台灣北部地區日本鰻鰻線族群遺傳結構之月別變化

Abstract

日本鰻，為降河洄游性魚類(Catadromous fish)，廣泛地分佈於東北亞：南起台灣、經中國大陸、韓國，北至日本沿岸。日本鰻的產卵場推測在馬里亞納島西方海域 (15°N,140°E) ，孵化後的柳葉鰻 (Lepocephalus) 順著北赤道洋流往西方向漂送，到了菲律賓東方海域轉向北方進入黑潮流域，接近東北亞國家的大陸棚水域時變態為玻璃鰻 (Glass eel)，到了河口域體表出現黑色素稱之為鰻線 (Elver)。日本鰻分佈水域廣，至今仍然懷疑可能不是單一族群。過去的研究發現日本鰻同功異構酵素(Isozyme)有地理傾斜現象，但粒線體DNA的研究結果卻發現族群遺傳結構沒有明顯的差異，最近以多型性較高的微衛星DNA研究不同地區的日本鰻族群時，卻發現可能有不同的產卵族群存在。 此外，在耳石日周輪的研究發現，每年冬天在台灣地區不同月份所捕獲的鰻線，是來自不同產卵群。為了解這些不同產卵群所生產的子代，其族群遺傳結構是否相同。本研究以微衛星DNA分析 2000年11月~2001年3月起連續四個月於台灣北部淡水河口採集的日本鰻鰻線的族群遺傳結構的時間變化，想驗證上述不同時間到達的鰻線，是否為不同族群。實驗結果發現七個微衛星DNA基因座上的等位基因頻率分布在月份間略有不同，但Overall FST值不顯著(P= 0.428)，顯示月份間族群遺傳結構分化程度並不明顯。因此，每個月份所到達的鰻線，仍屬於同一族群。綜合以上結果，我們認為台灣地區的鰻線，雖然是來自不同的產卵群，但是月份間的遺傳分化程度不明顯，所以不能斷定它們是來自不同的族群。

Japanese eel Anguilla japonica is a catadromous fish, widely distributed in northeastern Asia, south from Taiwan, through mainland China, Korea and north to Japan. The spawning ground of the eel is located in the west of Mariana Islands (15°N, 140°E). The leaf-like larve leptocephali are drifted westward with North Equatorial Current and then turn northward with Kuroshio near Philippines. At arriving continental shelf, they metamorphose to glass eel, and become pigmented elver in the estuaries. Japanese eel is presumed not a single population due to their broad distribution. Previous studies based on Isozymes analyses found a geographic decline in the population of Japanese eel, but the results from mitochondrial DNA analyses found no significant differences among population genetic structure and concluded that Japanese eel is a panmictic population. Moreover, recent study by microsatellite DNA indicates that they may have different spawning populations among different locations but the temporal genetic variation still remained controversial. According to the otolith daily growth increments study, backcalculated hatching date discovered that elvers recruit to Taiwan coast at different winter months were spawned by different stocks. Therefore, present study aims to use polymorphic microsatellite loci in the genomic DNA to examine the temporal variation of population genetic structure of the eel in Taiwan. The Japanese eel elvers were collected at 4 consecutive months between November 2000 and March 2001 from the estuary of Tanshui River in Taiwan. The mean observed heterozygosity (H0) and change in the total number of alleles were compared among months. The genetic differentiation indices indicate that the difference in genetic structure was not significant among months (FST=0.001, P= 0.482). This implies that the elvers recruited to the estuary of north Taiwan in the different months were coming from the same spawning population. The population structure of Japanese eel elvers appearing in northern Taiwan remained temporally constant in the same geographic region among months.