As early as 1921, the Ecological Society of America began opposing introduction of nonnative species in national parks, and in the late 1920s and 1930s, biologists both inside and outside the agency recognized the threat that introduced species posed for native fish. Still, many parks continued hatchery programs and nonnative fish planting well into the 1970s. Fish stocking in Devils Postpile ended in 1971. Even though fish are no longer stocked within the monument boundary, trout are stocked upstream on Forest Service land. In addition to the stocked fish, there is a successful breeding population which continues to attract fishermen to the valley.