原著論文 ORIGINAL PAPER

琵琶湖周辺の内湖における魚類相の変化と生息環境分析−在来魚の繁殖・生息の場としての生態的機能の復元に向けて−

美濃部博¹⁾²⁾・ 桑村邦彦¹⁾³⁾

1) Applied Research Division, Scientific Research Department, Lake Biwa Museum, 1091 Oroshimo, Kusatsu, Shiga, 525-0001, Japan

Hiroshi MINOBE¹⁾²⁾, Kunihiko KUWAMURA¹⁾³⁾: Analysis of the change in fish fauna and ecological environment in “naiko”around Lake Biwa Ecol. Civil. Eng. 4(1), 27-38, 2001.

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.

Key words: alien species, controlled water level, emerged plant zone, naiko, native fish

摘要

琵琶湖周辺の内湖の機能のうち，在来魚の繁殖・生息の場としての生態的機能に着目し，8内湖における魚類相の変化とその要因を考察した． 小津袋（守山市）の魚類相は1960年代から1990年代にかけて大きく変化しており，琵琶湖の固有種中心の魚類相から，固有種が姿を消し他の在来種に置き換わり，さらに現在では外来種であるブルーギルが優占していた．他の内湖の魚類相の状況も同様であった．西の湖と堅田内湖だけは他の内湖と異なり，在来種を含む均衡した魚類相を示していた． このような魚類相の変化要因としては，閉鎖性の高い水域における外来魚の在来魚に対する淘汰圧が大きいこと，および制限水位方式による洪水期（6〜10月）の水位低下により，在来魚の産卵場や仔稚魚の成育場である水深が浅くて奥行きのある抽水植物帯が干陸化したことが挙げられる． 今後，内湖の多様な魚類相を復元していくためには，沿岸の抽水植物帯の奥行きだけでなく，湛水した水生植物帯を維持するための水位条件が十分配慮されなければならない．なお，湖岸の大半が人工化されているにもかかわらず多様な魚類相を保つ堅田内湖の魚類の生息環境分析を進める必要がある．

2000年4月20日受付，2001年4月22日受理
2) e-mail: hass-m@mx.biwa.ne.jp
現住所：滋賀県土木交通部河港課 〒520-8577 大津市京町４-１-１
Present Address: River and Port Division, Department of Public Works and Transportation, Shiga Prefectural Government, 4-1-1 Kyomachi, Otsu, Shiga, 520-8577, Japan
3) 現住所：滋賀県農政水産部水産課 〒520-8577 大津市京町４-１-１
Present Address: Fisheries Management Division, Department of Agriculture and Fisheries, Shiga Prefectural Government, 4-1-1 Kyomachi, Otsu, Shiga, 520-8577, Japan