When do the yellow eels turn black and silver?

Yellow eels, which are the juvenile stage of the European eel (Anguilla anguilla) and American eel (Anguilla rostrata), undergo a transformation to black and silver as they prepare to migrate to their spawning grounds. This metamorphosis typically occurs when they reach sexual maturity, usually around 6 to 20 years of age, depending on environmental conditions. During this phase, they become known as silver eels, characterized by their darker coloration and changes in body shape and behavior, indicating they are ready to undertake the long journey to the Sargasso Sea to spawn.