(Seattle, WA) -- The Southern Resident orca population is at a 30-year low. The Seattle PI reports there are just 75 killer whales left in the Puget Sound's three Southern Resident pods. Annual census numbers also show that two killer whales are missing and are currently presumed dead. Scientists say the population decrease is due to pollution and a dwindling primary food supply. The whales subsist mainly on Chinook salmon.