臺灣櫻花鉤吻鮭在異質棲地環境下之族群動態與準滅絕風險

Abstract

氣候變遷預期將增加極端降雨與颱風洪水的發生頻率。此類事件可能在短時間內造成高山溪流魚類跨齡級死亡，並透過河床翻動與棲地結構重塑引發延遲的補充量不足與繁殖輸出下降。在此類尾部事件主導的族群動態下（以下稱尾端風險，tail risk），準滅絕風險可能呈現非線性放大；然而同一流域內不同支流族群之人口學韌性比較仍相對缺乏。本研究評估瀕危臺灣特有陸封鮭科魚類，臺灣櫻花鉤吻鮭（Oncorhynchus formosanus）於大甲溪上游兩條支流，七家灣溪（殘存核心族群）與合歡溪（再建立／復育族群）的族群動態與準滅絕風險。研究結合長期監測與擾動後追蹤資料，主要以低侵擾的浮潛視覺調查量化齡級相對豐度，並於合歡溪以同步電捕取樣作為參考基準，校正水下體長估計偏誤與體型相關之偵測率差異。人口學參數結合鱗片年齡判讀，並以自助法重抽樣將不確定性傳遞至年齡結構重建與季節存活率、補充量指數及繁殖輸出之估計。上述參數進一步納入雙狀態（颱風年／非颱風年）的隨機年齡結構矩陣模型，用以推估 20 年族群軌跡與不同颱風年機率（p = 0.35–0.65）下的準滅絕風險（定義為連續四年成熟雌魚少於 50 尾）。兩個族群在非颱風年皆呈現成長（λ ≈ 2.2），但在颱風年情境下轉為衰退（λ < 0.5），反映零齡魚與成魚存活率降低，以及颱風後補充量與繁殖輸出下滑。準滅絕風險隨颱風頻率急劇上升：當 p 由 0.35 增加至 0.65 時，七家灣溪由 0.6% 升至 56%，合歡溪由 0.2% 升至 18%。結果顯示，支流尺度的人口學韌性差異可能推翻殘存自然族群本質上較安全的假設，並支持流域尺度的管理策略，包括降低洪水期間跨齡級同步死亡、強化擾動後早期生活史之補充，以及維持多支流族群以分散極端事件造成的同步衰退風險。

Climate change is expected to increase the frequency of extreme rainfall and typhoon-driven floods. Such events can cause rapid mortality across fish age classes and generate delayed demographic deficits by restructuring streambeds and habitats. These “tail-risk” dynamics may elevate quasi-extinction risk nonlinearly, yet demographic resilience is rarely compared among tributary populations within the same watershed. Here, population dynamics and quasi-extinction risk were evaluated for the endangered Formosan landlocked salmon (Oncorhynchus formosanus) in two tributaries of Taiwan’s upper Dajia River: Qijiawan Creek (a remnant core population) and Hehuan Creek (a reintroduced population). Analyses integrated long-term monitoring with post-disturbance tracking, relying primarily on low-impact snorkel surveys to quantify age-class–specific relative abundance. In Hehuan Creek, paired enclosed electrofishing was additionally used to calibrate underwater length estimates and size-dependent detectability. Age structure was reconstructed using scale-based aging with bootstrap uncertainty propagation, yielding estimates of seasonal survival, recruitment indices, and fecundity. These parameters informed a two-state (typhoon vs. non-typhoon year) stochastic, age-structured matrix model to project 20-year trajectories and quasi-extinction risk (defined as fewer than 50 mature females for four consecutive years) across annual typhoon probabilities (p = 0.35–0.65). Both populations increased during non-typhoon years (λ ≈ 2.2) but declined during typhoon seasons (λ < 0.5), consistent with reduced survival of age-0 and adult classes and depressed recruitment and reproductive output following typhoons. Quasi-extinction risk rose sharply with typhoon frequency, increasing from 0.6% to 56% in Qijiawan and from 0.2% to 18% in Hehuan as p increased from 0.35 to 0.65. These results demonstrate that tributary-level differences in demographic resilience can overturn the assumption that remnant populations are inherently safer. Watershed-scale strategies are therefore supported, including (1) reducing synchronous flood mortality across age classes, (2) enhancing early-life recruitment after disturbance, and (3) maintaining multiple tributary populations to buffer against synchronized declines.