環境

In order to identify the distribution of freshwater turtles in the Tama River watershed and the Tokyo metropolitan area, we conducted (1) a visual observation in the river and (2) a collection of eyewitness information of turtles from citizens. In addition, to clarify chronological changes in the distribution of the turtles, we carried out (2) above and (3) a literature search. Based on the findings, we made chronological habitat distribution maps of native and invasive turtle species confirmed in this area. We also examined the current status of native turtles in the area and when alien turtles came to and how they have spread in this area.

The northern limit of distribution of Mauremys japonica on the Pacific side of Japan’s main island is said to be the Kanto region, but its distribution has only been identified in Chiba prefecture and not in any other prefectures in the region. Our study revealed that the population of Mauremys japonica inhabits the Tama River watershed. In any identified location, the population density was low and the population was vulnerable. This report is expected to be used as a resource to prevent the local extinction of Mauremys japonica.

Mauremys reevesii is considered not to have established its population in the Kanto region until after the Second World War, but there have been no studies to back this up. Since our study found that Mauremys reevesii was first observed in the 1960s, the possibility of it being an invasive species has further increased. The study also found that Trachemys scripta elegans is overwhelming native turtle species in both population size and distribution area. We hope our report will be utilized as a resource to promote conservation of native species and measures in response to invasive species.

Tomomi Kataoka, Kim Hyeonjin, Aimu Iwamoto