|
Vol. 45 No. 2, 2006
Use of the Sex Ratio as a Means of Resource Assessment for the Japanese Eel Anguilla japonica: A Case Study in the Kaoping River, Taiwan
Yu-San Han1 and Wann-Nian Tzeng2,*
1Division of Biotechnology and Pharmaceutical Research, National Health Research Institutes, Taipei, Taiwan 115
2Institute of Fisheries Science, College of Life Science, National Taiwan University, Taipei, Taiwan 106
Yu-San Han and Wann-Nian Tzeng (2006) A
negative relationship between the proportion of females and the
population density was found when compiling historical data of both
wild and cultured Japanese, European, and American eels. Based on the
relationship, the population status of the Japanese eel Anguilla
japonica was assessed. Japanese eel samples were collected from the
lower reaches of the Kaoping River in southwestern Taiwan from 1998 to
2002, and the population density was estimated by mark-recapture
experiments in 2001 and 2002. The sex ratio of the eel was skewed
towards females, accounting for 81.3%-88.3% in the total samples of
yellow and silver eels and for 63.6%-81% in the silver stage eels. The
significant female-skewness of the sex ratio validated that the
population of Japanese eels in the Kaoping River has declined to a very
low level in recent years. The population density of the Japanese eel
in the lower reaches of the Kaoping River was approximately 0.01 eels/m2
in 2001 and 2002. The decline in the Japanese eel population in the
Kaoping River has probably resulted from both overfishing of glass eels
in the estuary for aquaculture needs which severely influences
recruitment, and degradation of the growth habitat of the yellow eel
along the river. The sex ratio therefore is a fast and reliable
indicator for eel resource assessment.
Key words: Japanese eel (Anguilla japonica), Resource assessment, Sex ratio, Mark-recapture, Population density.
*Correspondence: Tel and Fax: 886-2-23639570. E-mail: wnt@ccms.ntu.edu.tw
|
