Did food resource shortage cause Ryukyu-ayu extinction on Okinawa-jima Island? A clue based on morphological approach

The Ryukyu-ayu population on Okinawa-jima Island went extinct suddenly in the late 1970s, but the cause remains unclear. Reports say that genetic diversity was not responsible, so we investigated food shortage as a probable driver. We examined pre-extinction specimens to look for territorial individuals, which have unique morphological characteristics and occur only in food-affluent conditions. We found that 42.1% ( n = 45) of individuals possessed longer dorsal fins than non-territorial fish. Combining this with another key characteristic (blackened membrane at posterior dorsal fin ray), we found that territorial fish probably constituted 35.5% which fell in the range of the expected proportion for territorial fish in the nominotypical subspecies and higher than that observed in the extant population on Amami-oshima Island. These findings imply that the food resources in the riverine environments of Okinawa-jima Island before the extinction event might be sufficient for Ryukyu-ayu to establish feeding territories. Therefore, food shortage did not cause this population extinction.