There are many islands in the southwestern seas of Korea. Each island has small independent streams and various freshwater fish live there. Because freshwater fish are distributed separately among islands, and also among streams, fish fauna of each island, and the geographical distribution of fish provide valuable data for research of the natural history of the island (Son and Song 1998). All streams in Jeju-do have steeper slopes towards the upstream, so there is severe erosion during heavy rainfall. In addition, most stream beds consist of volcanic rock formed by volcanic activities. Most streams are normally ephemeral streams and when it rains, most rainwater permeates through the ground so the environment is extremely poor for freshwater fish, excluding a few streams