Ecological Changes in the Lake Biwa Environment

Abstract

The ecosystem of Lake Biwa has been greatly affected by various kinds of human activities. The water level has been decreasing in the long run since the man-made Seta Weir was constructed at the sole outlet the Seta River in 1905 and the lake water level had been controlled artificially. Reclamation of shallow lagoons and attached lakes or naiko was conducted during the mid-1940s–1971 and led to the decrease in the water level. Levees along the lake shore, dams, and enclosing bands on the river flowing into the lake were constructed during the 1970s–1990s, affecting adversely the spawning of the indigenous fishes. Changes in the water-level control since 1992 have also affected the fish spawning. Invasive alien fishes, largemouth bass Micropterus salmoides, and bluegills Lepomis macrochirus invaded the lake in the mid-1960s and 1970s, respectively. Feeding on or competing with indigenous fishes and shrimps, the two fish species have propagated since the 1980s and had adverse affects on the indigenous fish communities which in turn changed the food web structure of the lake. Various kinds of alien species, such as emergent and floating-leaved plants, like Alternanthera philoxeroides, Gymnocoronis spilanthoides, and Ludwigia grandiflora spp. hexapetala, submerged macrophytes, invertebrates, fishes, and tetrapods and have invaded and increased in the lake and adjacent waters.