The Effect of Water Level Lowering in Miharu Dam Reservoir on the Reproduction of Largemouth Bass, Micropterus Salmoides

27 Mar The Effect of Water Level Lowering in Miharu Dam Reservoir on the Reproduction of Largemouth Bass, Micropterus Salmoides

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Abstract

さくら湖(三春ダム)において洪水調節を目的とした水位低下がオオクチバスの繁殖に与える影響について調査し,検討を行った. 試験湛水前から継続した捕獲調査の結果,オオクチバスは侵入直後に増加し,特に水位低下のない前貯水池において激増する傾向が認められた.侵入直後に増加したオオクチバスは,すぐに減少する傾向が全水域に共通して認められたが,オオクチバスが激増した前貯水池では,本種の被食魚が激減し,オオクチバスが減少してもその傾向が継続することが示された. 成魚の生殖腺分析,当歳魚の耳石輪紋計数の結果,さくら湖における繁殖期は,日平均水温が15℃から21℃に上昇する5月上旬から6月下旬であると推定された. 水位低下後に採取した当歳魚には,水位低下中の前貯水池で孵化した個体が含まれなかったことや,水位低下中の湖岸で孵化後20日程度の当歳魚の群れが水際で死亡していることが確認されるなど,オオクチバスの繁殖期に合わせた水位低下が本種の繁殖を抑制するものと考えられる.以上の結果から,日平均水温が15℃から21℃に上昇する時期に0,27m/day以上の速度で2.5m以上(約9日間以上)水位低下することは,本種の繁殖抑制につながることが推察された.

English Abstract

The effect of water level lowering for flood control on the reproduction of largemouth bass, Micropterus salmoides, was studied in Miharu Dam Reservoir, Fukushima Japan. The capture investigation was continued from the year of the first filling of water in the dam. The results indicated that largemouth bass increased immediately after invasion, especially in the Pre-Reservoir where water level was fixed. The increased largemouth bass then decreased in the next year, however the tendency in which fishes eaten by largemouth bass decreased was continued. Maturating and hatching season of largemouth bass was surveyed in the Pre-Reservoirs. The hatching date was a lyzed by the number of otolith rings, and maturity of go ds were determined by histological a lyses. These a lyses suggested that their spawning season was from early May to late June when water temperature rose from 15°C to 21°C. A lyses of otolith rings of juveniles indicated that hatching did not occur during the water level lowering period. Furthermore, it was observed that juveniles were dead at the edge of the water during the period. These results suggested that the water level lowering over 2.5m and at the speed over 0.27m/day, water temperature risen from 15°C to 21°C, suppressed the reproduction of largemouth bass.