Introduced Species Summary Project
Alewife (Alosa pseudoharengus)

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Common Name: Alewife (Gaspereau, Sawbelly, Spreau, Kyak, Kiack, River herring, Glut herring)

Scientific Name: Alosa pseudoharengus


Phylum or Division: Chordata
Class: Osteichthyes
Order: Clupeiformes
Family: Clupeidae

Identification: Adult alewife are typically 10 to 12 inches in length (25 to 30 cm), with a green back and silvery belly; they have a single black spot located behind the eye. The scales that line up in a row along the belly give it one of its common names, the sawbelly. The blue-back herring (A. aestivalis) is a physically similar species, and it is difficult to distinguish between the two. The only definable difference (alewife tend to have larger eyes, and blueback herrings have more "compressed" bodies, but these are difficult to enumerate unless directly comparing the two species in hand) is the color of the peritoneum in the two species. The abdominal cavity in the blueback herring is much darker, almost black, whereas the alewife has a paler abdominal cavity with some black spots. Misidentification between the two species may cause problems in identifying range and abundance.

Original Distribution: The alewife used to be a purely anadromous species, breeding in freshwater rivers but returning to the ocean to complete their life cycle. They were typically found from Newfoundland to the Carolinas, preferring depths of approximately 150 to 350 ft off the coast, and spawning populations were found among the tributaries at a maximum of about 100 miles inland.

Current Distribution: Although the means of introduction are still debated, the alewife seems to have entered the Great Lakes at about the time of canal building in the late 19th century. Perhaps using the Erie as a mode of transportation, the alewife range increased greatly as they entered the Great Lakes and from there became established in all five lakes; cold temperatures in the winter have been known to kill off large populations periodically, but typically the alewife can survive most winter temperatures even in the northern parts of Lake Superior. These introduced populations have forsaken the second part of the anadromous life cycle, and do not return to the sea as adults. Instead, they spend the entirety of their life in fresh water. There are also a number of separate isolated inland populations in Virginia, Kentucky and Tennessee.

Site and Date of Introduction: Alewife were first detected in Lake Ontario in 1873, Erie in 1931, Huron in 1933, Michigan in 1949, and finally Lake Superior in 1954. The Ontario population should be only considered as the first date on record, not necessarily the introduction date.

Modes of Introduction: There are three main introduction theories for the alewife. It was first recorded in Lake Ontario in 1873, and some believe that it was native to the lake, but spread to the others with the decline of Atlantic salmon and lake trout, two natural predators of the alewife. Others have suggested that it was introduced when Ontario was being stocked with American shad in the 1880s. The third theory contends that the alewife used the newly-built Erie canal as an opened introduction point, connecting the Atlantic with the Great Lakes. All the Southern lake populations were introduced as a result of intentional introduction.

If the alewife had been native to Ontario previous to the opening of the Erie canal, it would have had to have traveled up the St. Lawrence river from the Atlantic earlier in its natural history. Genetic evaluation, however, shows that the Great Lake populations and the Atlantic populations are similar enough that the introduction was a recent event, and that the introduced population probably proceeded through the Erie canal. Their introduction into the lakes would not have been possible if not for the over-fishing of the Atlantic salmon and lake trout, or the alewife's ability to survive living only in freshwater, contrary to its natural anadromous life cycle.

Reasons Why it has Become Established: The alewife is mostly a filter feeder, but has been known to be piscivorous, feeding on fry as large as 50mm. Before their introduction, the Great Lakes ecosystem functioned with Atlantic salmon as the main predator, with no dominant filter feeding species. As the Atlantic salmon populations declined, the alewife would have found a suitable ecosystem with no strong competition for food resources. Introduced sea lamprey populations may have contributed to the decline of native species that could have outcompeted the alewife.

The extreme temperatures of the lakes generally support populations off alewife - although Lake Superior can occasionally get too cold and kill of a populatoin. Average temperatures are also suitable for spawning, between 12 and 22.5 degrees Celsius. It is also important to note that Alewife were once an anadromous species, and its vagility was appropriate to this life cycle. A spawning female lays somewhere 100,000 eggs, as an adapatation to the hazards of moving downstream and into the open ocean. Landlocked populations, though, may not face the same perils, and so their survival rate would increase. Current landlocked populations suggest that, on average, a female lays 17,000-38,000 eggs while breeding, but upon first introduction from the Erie canal, the initial populatoin may have had a much larger reproductive rate, allowing for the establishment of the species.

The alewive's ability to adapt from an anadromous life cycle to a landlocked one was a key factor in their establishment. While anadromous populations prefer slowly moving waters, and lay their eggs on sandy or gravelly bottoms, the landlocked populations show no preference for breeding grounds. Similarly, landlocked alewife were able to move from being exclusive filter feeders to also feeding on copepods and larvae. Alewife are generalists, which pre-supposes them to invasion.

Ecological Role: alewife are important zooplanktivores. They feed extensively on zooplankton, as well as small insect and fish larvae. They have three different feeding methods: gulping, individual particulate feeding, and filtering. Gulping involves opening the mouth wider for larger objects, as opposed to particulate feeding, during which the alewife open their mouth a small ways. When filtering, the alewife leaves its mouth open and captures any zooplankton and other small organisms present in its feeding area. Alewife may be good competitors for this particular niche, based on their success at transforming the zooplankton community in the Great Lakes. They also seem to have out-competed any native zooplanktivore species.

Alewife serve as food for larger organisms, including Atlantic salmon and lake trout. Herons and other pescivorous birds, as well as otter, mink and other aquatic mammals are all alewife predators. In addition, humans have been known to consume A. pseudoharengus. There are no known large species, however, that depend on the alewife for food - its removal from the Great Lakes, in other words, would probably not be particularly detrimental to larger species.

There are also a number of parasites that have been found in alewife, including Acanthocephala, cestodes, trematodes and copepods.

Benefits: Alewives feed on zookplankton so extensively that they increase water clarity in the Great Lakes; this may, most of the time, serve as an attraction to tourists who want "purity" in the lakes, but this can cause large algal blooms from time to time. Alewives also serve as a food source for many predators, including the diminishing Atlantic salmon. As conservationists attempt to re-stock the Great Lakes with the once-native salmon, alewife may become an important resource. Humans also consume alewife, and states along the Eastern seaboard have taken measures to support dwindling populations of anadromous alewife. Maine, in particular, has seen a dramatic decrease in population sizes, and has made efforts to restore historical spawning runs. Fishing licenses for alewife, as well as the potential tourists they invite, can be beneficial for the local economy.

Threats: Alewives have fundamentally altered the Great Lakes ecosystem. Since their invasion, all trophic levels have been effected by their extensive predation of zooplankton. This ecosystem now, in some places, significantly revolves around the alewife. Native zooplanktonivores have been out-competed. Zooplankton are fed upon extensively, clarifying the water but also allowing for algal blooms. Any potential damage from the alewife has already been done, as integration into the ecosystem seems to have come to completion, at least if temperatures in the lake remain stable. Warming in Lake Superior may cause an increase in alewife populations, leading to greater changes in the ecosystem.

The large presence of alewife in an area has been shown to directly affect the biodiversity of that area, even during annual lows in the population size. In some places in the Great Lakes, fish populations have been shown to consist of nearly 75% alewife. Weinstein and Logan show clearly in their paper that a species with that amount of dominance drastically effects local diversity of an ecosystem. In particular, alewife feed on eggs and larvae of other fish species, a major method in which they outcompete other zooplanktivores.

There has been some suggestion that alewife carry the virus VEN, or viral erythrocytic necrosis, however this does not seem to be a particular threat, as VEN has not shown to be fatal. Species that feed on landlock alewife have shown to be thiamin deficient. As alewife dominate local communities, this may be an issue for predators that can find little else to feed on. Alewife have been shown to cause both early mortality syndrome (EMS) and Cayuga syndrome in lake trout and Atlantic salmon populations, and thiamin deficiency may be to blame.

Finally, mass die-offs in alewife populations occur periodically, and this can cause both aeshetic and hygenic problems for humans in the area as hundreds or thousands of alewife decay on the beaches. This, of course, also places economic strains on local economies that must pay to remove the fish, often by bulldozing them.

Control Level Diagnosis: "Minimal Priority" - Alewife have invaded the Great Lakes system, and the ecosystem is fundamentally changed. Alewife are so fundamental part of the ecosystem that removing them now could potentially do unforeseen damage along all trophic levels. As Atlantic salmon are re-introduced into the environment, increased predation may result and alewife populations will decline naturally. Alewife have been present in Lake Ontario for over 150 years; their presence is not only stabilized, but their status as "non-native" may become less clear as time goes on. If our definition of non-native is based in the exploration time period, the introduction of alewife came soon after that. In addition, it is not impossible that alewife could have arrived in the Great Lakes via other routes. Their introduction seems more "natural" as their range could have overlapped with the Great Lakes anyway. In addition, they are not causing major financial problems, and in fact can be beneficial to local economies.

This diagnosis, however, applies only to the Great Lakes and other areas that alewife have invaded. Plenty of freshwater lakes throughout the northeast are susceptible to introduction and subsequent invasion by alewife, and in places that have not suffered invasion yet, natural resource managers must be wary of accidental or inentional introductions by humans. If introduced, alewife can cause large-scale changes in the ecosystem, including the decline of native species, and overall biodiversity.

Control Method: Population reduction is essentially the only possible method of control in the Great Lakes at this point. Increasing the effort to re-introduce Atlantic salmon to the area would certainly help, as well as increasing support for native species that may feed on alewife eggs. Active culling may also be beneficial in reducing local populations, particularly if done during times when alewife are either congregated together for spawning, or in winter when they are more susceptible to cold temperatures.

Containment may also help in terms of legislating the distribution of Alewife. Vermont already has passed laws making transportation of alewife, dead or alive, illegal, but the law goes further to bar people from capturing them. Increased fishing of alewife, as long as their use is monitored, may be beneficial in reducing population size. Ensuring that alewife do not spread beyond their current range should be the focus, at this point, as removing them from the Great Lakes is unlikely.




  • Bronte, C.R., Selgeby, J.H. and Curtis, G.L. 1991. Distribution, abundance and biology of the alewife in U.S. waters of Lake Superior. Journal of Great Lakes Res. 17:304-313.
  • Ihssen, P.E., Martin, G. W., and Rodgers, D. W. 1992. Allozyme variation of Great Lakes alewife, Alosa pseudoharengus: genetic differentiation and affinities of a recent invader. Can. J. Fish. Aquat. Sci. 49:1770-1777.
  • Johansson, O.E., and O’Gorman, R. 1991. Roles of predation, food and temperature in structuring the epilimnetic zooplankton populations in Lake Ontario, 1981-1981. Trans Amer. Fish. Soc. 120:193-208.
  • Smith, S. H., 1970. Species interactions of the alewife in the Great Lakes. Amer. Fish. Soc. 99:754-765.
  • Stone, H.H., and Jessop, B.M. 1992. Seasonal distribution of river herring Alosa pseudoharengus and A. aestivalis off the Atlantic coast of Nova Scotia. U.S. Fish. Bull. 90:376-389.
  • Weinstein, Michael, and Dennis Logan. 1979. Diversity as a Measure of Alewife (Alosa psuedoharengus) Dominance in Southeastern Lake Ontario. Journal of Great Lakes Res. 5(2):139-143.


Photo Credits:

  • Alewife picture from
  • Alewife range adapted from

Author: Tim Bean
Last Edited: 11/18/02

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