利用性比及耳石元素指紋圖辨識高屏溪野生與養殖日本鰻(Anguilla japonica)之研究

Abstract

台灣河川中的日本鰻(Anguilla japonica)，因過度捕撈及棲地惡化，所以族群量日益減少，而且雌性比例偏高。但是2004年7月2日敏督利(Mindulle)颱風之後，高屏溪的鰻魚族群量卻突然升高，而且性比偏向雄性。推測其原因，可能是養殖池的鰻魚流入所造成的現象。為了瞭解高屏溪鰻魚族群結構變化的原因，本研究嘗試建立野生及養殖鰻之識別方法，並探討性比變化的原因以及養殖鳗進入河川之後的命運。 1998年5月起至2004年12月止，每個月在高屏溪河口進行野生鰻採樣，並且在2004年2月及8月分別在鹿港地區及高屏溪沿岸的養殖池採集養殖鳗。標本採集之後，便進行性別、外部形態(體長、體重、胸鰭長、眼徑及體色)、體長組成、體長-體重關係、肥滿度、以及生殖腺和肝臟指數等之測定，並利用電子微探儀(EPMA)及感應耦合電漿質譜儀(ICPMS)分析耳石的鍶鈣比(Sr/Ca ratio)及微量元素組成，然後利用變異數分析(ANOVA)及判別函數分析法(Discriminant function analysis)分辨野生鰻和養殖鰻，並比較颱風前、後高屏溪的日本鰻之差異。 結果發現野生鰻和養殖鳗的外部形態以及耳石的微細構造、Sr/Ca比和微量元素組成有顯著性的差異。野生鰻的族群密度低，雌性比例偏高；而養殖鰻的族群密度高，雄性比例也偏高，顯示鰻魚的性別分化與族群密度有非常密切的關係。另外，發現2004年冬季高屏溪河口的雄性銀鰻比例偏高，銀化程度與野生雄鰻沒有差異，顯示颱風時流入高屏溪的養殖鳗在天然環境中可以銀化，並準備進一步洄游至產卵場繁衍下一代。這些發現對於未來在河川中放流養殖鳗改善日本鰻的族群結構及其資源的復育將有莫大的參考價值。

Due to over-fishing and habitat degradation, the population of wild Japanese eel (Anguilla japonica) in most Taiwanese rivers declined since 1980, and its sex ratio extremely skewed to female. However, the population density of the eel in Kaoping River was found to be abruptly increased and the sex ratio of the eel skewed to males after Typhoon Mindulle, July 2nd, 2004. This may indicate that the population structure of the eel in the river was affected by the immigration of cultured eel resulting from the typhoon. In order to validate the relationship between the drastic change of the Japanese eel population structure in the Kaoping River and the typhoon, we attempted to establish the methods of discriminating cultured and wild eels as well as to understand the sex-determination mechanism of the eel and the fate of the eel escaped from the eel farm. The wild eels collected monthly in the Kaoping River estuary since May 1998 through December 2004, and cultured eels collected respectively from Lukang and Kaoping eel farms in February and August 2004 were examined. The sexes of the eel were determined, the biological characteristics including somatic length and weight, condition factor (CF), gonadosomatic index (GSI) and hepatosomatic index (HSI) were measured, skin tone, fin index(FI), ocular index(OI), length-weight relationship were analyzed, and otolith microstructure, Sr/Ca ratio and trace elemental composition were measured via EPMA and ICPMS. The data were analyzed with ANOVA and discriminant function analysis to discriminate wild and cultured Japanese eels. It was found that external morphology and the Sr/Ca ratios and trace elemental composition in otolith can effectively separate the wild eel from the culture eel escaped after typhoon in the Kaoping River. The wild eel was lower in density and dominated by female, but the intensively-cultured eel was dominated by male. This indicated that the sex determination of the eel was highly related to their population density. The silver eel captured in the river during winter in 2004 was dominated by males, and their GSI values were similar to wild ones, indicating that the cultured eels escaped from the eel farms during typhoon can silvering in natural environment. These findings suggest that it is possible to restock the Japanese eel by releasing the cultured eel in the river in stead of releasing hormone-induced eel in the sea in the past time in Taiwan.