Current Herpetology 30(1): 63–68, June 2011
© 2011 by The Herpetological Society of Japan

Reentery of Juvenile and Subadult Loggerhead Turtles into
Natal Waters of Japan
TAKASHI ISHIHARA1,2*,
NAOKI KAMEZAKI1,3,
YOSHIMASA MATSUZAWA2,
FUTOSHI IWAMOTO2,TATSUYA OSHIKA2,YOSHITAKA MIYAGATA2,CHIAKI EBISUI2,
AND SUGURU YAMASHITA2
1 Graduate School of Agricultural and Life

Sciences, The University of Tokyo, Yayoi,
Bunkyo, Tokyo 113–8657, JAPAN2 Sea Turtle Association of Japan, Nagaomotomachi, Hirakata, Osaka 573–0163,
JAPAN
3 Suma Aqualife Park Kobe, Suma, Kobe,
Hyogo 654-0049, JAPAN
Abstract: We measured the straight carapace
length (SCL) of 1392 loggerhead turtles of theJapanese aggregation that were incidentally captured between July 2002 and November 2009 bypound nets set near the eastern coast of Cape
Muroto, Kochi Prefecture, Japan. Each specimen was categorized as adult, subadult, or juvenile on the basis of their maturity status inferredfrom SCL. In the present sample, SCL exhibiteda unimodal distribution with the mode located inthe 740–749 mm class (x4±SD: 757±67 mm; range:
563–1050 mm). The majority of loggerhead turtles composing the Japanese aggregation was subadult (75.9%). The SCL of smaller specimens
suggested that reentery into Japanese waters ofindividuals once flown to the northeastern Pacificoccurs in the later juvenile stage. A unimodal
histogram implies that the major size classes in
SCL of turtles reentering Japanese waters range
from 560–749 mm. A comparison between theJapanese aggregation and the Mexican aggregation indicates that turtles at the later juvenile* Corresponding author. Tel: +81–72–864–0335;
Fax: +81–72–864–0535;
E-mail address: ishihara@umigame.org

stage start to migrate westward, taking only a few
years to cross the North Pacific to Japanesewaters.
Key words: Loggerhead turtle; North Pacific; Size
distribution; Migration; Japanese aggregation

Loggerhead turtles (Caretta caretta) are
widely distributed in the world’s temperate
oceans (Pritchard, 1997). Studies on the distribution of mitochondrial DNA (mtDNA)
haplotypes have revealed the presence of certain levels of reproductive isolation between
breeding assemblages of different oceans andof different hemispheres (Bowen et al., 1994;
Bowen, 2003). In both the Pacific and Atlantic oceans, nesting sites are mostly restricted towestern coasts. Hatchlings are thought tomigrate from their natal beaches to the easternwaters of the ocean with the aid of ocean
currents, and then re-migrate to the westernwaters (Bolten et al., 1998; Boyle et al., 2009).
Some authors argued that the turtles start remigration after growing to approximately
50 cm in curved carapace length in the North
Atlantic (Bjorndal et al., 2000) and approximately 70 cm in the South Pacific (Limpus and
Limpus, 2003).
Japan is known to be the sole breeding areafor loggerhead turtles in the North Pacific,
and has a number of nesting beaches along thecoastlines from Kanto District of Honshu to
the Ryukyu Archipelago (Kamezaki et al.,2003). Such strict limitation of nesting sites toJapan, along with the absence of haplotypesother than those found in hatchlings fromJapan, indicate that loggerhead turtles of theNorth Pacific belong to a single population(Bowen et al., 1995). Furthermore, eastward
and westward trans-Pacific migrations were
demonstrated by mark-recapture (Uchida andTeruya, 1991; Resendiz et al., 1998) and satellite tracking techniques (Nichols et al., 2000).
It is therefore assumed that the North Pacificloggerhead turtles hatch on Japanese beaches,
disperse eastward, and then return to Japanesewaters.
For the Japanese aggregation, the status of64

Current Herpetol. 30(1) 2011

nesting females has been relatively well studied. With respect to the status of non-nestingturtles, however, information is available only
from fragmented observations (e.g. Nomura,
1993; Miyawaki, 1994; Maeda and Kimura,
1995; Iwamoto et al., 2005; Tanaka andIshihara, 2006). In this study, we investigated
the size distribution of the Japanese aggregation of loggerhead turtles on the basis of a
relatively large sample from incidental capture. Our purposes were to estimate their age
structure and to infer stages in their life historyat which they re-migrate to Japanese waters
after dispersing to the western North Pacific.

MATERIALS AND METHODS
Loggerhead turtles, incidentally captured bythree large-size pound net fishery associations,were examined. The pound nets were set at a
depth of 35–78 m near the edge of the continental slope off the coast of Cape Muroto,southeast of Shikoku, Japan (33°15' N, 134°11'
E, Fig. 1). The continental slope is steep, leading to nutrition-rich marine upwelling of theNorth Pacific intermediate current ranging
from 500 to 1000 m deep (Fukasawa, 1998).
Each fishery association caught approximately 1500–2000 tons of fish per year, withyellowtail (Seriola quinqueradiata), spottedmackerel (Scomber australasicus), and bullet
tuna (Auxis rochei) being the main harvested
species. The pound nets also caught loggerhead
turtles and green turtles (Chelonia mydas) incidentally. Because the pound net is a set net
that is not relocated throughout the year, we
assumed that loggerhead turtles migrating
through Cape Muroto were sampled at random. The captured loggerhead turtles werebrought back to fishing ports where they
were measured for straight carapace length(SCL) and examined for various external
character states. Then the specimens werereleased from the fishing ports or off thepound nets within a day.
We classified those turtles into one of thefollowing four life history stages—hatchling,
as characterized by possession of an internalized yolk sac; juvenile, as the intermediatestage between hatchling and subadult; subadults, as characterized by the presence of
secondary sexual characteristics, but without
maturity; and adult, as showing full maturity.
For the North Pacific loggerhead populations,
SCL of hatchlings is shown to be 41.9±1.51 mm(x4±SD) (Matsuzawa and Sakamoto, 2002).
Likewise, secondary sexual characteristics ofmales reportedly occur from around 688 mm
in SCL (Ishihara et al., 2007), and SCL of
nesting females is reported to be 848±
50.8 mm (x4±SD) (Kamezaki et al., 1995). We
therefore regarded turtles with SCL ≤ 41.9 mm
as hatchling, 42.0–687 mm SCL as juvenile,688–847 mm SCL as subadult, and SCL ≥848 mm as adult. We did not take into accountsexual size differences because the size atmaturity does not differ significantly between
males and females of the North Pacific loggerhead turtles (Kamezaki, 2003; Ishihara, 2007).

RESULTS AND DISCUSSION

FIG. 1. Map of Japan and adjacent regions
showing the location of Cape Muroto, Kochi Prefecture, Shikoku Island, Japan. The three black circles
indicate the locations where pound nets were set.

A total of 1392 loggerhead turtles, incidentally captured from July 2002 to November
2009, were examined. SCL of these turtles
exhibited a unimodal distribution, with themode being located in the 740–749 mm class(x4±SD: 757±67 mm; range: 563–1050 mm;

ISHIHARA ET AL.—REENTRY OF LOGGERHEAD INTO NATAL WATERS

65

FIG. 2. Distribution of straight carapace lengths of loggerhead turtles captured around Cape Muroto(n=1392, x4±SD: 757±67, range: 563–1050 mm).

Fig. 2). In the whole sample, subadults accounted
for 75.9% (1056 specimens), followed byjuveniles (13.5%, 188 specimens) and adults(10.6%, 148 specimens) in that order, while
no hatchlings were included. The smallest
specimen was 563 mm in SCL, and obviously
represented the later phase of juvenile stage.
The sample of loggerhead turtles did notinclude individuals whose SCLs are smaller
than those of large juveniles. This result is
consistent with the current hypothesis that
hatchling loggerhead turtles, emerging fromJapanese beaches, immediately disperse to theeastern North Pacific instead of staying inwaters adjacent to Japan (see above). SCL
distribution in the present sample also indicates that loggerhead turtles from Japan, once
flown to the eastern North Pacific, neverreenter into the coastal waters of Japan until
they reach to the later phase of the juvenilestage (SCL>563 mm in our sample).
Toward a relatively large subadult class of
740–749 mm SCL, the number of individuals
almost consistently increased with an increaseof SCL. Conversely, the number of individuals
gradually decreased with an increase of SCL in
the classes larger than that with 740–749 mm
SCL (Fig. 2). This latter pattern may possiblyreflect scarcity of subadult and adult turtles
that newly reenter into the Japanese coastal
waters. Also, decrease of individuals withlarger SCL may possibly reflect mortality fromvarious causes, such as shark predation (Witzell, 1987), prevalence of diseases (George,1997), and fatal by-catches through various
fishery activities (Gilman et al., 2010; Peckham, 2007). Besides these, subadult and adultloggerhead turtles that have been “lost” fromaround the current sampling sites may also
include those that have left this area subsequent to their reentry. Analyses of recapturerecords and satellite tracking data for individuals once captured around the present study
sites are desired to verify this possibility, but as
far as the currently available information isconcerned (Sea Turtle Association of Japan,
unpublished), substantial contributions ofsuch individual re-migrations to the decline oflarger turtles (Fig. 2) are much unlikely.
On the other hand, increase of individuals
with larger SCLs, observed in juvenile classes
with smaller SCL in the putatively randomly
collected sample suggest that reentry into
Japanese coastal waters by loggerhead turtles
that had dispersed from Japan to the eastern
North Pacific largely involves individuals thatare yet immature but have substantially grown
up from the hatchling state. Taking generally
higher mortality in smaller turtles (National
Research Council, 1990) into considerations
higher frequency of reentry by juvenile turtles
just inferred from Fig. 2 histograms may still

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Current Herpetol. 30(1) 2011

suffer underestimation.
SCLs (x4±SD) of loggerhead turtles occurring in the eastern North Pacific off thewestern coast of Mexico (well-known nurseryarea for juvenile loggerhead turtles fromJapan: Bowen et al., 1995) reportedly correspond to those in the small to middle-sizedjuveniles, i.e., 469±130 mm (Ramirez-Cruz etal., 1991), 585±111 mm (Gardner and Nichols,
2001), or 619±21 mm (Seminoff et al., 2004).
For the Mexican aggregation of loggerhead
turtles, Gardner and Nichols (2001) also provided unimodal histograms of SCL with themode ranging from 500–700 mm, i.e., slightly
smaller than the modal range of SCL in
juveniles from our sampling sites. Considering
this modal range as the standard SCL range inindividuals just starting the westward transPacific migration, possession of slightly larger
carapaces by the putative reentering juveniles
in our sample (740–749 mm in modal range:
see above) could be attributed to growthduring migration. Several previous authors
yielded data from satellite tracking and inadvertent recaptures of marked individuals thatindicate that juvenile loggerhead turtles usually spend only a few years or even less fortrans-Pacific migration from Mexican to Japanese coastal waters (Resendiz et al., 1998;
Nichols et al., 2000; Polovina et al., 2000). On
the other hand, Parham and Zug (1997)
estimated growth rates of juvenile loggerhead
turtles with 500–599 and 600–699 mm SCL tobe 28–38 and 27–39 mm/yr, respectively. All
these data do not contradict with each other,and offer a circumstantial support to the ideathat the juvenile loggerhead turtles captured in
the coastal waters of Cape Muroto represent
the majority of size classes of the species reentering Japanese coastal waters just after transPacific migration from Mexican waters.

ACKNOWLEDGMENTS
We thank Takaoka, Shiina, and Mitsu
pound net fishery associations for their cooperation in collecting loggerhead turtles. We
also thank T. Maeda, K. Ebisui, Y. Yasuoka, K.

Hashimoto, S. Yamashita, M. Wakatsuki, S.
Ohara, Y. Kasai, H. Miyanohara, T. Nakamura,Y. Eguchi, K. Murai, K. Kusakabe, H. Tanaka,
K. Saito, K. Mori, M. Matsumoto, K. Sato, A.
Abe, T. Nakazatomi, and H. Mizuno for their
help in examining these turtles. This study was
supported by the Sea Turtle Association ofJapan, Research Fellowships of the Japan
Society for the Promotion of Science for
Young Scientists, the Western Pacific Regional
Fishery Management Council, the Keidanren
Nature Conservation Fund, Lush Japan Co.
Ltd., Patagonia Japan, the AEON Environmental Foundation, and the Mobile Communication Fund.

LITERATURE CITED
BJORNDAL, K. A., BOLTEN, A. B., AND MARTINS,
H. R. 2000. Somatic growth model of juvenile
loggerhead sea turtles Caretta caretta: durationof pelagic stage. Marine Ecology Progress
Series 202: 265–272.
BOLTEN, A. B., BJORNDAL, K. A., MARTINS, H. R.,
DELLINGER, T., BISCOITO, M. J., ENCALADA, S.
E., AND BOWEN, B. W. 1998. Transatlantic
developmental migrations of loggerhead sea
turtles demonstrated by mtDNA sequence analysis. Ecological Applications 8: 1–7.
BOWEN, B. W. 2003. What is a loggerhead turtle?
The genetic perspective. p. 7–27. In: A. B. Bolten
and B. E. Witherington (eds.), Loggerhead Sea
Turtles. Smithsonian Books, Washington, D.C.
BOWEN, B. W., ABREU-GROBOIS, F. A., BALAZS, G.
H., KAMEZAKI, N., LIMPUS, C. J., AND FERL, R.
J. 1995. Trans-Pacific migrations of the loggerhead turtle (Caretta caretta) demonstrated withmitochondrial DNA markers. Proceedings of
the National Academy of Sciences of the UnitedStates of America 92: 3731–3734.
BOWEN, B. W., KAMEZAKI, N., LIMPUS, C. J.,
HUGHES, G. R., MEYLAN, A. B., AND AVISE, J.
C. 1994. Global phylogeography of the loggerhead turtle (Caretta caretta) as indicated by
mitochondrial DNA haplotypes. Evolution 48:
1820–1828.
BOYLE, M. C., FITZSIMMONS, N. N., LIMPUS, C. J.,
KELEZ, S., VELEZ-ZUAZO, X., AND WAYCOTT,ISHIHARA ET AL.—REENTRY OF LOGGERHEAD INTO NATAL WATERS
M. 2009. Evidence for transoceanic migrations
by loggerhead sea turtles in the southern Pacific
Ocean. Proceedings of the Royal Society B 276:
1993–1999.
FUKASAWA, M. 1998. Kitataiheiyou no chusou
jyunkan [Circulation in the middle stratum of
the North Pacific]. p. 5–8. In: Anonymous (ed.),Abstracts of the Kaiyo Shin-sousui Meeting in
Kochi. Kaiyo Shin-sosui Riyo Kenkyu-kai, Tokyo.
(in Japanese)
GARDNER, S. C. AND NICHOLS, W. J. 2001. Assessment of sea turtle mortality rates in the Bahia
Magdalena region, Baja California Sur, Mexico.
Chelonian Conservation and Biology 4: 197–
199.
GEORGE, R. H. 1997. Health problems and diseases of sea turtles. p. 363–386. In: P. L. Lutz
and J. A. Musick (eds.), The Biology of Sea
Turtles. CRC Press, Boca Raton, Florida.
GILMAN, E., GEARHART, J., PRICE, B., ECKERT, S.,MILIKEN, H., WANG, J., SWIMMER, Y., SHIODE,
D., ABE, O., PECKHAM, S. H., CHALOUPKA, M.,
HALL, M., MANGEL, J., ALFARO-SHIGUETO,DALZELL, P., AND ISHIZAKI, A. 2010. Mitigating
sea turtle by-catch in coastal passive net fisheries.
Fish and Fisheries 11: 57–88.
ISHIHARA, T. 2007. The Study of Maturity Processof Loggerhead Turtles (Caretta caretta), Unpublished master’s thesis. University of Tokyo,
Tokyo. (in Japanese)
ISHIHARA, T., KAMEZAKI, N., YAMASAKI, C.,
YAMASHITA, S., AND HINO, A. 2007. Tail elongation with growth in loggerhead turtle, Caretta
caretta, male. Bulletin of the HerpetologicalSociety of Japan 2007: 78–79. (in Japanese)
IWAMOTO, F., ISHIHARA, T., HAYASHI, A., KINO,
M., WAKABAYASH, I., AND KAMEZAKI, N. 2005.
Occurrences of the loggerhead turtle, Caretta
caretta, on the northern coast of Mie prefecture
in Japan. Bulletin of the Herpetological Societyof Japan 2005: 109–111. (in Japanese)
KAMEZAKI, N. 2003. What is a loggerhead turtle?
The morphological perspective. p. 28–43. In: A.
B. Bolten and B. E. Witherington (eds.), Loggerhead Sea Turtles. Smithsonian Books, Washington, D.C.
KAMEZAKI, N., GOTO, K., MATSUZAWA, Y.,
NAKASHIMA, Y., OMUTA, K., AND SATO, K.

67

1995. Carapace length and width of the loggerhead turtle, Caretta caretta nested in the coast ofJapan. Umigame Newsletter of Japan (26): 12–
13. (in Japanese)
KAMEZAKI, N., MATSUZAWA, Y., ABE, O., ASAKAWA,
H., FUJII, T., GOTO, K., HAGINO, S., HAYAMI,
M., ISHII, M., IWAMOTO, T., KAMATA, T., KATO,
H., KODAMA, J., KONDO, Y., MIYAWAKI, I.,MIZOBUCHI, K., NAKAMURA, Y., NAKASHIMA,
Y., NARUSE, H., OMUTA, K., SAMEJIMA, M.,
SUGANUMA, H., TAKESHITA, H., TANAKA, T.,
TOJI, T., UEMATSU, M., YAMAMOTO, A., YAMATO,
T., AND WAKABAYASHI, I. 2003. Loggerhead
turtles nesting in Japan. p. 210–217. In: A. B.
Bolten and B. E. Witherington (eds.), Loggerhead
Sea Turtles. Smithsonian Books, Washington,
D.C.
LIMPUS, C. J. AND LIMPUS, D. J. 2003. Loggerhead
turtles in the Equatorial and Southern Pacificocean. p. 199–209. In: A. B. Bolten and B. E.
Witherington (eds.), Loggerhead Sea Turtles.
Smithsonian Books, Washington, D.C.
MAEDA, H. AND KIMURA, J. 1995. Third record of
the juvenile loggerhead turtle from Ogasawara
Islands, Japan. Umigame Newsletter of Japan(24): 5. (in Japanese)
MATSUZAWA, Y. AND SAKAMOTO, W. 2002. Effectof incubation temperature on body size ofloggerhead hatchlings. Umigame Newsletter ofJapan (55): 17–18. (in Japanese)
MIYAWAKI, I. 1994. Straight carapace length of asea turtle captured at the coast of Kushimoto,Japan. p. 75–80. In: N. Kamezaki, S. Yabuta,
and H. Suganuma (eds.), Nesting Beaches of
Sea Turtles in Japan. Sea Turtle Association ofJapan, Osaka. (in Japanese)
NATIONAL RESEARCH COUNCIL, C. O. S. T. C.
1990. Decline of the Sea Turtles: Causes andPrevention. National Academy Press, Washington, D.C.
NICHOLS, W. J., RESENDIZ, A., SEMINOFF, J. A.,
AND RESENDIZ, B. 2000. Transpacific migrationof a loggerhead turtle monitored by satellite
telemetry. Bulletin of Marine Science 67: 937–
947.
NOMURA, T. 1993. Record of stranded juvenile
loggerhead turtles during winter period (1992–
1993) on the coast of Niigata Prefecture, Japan.

68

Umigame Newsletter of Japan (16): 12–13. (inJapanese)
PARHAM, J. F. AND ZUG, G. R. 1997. Age andgrowth of loggerhead sea turtles (Caretta
caretta) of coastal Georgia: an assessment ofskeletochronological age-estimates. Bulletin ofMarine Science 61: 287–394.
PECKHAM, S. H., MALDONADO-DIAZ, D., WALLI,
A., RUIZ, G., CROWDER, L. B., AND NICHOLS, W.
J. 2007. Small-scale fisheries bycatch jeopardizes endangered Pacific loggerhead turtles.
PLoS One 2: e1041. doi:10.1371/journal.pone.
0001041.
POLOVINA, J. J., KOBAYASHI, D. R., PARKER, D.
M., SEKI, M. P., AND BALAZS, G. H. 2000. Turtles
on the edge: movement of loggerhead turtles(Caretta caretta) along oceanic fronts, spanning
longline fishing grounds in the central North
Pacific, 1997–1998. Fisheries Oceanography 9:71–82.
PRITCHARD, P. C. H. 1997. Evolution, Phylogeny,
and current status. p. 1–28. In: P. L. Lutz, and J.
A. Musick (eds.), The Biology of Sea Turtles.
CRC Press, Boca Raton, Florida.
RAMIREZ-CRUZ, J. C., PENA-RAMIREZ, I., AND
VILLANUEVA-FLORES, D. 1991. Distribucion yabundancia de la tortuga perica, Caretta caretta
Linnaeus (1758), en la costa occidental de Baja
California Sur, Mexico. Archelon 1: 1–4.
RESENDIZ, A., JIMENEZ DE RESENDIZ, B. A.,

Current Herpetol. 30(1) 2011
NICHOLS, W. J., SEMINOFF, J. A., AND
KAMEZAKI, N. 1998. First confirmed east-west
Transpacific movement of a loggerhead sea
turtle, Caretta caretta, released in Baja California, Mexico. Pacific Science 52: 151–153.
SEMINOFF, J. A., RESENDIZ, A., RESENDIZ, B., ANDNICHOLS, W. J. 2004. Occurrence of loggerhead
sea turtles (Caretta caretta) in the Gulf ofCalifornia, Mexico: Evidence of life-history
variation in the Pacific Ocean. Herpetological
Review 35: 24–27.
TANAKA, K. AND ISHIHARA, T. 2006. A record oftail elongation of a male loggerhead turtle
recaptured on his way to maturation. Umigame
Newsletter of Japan (68): 10. (in Japanese)
UCHIDA, S. AND TERUYA, H. 1991. A) Transpacificmigration of a tagged loggerhead, Caretta caretta.
B) Tag-return result of loggerhead released from
Okinawa Islands, Japan. p. 169–182. In: I.
Uchida (ed.), International Symposium on Sea
Turtles ’88 in Japan. Himeji City Aquarium,
Himeji City, Japan.
WITZELL, W. N. 1987. Selective predation on large
cheloniid sea turtles by tiger sharks (Galeocerdo
cuvier). Japanese Journal of Herpetology 12:
22–29.

Accepted: 19 December 2010