攔砂壩對烏石坑溪生態影響及魚道效用之評估

Environmental Impact Assessment of Check Dams with Evaluation of the Function of Fishways in the Wushykeng Creek

從1995年7月至1997年5月止，評估烏石坑溪7座攔砂壩對水域物理、化學環境及魚群的影響和9座魚道之功用。攔砂壩上方填滿砂石，使得河床坡度減緩，流水面加寬，水深變淺，流速變慢，溶氧量減少、水溫升高，此現象在乾季較明顯。攔砂壩下方河床及兩岸侵蝕嚴重，水濁度及總固體溶解量增加，這種現象以雨季較為明顯。烏石坑溪共記錄10魚種，以台灣鏟頜魚為最優勢種。高攔砂壩構成魚類洄游障礙，依攔砂壩上下水域魚群月別體長頻度分布的資料，台灣鏟頜魚成魚在雨季末（10月）可經由現有之水池式魚道上溯洄游，而雨季初（3月）下移。台灣間爬岩鰍（Hemimyzon formosanum）與台灣鏟頜魚（Varicorhinus barbatulus）同時會利用魚道。其他6魚種包括褐吻鋛虎（Rhinogobius brunneus）、短吻鋛虎（Rhinogobius rubromaculatus）、台灣馬口魚（Candidia barbata）、台灣石𠕇（Acrossocheilus parodoxus）、粗首𠊠（Zacco pachycephalus）及脂鮠（Leiocassis adiposalis）所得資料顯示可經魚道或攔砂壩上溯的可能性。為了減輕攔砂壩長期對河川生態的影響及其魚道功能不彰，建議以連續性矮壩替代高壩為最佳之解決方案。 Between July 1995 and May 1997, a survey was conducted to assess the impacts of seven check dams on the Wushykeng Creek, and to evaluate the effectiveness of their fishways for fish upstream migration. The check dams caused the formation of rock and gravel piles in the immediately upstream areas and deep excavation in the immediately downstream areas. The former resulted in decreases in stream slope, surface stream flow, water depth, water flow velocity and dissolved solid content of the stream water, particularly in the raining season. The latter resulted in erosion of stream bed and shores, and increases in turbidity and dissolved solid content of the stream water, particularly in the raining season. The dams became barricades for the upstream migration of fishes. For 10 species of fishes found in the creek, there were circumstantial evidences that Varicorhinus barbatulus, Acrossocheilus parodoxus, Candidia barbata, Zacco pachycephalus, Rhinogobius brunneus, Rhinogobius rubromaculatus and Hemimyzon formosanum were able to pass the existing pool-type fishways for upstream migration. However, most of the fishways have been functionally damaged or destroyed for lack of maintenance. For a long-term and cost-effective consideration, serial low-dams to replace the present high rise check dams is recommended.