Post-nesting females were distributed mostly in the ECS and Tsushima Strait, concentrated in three very narrow habitats during the summer and widely scattered in the ECS in winter. Highly used areas in summer have relatively calm flow on the margins of strong currents and wintering area is SST > 18°C on the continental shelf. Also, turtles that used Tsushima where is the northernmost of three subareas were the largest in body size.
Loggerheads had a distinct distribution pattern that inhabitation in narrow three regions during the summer and wider distribution during the winter. The summer subareas defined in this study used by loggerheads including males in previous studies (Saito et al. 2015, Kume et al. 2017, Oki et al. 2019). However, these studies have not examined the reasons for loggerheads settling in these confined regions or dispersing in winter. Hatase et al. (2011) suggested that foraging areas of loggerheads are determined by their growth before maturation, however, this hypothesis explained their foraging dichotomy, oceanic or neritic, not distribution within the ECS.
Loggerheads stayed on the margins of strong northward streams in summer and winter. Current velocities of highly used areas were calmer than nearby northward currents but weren’t always calmer than entire home ranges (Fig. 4). Tsushima and Jeju subarea are located on the edge of the Tsushima Warm Current and the Jeju Warm Current, respectively. The Tsushima Warm Current, strong northward current flows toward the Sea of Japan. The Jeju Warm Current, which flows west of Jeju Island, is calmer than the Tsushima Current, however, it flows into the Yellow Sea where SST is lower (Ichikawa and Beardsley 2002, Lie et al. 2001). Loggerheads nesting in Florida do not use the fast-flowing Gulf Stream, instead mainly using the edge of the stream, likely to avoid drifting into the nutrient-poor and lethally cold winter water at high latitudes (Hoffman & Fritts 1982, Hawkes et al. 2007). It takes extra energy to swim against these strong currents to avoid drifting into the northern area. Therefore, it can be presumed that the loggerheads nesting in Japan use slower waters to save energy. The mean SST of Tsushima and Jeju began to drop in October and respectively went down to 15.6 and 13.5°C in February, whereas Taiwan and c-ECS maintained above 20°C during the winter (Fig. 4). Most post-nesting females settling at Tsushima and Jeju may have migrated to warmer c-ECS to avoid cold water. Wintering migration of loggerhead is known in the Mediterranean and North Atlantic Ocean (NAO; Hawkes et al. 2007, Mansfield et al. 2009, Bentivegna 2002). Loggerheads in the NAO were distributed at lowest SST 14°C and typically around 18–26°C (Hawkes et al. 2011, Arendt et al. 2012). As the cold water from the Yellow Sea extends (Fig. 4), Jeju would become unsuitable habitat for turtles. The mean SST at Tsushima is about 15°C in February, almost the same level as the water temperature range of their distribution area in the NAO reported in previous studies, but the Japanese loggerheads were hardly distributed. It is likely that females did not prefer to overwinter in the Tsushima Strait because the water temperature of c-ECS was warmer during this period. It was clear that Japanese loggerheads, like the Atlantic population, utilize the warmer regions.
Individuals stayed on the flat continental shelf or at shelf edge region at a depth of around 100 m (Fig. 3). The continental slope and shelf edge has high productivity due to the upwellings and may provide many foraging opportunities. Hatase et al. (2007) indicated that turtles staying at the continental shelf edge of the ECS dived mainly 100-150m during the day and estimated she foraged benthic prey. All the areas identified in this study seem to have suitable depth for searching benthic prey.
Loggerheads didn’t show consistent trend in Salinity and Chla value in this study. Both Salinity and Chla affect primary productivity. Jeju where has low Salinity and high Chla may be influenced by the nutrient rich freshwater mass flowing in from the Changjiang River, while Chla in other areas are low. Loggerheads using pelagic habitat are known to concentrate in areas have high primary production (Zbinden et al. 2008, Kobayashi et al. 2008, 2011). While neritic foragers don’t show consistent preference for highly productive areas (Iverson et al. 2019, Eder et al. 2012). High chlorophyll concentration influences the distribution and amount of planktonic prey such as jellyfish (Xu et al. 2013), but possibly may not coincide with abundance of benthic prey.
The post-nesting loggerheads showed that habitat segregation according to their body size not only in coarse scale such as oceanic or neritic, but possibly in fine scale. Habitat segregation that larger females settle in neritic habitat and smaller migrate to open ocean is observed in several studies (Hatase et al. 2002b, 2007, Nobetsu et al. 2003, Oki et al. 2019, Okuyama et al. 2022). Within neritic habitat, however, dividing of settlement area based on their body size has not been reported. Although limited in number, Tsushima turtles had a larger body size than other areas. Tsushima has deeper and faster current than other areas, so large individuals with better capability of swimming and diving might be of advantage. Alternatively, if turtles have foraging site fidelity over a long period, Tsushima might have more abundant prey than other areas. More individuals should be tracked across multiple years to determine whether there is habitat selection within fine scale.
The ECS is an important fishing ground for surrounding countries and multiple fishing gears are operated (Yu et al. 2007, Zhang et al. 2016, Sumaila 2019, Tah et al. 2019). Due to the competition for resources in the ECS, fisheries management is not very advanced, and the knowledge of non-target species bycatch is scarce. (Yang et al. 2022, Tah et al. 2019). Loggerheads bycatch at the coast of Taiwan and Korea, which were identified as highly used areas for loggerheads in this study has been reported (Kobayashi et al. 2011, Kim et al. 2021). Although there are few studies on bycatch in the ECS, the effect of sea turtle bycatch on the population may be significant. We clarified some of the characteristics of foraging habitat for the post-nesting migration of loggerheads in southern Japan, and these results contribute to the comprehensive management of the ECS ecosystem. However, other elements as well as environmental features should be considered as contributing factors on distribution of the Northwest Pacific loggerheads. It is necessary to investigate the habitat or consistency of site fidelity of immature and mature males as well as other nesting population.