Investigating ENSO effects on the spawning migration of Japanese anchovy in the East China Sea using a behavior-hydrodynamic model

每年冬春之交，攝食區域位於中國東海的日本鯷成魚會洄游至位於台灣沿海的產卵場，其仔稚魚（魩仔魚）則會成為台灣沿海漁業的重要標的之一。過去研究顯示日本鯷仔稚魚的單位努力漁獲量（Catch-per-unit effort）的年間變異與聖嬰-南方震盪現象（El Niño-Southern Oscillation, ENSO）有著負相關性；然而對於ENSO如何影響洋流等物裡環境進而影響魩仔魚的再補充量的詳細機制，目前仍不清楚。為了釐清此問題，本研究利用物裡洋流模型（hydrodynamic model）模擬過往年份的中國東海及台灣海峽的冬季流場，同時利用個體模型（Individual-based model）來模擬日本鯷成魚的的洄游行為並記錄其在模擬流場中的洄游過程。比較不同ENSO強度下的模擬結果，顯示到達台灣沿海海域的成魚數量與被東北季風驅動的中國沿岸流（China Coastal Current）強度呈現正相關。當強度越大的聖嬰(反聖嬰)現象發生時，東北季風減弱(增強)的情形也越明顯，進而影響中國沿岸流使之減弱(增強)，最終導致到達台灣海域的日本鯷成魚數量減少(增加)。ENSO、東北季風、中國沿岸流以及日本鯷的生殖洄游行為之間的關聯顯示了大尺度的環境變異可能藉由影響中尺度的環境物理因子傳遞至海洋生物進而影響其生殖行為。結果顯示，考慮洄游物種的漁業政策時，洄游過程所造成的影響不可忽略。Mature individuals of Japanese anchovy (Engraulis japonicus) migrate from the East China Sea to the coastal water of northern Taiwan to spawn during winter. The anchovy larvae then form an important fishery resource for Taiwan. Previous studies indicate that the interannual variation of anchovy larvae catch per unit effort data exhibited a negative correlation with ENSO; however, the detail underlying mechanism explaining how ENSO affects ocean currents or environmental conditions, which in turn determine the fluctuations of anchovy larvae, remains unclear. To bridge the knowledge gap, we construct historical circulation patterns in the East China Sea and the Taiwan Strait by hydrodynamic models, and use individual-based model to represent fish behavior for simulating the migration process of Japanese anchovy. Comparing model results under various ENSO states (strong El Niño (La Niña) and moderate El Niño (La Niña) states), we find that the fluxes of Japanese anchovy populations toward Taiwan is positively correlated with the strength of China Coastal Current, which is driven by the East Asian Winter Monsoon. Specifically, in El Niño (La Niña) states, the East Asian Winter Monsoon decreases (increases) in boreal winter and weakens (strengthens) the China Coastal Current and in turn decreases (increases) the fluxes of Japanese anchovy populations migrating toward the coastal water of Taiwan. The connection between the ENSO, East Asian Winter Monsoon, China Coastal Current, and spawning migration of Japanese anchovy suggests that large-scale environmental changes may affect reproductive biology of species through meso-scale physical environmental factors. The results also indicate that the dynamics of migration processes should be considered in fishery managements