Precocious Silvering of Farmed Eels with Special Reference to Their Evidence for Genotypic Sex Determination

In the present investigation, developmental stages of gonads from the hatchery specimens of European yellow and silver eels (Anguilla anguilla L.) were examined. Based on histological observations of gonads, the total collected specimens of farmed eel was classified into four groups i.e., undifferentiated sex, intersex, males and females, respectively. The obtained results revealed that the total catch specimens of yellow eels (N=36)contain 44.44% males, 41.67% females,2.78% undifferentiated sex, and 11.11% intersex; whereas, silver eels (N=36) includes 83.33% male, 0.0% females, 11.11% intersex and 5.56% undifferentiated sex in decreasing order, respectively. The abundance of males in hatcheries and greenhouses had concluded that Environmental Sex Determination (ESD) mechanism was found to have predominateon Genotypic Sex Determination (GSD) at future stages. The skewed sex ratio's of silver eel in hatcheries and greenhouses can possibly be explained by differences in growth and silvering strategies between the sexes, which result in an earlier culling of the females by the farmer. Therefore, while studying the sex differentiation mechanism of eel fishes, only yellow eel population was seems to be considered. Thence, based on the observation that the ratio of males vs. females in the yellow eel group was 50:50, which revealed that Genotypic Sex Determination (GSD) is prevailing at this stage.