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There are no records of nesting olive ridley turtles in the Japanese archipelago, which is mainly situated in temperate water. However, >50 bycatch and stranding records of olive ridley turtles exist from 1957-2015 (Nishimura et al. 1972; Kuroyanagi & Masuda 1992; Suganuma 1994; Kamezaki et al. 1995; Kuramoto & Ishii 2003; Tsutsumi & Hashimoto 2003; Kino & Kawazu 2014; Shimura et al. 2017; Fig. 1, Table 1). Suganuma (1994) noted that the majority of the bycatch occurred during summer and autumn when the water temperature is high (20-30 °C; Table 1). In contrast, stranding records were concentrated during winter (Table 1). These results suggested that olive ridley turtles utilize Japanese coastal waters for seasonal foraging and are unable to tolerate the low water temperatures during winter (Suganuma 1994). In the Pacific coast of northeastern Japan (>35 °N), which is the cool-temperate region with lower water temperature (4- 20 °C), there is no evidence that olive ridleys occurred previously, with the exception of one bycaught turtle from oceanic waters recorded in area 545 of the fishing grounds (42.50-43.00 °N, 152.50-153.00 °E) (Suganuma 1994).

Olive ridley turtles are omnivores that feed on gelatinous prey, benthic animals and macroalgae (Bjorndal 1997; Peavey et al. 2017). In Japan, benthic animals such as crustaceans, snails, bivalves and sea urchins were found in the feces of two olive ridley turtles stranded in Ise Bay (Kuroyanagi & Masuda 1992; Fig. 1). However, no other diet information has been reported in Japan until this study. Here, we describe the fecal contents of the first olive ridley bycatch record from neritic waters of the cool-temperate Sanriku Coast, northeastern Japan.

The Sanriku Coast is one of the most productive areas in Japan because of the mixing of waters from the Tsugaru Warm Current, the cold nutrient-rich Oyashio Current, and a branch of the warm Kuroshio Current (Hanawa & Mitsudera 1987; Sugimoto & Tameishi 1992; Fig. 1). Since 2005, the incidental capture of sea turtles by commercial set nets has been surveyed in this area, and 456 loggerhead (Caretta caretta), 145 green (Chelonia mydas), 10 leatherback (Dermochelys coriacea), six black (C. mydas agassizii), and one hawksbill (Eretmochelys imbricata) turtles were captured during June to November from 2005 to 2019. This result indicated that the Sanriku Coast is a summer-restricted habitat for sea turtles (Fukuoka et al. 2015; Narazaki et al. 2015; Sato et al. 2016).

On 21 July 2016, one olive ridley turtle was incidentally captured by a set net at Funakoshi Bay, Iwate prefecture, Japan (39.40 °N, 141.97 °E; Fig.1). The turtle was identified as Lepidochelys olivacea based on diagnostic features including the presence of six pairs of lateral scutes, olive-grey scute and skin color, circular shape of the carapace, and two pairs of prefrontal scales (Figs. 2a, 2b; Márquez 1990). The water temperature of Funakoshi Bay at 3m depth was 16.9 °C when the turtle was captured.

Figure 1. Study site on the Japanese archipelago and locations of olive ridley bycatch (filled markers) and stranding (open markers) records. The records of unknown locations and unknown capture types were not shown. Difference in marker shape means captured alive (circles), dead (squares), and unknown (triangles), respectively. The red circle indicates the capture location in this study. Orange solid and blue dashed lines illustrate warm and cold currents, respectively. Numbered currents show 1: branch of the warm Kuroshio Current, 2: cold Oyashio Current, and 3: Tsugaru Warm Current. Inset is a magnification of the study site (open rectangle). A star marks the International Coastal Research Center (ICRC).

Table 1. Olive ridley turtle occurrences in Japan. UN = unknown. For size, (S) is straight carapace length, (C) is curved carapace length, and (U) is unknown if straight or curved carapace length.

Figure 2. An olive ridley turtle captured by a commercial set net along the Sanriku Coast, Japan. (a) Carapace, and (b) head of the turtle. Six pairs of lateral scutes and two pairs of prefrontal scales exist on the carapace and head, respectively. (c) The turtle floating on the water surface. (d) Lacerations (two black arrows) on the plastron.

The olive ridley turtle was found alive but emaciated and in captivity it always floated on the water surface in the tank (Fig. 2c). In addition, the turtle had lacerations on its plastron; unfortunately, we could not identify their cause (Fig. 2d). Therefore, it was transferred to an indoor tank (1.5 × 1.2 × 0.6 m) at the International Coastal Research Center (ICRC, Fig. 1), Atmosphere and Ocean Research Institute, the University of Tokyo, Otsuchi town, Iwate prefecture, Japan. Straight carapace length (SCL) and straight carapace width (SCW) were measured to the nearest 0.1 cm with calipers. Curved carapace length (CCL) and curved carapace width (CCW) were measured to the nearest 0.1 cm with a flexible tape measure. Body mass (BM) was measured to the nearest 0.5 kg with a hanging scale. When the olive ridley turtle was captured, SCL, CCL, SCW, CCW and BM were 50.6 cm, 53.0 cm, 46.3 cm, 56.1 cm and 16.0 kg, respectively. A sexually mature olive ridley turtle has been estimated to have an SCL of 60 cm approximately, typically a minimum of 52.5 cm and maximum of 75 cm (Hirth 1980; Zug et al. 2006). Hence, the olive ridley turtle captured at the Sanriku Coast was considered to be a large juvenile. This is a common size for previous records of olive ridley turtles in other parts of Japan (40-60 cm SCL, Table 1). Two Planes crabs, Planes major (male and female), which are frequently attached to loggerhead and olive ridley turtles (Frick & Pfaller 2013; Pfaller et al. 2014) were found on the tail of the olive ridley turtle when the turtle was captured.

For the first week after capture, the water temperature of the tank was set at 25 °C using a thermo-controller (REI-SEA TC-101, Iwaki Co, Tokyo, Japan). Afterwards, the turtle was transferred to an outdoor tank (3.6 × 1.5 × 1.0 m) at the same institute. The outdoor tank water temperature was maintained >25 °C (approximate range: 25-30 °C) due to the warm air temperatures heating the water. After being in water >25 °C for a week, the turtle became active and started diving and eating fish fillets, squid, and mussels. However, the pump system of ICRC failed in September, and the turtle died on 20 September 2016 when the water temperature dropped rapidly <20 °C (approximately range: 15-20 °C) because of low air temperatures around 15 °C.

During the first week of the captive period, we sampled and analyzed feces. The tank that housed the olive ridley turtle was checked daily, and feces found in the tank were collected using a net. To collect as much fecal matter as possible, we did not flush the water in the tank during the feces sampling period. After the turtle was transferred to the outdoor tank, we stopped the sampling because some algae and marine debris were contaminating the tank water through the seawater pump system. Following Fukuoka et al. (2016), items within the fecal samples were classified as diet items, natural debris, and artificial debris. Diet items were identified to the lowest taxonomic level possible by visual examination. Wet mass of each sample was weighed to 0.1 g using a digital scale. Of a total of 62.9 g of feces sample, 52.7 g of Salpidae was present, representing 83.8% of the total mass, and 6.7 g of artificial debris was identified, representing 10.7% of the total mass (Table 2). Brown, green and red algae were also found in the feces but only in small amounts (<2 g).

Table 2. Wet mass (grams and % of total mass) of different taxa and other materials in the sample of Lepidochelys olivacea feces.

The fecal analysis indicated that prior to capture, the olive ridley turtle mainly fed on floating materials such as salps. Salpidae are commonly found in the gut contents of olive ridley turtles in the Pacific Ocean (Bjorndal 1997; Wedemeyer-Strombel et al. 2015), although this was not found in a previous study of two olive ridley turtles from Ise Bay, along the southern coast of Japan (Kuroyanagi & Masuda 1992). Salps are present and available in both neritic and oceanic waters (Henschke et al. 2016). Planes crabs are typically found drifting in pelagic/oceanic waters (Frick & Pfaller 2013; Pfaller et al. 2014). Because Planes crabs were found on the captured olive ridley, and salps were determined to be part of the olive ridley turtle’s diet, we suggest that this turtle utilized oceanic waters before migrating to the Sanriku Coast. Because a branch of the Kuroshio Current flows northwards off the Sanriku Coast (Kawai 1972), this olive ridley turtle may have travelled north following this current.

A previous study reported that artificial debris, which was the second most abundant item in this study, was frequently ingested by sea turtles in the Pacific Ocean (Wedemeyer-Strombel et al. 2015). According to gut content analyses, loggerhead and green turtles migrating to the Sanriku Coast display a high frequency of occurrence of debris ingestion (>80%; Fukuoka et al. 2016). Artificial debris ingestion by the olive ridley in this study confirmed that three sea turtle species along the Sanriku Coast may ingest debris. A recent study addressed sub-lethal effects of debris ingestion such as body condition and transfer of toxic compounds (Clukey et al. 2018). Hence, we consider it necessary to conduct such a study along the Sanriku Coast where sea turtles ingest artificial debris.

The present study is the most recent and northernmost record of an olive ridley turtle along the Pacific coast of Japan, approximately 500 km north from the previous record in Kanto district (Fig. 1, Table 1). The Sanriku Coast is highly productive; however, the period when water temperatures exceed 20 °C, which is considered a suitable temperature for olive ridley turtles, is shorter than two months annually (Sato et al. 2007). When the turtle was captured in water temperatures of 16.9 °C, it was emaciated. The turtle became active and started diving when the water temperature was maintained >25 °C, suggesting that this individual experienced hypothermia because of low water temperatures when it was captured. Therefore, it is inferred that the northern limit of seasonal foraging areas of olive ridley turtles is the southern coastal waters of Japan, and along the Sanriku Coast is considered unfavorable habitat for the olive ridley turtle even during the summer.

It is reported that water temperatures off the Sanriku Coast have risen 0.68 °C during the last 100 years (The Japan Meteorological Agency 2017). In July 2016 when this olive ridley turtle was captured, water temperature off the Sanriku Coast was 2-4 °C warmer than the average water temperature from 1986 to 2016 (The Japan Meteorological Agency 2016). We cannot associate the relationship between this northernmost bycatch of an olive ridley turtle and rising ocean temperatures because it is only one record. However, it has been reported that many marine mammal species display poleward distribution shifts in their geographical ranges, in relation to rising water temperatures (Tasker 2008; Poloczanska et al. 2013; Kleisner et al. 2017). Moreover, some studies predicted that sea turtles also change their habitat accompanied by rising water temperature (Chaloupka et al. 2008; Witt et al. 2010; Hazen et al. 2012). Further monitoring, and continued bycatch surveys in Japan in the northern limits of marginal habitat for olive ridley turtles, is important to examine whether this species is shifting their distribution as water temperatures increase around the globe.

Acknowledgements. The present study was incidentally conducted during a tag and release program where loggerhead and green sea turtles were caught by commercial set nets as bycatch in the Sanriku Coast and then turned in by fisherman to researchers. The program was performed in accordance with the guidelines of the Animal Ethic Committee of the University of Tokyo, and the protocol of this study was approved by the committee (P 16-5). We are grateful to all volunteers from the Fisheries Cooperative Association of Funakoshi Bay, who turned over the sea turtle in this study. Special thanks to K. Kameda for assistance with species identification of the turtle. We also thank N. Ohtsuchi for identifying Planes crabs as Planes major. We acknowledge Y. Nishibe and K. Nakamoto for their assistance with feces analysis. We also thank M. Sato who assisted with the transfer of the turtle from the port to the institute. The present study was performed under the Cooperative Program of Atmosphere and Ocean Research Institute, the University of Tokyo. We acknowledge Maptool <https://seaturtle.org/maptool/> used to generate the map of the study site. The field survey was financially supported by the Japanese Society for the Promotion of Science (16J06542) and Tohoku Ecosystem-Associated Marine Science (TEAMS). We would like to thank Editage (www.editage.jp) for editing this document in English.

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