A Cessna U206F operating on a Part 135 aerial survey flight was reported missing. The aircraft was monitoring deer migration along the Snake River, first by homing in on radio collars while flying at 2,000 feet agl and then spiraling down to 500 feet agl to visually identify the herd. The aircraft took off at about 11:45 and was last heard from at about 12:00. When it did not land as scheduled at 15:00, a search was begun. The search was suspended after six days and the pilot is presumed dead.