結合耳石日周輪與東亞沿岸流分析日本鰻苗的輸送路徑

Abstract

日本鰻 (Anguilla japonica) 為東北亞國家的重要養殖魚類，日韓雖有成功人工繁殖鰻苗但生產費昂貴難以應付大量需求，所需魚苗仍來自於野外採捕。根據前人的研究，日本鰻產卵場應該在馬里亞納海脊的西側，因為東側未出現柳葉鰻。柳葉鰻隨北赤道洋流、黑潮、台灣海峽流、台灣暖流和黃海暖流等漂送到台灣、日本、中國、韓國等地區。前人研究發現，每個地區的主漁期並不一樣，台灣是11-2月、日本是12-4月、韓國是1-5月和中國是12-5月，漁期不一的主要原因是河口溫度和運輸路徑長短不一。由於複雜的環境因素，日本鰻鰻苗的詳細漂送路徑仍未知。前人研究發現，柳葉鰻耳石會每天沉積一輪，因此可以透過輪數得知鰻苗日齡。 本研究綜合利用(1)耳石日周輪定齡; (2)東亞沿岸流; 以及 (3)各地主漁期之方法，來解析日本鰻鰻苗在東亞沿岸的漂送路徑。收集台灣、中國、日本和韓國的鰻苗樣本，並根據上述資料設定研究假說: 柳葉期與漂送距離成正比，距離產卵場越遠，柳葉期越長;依此推測日本鰻苗在東亞地區有多個擴散路徑存在。 研究結果發現，台灣鰻苗的平均日齡最短，而鴨綠江的是最長的。從採樣點採集的玻璃鰻的平均日齡沿運輸距離皆呈上升趨勢，但漁期越晚的地點，鰻苗耳石平均日齡低估的程度就越高。這5條可能的輸送路徑分別是: (1)主流黑潮運送的宜蘭-種子島-三河灣-相模川-千葉縣; (2)台灣海峽流運送的屏東-淡水和藉沿岸流將鰻苗運送到廣東; (3) 台灣暖流運送的宜蘭-福鼎-閩江-溫州-寧波-上海; (4) 經黑潮後分出黃海暖流運送的宜蘭-濟洲島-韓國錦江-鴨綠江以及(5) 福建北部的鰻苗可能經由沿岸流運送到福建南部和象山(廣東)。本研究結果闡明了日本鰻苗在東亞的輸送機制，對於日本鰻苗的資源分布與管理，具有重要的意義。

Anguilla japonica is an important feeding aquaculture fish in Northeast Asian countries. Although the artificial reproduction of cultured A. japonica eel was succeeded in Japan and Korea, the production cost is too high for huge demand of eel. Therefore, all the fry need to capture from the wild. According to previous research, the spawning ground should be on the west side of the western Mariana Ridge due to glass eels never appear on the east side of western Mariana Ridge. The larvae of A. japonica were passively transported by the North Equatorial Current, the Kuroshio, the Taiwan warm Current, the Taiwan Strait Current, the Tsushima Warm Current, the Jeju Warm Current, and the Yellow Sea Warm Current to Korea, China, Taiwan and Japan. Previous studies have found that the main fishing seasons in each region are different, Taiwan is from November to February, Japan is from December to April, South Korea is from January to May and China is from December to May. The main reason for the different fishing seasons is the differences of estuary temperature and the distance of the transportation route. Due to complex environmental factors, the detailed drifting pathway of the larvae is not well understood. Previous studies show that the growth increments in otoliths of larvae were found to be deposited daily, thus the daily age of glass eels can be examined through the number of increment of otolith. This study combing information from (1) otolith increments, (2) East Asian coastal currents and (3) local main fishing seasons to analyze the drifting path of A. japonica glass eels on the East Asian coast. Samples were collected from Taiwan, China, Japan, and South Korea. The hypothesis of this study was based on the above data: (1) the time that larvae had drifted is proportional to the number of the increment in the otolith, the farther from the spawning ground, the longer drifting time and larval duration it would have. Thus, it is speculated that there were multiple diffuse paths for A. japonica in East Asia. Our results find that the glass eels captured from Taiwan have the smallest mean larval duration and glass eels captured from Yalu River have the biggest mean larval duration. The mean larval duration of glass eel from sample sites is on an upward trend along the transport distance. The later the recruitment of glass eel at the sampling site, the age of the glass eel will be underestimated easier. Five possible routes were found : (1) The main Kuroshio transport glass eel from Taiwan I-Lan to Japan Tanega-Shima Island, Mikawa Bay, Sagami River and Ichinomiya River; (2) Taiwan Strait Current transport glass eel from Taiwan Ping-Tung then to Taiwan Dan-Shui, and to Xiang-Shan by ESCC; (3) Taiwan Warm Current transport glass eel from I-Lan to China Fu-Ding, Min River, Wen-Zhou, Ning-Bo and Shang-Hai; (4) Yellow Sea Warm Current transport glass eel from I-Lan to Korea Je-Ju Island, Geum River and China Yalu River and (5) glass eels in northern Fu-Jian may transport by the ESCC from Fu-Ding to southern Fu-Jian and Xiang-Shan (Guang-Dong). The results of this study clarify the transport mechanism of A. japonica in East Asia which is meaningful for the distribution and management of A. japonica in East Asia.