Abstract
Using an allozyme marker (i.e. MPI*), we traced the mixing rate of the landlocked ayu-juveniles from Lake Biwa released in the Yasuda River and the Ioki River, inhabited by a native amphidromous form. We also estimated the population density based on under-water observations. The percentage of the landlocked form was remarkably high in the upper stream and very low in the lower stream in June. The percentage of the landlocked form decreased remarkably in the upper stream in both rivers in August. The percentage of the landlocked form decreased to almost zero in December. These seasonal changes in the mixing rate of the landlocked form suggested a difference in the migration pattern of the landlocked form and the amphidromous form in the river. The number of the amphidromous form coming up to the river from the sea was estimated using the landlocked form's percentage and the population density observed at each location during the early fishing season.
