内湖の生物多様性保全・修復に向けて (特集論文 I 水辺エコトーンにおける生物多様性と生業活動・コモンズの変容に関する研究)

Abstract

Lake Biwa is the largest lake in Japan, as well as one of the world's few truly ancient lakes. More than 1000 animal and plant species have so far been reported, in which at least 61 species are endemic. Most of the animal and plant species of the lake, excluding planktonic species, inhabit the littoral and wetland areas, i.e., lagoons or attached lakes, called as “naikos”, around the lake. All of the naikos were covered with Phragmites reed beds and aquatic plants, and used to be nursery areas of the indigenous cyprinid fishes of the lake. However, many of these naiko s disappeared or shrunk due to land reclamation during past decades. The total area of the naikos used to be 2902 hectares in 1940, but today it has diminished to 425 hectares. This is one of the reasons why the significant biodiversity of Lake Biwa has been threatened. Recent studies on the biodiversity of naikos reveal that nearly 60 species of the endangered and rare plants are still found in the Phragmite s reed beds and aquatic plant zones surrounding the 23 remaining naikos . Three dimension analysis of the aerial photos suggested that these limocoline plants require floods, i.e., fluctuation in water level to grow. For the fish, 31 indigenous species, half the fish fauna of the lake, were still found from the remaining naikos. But, two invasive alien fishes, i.e., bluegill, Lepomis macrochirus, and largemouth bass, Macropterus salmoides, threatened the indigenous fish fauna of naikos, because they not only eat native fishes a lot but also their larvae feed on the same food as native fish larvae. We found that the nursery habitat of these alien fishes slightly differ from that of the indigenous fishes. Thus, conservation and reconstruction of favorite nursery habitat of the indigenous fishes are urgently required, in order to get rid of these alien fishes as the local government had already been conducted. Investigation of detailed distribution of the endangered and rare plants of respective naikos can indicate where should be conserved, and where should be restored. For the conservation and rehabilitation of biodiversity in Lake Biwa and its naikos, it is required to set the goals of restored fauna and flora of respective naikos, based on their local and ecological characteristics. The biota and topogeography about forty or fifty years ago, when human impact of the nature was not so high, will provide useful information to set the restoration goals.