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 eeis/m(2) 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.
