Common Name: Alewife (Gaspereau, Sawbelly, Spreau, Kyak, Kiack,
River herring, Glut herring)
Scientific Name: Alosa pseudoharengus
or Division: Chordata
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 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.
- Alewife picture from http://www.nmfs.noaa.gov/habitat/habitatprotection/images/alewife.jpg
- Alewife range adapted from