The alewife is native to the Atlantic Coast and entered the Great Lakes through the Welland Canal. They first arrived in Lake Michigan by 1949 and in Lake Superior in 1954. After sea lampreys had eliminated most of Lake Michigan's large predator fish, the population of alewives exploded throughout Lake Michigan. During the early summer, these small fish spawn in harbors and nearshore waters, disappearing by late fall to feed off the bottom in the central depths of the lake. They migrate shoreward again in mid-March and April, completing the yearly cycle.
Alewives swim in dense schools and have been the major prey of the Lake Michigan's trout and salmon.
Tremendous numbers of these small, silvery ocean fish never developed in Lake Superior like they did in Lakes Michigan and Huron. Perhaps Superior's waters are too cold, or perhaps enough predator fish survived the sea lamprey invasion there to keep the alewife population in check. Because they are saltwater fish, alewives are not well adapted to life in fresh water. In fresh water, the salt concentration in a fish's body is higher than in the surrounding water. This causes water to leak into the cells of the fish.