By the time Sawyer took over as head keeper on the SDFFA waters, natural regeneration of wild trout had dropped markedly due to the decline of the water meadows, increased pollution and large amounts of silt running into the river from surrounding farm land and the Army's tank manoeuvres on Salisbury Plain. By the early 1930s the fishery could no longer sustain a wild trout population large enough to meet the demands of fishermen. Frank Sawyer determined through detailed observation of the river that the mortality of eggs was virtually 100%. However, those few fry that did hatch had a reasonable chance of survival. Frank Sawyer introduced a large scale programme to stock the river with trout fry. Mature wild trout were caught from the Nine Mile River and other tributaries, stripped of their eggs and milt, and then returned to the river. The fertilised eggs were then reared in a purpose-built hatchery designed by Sawyer before being released into the river. Some 100,000 fry were stocked in the river each year from 1930 to 1953 and resulted in around 2,000 large fish being caught each year.