Larval drift of Odorrana splendida, a frog endemic to Amami-Oshima Island

Odorrana splendida is an endangered frog endemic to Amami-Oshima Island that is listed as a natural monument by the Kagoshima Prefecture government. While some studies have examined the ecology of adult O. splendida, few studies have examined its larvae. Therefore, this study examined larval drift by tracking fluorescently tagged larvae in a natural stream. We tagged 283 individuals, released them in a pool in the head stream, and tracked them for 2 years in 13 surveys at 1-3-month intervals. We divided the 303-m-long stream into 6-m-long sections and counted the number of larvae within each section. Over the 2 years, we found 9,642 individuals. Tagged larvae were found until day 736. The farthest drift distances from the release pool were 56-62, 62-68, and 128-134 m on days 29, 261, and 320-682, respectively. The estimated maximum drift distance did not vary markedly within a year, as it was 85.3 m on day 29 and 89.9 m on day 320. These results suggest that larval O. splendida in the stream studied drift as far as 130 m in a year and, typically, over 85-95 m. The drift distance did not extend much after day 29 and was not proportional to the corresponding time period. In addition, the proportion of tagged individuals remaining in the release pool increased as time passed. Our results suggest that while O. splendida tadpoles do drift from the original pool, the drift occurs during their early stage, during rare events, such as heavy rain. The survival rates of drifted larvae might be low or the drift rates of tadpoles downstream might be high. This study showed the need to consider the downstream environment as well as that around oviposition sites for conserving the habitat of O. splendida larvae.