Shortly after hatching in freshwater streams, lamprey larvae or ammocoetes drift downstream into areas of low velocity and fine substrates where they burrow, and live as filter feeders for up to 7 years. Metamorphosis to macropthalmia (juvenile phase) occurs gradually over several months from July to November before their migration to the ocean in the winter and early spring. During this transformation they develop eyes and teeth. As adults in the ocean, Pacific Lamprey are parasitic and feed on the body fluids and blood of marine fishes. After spending 1 to 3 years in the marine environment, Pacific Lamprey stop feeding and migrate back to freshwater between February and June. They overwinter in fresh water until they spawn the following year between March and July. After spawning, lamprey die within days.