This photograph shows a group of newly hatched young trout, called alevins, each about 16 millimeters in length. Once hatched, the alevins remain within the river gravel and attached to their yolk sacs which provide them with food, via a rich network of blood vessels, for the first three to four weeks of life. Once the contents of the yolk sac have been completely absorbed and the young fish have developed fins and other adult features, they are know as fry and swim up from the gravel to begin catching small live prey.