Twenty-year changes in the composition of a mixed stock of foraging green turtles in the Yaeyama Islands of Japan

Abstract

Recent studies have shown an increasing mean straight carapace length (SCL) of foraging green turtles Chelonia mydas around the Yaeyama Islands, Japan, compared to that of 20 yr ago. This trend is attributed to reduced juvenile mortality resulting from a decline in sea turtle fisheries. In this Yaeyama foraging aggregation, the proportions of turtles originating from different populations (genetic stocks) are known to vary according to size; for turtles originating from the Ryukyu Archipelago and Ogasawara (Japanese populations), the proportion in the 50-70 cm SCL size class is higher than that in the <50 cm SCL size class. Here, we compared the natal origin compositions of the Yaeyama foraging aggregation from 2 sampling periods (1997-1999 and 2016-2018) across 2 size categories (<55 and >55 cm SCL) and investigated the relationship between the size increase and changes in natal origins. The results showed that the proportion of the Japanese populations has recently increased compared to that 20 yr ago. The size distribution, based on the natal origin of each turtle in 2016-2018 estimated using single nucleotide polymorphisms, showed that the proportion of populations at latitudes lower than Japan with a ≥55 cm SCL decreased substantially, whereas the proportion of the Japanese populations remained stable between size categories. These results suggest a link between the size increase and the increase in Japanese turtles. This study also provides the detailed composition of the Yaeyama foraging aggregation, including the size distributions and sex ratios of contributing populations.