颱風豪雨使洪水挾帶集水區大量崩塌土砂與漂流木進入水庫，形成水庫淤積問題，導致水庫有效庫容逐漸減少。排除庫區沉積物以維持既有水庫庫容，最有效的方法除水力排砂外要屬機械清淤，爰水庫管理當局自2012年起辦理曾文水庫壩前清淤作業，並將清淤土砂暫置於下游河道，利用洪水將土砂沖往下游以解決土砂去化問題，亦稱為「土砂回歸河道」操作。
本研究地點位於曾文溪主流上之曾文一號橋(中正橋)、麻善大橋及出海口共4處採樣站，蒐集1995年、2004年及2016-2022年的魚類調查數據、河床底質及水質數據，以魚類個體生態矩陣、多樣性及均勻度指數等分析方法，分析曾文水庫放淤是否對大壩下游曾文溪主流上之魚類群落組成產生影響。研究發現，放淤後之魚類捕獲數量、多樣性指數及曾文一號橋(中正橋)站特有種種數的變化，皆呈現放淤後下降並於隔年放淤前恢復之趨勢；各採樣站多樣性指數2004年高於2016-2022年，研判實施「土砂回歸河道」操作可能對多樣性產生負面影響；魚類個體生態矩陣結果顯示，曾文一號橋(中正橋)站於底質考量，魚類群落組成受放淤影響，麻善大橋及出海口站於底質及水質考量皆無明顯受放淤影響，亦無明顯受底質及水質變化影響，研判為(1)麻善大橋站及出海口站原魚類群落組成即以偏好細顆粒底質為主，放淤未對其組成造成影響；(2)放淤後底質及水質變化可能需一定時間才反應在魚類群落組成的改變；(3)水質採樣與魚類調查時間差及水質數據較易變動，皆有可能導致分析之誤差。
研究結果顯示，放淤仍對魚類群落造成一定程度的負面影響，並考驗魚類群落對棲地環境干擾的耐受度及適應力。本研究建議以長時間定期、不定期進行魚類生態調查，建立更完整且可靠的魚類資料庫，在魚類個體生態矩陣分析上可較準確評估魚類群落組成的變化，以隨時監測放淤對魚類群落影響程度；支流之生態調查亦為重要，以維持支流在放淤期間提供魚類避難所的功能；另建議根據長期魚類生態調查結果，結合年度出流量及放淤排砂量，評估對魚類生態在適當干擾程度內之最大放淤排砂量體，以推估暫置場土砂最大暫置量，減少因放淤及土砂回歸河道之操作影響魚類生態。

To realize the impact of the sediment flushing operation on fish community composition, this study uses fish autecology matrix, diversity and evenness index methods to analyze fish, substrate, and water quality data which are collected from 4 sample stations located in the Zengwen River from 1995, 2004 and 2016 to 2022. It can be found that the number of fish caught, the diversity index and the number of endemic species show a trend of decreasing after flushing and recovering in the next year before flushing. The diversity index of each sample station was higher in 2004 than in 2016 to 2022. It is thought that the sediment replenishment operation may have a negative impact on the fish community. According to the result of the fish autecology matrix, the fish community composition in the Zengwen No.1 Bridge station is affected by flushing and the changes in substrate. The fish community composition in the Mashan Bridge station and the estuary station is not significantly affected by flushing and the changes in substrate and water quality. It is thought that (1) the fish community composition in the Mashan Bridge station and the estuary station prefer fine-grained substrate originally, so the sediment flushing has not affected its composition; (2) it may take time to see the changes in fish community composition resulted by substrate and water quality after flushing; (3) it is considered that the time difference between fish caught and water quality data, and the inconstant water quality may lead to error analysis.

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