The Sea of Okhotsk is semi-enclosed and supports large populations of fish and shellfish, together with high concentrations of larger marine creatures, including the critically endangered western population of Pacific gray whales. It is an important bird and fish migration route, especially for salmon heading to and from their spawning grounds in the rivers of eastern Russia. Overfishing and illegal fishing are widespread, and the region is being rapidly developed for oil production, all of which threatens marine life.