Nippon Suisan Gakkaishi 77(3), 387 401 (2011)
琵琶湖沿岸のヨシ帯におけるニゴロブナCarassius auratus grandoculis の初期生態とその環境への適応 The early life ecology of nigorobuna Carassius auratus grandoculis was investigated in order to determine the requirements for developing breeding grounds and eŠective release sites for its stock enhancement in Lake Biwa.Larval nigorobuna within the well-developed reed zone swarmed actively near the shoreline immediately after release and began to leave the reed zone when they grew to the juvenile stage of approximately 16 mm in standard length. Zooplankton used as food for the larvae were abundant near the shoreline in the reed zone, but there was
Reed zones indispensable for the development and early growth of nigorobuna Carassius auratus grandoculis in Lake Biwa are on the decline. In order to examine eŠective release strategies for thisˆsh independent of reed zone, a follow-up survey of otolith marked juvenile nigorobuna with ALC released into reed zones in June to July was conducted. As a result, growth of nigorobuna was as good as in the year with high water temperature from August to October. Nigorobuna extended the distribution along with the thermocline when the water temperature fell gradually toward winter, and expanded the distribution toward the bottom of the lake when the water temperature diŠerence of the upper and lower layers of the thermocline was less than about 3.5 degrees. High survival was expected by releasing young nigorobuna of body weight ca. 20 g (standard length ca. 85 mm) to the o‹ng during this period.
The ALC multiple marking technique to evaluate eŠective stocking method of nigorobuna Carassius auratus grandoculis in Lake Biwa was examined. Fluorescence multiple rings corresponding to the marking times were clearly identiˆed in the lapillus marked in the eyed-stage of eggs and at intervals of four days or more forˆve-day-
EŠective 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 stockedˆsh in the lake was observed in cases of 1) release to the reed-free lake shore, 2) reduction of the depth of ‰ooded reed zone according to draw down of water level by artiˆcial 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.
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.
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