River-Lab 5 Guide Manual
ALEWIFE (Pomolobus pseudoharengus)
Also known as “bonies”related to American shad
Alewives are medium-size,
slender fish that spend most of their
adult lives in coastal salt water and
come up into rivers to spawn. They
contribute to the health and productivity
of both the basin system and coastal
water by providing food for larger
organisms and by eating plankton.
The alewife’s contribution in fresh water is mainly food, in the form of their very
plentiful eggs and young and some adults. Alewives are eaten as adults more in salt
water than during their briefer time upriver. They are an important food for the saltwater
striped bass. They are also eaten by people, as a dish called pickled herring.
Both young and adult alewives consume plankton, the microscopic organisms that
make a rich “soup” of sunlit areas of rivers, estuaries, and coastal water. All systems
benefit from consumption of the highly reproductive plankton communities. If
unconsumed and left to decay, these organisms could use up the oxygen supply in the
water. Consumers that convert plankton to larger, efficient-sized meals provide a double
benefit to the basin system of life.
Alewives are bluish along their upper backs, with silvery sides and undersides.
Their scales are large, coin-shaped, and iridescent. They have slightly protruding lower
jaws and sieve-like gill “rakers”ideal for gathering and straining the plankton they eat.
Full grown (10 – 14 inch) females are slightly larger than males.
Alewives exhibit an extraordinary adaptation in their ability to migrate from their
normal saltwater habitats to spawn in the freshwater streams of their birth. This
anadromous migration, from salt to fresh water, is a truly amazing feat, requiring
adjustments to changes in the water’s salinity, temperature, and other factors. Alewives
also change color, matching their skin pigment to the bottom of the streambed.
Alewives (and shad) that survive their spawning run will repeat it for several
years. This pattern differs from another well-known anadromous fish, the salmon, which
dies after spawning. Mature alewives (age 3 and over) make their spawning runs upriver
in “schools” of their own age group. The oldest group (6–8 years) runs up first, at the
end of April, signaled by the warmer, brackish water flowing into the sea. The youngest
fish run in June.
© 1999 Mill River Wetland Committee, Inc.
5GM - 26
River-Lab 5 Guide Manual
ALEWIVES, cont’d.
Only the most fit will survive their long and difficult journey upstream. The
alewives do not eat on this run, and the hazards they face are many. They must swim
against the current, which is often swollen by spring rains. Driven by instinct to reach the
area of their birth, they may beat themselves (even to death) against rocks, dams, or other
obstacles on their way. Freshwater fungi attach to the bruised areas of their skin. In their
stressed condition, alewives are also more vulnerable to other forms of disease or
infection.
On their trip upstream, alewives stop periodically to roil (circle) in small groups
in shallow, sunlit areas. Their intense roiling may help the alewives adapt to changes in
the water, or it may be a “mating dance,” or both. Churning and thrashing, some fish
break the water’s surface, flashing their silver underbellies in the sun. The run is usually
accompanied by hungry fishing birds, such as gulls and night herons, which find the
totally preoccupied and unwary alewives easy prey. Hovering herons grab the larger fish,
while the gulls tear open the alewive’s bellies in search of roe (eggs). Freshwater roeeating fish, such as suckers, also follow the run upstream, ready for the moment when the
alewives spawn.
Their homing instinct leads the alewives to the places where they were born.
Spawning usually takes place in a quiet, pond-like backwater corner of a river. The
female averages 10,000 to 100,000 heavy, sticky eggs, which drop to the bottom.
Suckers rush in with their siphon-like mouths to slurp up as much of the spawn as they
can.
The exhausted and starved alewife parents eat freshwater plankton, such as algae
and seed shrimp, in preparation for their return trip to the coast. They have an easier trip
downstream and spend time recovering in the estuary, where they feed on shrimp and
small fish, including elvers (young eels). Any white patches of fungus on their skin will
disappear within two weeks in the salt water. After their recovery, they resume their
normal diet of saltwater zooplankton.
The young fish fry that have hatched upstream are busy consuming freshwater
plankton and dodging predators such as bigger fish and turtles. Their small size (2–4
inches) suits kingfishers, who cannot handle the adult alewives. Surviving fry stay in the
river until fall, when they migrate to the salt water at the coast. They remain in the
coastal waters until they mature in about three years. This remarkable fish then continues
its amazing life cycle by embarking on its first spring run back up into the river of its
birth. The large number of companion alewives in its school will ensure that spawning
takes place in that stream and that a new generation of alewives will provide the benefits
of their species to that basin.
5GM – 27
© 1999 Mill River Wetland Committee, Inc.