Sockeye are an anadromous species of salmon. They are the third most abundant species of the Pacific salmon. The fish are native to the western United States and are found in the Northern Pacific Ocean and rivers that flow into it. Sockeye begin in fresh water rivers and lakes and spend as many as four years there before migrating to the ocean. The salmon may spend one to four years maturing in the ocean feeding primarily on zooplankton and shrimp. They return to spawn in summer. The spawning salmon take on a bright red appearance with a green head. During the other phases of their life, including while they are in the ocean, they are silver with a bluish back, which gives rise to them being called "bluebacks". Sockeye spawn only once and then die. The landlocked version of the Sockeye is called a Kokanee. Some Sockeye populations are currently listed under the U.S. Endangered Species Act.