Abstract
"Naiko" is a Japanese term to indicate small and shallow satellite lakes around Lake Biwa, being usually connected to the lake by channels. Several naiko were surveyed on their ecological functions especially for native fishes, and were compared to clarify factors causing the change of fish communities in naiko. In Ozubukuro-naiko, Moriyama City, fishes endemic to Lake Biwa dominated in the fish community in 1960's, became less abundant than other native fish in 1970's, and were replaced by the alien species Lepomis macrochirus until 1998. Similar faunal changes occurred also in other naiko except Katata-naiko which is still inhabited by abundant native fishes in spite of artificial alternation of its coast. Water level in naiko is probably an important factor relating to the drastic changes in fish fauna in the most naiko. Controlled low water level during the flood season caused drying-up of vegetated area (emerged plant zone) in the shallow coast, and possibly diminished ecological functions of the zone as spawning ground and habitat for native fishes. Thus, the native fish fauna is plausibly sacrificed by two factors, i. e., an invasion of alien species (L. macrochirus) and the unnatural water level during the rainy season for flood control. These two factors should be primarily taken in account in the future plan for natural conservation of naiko. Further investigation on the exceptional case in Katata-naiko is required for elucidating factors enabling native fishes to re-inhabit other naiko.
