Before the advent of the commercial fishing industry, the millions of fish that are now caught in the ocean would return to freshwater to die, and their carcasses provided a rich source of nutrients for the freshwater ecosystem. These nutrients sustained the growth of the microscopic plants and animals on which the young sockeye fed before migrating to the ocean as smolts. Historical abundance of feed organisms was therefore much greater than today, when a century of heavy fishing has reduced freshwater productivity, and many salmon stocks, to a fraction of their previous levels. Lake enrichment replaces some of these nutrients to partially restore or enhance the freshwater productivity of sockeye nursery lakes. After hatching in the gravel of their stream and lakeshore spawning beds in the spring, sockeye salmon fry move into their nursery lakes and feed on plankton for one or two years, until they migrate to the ocean as smolts.
Sockeye salmon are the only salmon with a strict lake rearing requirement.
This freshwater phase of their life cycle is critical because mortalities are high in the lakes and their size as smolts migrating to sea directly affects their survival rate in the ocean. Like any plants, phytoplankton need nutrients and light for growth. Phytoplankton growth rates are limited by the amount of nutrients in the surface waters of the lakes. By making small, regular additions of nutrients throughout the growing season, phytoplankton growth rates are increased and more food is available for the zooplankton. More zooplankton means more food for the young sockeye.