The importance of the reed zone as a release region for larval and/or juvenile nigorobuna Carassius auratus grandoculis in Lake Biwa, and problems for stock enhancement

Effective release strategies for nigorobuna Carassius auratus grandoculis in Lake Biwa were investigated by release-recapture experiments of otolith marked nigorobuna with ALC. As a result, nigorobuna arrived at the juvenile stage exceeding approximately 16 mm in standard length and released to the Phragmites australis reed zone showed a rapid increase in survival rate. A remarkable decrease in the survival rate of stocked fish in the lake was observed in cases of 1) release to the reed-free lake shore, 2) reduction of the depth of flooded reed zone according to draw down of water level by artificial manipulation, 3) growth retardation of reed in spring by excessive cutting management of reed in winter, and 4) excessive release of nigorobuna larvae to a limited reed zone. Larvae-release techniques which overcome the problems and reed zone-independent release methods for nigorobuna should be developed in the future.