日本鰻鰻苗體長變異與洄游路徑之環境因子的關係

Abstract

日本鰻 (Anguilla japonica) 為東亞重要經濟魚種，由於無法有效率的人工繁殖，因此極度仰賴漁業捕撈，而自 1980年以來，資源量有急劇下降的趨勢，捕獲量年間的波動也對產業有巨大的影響。根據過去日本鰻生活史調查指出，其於夏季洄游至西太平洋產卵場繁殖，柳葉鰻隨著北赤道洋流及黑潮輸送至東亞國家，並進入河川生長。因此過去研究指出洋流流場的年間變異，影響洄游的輸送路徑，是導致補充量的波動或極端減少的事件可能原因。然而研究大多僅探討海洋環境對鰻苗的物理性影響，並未以生物角度進行評估野。因此本研究以日本鰻鰻苗體長作為指標，探討環境因子與鰻苗的關係。於2010-2018年間至宜蘭河河口採集鰻苗，並記錄體長，再以數值模擬方式建構出所採集之鰻苗可能的洄游路徑，根據洄游路徑中鰻苗隨著洋流的時空分布，擷取當下海域上衛星遙測所得之環境因子資料。最後以廣義加成性模型 (Generalized additive model) 分析環境因子與鰻苗體長變異的關係。研究結果計算出的最佳模型對於體長變異的解釋率為57.3%，且日本鰻鰻苗在聖嬰年、產卵場溫度於21至24.5度、洄游路徑溫度高於24度，以及產卵場葉綠素濃度於0.03-0.07 mg/m3、洄游路徑累積葉綠素濃度高於13 mg/m3、產卵場鹽度介於35.1至35.3 psu 時，會有較大體長。此外，產卵場的環境與洄游路徑環境相比，對於體長有較大的影響。總結研究結果，環境因子對於鰻苗體長的影響大致與過去養殖實驗研究的結果符合，然而關於產卵場溫度的影響則有所不確定，有待未來研究詳細探討。在聖嬰年出現較長體長，則可能與鰻苗在洋流輸送的時間較長，以致生長的日齡較長，因此體長較長。另外，日本鰻及其他鰻鱺屬物種在產卵時具有同步生殖行為 (synchronized spawning)，因此在產卵場中的幼魚族群量大小，可能影響種內或種間競爭食物資源的強度，為影響本研究結果的潛在因素。本研究以與過去不同的方法及觀點探討海洋環境對於日本鰻鰻苗的影響，未來研究結果也可用以探討日本鰻資源量波動的機制。

Japanese eel (Anguilla japonica) is an important commercial freshwater fish in East Asia. However, eel aquaculture is a typical capture‐based aquaculture. Recruitment of Japanese eel has declined since 1980s, and large interannual fluctuations were also observed. The causes of decline or extremely poor recruitment events remain unclear. By using numerical modelling method to reproduce larval migration routes, previous studies showed that the inter-annual variability of ocean circulation may influence the migration success of Japanese eel. However, the effect of environmental factor on wild larvae is less discussed by biological points of view. In this study, a recent 8–yr total length data of Japanese glass eel was used as representative of eel larval condition to evaluate the effect of environment by generalized additive model. And the environment experienced by eel larvae was based on a coupled biological physical model which were the used to estimate the migration routes. The result of generalized additive model showed larger total length of Japanese glass eel were observed at 21-24.5°C for spawning area temperature, higher than 24°C for migration routes temperature, 0.03-0.07 mg/m3 for spawning area chlorophyll concentration, higher than 13 mg/m3 for accumulated migration routes chlorophyll concentration with increasing trend, 35.1-35.3 psu for spawning area salinity, and during El Niño year. The environment of spawning area has greater effect on glass eel total length. The results correspond to previous studies in laboratory experiment, except the effect of spawning area temperature. The synchronized spawning behavior in Japanese eel may cause the density-dependent effect which potentially influence the result of this study. This study provided a different perspective of environmental effect on Japanese larvae, and may help to understand the cause of recruitment fluctuation.