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Adult male and female salmon lay there fertilized eggs in a redd(gravel nest) located in a streambed or along a lakeshore. The female salmon is responsible for digging the redd using undulating(sweeping) movements of her tail, while the male protects the female from intruders(other potential spawners). The embryos(fertilized eggs) hatch and develop into alevins(yolk-sac fry) which live off their nutrient rich yolk-sac that is attached to their underside. The young alevins will emerge as fry from the redd when they absorb their nutrient rich yolk-sac and start actively searching for food. Alevins that emerge from the redd before they completely absorb their yolk-sac are commonly called button-up fry. The small vulnerable fry will start to develop parr marks(vertical bars) on their sides as they start to feed and grow. The parr marks generally last a few months to years depending on the species of salmonid. Resident species of cutthroat trout retain their parr marks throughout life. After a period of growth in fresh water, anadromous species will start their downstream migration to the sea. The silvery smolts will undergo a series of physiological and morphological changes that allow them to acclimate(adjust) to the salt water conditions of the new marine environment. Once in the ocean, the smolts will feed and grow to fully developed adult salmon. Some salmon called jacks/jennies(precocious) will return to their natal stream to spawn(reproduce) earlier than the rest of their age-class. Salmon that have spawned are called kelts. Most salmon spawn only once during their lifetime(semelparous), although some salmonids, mainly steelhead, are iteroparous, or have the ability to spawn more than once.