Coastal Marine Science 42(1): 8–11, 2019

Short Note

First record of a known-age juvenile
loggerhead turtle (Caretta caretta) at the
Sanriku Coast in the Northwest Pacific Ocean
Takuya Fukuoka1*†, Kazuyoshi Omuta2†, Toshio Hidaka2 and Katsufumi Sato3
1

International Coastal Research Center, The University of Tokyo, 1–19–8 Akahama, Otsuchi, Iwate, 028–1102, Japan
Yakushima Sea Turtle Research Group, Yakushima Umigame-kan, 489–8 Nagata, Yakushima, Kagoshima 891–4201, Japan
3
Atmosphere and Ocean Research Institute, The University of Tokyo, 5–1–5 Kashiwanoha, Kashiwa, Chiba, 277–8564, Japan* E-mail: t.fukuoka2@gmail.com†
Equal contribution
2

Received 25 January 2019; Accepted 26 April 2019
Abstract ― Loggerhead turtles (Caretta caretta) in the North Pacific Ocean are primarily born in the southern coast of the JapaneseArchipelago. After a developmental period of several years in Central and East Pacific Oceans, they re-migrate to Japanese waters at a larger juvenile stage. However, the exact duration until re-migrating to Japanese waters is not known because of methodological limitations of current age estimation among wild sea turtles. On 27 August 2018, a juvenile loggerhead turtle(straight carapace length: 60.8 cm, body mass: 35.0 kg) with a passive integrated transponder (PIT) tag was incidentally captured
by a set net at the Sanriku Coast, Northeastern Japan (39°22′21″N, 141°58′41″E). The PIT tag number revealed that it is a 10-year
old turtle born in Yakushima Island (30°24′21″N, 130°25′58″E) in 2008. Total 1,095 loggerhead hatchlings with PIT tags were released from Yakushima Island in 2008. To the best of our knowledge, this is the first record of a known-age and non-headstarted loggerhead turtle in the North Pacific population. The tagged loggerhead turtle provides important knowledge about
the pelagic stage duration of this species in the North Pacific population.
Key words:

Bycatch, Passive integrated transponder (PIT) tag, Mark-recapture, Known-age turtle, Temperate habitat

Introduction
Sea turtles are typically long-lived, slow-growing, andwide-ranging, occupying multiple habitats over the course of
their development (Musick and Limpus 1997, Happell et al.
2003, Avens and Snover 2013). For example, loggerhead turtles (Caretta caretta) in the North Pacific Ocean are primarily born in the southern coast of Japan, from Ryukyu Archipelago to Kanto District (Kamezaki et al. 2003, Fig. 1). Loggerhead turtles are then dispersed eastward, transported by
the Kuroshio and North Pacific Currents, reaching as far as
the Central and East Pacific Oceans (Bowen et al. 1995, Hatase 2012). After a developmental period in these oceans,these turtles re-migrate to Japanese waters at the later juvenile stage (straight carapace length (SCL) 60–70 cm, Ishihara
et al. 2011) for subsequent growth and reproduction when
they reach sexual maturity (SCL 70–100 cm, Kamezaki2003). However, the timing of these events is currently uncertain.
To elucidate the duration of each life stage, several
methods of age estimation (e.g., skeletochronology andmark-recapture using external tags) have been suggested for
sea turtles (reviewed by Chaloupka and Musick 1997, Avens

8and Snover 2013). However, these methods have some limitations (Chaloupka and Musick 1997). For example, skeletochronology analysis, which is frequently used as an age estimation method of sea turtles, assumes that bony rings on thehumerus are created annually as lines of arrested growth(Castanet et al. 1994). Although verification studies havebeen conducted (Zug et al. 1986, Zug 1990, Avens andGoshe 2007), the rings are not fully understood. Moreover,resorption of early growth marks has been observed in larger(older) turtles, making it difficult to obtain direct age estimates (Zug et al. 1986). Mark-recapture studies using external tags (e.g., plastic and Inconel) can determine the absolute
age of turtles; however, these studies are limited to headstarting turtles or turtles in the later life stages (reviewed by
Avens and Snover 2013) because external tags (2.5–4.5 cm)
are too big to attach to hatchlings (SCL of approximately
5 cm). In addition, the loss of external tags is also a problem
with this method (Chaloupka and Musick 1997, Nishizawa et
al. 2018). As a consequence, an accurate age assessment
method for wild turtles has not been established. Therefore,
the exact duration that it takes for turtles born in the south
coast of Japan to re-migrate to Japanese waters is still unknown. This problem makes it challenging to understand thelife history of North Pacific loggerhead turtles.

Fukuoka T. et al.: First record of a known-age juvenile loggerhead turtle

sured using calipers, and their curved carapace length (CCL)
and curved carapace width (CCW) were measured using atape measure with an accuracy of 0.1 cm. Their body mass(BM) was measured using a hanging scale with an accuracyof 0.5 kg. From 2014, the PIT tag information (15-digit code)
was detected using an ARE H5 reader (Trovan Ltd) fromoutside the body of turtles.

Results and Discussion

Fig. 1. Study site in the Japanese Archipelago. The coastline
shown in bold illustrates the major nesting site of loggerhead
turtles (Kamezaki et al. 2003). The arrow and star indicate the release and bycatch locations of a PIT tag-injected loggerhead turtle, respectively.

To elucidate this aspect, the Yakushima Sea Turtle Research Group has been injected passive internal transponder(PIT) tags for almost 20 years into loggerhead hatchlings in
the Yakushima Island, which is the largest rookery of loggerhead turtles in the North Pacific population (Fig. 1). In thisnote, we report the first bycatch record of a loggerhead turtle
with a PIT tag in the Sanriku Coast (Fig. 1), which is a summer-restricted (from June to October) foraging area for juvenile- to adult-sized loggerhead turtles (Narazaki et al. 2015).

Materials and Methods
Injecting PIT tags to hatchlings at the Yakushima Island
Since 1999, PIT tags were injected into loggerhead
hatchlings on the Yakushima Island (30°24′21″N,
130°25′58″E, Fig. 1). When newly emerged hatchlings were
found, they were collected and transferred to the Yakushima
Umigame-Kan, Yakushima Town, Kagoshima Prefecture,
Japan. The PIT tags (ID-100A, 2 mm in diameter, 11 mm inlength, Trovan Ltd, Douglas, UK) were injected into their abdominal cavity on the left side (observer’s right). The injected hatchlings were then immediately released to the sea.
The duration from collection to release was less than 2 hours.

Bycatch survey at the Sanriku Coast
Since 2005, a year-round bycatch survey has been conducted in the Sanriku Coast (38°55′ to 39°40′N, 141°40′ to
142°05′E; Fig. 1). The captured turtles were transferred to an
outdoor tank at the International Coastal Research Center(ICRC), Atmosphere and Ocean Research Institute, the University of Tokyo, Otsuchi Town, Iwate Prefecture, Japan.
Their SCL and straight carapace width (SCW) were mea-

From 1999 to 2017, 10,256 loggerhead hatchlings with
the PIT tags were released from the Yakushima Island. In the
Sanriku Coast, we attempted to detect the PIT tags in 210
loggerhead turtles (SCL 51.6–92.0 cm) during 2014–2018.
On August 27, 2018, one loggerhead turtle with a PIT
tag (Fig. 2) was incidentally captured by a commercial set
net at Otsuchi Bay, Iwate Prefecture (39°22′21″N,
141°58′41″E). The other 209 turtles have no PIT or externaltags attached by researchers at the nesting sites. The 15-digit
number of the PIT tag revealed that this turtle was born in2008 (10 years old). A total of 1,095 loggerhead hatchlings
with the PIT tags were released from the Yakushima Island in
2008. To the best of our knowledge, this is the first record of
the known-age and non-head-started juvenile loggerhead turtle in the North Pacific population. A previous mtDNA analysis suggested that the loggerhead turtles migrating to theSanriku Coast are mainly derived from the Yakushima Island(Nishizawa et al. 2014). The identification of a tagged turtle
supports this estimation. The PIT tag was located on the rightside (observer’s left) of the abdominal cavity, which is the
opposite side to the injected location in the Yakushima Island(Fig. 2b). Previous studies have suggested that the PIT tags
move within the body of an individual (Balazs 1999, Kamezaki and Kuroyanagi 2004). Hence, the PIT tag found in
this turtle might have moved within its body during growth.
The SCL, CCL, SCW, CCW, and BM of this turtle were
60.8 cm, 64.4 cm, 51.6 cm, 63.5 cm, and 35.0 kg, respectively.
The minimum SCL of nesting females in Japan is 69 cm (Kamezaki 2003), indicating that this turtle was still a juvenile.
Captive studies of loggerhead turtles maintained under favorable conditions yielded growth trajectories that allowed theturtles to attain the adult size in as little as 6–7 years(Caldwell 1962, Uchida 1967). Our result demonstrated thatwild turtles display much slower growth rate than captive
turtles do under favorable conditions.
It has been reported that juvenile loggerhead turtles with
the SCL of approximately 60–70 cm re-migrate to Japanesewaters (Ishihara et al. 2011) and that this growth takes morethan 16 years estimated using a skeletochronology analysis(Ishihara 2011). The turtle in the current study (SCL 60.8 cm)
provided evidence that loggerhead turtles re-migrate to Japanese waters much faster than previously thought. One possi-

9Coastal Marine Science 42

Fig. 2. (a) Dorsal and (b) ventral sides of the PIT tag-injected loggerhead turtle captured by a commercial set net along the Sanriku Coast,
Japan. The white circle and the area enclosed by a dashed line in (b) indicate the injected and read locations of a PIT tag, respectively.

ble explanation is that the turtle in the current study exhibited
fast growth; however, one cannot rule out the limitation ofskeletochronology age estimation (i.e. resorption of early
growth marks). If recapture data of PIT-tagged turtles are accumulated in the future, the exact age of these turtles until remigration to Japanese waters will become clear.
If the PIT tags are injected into hatchlings of wild turtles, their exact age can be determined (Gibbons and Andrews 2004) although their recovery ratio is low (Avens andSnover 2013). Therefore, this method will provide helpful information to validate and/or improve accuracy of various age
estimations and absolute age determination. Furthermore, thePIT tags can also help determine the birth place of tagged
turtles, providing valuable data for molecular studies such as
mtDNA analysis. Since 1999, more than 10,000 hatchlings
have been injected the PIT tags and released from theYakushima Island, with the assumption that some of them
have re-migrated to Japanese waters in recent years or will in
the near future. In Japan, while several volunteer and scientist groups are investigating sea turtles in various areas, thePIT tags are checked by only a few groups. In addition, sharing of injected location (e.g. front flipper and abdominal cavity) and number-related information of PIT tags remain insufficient compared with those of the external tags. Hence,
introducing the PIT tag reader at various study sites, unification of injected location, and management of tag information
can help detect more known-age loggerhead turtles. The PIT
tag information will contribute to the understanding of life
history traits, such as duration of each life stage, somatic
growth rate, and migration pattern of loggerhead turtle in the
North Pacific population.

Acknowledgments
This study was conducted as a part of a bycatch survey in which
sea turtles caught by fisherman along the Sanriku Coast were brought
to researchers. The program was performed in accordance with the10guidelines of the Animal Ethic Committee of the University of Tokyo,
and the protocol of study was approved by the committee (P18-5). We
are grateful to all volunteers from the Yakushima Sea Turtle ResearchGroup, who injected the PIT tags to the loggerhead hatchlings. Special thanks to volunteers from the Fisheries Cooperative Associationof Shin-Otsuchi Bay, who provided us with the wild-caught sea turtle.
We also thank C. Kinoshita, M. Miyayama, M. Shinmura, K. Sakamoto for assistance in the bycatch survey along the Sanriku Coast.
The present study was performed under the Cooperative Program ofAtmosphere and Ocean Research Institute, the University of Tokyo.
This study was financially supported by Keidanren Nature Conservation Fund (KNCF), Tohoku Ecosystem-Associated Marine Science(TEAMS), and A School of Marine Sciences and Local Hopes in Sanriku. We thank two anonymous reviewers for their constructive comments and recommendations. We would like to thank Editage (www.
editage.jp) for English language editing.

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