Abstract
We examined spawning condition of the Ryukyu-ayu Plecoglossus altivelis ryukyuensis which is endangered through habitat destruction and decreasing population size and harbored in the Amami-oshima Island in order to contribute to making an effective conservation program. Investigations were carried out in the Yakugachi River which provides the largest habitat for the subspecies, and their eggs attached to substrates occurred only in a reach at 2.4 km away from the river mouth. The principal component analysis which was conducted for environmental variables including water depth, flow velocity, substrate size and silt sedimentation drew the property of spawning site on a transitional zone between riffle and run. The logistic regression analysis conducted for the environmental variables around the spawning site factored out the substratum adhesion as a determinant variable to explain presence/absence of spawning ground. A lot of net-spinning caddis larvae were observed in the spawning site of the Ryukyu-ayu, which has a potential to prevent the fish from spawning because of their substratum adhesion. As for water depth and flow velocity no other reaches within the Yakugachi River satisfied the preferable conditions for spawning, and likewise spawning sites were not realized in the neighboring small stream, the Yanma River, in which the Ryukyu-ayu sometimes establishes inconsistent population. The Yakugachi River population of the Ryukyu-ayu has a role to be a donor within a meta-population structure and therefore should be given priority of conservation. In conclusion, the restoration of spawning site is effective to avoid the risk of local extinction. Cultivation of streambed by handworks using shovels and hoes is practical and recommended from the view point of cost performance.
