Diet of the Ryukyu-ayu Plecoglossus altivelis ryukyuensis in subtropical rivers of Amami-oshima Island, southern Japan revealed by gut content analysis and inferred from stable isotope signatures

Abstract

The diet of the critically endangered Ryukyu-ayu Plecoglossus altivelis ryukyuensis in the middle reaches of the Kawauchi, Sumiyo, and Yakugachi Rivers, Amami-oshima Island, southern Japan was examined by gut content analysis and inferred from stable isotope signatures. No clear differences in diet composition were apparent among the rivers. Microalgae, such as diatoms and cyanobacteria, generally considered as important food, were only rarely observed in gut contents, whereas detritus was almost exclusively found. However, stable isotope signatures indicated that detritus was not directly utilized as a nutritional source, indicating that food availability was an issue for P. a.ryukyuensis.