Moriyama dyke, situated at the northern part of Lake Nakaumi, generates two different large water bodies, one being connected directly with the Sea of Japan, and the other being disconnected. The author compared the fish fauna collected with the set nets settled at both connected (Shimo-ubeo) and disconnected region (Honjou area), and found that fish fauna differed significantly between them. Some species such as swimming crabs, squids and octopuses seldom or never occurred in the catches of the set nets in Honjou area, whereas they frequently appeared in Shimo-ubeo region. Such phenomenon suggests that there occurred a drastic change in fish fauna in Honjou area.