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Alewife – Invasive Species – Part 61

Invasive Species in USA Waterways - Part 61


Alosa pseudoharengus

The Alewife is an invasive species of herring found in North America. A typical North American Shad species. The native range is the Atlantic Coast from Red Bay to South Carolina. This is a fish that migrates up the river from the sea to spawn. As an adult this fish can be found in the North West Atlantic Ocean, moving into estuaries before swimming upstream to breed in freshwater habitats. However, some populations of the Alewife are entirely freshwater. Other names include Kiack, Kyack, Gaspereau, Gasparot, LY, Sawbelly or Mooneye.

Invasive Species Definition

The definition of an invasive species is any species that is not native to our ecosystems and cause harm when introduced to the ecosystems. Furthermore, these may include amphibians, plants, insects, fish, fungus, bacteria and more. Impacts on the environment may cause economic loss or affect human health.

While, Invasive species tend to grow and reproduce quickly and spread aggressively with the potential to cause harm on the environment, economy or even human health. Therefore, given the label “invasive”.

Description of the Alewife


Has a Deep-bodied Body Shape, which is laterally compressed. The average length is ten inches (25cm) with a maximum length of sixteen inches (40cm). While, inland populations seldom reach 25cm.

Has a bluish grey to green back with silvery sides with horizontal darker stripes. Feature a dark humeral spot on the shoulder which is normally followed with a series of smaller spots behind it. Most visible with larger fish.

A lower jaw rises steeply within the mouth, presenting a series of minute teeth in the front of the jaw, which disappear with age. While, the lower gill rakers increase with age. The dorsal fin is composed of 17 – 21 soft rays, the anal fin has 20 -23 soft rays. The pectoral fin has 14 – 17 soft rays, while there are 29 – 31 ventral scutes.

Biology of the Alewife

Generally, this fish species lives up to four years of age. However, the maximum reported age is nine years old.

While, some Alewives are predominately sea fish, migrating to freshwater to spawn, others are exclusively freshwater fish. Meanwhile, the Alewife prefers slow-moving shallow sections of rivers or streams or lakes.

At the time that the adult Alewives migrate upstream in spring, is normally when they are scooped out of shallow, constricted areas with dip nets. Being the preferred bait for the spring lobster fisheries. There habitats may be affected by Didymo growths or Killer Algae blooms.


The Alewife adult migrates up stream in October to spawn in lakes, rivers and streams. Spawning is from October to April with predominate spawning activity occurring in January and February. Spawning is in offshore marine waters. Extremely fertile with the number of eggs ranging from 22 000 to 122 000 per female.

Eggs are attached to various aquatic vegetation including the Common Water Hyacinth, Eurasian Water Milfoil, Curly-Leaf Pondweed and Common Reed, which the fry then use to hide in. The fry descends in summer and autumn as late as November or December. Spawning is either diurnal or nocturnal. Meanwhile, the greatest activity is at night. With spawning activity ceasing with temperatures above 28°C.

Distribution of the Alewife

This fish is an estuarine species capable of surviving in marine, freshwater or brackish water. Occurring in open water over all types of substrate. Schooling adults migrate to spawn. Usually at depths of 56m – 110m.

Effect on Ecosystem

A lake, river or reservoirs food web can be restructured with the presence of the Alewife. Thereby, leaving less food for native species. Specifically, seen by the disappearance of the native Whitefish, Lake Herring and Chub Species from the Great Lakes. The Bloater from Lake Michigan evolved and shifted habitat and diet as a result of competition from the Alewife.

While, these fish contain high levels of thiaminase, reducing absorption and assimilation of thiamin in predators. Thereby, reducing body condition, swim performance and other potential impacts. Further, resulting in periodic large-scale die-offs littering the beaches with rotting fish which can pose a health hazard.

Introduction and Spread in the USA

Alewives colonized the Great Lakes and became most abundant in Lake Huron and Lake Michigan. Reaching the peak abundance from 1950s through to the 1980s. Lake of top predators in the lakes left Alewives populations unchecked. Overfishing and the invasion of the Sea Lamprey caused the wipe out of the lake trout, the alewives main predator.

While, seasonal die-offs of the alewives on shorelines of the Great Lakes called for the attempted introduction of the Pacific Salmon, which led to popular fishery with many sport anglers. Even so, the Alewives remained implicated in the decline of many native Great Lake species.

Alewife populations have decreased within much of their range. While, several threats have contributed to their decline including loss of habitat due to decreased spawning areas from the construction of dams and other impediments to migration, habitat degradation, fishing and increased predation due to recovering striped bass populations.

Currently, these fish occur in various states in the United States including Alabama, Colorado, Georgia, Illinois, Indiana, Kentucky, Maine, Maryland, Michigan, Minnesota, Nebraska, New Hampshire, New York, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Vermont, Virginia, West Virginia and Wisconsin.

Management of the Sawbelly

The Alewife became a popular fish for baiting in the spring lobster and crab fisheries and eaten by humans, fresh, dried or salted, smoked or frozen, eaten fried. Or even used as pet food. While, population reduction of the invasive species is the only possible method of control in the Great lakes.

As well as, culling that could be beneficial in reducing local populations especially in spawning times when they congregate or during winter when they are susceptible to cold temperatures. While, containment of the species may also help with the distribution.

Some states have passes laws making transportation of alewife, dead or alive illegal. While, the law goes farther to bar people from capturing them. Meanwhile, the continuation of fishing for alewife may be beneficial especially if monitored for reducing population size. Therefore, ensuring that Alewife do not spread beyond their current range.

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