With a body the size of a small child and a wingspan of up to two meters, the Blakiston's fish owl (Bubo blakistoni) is the largest owl in the world. It is also one of the rarest, shiest and least studied. But that didn't stop a team of researchers from the Wildlife Conservation Society (WCS), University of Minnesota and the Russian Academy of Sciences from trekking into the Russian Far East in the middle of winter to study these endangered birds. There they found a complex system of large owls, gigantic trees and commercially valuable salmon, all of which in many ways depend on one another. Their research, collected under the collaborative banner of the Blakiston's Fish Owl Project, was published online this month in the journal Oryx.

The researchers studied how Blakiston's fish owls nested and foraged over more than 20,000 square kilometers in Primorye, Russia, along the country's eastern coast and the Sea of Japan. Over the course of a year, they found that the owls relied on large, old-growth trees, mostly in riparian areas that bordered rivers and streams. Those old trees were the only ones big enough to house the birds, which use cavities inside the trunks to breed and nest. Many such forests in Russia are threatened by logging, but the researchers say the presence of the owls was an indication that the area they studied remains healthy.

The trees also have other roles: The living forest provides critical habitat for Amur tigers, Asiatic black bears and wild boars, among other endangered mammals. And when the trees die, they fall into and over rivers, leaving large woody debris that helps make the rivers more habitable for eight different species of salmon and trout, creating both fast-moving channels and slow-moving deep pools that support the fish at various stages in their life cycles. These fish, meanwhile, provide the main source of sustenance for the owls. (Cue "the circle of life" right about now.)

In a press release lead author Jonathan Slaght, of the WCS, said the presence of Blakiston's fish owls "is a clear indicator of the health of the forests, rivers and salmon populations." Slaght and his co-authors argue for greater protection for old-growth forests—specifically the riverbank areas in riparian zones—as they will help ensure the survival of the owls and the many other species that are found inside. This is especially important in the Primorye region, which used to be isolated but has now been crisscrossed with logging roads. They say protecting the forests could help to preserve salmon supplies, which in turn would support the economy.

Even if the protections are put in place, Blakiston's fish owls will continue to face other threats—such as illegal hunting and accidental deaths in traps meant for small mammals—across their range, which also includes North Korea, China and Japan. The Japanese population is probably the most endangered, with just 35 breeding pairs and a history of inbreeding, according to a paper published in 2009 in the Journal of Raptor Research. The IUCN Red List of Threatened Species estimates the Primorye population at 250 to 400 birds and the total population across the species' range at between 1,000 and 2,500 mature individuals.

Photo: Hiyashi Haka, used under Creative Commons license