For reproduction, sea lampreys swim upstream and spawn eggs on substrate rocks and gravel at the bottom of freshwater lakes and rivers. Reproduction normally occurs from April through June. These fish usually swim to a tributary river of the large freshwater lake or ocean environment where they lived during their free-swimming stage.
Sea lampreys migrate upstream in large groups. While spawning, sea lamprey groups gather together and form into a large ball. Adult sea lampreys die once they lay eggs.
The sedimentary stage of a sea lamprey's life cycle lasts approximately three to six years before the sea lamprey fully matures. Incubation is the beginning of this stage. The incubation period ranges from seven to 17 days, depending on the water temperature. Colder temperatures lengthen the incubation period.
During this period, sea lamprey larvae burrow into the floors of slow-moving streams and consume vegetation as the primary part of their diets.Sea lampreys must live in freshwater rivers while they are young, but they can live in saltwater once they are adults.
After their sedimentary stage, fully mature sea lampreys emerge from their burrows and swim to the nearest freshwater or marine body of water. Adult sea lamprey have eel-like bodies and grow to 20 inches long. This migration normally occurs from September through May. During the downstream migration, young adult sea lampreys swim in groups to their destination. If low flows or dams slow down the migration of sea lampreys, these fish will latch onto freshwater fish as nourishment. Sea lampreys do not usually lethally prey on freshwater fish during their downstream migration.
Sea lampreys spend 12 to 20 months of their adult lives in the free-swimming stage in a large freshwater body or saltwater environment. During the free-swimming stage, sea lampreys prey on fish. Sea lampreys latch onto fish with their suction-cup-shaped mouth, creating a hole on side of the fish with their tongues. These parasites feed on the fish's blood and tissue, causing round wounds on the fish's side. Sea lampreys' saliva contains an anti-coagulant, which keeps the wounds open for up to several weeks.