台灣北部地區日本鰻鰻線族群遺傳結構之長期變化

Abstract

日本鰻，降海洄游性魚類 (Catadromous fish)，廣泛分部在西太平洋地區，南起台灣、經中國、韓國、再到日本。日本鰻的產卵場推測是在馬里亞納島西方海域(13-15°N,142-143°E之間)。日本鰻每年4-11月產卵，孵化的柳葉鰻 (Lepocehalus) 順著北赤道洋流 (North Equatorial Current) 先向西到達呂宋島東方，後再順著黑潮北上，當其接近東亞大陸大陸棚時，柳葉鰻會變態成玻璃鰻 (Glass eel)，方便脫離潮流並進入河口，到了河口域後體表會出現黑色素沉積，俗稱鰻線 (Elver)。隨後經4-10年成長，渡過黃鰻 (Yellow eel)、銀鰻 (Silver eel)時期，生殖腺開始成熟，再回到深海產卵，產卵後死亡。 日本鰻 (Anguilla japonica) 為台灣重要經濟魚種之一，但其資源量隨著養鰻產業的蓬勃發展，自1970年代後即快速的下降。過去的研究Tseng et al. (2003) 利用微衛星DNA分析1997-1999的日本鰻族群遺傳結構，發現日本鰻年間分化不顯著，但異型結合子觀測值 (Ho) 似乎有逐年下降的趨勢。本研究以微衛星DNA分析取自台灣北部連續從1986年到2007年之間的日本鰻樣本，想驗證日本鰻的資源量大量縮減是否影響了其族群遺傳結構的組成與變異性；不同年間到達台灣的鰻線彼此間有何關係，是否以其生活史做週期性的變化；同時也可驗證日本鰻的異型結合子觀測值(Ho)是否有逐年下降的趨勢。

本實驗共分析1150尾鰻線標本，進行日本鰻族群遺傳結構的解析，結果發現日本鰻年與年間族群之總分化指數雖不高 (Fst=0.004)，但卻有顯著差異性(p < 0.001)。在Phylogenetic tree 結果發現整體Bootstrap值皆偏低，整各族群呈現低度分化，且之間相互沒有關聯性。在時間隔離Isolation By Time (IBT) 的測試中p =0.249並不顯著。近二十年來異型結合子觀測值(Ho)與總基因型總數皆無明顯下降的趨勢，在區間內呈現波動。根據本研究結果得知日本鰻族群遺傳結構在近20年並無明顯變遷，族群多樣性程度亦無降低。

The 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 (13-15°N, 142-143°E). The leaf-like larvae leptocephali are drifted westward with North Equatorial Current and then turn northward with Kuroshio Current near Philippines. At arriving continental shelf, they metamorphose to glass eels, and become pigmented elvers in the estuaries. The elvers spend 4-10 years growing through yellow eel stage and silver eel stage, after that time they go seawater spawning migration, lay eggs in the deep sea and die.

Anguilla japonina is an important economical fish in Taiwan, its resource however fell down after 1970s. Tseng et al. (2003) used polymorphic microsatellite loci in the genomic DNA to examine the variation of population genetic structure of the eel in north Taiwan during 1997-1999. The results showed no genetic differentiation among 1997-1999, but the mean observed heterozygosity (Ho) declined among those years. In the present study we use 6 polymorphic microsatellite loci to analyze long term population genetic structure of Japanese eel among 1986-2007. We test first：If the eel resource declined, would their genetic structure and variation be affected? Second：What is the correlations among year cohorts? Do their genetic structures follow their life cycle? Third：Dose the mean observed heterozygosity (Ho) decline in the latest twenty years?

In this study we used a total of 1150 Anguilla japonica elvers for analysis of population genetic structure. The results showed very low genetic differentiation indices among years but with significant difference (Fst=0.004, p<0.001). In phylogenetic tree result, all bootstrap values were low, meaning low population differentiation among samples. The isolation by time (IBT) test showed no significance (p=0.249). The mean observed heterozygosity (Ho) and total number of alleles do not dramatically decline in the latest twenty years. In conclusion, the population genetic structure of the Japanese eel has no significant changes in the latest twenty years, and its genetic diversity keeps stable.