Alewife (Alosa pseudoharengus)

Description: They were first introduced to the rest of the Great Lakes through the Welland Canal in Ontario and were found in Lake Erie in 1931. Populations are currently established in all of the Great Lakes.

Identification: Silver with blue to blue-green shine on back; white below; small dark spot on the shoulder. Mouth terminal extends to below the front of the eye. Upper jaw bones are wide and rounded, lower jaw is protruding. Adipose eyelids are present. Ridge of sharp, sawtoothed scales on the belly. No long filament at the end of the dorsal fin. Body length up to 15 inches, usually less than 10 inches in landlocked populations.

Habitat: Native to Lake Ontario. Open water over all bottom types and at all depths. Usually found in deep waters except in the summer.

Reproduction: Alewives spawn in summer at night in shallows over sand or gravel, forming small whirling groups and releasing tens of thousands of eggs. Hatching occurs in three to six days. They do not provide any parental care and return to deep waters for the winter after spawning.

Impact and Damage: Alewives often die in large numbers and wash up on the shore of the Great Lakes, their rotting carcasses littering beaches. Alewives have also been known to decrease native fish populations such as lake trout by eating their eggs and competing for food. They can also create issues for lake trout that feed on alewives due to a chemical in the alewife tissues that creates a vitamin deficiency in newly hatched lake trout.

Similar species: Blueback shad (Alosa aestivalis), which can be distinguished by its abdominal cavity. Blueback shad's are much darker and almost black.

Monitoring and rapid response: The state of Michigan introduced Coho and Chinook salmon to the Great Lakes in the 1960s to control alewife populations. This has been proved to be a fairly successful method of alewife control.

Credits: The information contained in this factsheet was provided by the Shedd Aquarium and the Global Invasive Species Database.

Photos (T-B) courtesy of The New Jersey Department of Environmental Protection, Luc Gagnon, and the Washington Department of Fish and Wildlife.

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