2022 Volume 10 Pages 35-41
In 2019, a sudden decline in the population of Ryukyu-ayu (Plecoglossus altivelis ryukyuensis) was recorded on Amami-oshima Island, coinciding with a rapid increase in the number of migratory great cormorants (Phalacrocorax carbo). Given that selective foraging by great cormorants poses the potential risk of causing Ryukyu-ayu extinction, the precautionary principle was applied to protect this endangered species from these piscivorous avian predators. Damage control measures designed to limit the activity of native animals should be non-invasive, and in this study, black nylon strings were pulled across streams to deter great cormorants from gaining access to the spawning grounds of the Ryukyu-ayu. Subsequent under-water censuses in four major streams, based on snorkeling observations, revealed that a rapid reduction in the numbers of Ryukyu-ayu was averted using the line technique, which prevented great cormorants from feeding on Ryukyu-ayu. Consequently, the preventative measures taken against the great cormorant enabled the Ryukyu-ayu to spawn and reproduce without any apparent negative effects on the dynamics of the great cormorant. However, these measures did have one unintentional consequence, in that waterfowl became entangled in one of the strings. Nevertheless, despite the need for further modification, our approach using the line technique was proved to be effective in protecting populations of Ryukyu-ayu on Amami-oshima Island.
