Satellite tracking immature loggerhead turtles in temperate and subarctic ocean habitats around the Sea of Japan*

The distribution limits of North Pacific loggerhead turtles, Caretta caretta, within the Sea of Japan are unknown, but are suspected to be linked to a decrease in winter sea surface temperature. Immature loggerheads in the Sea of Japan are thought to follow the prevailing current via a southern route, or to leave from the north with extensions of the warm current, or are unable to leave when winter approaches. Here, we investigated the migrations of 30 immature captive-reared turtles of 26.5 to 75.2 cm SCL at 2–6 years of age released off the central Sea of Japan in summer of 2011 using satellite telemetry to ascertain 1) the seasonal movements of immature loggerheads and 2) their behavior in the subarctic zone. Nineteen turtles successfully left the Sea of Japan, mainly via the Soya Strait traveling into the North Pacific; however, eleven did not leave and may have died due to stranding during the winter. Their excursion paths were similar to migrations of wild turtles. The presence of exit routes suggests that immature loggerheads entering the Sea of Japan can disperse successfully and contribute to the North Pacific subpopulation. During the summer of 2011, the turtles that left the Sea of Japan experienced water temperatures that were relatively wide ranging and lower than turtles that stayed in Sea of Japan. The sea surface temperatures (SSTs) experienced by the turtles that left the Sea of Japan were significantly different from those of the time percent temperatures (TPTs) (range: 17–35 °C vs. 5–35 °C, mode: 23–25 °C vs. 10–15 °C). The relatively lower TPTs may be due to their active diving behaviors. In contrast, the SSTs and TPTs (range: 10–35 °C, mode: 21–23 °C) experienced by the turtles that stayed in the Sea of Japan were almost identical. Success in leaving the Sea of Japan could be related to a higher frequency of dives, although further research is required to confirm this. * Citation: Saito, T, M. Kurita, H. Okamoto, Y. Kakizoe, D. Parker, D. Briscoe, M. Rice, J. Polovina & G. Balazs. 2018. Satellite tracking immature loggerhead turtles in temperate and subarctic ocean habitats around the Sea of Japan, Micronesica 2018-03, 20 pp. Published online 30 April 2018. http://micronesica.org/volumes/2018 Open access; Creative Commons Attribution-NonCommercial-NoDerivs License. † Corresponding author. Micronesica 2018-03 2 Introduction Loggerhead turtles, Caretta caretta, are a threatened species listed as “Vulnerable“ under the IUCN Red List categories (Casale & Tucker 2015) and require a global conservation effort. Loggerhead turtles nest from temperate to sub-tropical areas, with the North Pacific subpopulation of loggerheads utilizing the southern Japanese Archipelago coast as their only reproductive area (Kamezaki et al. 2003). Turtle nesting beaches occur on both the Pacific and Sea of Japan coasts of Japan. The region from the Yaeyama Group located in southwestern Japan to the western North Pacific coast of Yamamoto-cho, Miyagi Prefecture, and the Sea of Japan coast of Ishikawa Prefecture in the northern Noto Peninsula are known recent nesting areas of loggerhead turtles. However, substantial nestings of Japanese loggerhead turtles also occur along the Pacific coast (Kamezaki et al. 2003; Figure 1). Loggerhead turtles are known to often strand along the Sea of Japan coast due to cold stun (Nishimura 1967); however, their numbers have not been quantified. Cold stun is a phenomenon in which sea turtles can not regulate buoyancy as the water temperature decreases and passively float on the sea surface. It is difficult to study mortality rates of loggerhead turtles resulting from cold stun. Epperly et al. (1996) estimated the mortality of sea turtles, mainly loggerhead turtles, in winter along the northern coast of North Carolina from at-sea monitoring and beach stranding records. The proportion of deaths from bycatch in the winter trawl fishery was estimated to be 7–13%, with the majority estimated to be due to other factors. In the Atlantic, there are reports of cold stun on the US East Coast (e.g. Meylan & Sadove 1986; Morreale et al. 1992; Still et al. 2005; McMichael et al. 2008; Anderson et al. 2011) and Spain (Bellido et al. 2010). Kuramoto & Ishii (2003) reported the passive drifting of five species of sea turtles including loggerheads in Fukuoka Prefecture, near the west entrance of the Sea of Japan. Ichisawa et al. (2014) recorded two stranded and one bycaught loggerhead turtles near the coasts of Tottori Prefecture. Honma et al. (2010) recorded more than 70 loggerheads stranded along the coasts of Niigata Prefecture and Sado Island after 1830. Furthermore, Kharin (2008) mentioned finding sea turtles, including loggerheads, in the Sea of Japan‘s Russian waters. Recently, Ishihara et al. (2017) found 119 post-hatchling loggerhead turtles, approximately 10 cm in Standard Carapace Length (SCL), washed ashore or obtained as bycatch in the Sea of Japan from September 2012 to April 2013. Mixed stock analysis demonstrated that their natal orgins were mainly southern Japan, including Okinawa Islands, Okinoerabu Island, and Yakushima Island. Furthermore, tagged loggerhead turtles, 1 to 2 years of age, released offshore of Okinawa Island were recaptured in the Sea of Japan (Uchida & Teruya 1991). This also shows that loggerhead turtles from distant places, like Okinawa Island, can enter the Sea of Japan. Nishimura (1969) stated that the most significant factors controlling animal distribution in the Sea of Japan were the strong influx of the Tsushima Warm Current from the south in the summer to autumn and the winter thermohaline convection and southward drift current induced by the cold and dry northwest monsoon. Moreover, the periodic winter wind in this area is closely related to severe water temperature decreases. Many sea turtles that were carried to the Sea of Japan by the Tsushima Warm Current might not be able to endure the decreased water temperature during winter, weaken, and drift ashore to die (Nishimura 1967). Some hatchlings born at the nests that are established over a wide area from further south, including these sites, enter into the Sea of Japan with the Tsushima Warm Current and might not be dispersed effectively. Previous studies regarding seasonal migration of loggerhead turtles have suggested that they migrate in response to a drop in sea surface temperature (SST) (Polovina et al. 2006, Kobayashi et al. 2008, Mansfield et al. 2009, Arendt et al. 2012, Saito et al. 2015). Polovina et al. (2000, 2004, 2006) identified that immature and adult loggerhead turtles in the North Pacific Ocean occupied specific areas of the pelagic environment, which is possibly related to SSTs and/or mechanisms of ocean productivity and retention of zooplankton prey. Similarly, the movement of turtles in the Sea of Japan may involve cues from such hydrographic conditions in these regions. Saito et al: Tracking loggerhead turtles around the Japan Sea 3 Figure 1. Map of study sites. Location of release and recapture sites of the loggerhead turtles. Dashed lines: northern and southern limits of turtle nesting. Gray lines and arrows: typical year-round tracks of the Kuroshio and Tsushima Warm Currents with directions of current flow. Open star: release site. Micronesica 2018-03 4 Satellite telemetry has made it possible to monitor the movements of turtles over large areas, which can assist in researching their life history in more detail. Extensive research has been performed on hundreds of loggerhead turtles raised by the Port of Nagoya Public Aquarium (PNPA), that were released and tracked in pelagic waters on the Pacific east coast of Japan (Polovina et al. 2006, Kobayashi et al. 2008). In contrast, no satellite tracking studies have been conducted on loggerhead turtles along the western Japan coast and within the Sea of Japan. In the present study, 30 immature loggerheads were tracked in the Sea of Japan using satellite telemetry. This study is unique, because it involved the release of immature loggerheads with Argos-linked transmitters that were smaller and younger than turtles caught using longlines. The only way to obtain such very young turtles for the present study was to use captive-reared turtles (Parker et al. 2005). It is unknown whether nesting in this region and the initial influx of hatchlings into the Sea of Japan contributes to the North Pacific subpopulation of loggerheads or if they die due to low water temperature in winter. There are three possible outcomes for loggerheads in the Sea of Japan: 1) Leave the Sea of Japan via a southern route to the Pacific Ocean; 2) Follow the Tsushima Warm Current and exit the Sea of Japan to the North; or 3) Remain in the Sea of Japan and suffer the consequences of lethal low temperatures. The aims of the present study were to acertain the seasonal migratory route of immature Japanese loggerheads from the Sea of Japan to the western North Pacific Ocean and clarify how they behave in the temperate to subarctic habitat zone. Materials and Methods TURTLE INFORMATION A total of 24 immature loggerhead turtles <50 cm SCL (range: 26.5 to 37.7 cm SCL) at 2 years of age and six turtles >50 cm SCL (range: 57.7 to 75.2 cm SCL) at 3–6 years of age were used (Tables 1, 2). These turtles were obtained from the PNPA captive breeding and rearing program. All turtles were healthy and vigorous when released. Prior to release, the SCL and body mass (BM) of each turtle were measured to the nearest 0.1 cm and 0.1 kg, respectively. It was not possible to identify the sex of immature turtles at this stage based on external morphology (i.e., tail length or presence of claws on the front flippers; Limpus & Limpus 2003). SATELLITE TAG ATTACHMENT The transmitters were the smallest available for diving animals and, hence, the minimum size possible to track our loggerheads was 26.5 cm in straight carapace length (SCL). Small-sized satellite tags for hatchings are not yet readily