The alewife (Alosa pseudoharengus) is an anadromous species of herring found in North America. It is one of the "typical" North American shads, attributed to the subgenus Pomolobus of the genus Alosa. As an adult it is a marine species found in the northern West Atlantic Ocean, moving into estuaries before swimming upstream to breed in fresh water habitats, but some populations live entirely in fresh water. It is best known for its invasion of the Great Lakes by using the Welland Canal to bypass Niagara Falls. Here its population surged, peaking between the 1950s and 1980s to the detriment of many native species of fish, and Pacific salmon were introduced to control them. As a marine fish, the alewife is a US National Marine Fisheries Service "Species of Concern".