Abstract
Stocking with fish from non-native populations may lead to spawning intrusion for native fish. We observed the spawning of ayu Plecoglossus altivelis with experimentally mixed fish groups, amphidromous×land-locked forms, and diploid×triploid. There was no difference in the courtship behavior among them, and cross-mating between forms and between ploidities occurred promiscuously. Spawning intrusion by transplanted ayu is considered to be inevitable, unless there is complete separation in the spawning period between transplanted and native fish.
