Thousands of salmon have begun their lives not in sparkling mountain streams but in plastic trays stacked 16 high in a building. The Merced River Hatchery, near Snelling, has assisted Mother Nature since 1970. It removes eggs from adults that have returned after a few years in the Pacific Ocean, then rears the young until they are ready for their own journey to the sea. Upgrading the hatchery is part of a plan the Merced Irrigation District devised in response to a state proposal to sharply increase releases from Lake McClure.