原著論文　ORIGINAL PAPER

河口閉塞による感潮域魚類相への影響

鳥居 高志^１）*･塩根 嗣理^１）･加藤 憲一^２）･杉浦 幸彦^３）･黒川 忠之^３）･大野 正博^３） ･大城 朝一^４）･新垣 敏一^４）

１) いであ株式会社　沖縄支社　〒900-0003　沖縄県那覇市安謝2-6-19
２) いであ株式会社　本社　〒154-8585　東京都世田谷区駒沢3-15-1
３) いであ株式会社　国土環境研究所　〒224-0025 神奈川県横浜市都筑区早渕2-2-2
４) 内閣府沖縄総合事務局北部ダム統合管理事務所　〒905-0019 沖縄県名護市大北3-19-8

Takashi TORII^１）*, Hidemasa SHIONE^１）, Kenichi KATO^２）, Yukihiko SUGIURA^３）, Tadayuki KUROKAWA^３）, Masahiro OONO^３）, Tomokazu OSHIRO^４）, Toshikazu ARAKAKI^４）: Effects of river mouth closure on fish fauna in tidal river areas. Ecol. Civil Eng. 13(2), 123-139, 2011.

1) IDEA Consultants, Inc., Okinawa Branch, 2-6-19 Aja, Naha, Okinawa 900-0003, Japan
2) IDEA Consultants, Inc., Head Office, 3-15-1 Komazawa, Setagaya-ku, Tokyo 154-8585, Japan
3) IDEA Consultants, Inc., Institute of Environmental Informatics, 2-2-2 Hayabuchi, Tsuzuki, Yokohama 224-0025, Japan
4) Okinawa General Bureau North Dam Integrated Control Office, Cabinet Office, Government of Japan, Okita, Nago, Okinawa, 905-0019, 3-19-8, Japan

Abstract: Effects of river mouth closure on fish fauna were studied in 11 rivers in the northern region of Okinawa Island. We investigated the occurrence of fish species and recorded physical environmental variables, i.e., water level, flow velocity, water temperature, and the width of the river mouth at high water, in the tidal river areas over four seasons. There were significantly fewer fish species in rivers with completely closed mouths than in those with open or incompletely closed mouths, particularly those species depending strongly on the sea and tidal river areas. This was presumed to be due to low salinity and obstruction of fish migration. Among the rivers with completely closed mouths, marine fish species occurred only in the Zatsun River, where the salinity changed with the tide. This shows that marine fish species are able to survive in tidal rivers, even when the mouth is completely closed. In two rivers with completely closed mouths, no larvae or juveniles of Rhinogobius species were found in the spring season, suggesting that the closed mouths obstructed the migration of these species. There were significant positive correlations between the number of fish species and the range of water-level fluctuation and the width of the river mouth at high water, two indications of the degree of river mouth closure. We suggest that the water-level range and the width of the river mouth at high water provide valid indices of the effects of river mouth closure on fish fauna.

Key words: fish fauna, Okinawa Island, river mouth closure, tidal area

摘要

河口閉塞が感潮域魚類相に及ぼす影響を把握することを目的に,沖縄島北部の11河川感潮域で,魚類の出現状況と河川環境(水位,流速,水温,塩分,開口幅)について,四季の調査を行った.
｢閉塞なし｣,｢部分閉塞｣,｢完全閉塞｣の河川間で比較すると,｢完全閉塞｣の河川で,魚類の出現種数が少ない傾向がみられ,特に海域及び感潮域-の依存性の強い魚類の種数が少なかった.原因として,河川水の低塩分化と魚類の侵入阻害が考えられた.｢完全閉塞｣の河川のうち,潮汐に応じて塩分が変動した座津武川のみで,海域依存性の強い魚類が確認された.このことから,｢完全閉塞｣の河川であっても,潮汐による影響を受ける場合は,海域依存性の強い魚類が河川内で生存することが可能であると考えられた.
また,ヨシノポリ属は,春季に完全閉塞していた2河川で,遡上個体が確認されなかった.このことから河口閉塞がヨシノポリ属の新規加入を阻害することが示唆された.
魚類の出現種数は,四季を通じて,朔望平均満潮位における河川開口幅および水位変動幅と正の相関にあった.河川開口幅および水位変動幅は閉塞状況に関係しており,河口閉塞による魚類の出現状況への影響を示す指標となると考えられた.

2010年6月15日受付, 2010年12月3日受理
* e-mail: ttorii@ideacon.co.jp