Until the early 19th century the life cycle of the Atlantic salmon was not understood and juvenile stages were thought to be different species. Females dig a depression in the gravel into which eggs and sperm are released simultaneously. The first juvenile stage (alevins) hatch and remain in the gravel, feeding on yolk sacs that are attached to the body. The next stage (fry) feed on microscopic particles in the stream. When vertical markings appear on the young fish's body the juveniles are known as parr. This stage remains in the river for two to six years before they transform into 'smolt'. A silvery colouration develops and complex internal changes occur to allow survival in salt water. Adult Atlantic salmon spend most of their lives at sea where they roam vast distances in small groups in search of food. At sea their diet consists of squid, shrimp and small fish such as herring or cod. After one or more years the salmon return to their birthplace in order to spawn, and do not eat during this phase of their life cycle. It appears that an olfactory sense (sense of smell) enables the salmon to identify their exact natal location and they are able to leap vertical distances of up to an amazing 12 feet in their endeavour to return there.