Potential Contributions by Escaped Cultured Eels to the Wild Population of Japanese Eel Anguilla japonica in the Kao-Ping River

Abstract

Due to Typhoon Mindulle a lots of cultured Japanese eel Anguilla japonica escaped from aquaculture ponds to the Kao-Ping River in July, 2004. To evaluate whether the escaped cultured eels contributed to the wild silver eel population, the compositions in age, sex ratio, and migratory histories indicated by otolith Sr:Ca ratios of the silver eels in the river were examined from during 1999 to 2006. Mean ages were younger in 2004 to 2006 than in 1999 to 2003 and the sex ratio of the eels had significantly changed from female-dominated before 2004 to male-dominated in 2004 and 2005 (Test of homogeneity, χ(superscript 1 subscript 2) = 69.5 and 103.5, all p ＜ 0.001). However, after 2006 the sex ratio reverted to their previous conditions (Test of homogeneity, χ(superscript 5 subscript 2) = 8.22, p ＞ 0.1). The dominant migratory types of the silver eels also significantly changed from brackish-water type in 1999 ~ 2003 to freshwater type in 2004 ~ 2006 (Goodness of fit, χ(superscript 1 subscript 2) = 252, p ＜ 0.001). This indicated that the effects of escaped eels on the wild eel population were of not more than two years duration. Some escaped eels have silvered and left the river to potentially contribute to the wild spawning stock within two years.