World's largest owls depend of health of Russian forests for survival Blakiston's fish owl. Credit: Jonathan C. Slaght, WCS Russia. VLADIVOSTOK, Russia, Aug. 15 (UPI) -- A study of one of the world's rarest owls found it to be a key indicator of the health of some of the last great forests of Russia's Far East, scientists say. Blakiston's fish owl, the world's largest owl with a 6-foot wingspan, relies on old-growth forests along streams for both breeding and to support healthy populations of their favorite prey, salmon, they said. Researchers from the Wildlife Conservation Society and the University of Minnesota said large trees provide breeding cavities for the enormous bird, and when these dead, massive trees fall into adjacent streams they create water flow patterns providing important microhabitats critical to salmon in different developmental stages. Large old trees and old-growth forest were the primary distinguishing characteristics of both nesting and foraging sites of the endangered owls, they said, and management and conservation of old-growth forests is essential for sustaining the species. "Blakiston's fish owl is a clear indicator of the health of the forests, rivers, and salmon populations," lead author Jonathan Slaght of the Wildlife Conservation Society said. Retention of habitat for fish owls will also maintain habitat for many other species associated with riparian old-growth forests in the Russian Far East. The owl, listed as Endangered by the International Union for the Conservation of Nature, is restricted to forest areas in Russia, China, Japan and possibly North Korea, the researchers said.