21.1 (med)Neritic: 17

41.9–90.5

Neritic winter: 9.0–21.5

Neritic winter: 18.7 ± 2.8

Oceanic: 6

SCL

Neritic spring: 13.6–24.4

Neritic spting:1 8.6 ± 2.3

Neritic summer: 22.9–29.3

Neritic summer: 25.5 ± 1.6Group 1: 7

84.3–103.2

Group 2: 3
IndianYear round: 20.8–28.81

Group 1: 9

54–77

Neritic fall: 10.0–25.3

Neritic fall: 19.5 ± 2.6

Oceanic winter: 15.5–21.0

Oceanic winter: 17.1 ± 1.4

Oceanic spring: 12.6–26.4

Oceanic spring: 18.7 ± 2.0

Oceanic summer: 16.7–28.1

Oceanic summer: 22.8 ± 2.6

Oceanic fall: 13.2–24.4

Oceanic fall: 19.3 ± 3.0Group 1: 27.6–30.02

Group 1: 28.8 ± 0.9Group 2: 26.6–28.52

Group 2: 27.4 ± 1.0

17.2–30.1

Group 1: 27.61 ± 1.273

Group 2: 4

Group 2: 21.65 ± 1.033

Group 3: 5

Group 3: 27.25 ± 1.443

10

74.6–85.0

13.0–29.0

Neritic: 21.3 ± 1.5

Hawkes et al., 2011

Mansfield et al., 2009

Hart et al., 2012
Dalleau et al., 2014

Casale et al., 2012b

Oceanic: 22.0 ± 2.4
Mediterranean

7

47–66.5

Pacific

Group 1:23

51–89

Group 2:40

59–91

Pacific

Warm group: 3

41.0–81.0

10.1–29.7

21.2 ± 2.6

Casale et al., 2012aGroup 1: 19.7 ± 1.8

Peckham et al., 2011

Group 2: 24.0 ± 1.9
14.0–25.0

Cold group: 6

Warm group = 20.7

Polovina et al., 2000

Cold group = 17.6

Pacific

105

25.6–89.1 SCL

11.0–21.6

16.3 ± 1.6

Howell et al., 2008

Pacific

188

23–81 SCL

16.7–21.0

SCL > 50 cm: 19.0

Abecassis et al., 2013

Pacific

8

35.5–59.0

18.5–22.4

20.3 ± 1.5

Allen et al., 2013

Pacific

231

23.3–83.0 SCL

10.0–28.7

17.6 (med) ± 2.2

Briscoe et al., 2016a

SCL < 35 cm: 17.6

Sample size (n) is the number of turtles. Mean/median and SD were often computed using the number of data points rather than the number of turtles: 1 ranges of medians.
2 Ranges of means. 3 Interquartile range. Size is reported in curved carapace length unless it was noted as straight carapace length (SCL). Groups in the publications wereconverted into numeric groups (see the original papers for the details).

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distribution data combined with loggerhead growth rates will
provide useful information about juvenile loggerheads in eachof these regions. Because El Niño events strongly affect theseregions, influence of changes in SST and ocean productivity onthese populations should be examined.

shown for other marine turtle species (e.g., leatherback turtles,
Dermochelys coriacea; Benson et al., 2011). A broad analysisof relationships between presence/absence of loggerheads andenvironmental variables may provide insights into what variables
are important in predicting presence of loggerheads.
A study in the Atlantic Ocean indicated fidelity of juvenile
loggerheads to foraging sites over years and another study
indicated a large proportion of transients within a foraging
site (Sasso et al., 2006; Mansfield et al., 2009). The observed
coexistence of transients and philopatry to foraging areas
suggested that multiple foraging strategies exist within aloggerhead population. Consequently, it is possible that somejuvenile loggerheads in the BCP foraging ground may moveto high seas as such behavior has been observed in the
Atlantic (Mansfield et al., 2009). These ‘transient’ loggerheads
from nearshore BCP, as well as those from the pelagic CNP,
may be found within the southern California area whenenvironmental conditions are optimal. Particle tracking, regularsurveys, stable isotope analyses, and satellite telemetry withcareful examination of oceanographic conditions would help
elucidate the mechanisms by which loggerheads enter thesouthern California waters and their movements within the
North Pacific.
Effects of changing ocean climate and local ocean conditions
on loggerhead distributions should be examined closely. Changes
in species compositions as a response to El Niño and otherlocal ocean conditions have been reported globally (e.g., Benson
et al., 2002; Wernberg et al., 2012). The observed high density ofloggerheads in 2015–2016 also was likely caused by the warmingof the Eastern Pacific, as we described in the Environmental
Conditions section above. As the ocean temperature is expected
to increase in the coming years, occurrence of loggerheads in thearea may become more common.

CONCLUSION
Considering all the available data sources on loggerheads in the
Northeast Pacific, we found that a large number of multiple
cohorts of loggerheads exist in the region. Spatial distributionsof loggerheads appeared not to associate with a particularoceanographic feature, including SST or meso-scale eddies, as
was suggested by studies from different regions. Consequently,
generalization of findings about association of loggerheads toenvironmental features is not warranted. With the expectedchange in the ocean climate, we expect to see loggerheads
and other exotic species in the region more frequently. In
order to monitor the effects of changing environment on
distributions of various species in the area, aerial surveys
need to be conducted during thermally anomalous years aswell as non-anomalous years in order to determine interannual variability in density, abundance, and distribution ofloggerheads in the region. Multi-year dedicated surveys in thesurrounding areas and concurrent telemetry studies are alsoneeded to determine how loggerheads use this area. Longterm studies need to be conducted to help elucidate the effectsof oceanographic variability and climate shifts on loggerhead
distribution and movements. Detailed studies at various locations
using a wide variety of research techniques are necessary to
elucidate demography and ecology of loggerheads within apopulation.

Size Range of Loggerheads in the
CCLME

AUTHOR CONTRIBUTIONS

The observed size range of loggerheads indicated the presenceof multiple life stages off southern California and in the CCLME
in general (Figure 5). Stranding records encompassed very small
individuals of the same sizes as those caught in the CADGN
fishery, as well as large individuals of the same sizes as those foundin the BCP foraging ground (71.8 ± 0.21, n = 1918, Peckham
et al., 2008; 65.9 ± 1.23, n = 146, Turner Tomaszewicz et al.,2015). Further, we captured two juvenile loggerheads off southern
California with CCLs of 22.3 and 61.9 cm in two consecutive
days, the smallest of which was about 1-year-old, whereas thelargest may have been 20 years or older (Turner Tomaszewicz
et al., 2015). Together, these data underscore the fact that multiple
cohorts of loggerheads exist in the CCLME.
Size distributions of loggerheads along the west coast of North
America are comparable to those found along the west coastof South America where loggerheads originate from Australia
and New Caledonia (Alfaro Shigueto et al., 2008; Boyle et al.,2009; Donoso and Dutton, 2010). The minimum size found in
our study (22 cm) was also similar to that reported from theeastern South Pacific (26 cm; Alfaro-Shigueto et al., 2004). Size

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TE, JS, and SB conceived the study, secured funding,and collected data. SM and CW provided oceanographic
information. RL dealt with permit, stranding data and sighting
database, budget, and other administrative tasks. TE drafted themanuscript. TE, JS, SB, SM, CW, and RL edited the manuscript.

ACKNOWLEDGMENTS
We thank Nicki Beaulieu, Joel Schumacher, Camryn Allen, Erin
LaCasella, Dan Prosperi, Jim Carretta, Lisa Komoroske, Morgane
Lauf, and Dan Lawson for their keen eyes on airplanes aswell as other duties, related to this study, including stranding
responses and data entry. We also thank pilots (Shanae Coker,
Rick De Triquet, Kevin Doremus, Chris Kerns, Robert Mitchell,
and Sandor Silagi) of NOAA Office of Marine and Aviation
Operations for their excellent aviation skills. Commander
Michelle Kemp at San Nicolas Island generously allowed us
to use Outlying Field San Nicolas Island. We also appreciated12

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Loggerheads in the California Current

those who responded to strandings, enduring nasty smells forthe sake of science. Thanks also to those who reported turtle
sightings to the SWFSC Sea Turtle Sighting Hotline (swfsc.turtlesightings@noaa.gov) and to Lynn Massey for entering sightings
data to the database. Karin Forney and Elizabeth Becker
generously shared R code to analyze sightings data. Al Jacksonkindly responded to our requests for sightings data from various
marine mammal surveys. Suzy Kohin shared the DGN observer
data. Calandra Turner Tomaszewicz kindly shared her raw turtle
size data with us. Jim Carretta, Christina Fahy, and Penny Ruvelas

provided constructive comments to the manuscript. Aerial andshipboard surveys were conducted under the NMFS permit
numbers 774-1714 and 14097, respectively.

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Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Copyright © 2018 Eguchi, McClatchie, Wilson, Benson, LeRoux and Seminoff. Thisis an open-access article distributed under the terms of the Creative Commons
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