The Guide-flow Fishways and the Brook-type Fishways at the Gara Rivermouth Barrage comparison of Function During Migration of Ayu Fry

27 Mar The Guide-flow Fishways and the Brook-type Fishways at the Gara Rivermouth Barrage comparison of Function During Migration of Ayu Fry

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Abstract

長良川河口堰の呼び水式魚道(左岸側と右岸側)とせせらぎ魚道について,建設省中部地方建設局,水資源開発公団中部支社のモニタリング調査結果からその機能を比較した.1995年から1999年までの5年間の一日当たりの平均遡上尾数を比較すると,右岸呼び水魚道が他の魚道と比較して遡上尾数が少ない傾向が見られた.左岸呼び水とせせらぎ魚道の遡上尾数については有意差がなかった。しかしながら,せせらぎ魚道は集中した日に多くの稚アユが遡上し,一日の中でも短時間に多数の稚アユが遡上した.またせせらぎ魚道は他の魚道と比べ遡上の最大時が潮汐周期の小潮時とよく一致した.自然状態の河川において,小潮時には弱混合により塩水楔の遡上領域が上流側に移動する.せせらぎ魚道はこれと類似した環境が作り出されることが示唆された. 長良川河口堰からの流出水の量と遡上との関係を見ると,呼び水魚道では流出水が170m3/sec付近を最大に遡上尾数は減少した.せせらぎ魚道についてはこの傾向は見られず流出量の増加とともに遡上尾数が増加した.呼び水式魚道については,稚アユを誘引する呼び水の効果が,流出水の増加により弱められたものと考えられる.せせらぎ魚道は集中的な遡上というアコの遡上生態に即した機能をもち,河口域の魚道として呼び水式魚道よりも機能的に勝っていることが明らかになった.呼び水式魚道と比較して,せせらぎ魚道は開口部が小さく,必要流量も呼び水式魚道より少ない.このことは,治水,利水の両面からもその有効性が高い事を意味している.また,今後の課題として,魚道開口部での滞留による遡上の遅れに関しての検討が必要である.

English Abstract

The daily total number of ayu fry under upstream migration was compared among the three fishways of different types at the gara Rivermouth Barrage, based on the monitoring data taken over a five-year period from 1995 to 1999 by the Japanese Ministry of Construction. The number of migrating fry through the guide-flow fishway on the right side tended to be fewer than the other two fishways. There was no significant difference in the number of migrating fry between the guide-flow fishway on the left side and the brook-type fishway. It should be noted, however, that temporal concentration of upstream migration was observed both hourly and daily in the brook-type fishway. The peak date of the migration corresponded to the neap tide period. This correspondence was only observed in the brook-type fishway. Under tural conditions, the water at the mouths of rivers becomes brackish at the neap tide and the range of brackish water extends upstream farther as the tide advances. The data suggests that the environment created in the watercourse of the brook-type fishway resembled tural conditions. Relations between the number of migrating ayu fry through the guide-flow fishway and the total water flow at the barrage showed that the upstream migration of ayu fry reached the maximum when the total water flow was near 170 m3/sec and decreased when the flow exceeded this volume. It is likely that the effect of the guide-flow is weakened as the water flow increases at the barrage. In contrast, the number of migrating ayu fry through the brook-type fishway did not show such a decrease and increased with increasing total water flow. These results suggested that the brook-type fishway surpassed the guide-flow fishway in its function.