The Alewife is an exotic invasive fish that proliferated in Lake Huron and is now a key prey fish for salmon and walleye.

The alewife (Alosa pseudoharengus) is an ocean fish native to the Atlantic that can survive in fresh water. While they may have made their way up to Lake Ontario on their own, they moved through the Welland Canal, appearing in lake Erie in 1931, in Lake Huron in 1933, in Lake Michigan in 1949, and finally in Lake Superior in 1954. They reached their peak abundance by the 1950s and 1960s, growing in number unchecked because their top predator, the lake Trout, was almost wiped out by another exotic invader, the Sea Lamprey. In those times, some parts of the Great Lakes had fish populations that were 75% Alewives. Alewives live 6 or 7 years and breed at 2 years, spawning 10,000 to 12,000 eggs per female in streams feeding into the Lakes. Alewives feed on zooplankton and small crustaceans, impacting the Lake Herring and Whitefish that prey on similar food sources.

To reduce the Alewife population Coho and later Chinook Salmon were introduced into the Great Lakes to act as predators. This caused the development of a salmon/alewife fish community, popular with many sport anglers. Alewives occasionally have mass die-offs in the Great lakes where the piles of dead Alewives on the shore create a smelly nuisance so large that they have to be removed by bulldozers. In the 21st century, the alewife population has declined in the Great Lakes as the predator species such as Lake Trout and Walleye have increased to the point that there is concern about the Salmon fishery. Another issue regarding Alewives and Salmonids (trout and salmon) is that Alewives contain an enzyme called thiaminase that breaks down thiamine (vitamin B1) causing embryonic mortality in the salmonid fry. Scientists are monitoring the Lake Huron fishery to determine what the long-term sustainability of fish species population mix will be over time, and what adjustments, if any are needed.