1Shiga Prefecture Fisheries Experiment Station, Hikone, Shiga 522-0057, 2Faculty of Marine Science, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, 3Stock Enhancement Center of Lake Biwa, Kusatsu, Shiga 525-0005, Japan
Nigorobuna Carassius auratus grandoculis recaptured by gill nets with specific ALC marks for different release conditions were investigated to evaluate the effectiveness of the stock enhancement program in Lake Biwa. Recapture rate and cost-benefit ratio (B/C) were estimated to be high at 9.6% and 12.0 for releases of larvae and/or juveniles of 16.5 mm in standard length (SL) in large-scale reed zones in June. The equivalent values for releases of juveniles of 83.9 mm in SL to the offing in October were also estimated to be high at 12.7% and 2.2. In contrast, B/C values were estimated to be less than 1.0 for releases of larvae and/or juveniles outside of the reed zones and releases to the offing of juveniles smaller than 58.8 mm in SL. We suggest that releasing juveniles of ca. 85 mm in SL independent from flooded reed areas could work as a tentative management tool to complement natural recruitment, however, recovery of the flooded reed zone which has decreased in area is indispensable to enhance the nigorobuna stock.