Lake Michigan, one of North America’s Great Lakes, supports diverse aquatic life. It is home to numerous fish species, whose populations change due to environmental and biological influences. Understanding the dominant fish species provides insight into the lake’s complex food web and overall ecological dynamics.
Identifying the Most Abundant Fish
The most abundant fish in Lake Michigan is the Alewife (Alosa pseudoharengus). This small, herring-like fish typically has a silvery body with a greyish-green back and faint darker stripes. A single, dull black spot behind its head, at eye level, is a distinguishing mark. Its head is broad and triangular, and its body is deep and laterally compressed. Its eyes are large, and its scales are easily shed, with those along the belly forming a serrated, saw-like edge, earning it the common name “sawbelly.”
Alewives in landlocked populations mature at a smaller size, rarely exceeding 25 centimeters (approximately 10 inches), though they can reach up to 38 centimeters (about 15 inches). They can live for up to 11 years in the Great Lakes. During daylight hours, Alewives often form schools in open water, dispersing towards nearshore areas at night. Their abundance and role as a key forage species classify them as the most common fish in Lake Michigan.
Understanding Its Ecological Success
The dominance of the Alewife in Lake Michigan stems from a combination of biological traits and historical ecological shifts. Alewives were first reported in Lake Michigan in 1949, spreading through the Great Lakes via the Welland Canal following a decline in native predators. Their populations rapidly expanded in the 1950s and 1960s, significantly changing the lake’s fish community.
Alewives are opportunistic feeders, consuming zooplankton, aquatic insects, and the eggs and larvae of other fish. They actively select for larger zooplankton, altering the zooplankton community structure in the Great Lakes. Their reproductive strategy supports their abundance; females can deposit 10,000 to 360,000 non-adhesive eggs, scattered over various substrates. Spawning occurs in spring when water temperatures rise above approximately 15°C (59°F).
The species adapts to the lake’s conditions, surviving in a wide temperature range between 3°C and 31°C (37°F to 88°F), though they prefer waters between 16°C and 20°C (61°F to 68°F). This thermal tolerance allows them to occupy various depths and areas throughout the year, overwintering in deep offshore waters and moving to shallower areas for spawning. Young Alewives’ ability to grow rapidly and achieve sufficient size before winter is important for their survival through colder months.