Alewives are a fascinating type of fish, known for their unique life cycle that bridges both freshwater and saltwater environments. Their presence, whether native or introduced, significantly influences aquatic communities.
Defining Alewives
The alewife, scientifically known as Alosa pseudoharengus, is a member of the herring family, Clupeidae, which also includes shad. This species is characterized by its slender, silver body with a grayish-green back and lighter belly, often displaying varied metallic hues. Alewives typically grow up to 38 centimeters (about 15 inches) in total length, though landlocked populations are generally smaller, usually less than 25 centimeters (about 10 inches).
A distinctive feature of the alewife is a single dark spot located behind its head, near the gill cover. They possess large eyes, with the eye width often exceeding the snout length, and their scales are easily dislodged. The midline of their belly forms a sharp, saw-toothed ridge, earning them the nickname “sawbelly”.
Habitat and Migration
Alewives are native to the Atlantic coast of North America, historically ranging from South Carolina north to Labrador, Nova Scotia, and northeastern Newfoundland. They are anadromous, meaning they spend the majority of their adult lives in saltwater, primarily the ocean, but undertake significant migrations to freshwater for spawning. While most are anadromous, some populations have become landlocked, completing their entire life cycle in freshwater systems, including the Great Lakes. The spring migration of adult alewives progresses from south to north, with timing influenced by rising water temperatures, typically beginning in late February in southern regions and extending through June in the north.
Life Cycle and Reproduction
The life cycle of an alewife begins with a remarkable journey from the ocean to freshwater spawning grounds. Spawning typically occurs in slow-moving, shallow sections of freshwater habitats, including lakes, ponds, and quiet backwaters of rivers.
Females can produce a large number of eggs, ranging from about 60,000 to 360,000, depending on their size. After the adhesive eggs are laid, they typically hatch within three to six days, depending on water temperature. The newly hatched larvae and juvenile alewives remain in these freshwater nursery areas for several months, growing rapidly and feeding primarily on zooplankton. As temperatures drop in late summer and fall, juveniles migrate downstream to the ocean, where they will mature over the next three to five years before returning to spawn themselves. Many alewives are repeat spawners, returning to their natal streams annually for several years.
Ecological Role
Alewives play a complex and significant role in the ecosystems they inhabit. In their native ranges, they are a crucial forage fish, serving as a food source for a wide array of predators, including larger fish like striped bass, lake trout, and salmon, as well as various birds such as osprey and cormorants, and marine mammals. Their migrations also transport marine-derived nutrients into freshwater ecosystems.
When introduced to non-native environments, particularly the Great Lakes, alewives can become an invasive species with detrimental effects. They compete intensely with native fish species for food resources, primarily zooplankton, and can even prey on the eggs and larvae of native fish like yellow perch and lake trout. The presence of alewives has been linked to declines in native planktivorous fish populations and disruptions in the zooplankton community structure. Furthermore, alewives contain an enzyme that can cause thiamine deficiency in predators, such as lake trout and salmon, which consume them in large quantities, leading to reproductive issues and early mortality in their offspring.