Stocking effectiveness of nigorobuna Carassius auratus grandoculis in Lake Biwa

Abstract

Nigorobuna Carassius auratus grandoculis recaptured by gill nets with speciˆc ALC marks for diŠerent release conditions were investigated to evaluate the eŠectiveness of the stock enhancement program in Lake Biwa. Recapture rate and cost-beneˆt 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 o‹ng 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 o‹ng of juveniles smaller than 58.8 mm in SL. We suggest that releasing juveniles of ca. 85 mm in SL independent from ‰ooded reed areas could work as a tentative management tool to complement natural recruitment, however, recovery of the ‰ooded reed zone which has decreased in area is indispensable to enhance the nigorobuna stock.