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
The early life ecology of nigorobuna Carassius auratus grandoculis was investigated in order to determine the requirements for developing breeding grounds and effective 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 remarkably little dissolved oxygen. It was confirmed that the larvae have a high hypoxia tolerance and due to their low body specific gravity could easily rise to the water surface where oxygen readily dissolved. The results indicate adaptation of the larvae to the hypoxic environmental condition. It was thought that hypoxic water areas function as feeding sites and as refuges for the larvae.