The alewife, Alosa pseudoharengus, known as gaspereau in Atlantic Canada, is a small (about 30 cm long, 400 g) fish in of the shad/river herring genus Alosa that occurs in two forms: anadromous alewife spend most of their life in the western Altantic ocean but migrate into freshwater rivers in order to spawn, and the somewhat smaller landlocked form lives its full life in freshwater lakes. They are opportunistic feeders that eat zooplankton, insect larvae and adults, small fish, fish eggs, and can filter-feed through their gillrakers. The native range of alewife extends along the Atlantic seaboard from Labrador, Canada to North Carolina, USA. In its native range it is a valued food fish, and in some places anadromous populations are in danger of decline from overfishing, dams which block alewife access to spawning zones, and pollution. The US National Marine Fisheries Service classifies alewife as a “species of concern.” However, alewife can also invade land-locked lakes and waterways, and in this form can overpopulate, affecting populations of native fish and causing sanitary problems with large scale die-offs. A well-known example is the invasion of alewifes into the great lakes (especially Huron and Michigan) between 1930-1950, after the completion of the Welland canal.

(CABI 2011; Faria, Weiss and Alexandrino, 2006; Fisheries and Oceans Canada, 2011; NOAA 2009; WIkipedia 2012)