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Alewife (Alosa pseudoharengus)
Photo credit: Duane Raver, U.S. Fish and Wildlife Service,


(Alosa pseudoharengus)

Report this Species!

If you believe you have found this species in its invasive range in Pennsylvania, please report your findings to iMapInvasives by submitting an observation record.

Species at a Glance

The alewife is a small forage fish that belongs to the herring family. It migrates long distances from the ocean to fresh water to reproduce in the spring and returns to the ocean in the fall; however, some populations have become landlocked and are restricted to inland waters.


Alewife size can range from up to 38 cm (15 in) in coastal populations to less than 25 cm (10 in) in inland populations. Their typically silver body is small, slender, and laterally compressed, but individuals entering freshwater often have a copper-sheen color. Eyes are relatively large, with an obvious eyelid. A single black spot is present on each side just behind the head. A row of scales, called scutes, which form a sharp edge along the mid-line of the belly, are responsible for its nickname “sawback”.

Similar Species

The alewife is closely related to the blueback herring (Alosa aestivalis), which is native in parts of the lower Susquehanna River. Alewives are lighter in color than bluebacks, and their body is more strongly compressed and less elongated. The most distinguishing characteristic between these species is the color of their peritoneum, or the lining of the abdominal cavity. An alewife’s peritoneum is pale with dusky spots, while the blueback herring’s is black to dusky in color.


This pelagic species is found in marine waters or open lake waters, except during the breeding season, when they can be found in large rivers, small streams, ponds, and large lakes over a wide range of substrates, including gravel, sand, detritus, and submerged vegetation. Because alewives are sensitive to the osmotic stresses of freshwater, disturbances such as severe changes in water temperature can cause the fish to die, with large numbers washing up on beaches.


The Welland Canal most likely gave the alewife access to the Great Lakes and it has continued to spread through waterway connections. Because it looks very similar to native members of the herring family, accidental transfer is possible through fish stockings or release of live bait. It has also been intentionally introduced by some state agencies into inland lakes to increase the forage base for popular sport fish.


Native to the Atlantic coast and its tributaries from South Carolina northward, alewives are found from Nova Scotia to the Carolinas, with landlocked populations in the Great Lakes and in lakes and ponds along the East Coast. In Pennsylvania, introduced populations of alewives can be found in Lake Erie, as well as many inland lakes and reservoirs across the state–in addition to its native range in the Delaware and Susquehanna rivers.


Note: Distribution data for this species may have changed since the publication of Pennsylvania's Field Guide to Aquatic Invasive Species (Second Edition 2015), the source of information for this description.

*To learn more about the native/invasive range of alewife in Pennsylvania, check out the information available for this species on our Invasive Here but Not There page.

Environmental Impacts

Alewives feed primarily on zooplankton, which puts them in direct competition with native fish and invertebrates for limited food resources. Large densities of alewife in Lake Erie have been blamed for the disappearance of native salmonid species.


Information for this species profile comes from the Pennsylvania's Field Guide to Aquatic Invasive Species (Second Edition 2015).

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