Yes, the sex of eels is not simply male and female, but is controlled by environmental factors and density.

When the population is large and food is insufficient, it will become a male fish. On the contrary, the proportion of female fish will increase, the color of male fish will be brighter and the color of female fish will be darker. The dorsal fin of male fish is relatively large, and the posterior ends of dorsal fin and anal fin are relatively sharp, while the dorsal fin of female fish is relatively small, and the posterior ends of dorsal fin and anal fin are relatively round and blunt. The abdomen of male fish is not full, while that of female fish is full. The base of filiform pectoral fin of male fish is obviously orange-red, while that of female fish is not.