Using sex ratio and otolith elemental fingerprints to discriminate wild and cultured Japanese eel(Anguilla japonica) in Kaoping River of southern Taiwan

• Main Authors: Yu-wei Chu, 曲有為
• Other Authors: 曾萬年
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/08909672710830045671

碩士 === 國立臺灣大學 === 漁業科學研究所 === 93 === 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.