ENSO事件對日本鰻柳葉期長短之影響

Abstract

日本鰻為東北亞國家的重要養殖魚類，然而所需之鰻苗皆來自於野外採捕。自1970 年代以來，日本鰻的資源量急遽下降，導致鰻苗產量減少，近年來玻璃鰻之交易價格更屢創新高，使得鰻魚養殖業面臨嚴重的困境。因此，瞭解日本鰻的早期生活史及其入添動態，可提供鰻苗資源的利用、管理、保育以及未來更進一步研究作為重要的參考依據。根據過去的研究發現，一般而言，日本鰻玻璃鰻的全長在聖嬰現象時大多位在趨勢線的上方，在反聖嬰現象時則大多位在趨勢線 的下方。換言之，日本鰻玻璃鰻之體長的年間波動變化，與聖嬰及反聖嬰現象互有關聯。因此，本研究之目的為進一步探討聖嬰-南方濤動事件與日本鰻玻璃鰻漂送時間長短之關係。本研究所使用之樣本為自2002-2013 年中，聖嬰、反聖嬰及正常事件各挑選三個年度，每個年度的11 月份至翌年3 月份在台灣及中國大陸河口所採集之日本鰻玻璃鰻。測量其體長全長及色素階段後，再進行耳石的採集，藉由耳石所記載之日周輪判讀其自孵化至發生變態所歷經之天數。分析結果 發現，日本鰻玻璃鰻平均體長在VA 時，正常年之體長為最長（56.7±2.6）、反聖嬰年之體長為最短（55.4±2.5）；而在VB 時，聖嬰年及正常年之體長為最長（55.6±2.7 及55.6±2.6）、反聖嬰年之體長為最短（54.8±2.4）。在耳石日周輪部分，在各色素階段皆呈現聖嬰年之日周輪為最長（146.11±12.4）、正常年及反聖嬰年之日周輪為最短的趨勢（127.22±6.53 及127.10±8.49）。此結果顯示，日本鰻玻 璃鰻之日周輪的年間波動變化，亦受到聖嬰及反聖嬰現象之影響。此外，根據浮標的模式模擬結果，發現浮標在北緯14°東經139°的位置釋放時，漂流至台灣東部河口所需時間顯著較長(Duncan, p < 0.05)；反之，在浮標北緯13°東經139°的位置釋放時，所需時間顯著較短(Duncan, p < 0.05)。意即日本鰻玻璃鰻之漂送時間與其漂送起始緯度之不同有明顯的關聯。總結而言，聖嬰-南方濤動事件造成北赤道洋流分支緯度之波動，會使洋流輸送速度發生改變，可能影響日本鰻之漂送動態。然而，本研究更進一步發現，聖嬰-南方濤動事件對於日本鰻玻璃鰻之入添動態所造成之最關鍵影響為改變其漂送之起始位置，進而影響柳葉鰻期長短，乃至鰻苗到達河口時體型。

The Japanese eel Anguilla japonica (Temminck & Schlegel) is an important aquaculture species in East Asia countries. However all of the eel fry needed for aquaculture depends on the wild capture. The Japanese eel stock has declined rapidly since 1970s, and the price of A. japonica glass eels has a large increase, thus the eel aquaculture industry is facing serious problems. Therefore, understanding the recruitment dynamics of the Japanese eel is expected to lead to proposals for the utilization, preservation, management, and further study of A. japonica glass eels. On the basis of previous experiments in our laboratory, the total length (TL) of A. japonica glass eels during El Nino periods is usually above the regression line. In contrast, the TL of A. japonica glass eels during La Nina periods is usually below the regression line. In other words, the fluctuation of mean TL oscillation appears to be related to El Nino and La Nina events. Thus, this study aimed to elucidate the relationship between ENSO events and Japanese eel larval duration. This study used the A. japonica glass eels that had been collected from estuaries in Taiwan and China between 2002 and 2013, encompassing three El Nino, three normal, and three La Nina events. After measuring the TL and observing the pigmentation stages of the eel larvae, the otoliths were extracted. The daily increments on the otoliths provide information about larval duration. The results showed that the total length (TL) was longest for A. japonica glass eels during stage VA in normal years (56.7±2.6), while the shortest TL was recorded in La Nina year (55.4±2.5). During stage VB, TL was longest in El Nino years and shortest in La Nina years (55.6±2.7 and 55.6±2.6). The results of the otolith daily increments indicated that, for each pigmentation stage, LD was longer in El Nino years (146.11±12.4) and shorter in normal/ La Nina years (127.22±6.5 and 127.10±8.49). These results indicate that fluctuations in mean TL and mean LD oscillation appear to be related to El Nino and La Nina events. In addition, the tracer experiments indicated that tracers released at 139°E and 14°N took longer to drift to the east coast of Taiwan (Duncan, p < 0.05), whereas tracers released from 139°E and 13°N required a significantly shorter time (Duncan, p < 0.05). This result showed that the A. japonica glass eels drift from different latitude might cause effect on their drifting time. In conclusion, ENSO events are correlate to the latitude shifting of the North Equatorial Current (NEC) bifurcation, then impact the transportation velocity of NEC current and Kuroshio; this might cause some effects to the recruitment dynamics of Japanese eel. However, this study further display that the changing of the starting latitude where the A. japonica drift from was the critical factor which caused by ENSO events that influence the larval duration, thus influence the size of the estuarine glass eels of Japanese eel.