1Shiga Prefecture Fisheries Experiment Station, Hikone, Shiga 522-0057, 2Faculty of Marine Science, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, Japan
Near the shoreline of the well-developed reed zone of Lake Biwa, the dissolved oxygen concentration is remarkably low, but the water temperature is high and microcrustaceans which are preyed on by larval nigorobuna Carassius auratus grandoculis are abundant. Larval nigorobuna aggregate actively around this water area. Factors affecting this aggregation behavior were examined using two kinds of avoidance/preference test apparatus. Our examinations proved that larval nigorobuna do not avoid hypoxic water, but prefer high-temperature water, avoid heavily shaded environments, are strongly attracted to microcrustacean, and prefers the shoreline water containing an abundance of microcrustaceans in the reed zone over the outer-edge water with few microcrustaceans. It is important to take account of the ecological and environmental factors for developing breeding grounds of nigorobuna and selecting effective release sites for its stock enhancement in Lake Biwa.