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
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 effective release strategies for this fish 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 difference 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 offing during this period.