What Are River Herring?
Two Species: Alewife and Blueback Herring
Two species of fish — the alewife (Alosa pseudoharengus) and the blueback herring (A. aestivalis) — are known collectively as river herring. The two species look very similar and are difficult to distinguish from each other. They are grayish green on top and silvery on the sides, and they grow as long as approximately 16 inches. Adults generally live in the ocean for 2 years (mid-Atlantic states) to 4 years (Northeast states) before returning to rivers to spawn. While some adults die after spawning, most return to the ocean until the following year’s spawning migration. Alewife and blueback herring can live up to 8 years.
River herring are an important link in the food webs of freshwater, estuarine, and marine ecosystems. In rivers, they are eaten by bass, trout, salmon, other fish, ospreys, herons, eagles, kingfishers, cormorants, and aquatic mammals. In the ocean, they provide forage for predators including bluefish, weakfish, striped bass, cod, pollock, silver hake, endangered Atlantic salmon, seabirds, and marine mammals.
River herring can be found along the Atlantic coast of North America from the Southern Gulf of St. Lawrence, Canada, to the southeastern United States. The coastal ranges of the two species overlap. Blueback herring range from Nova Scotia south to the St. John’s River in Florida, while alewife range from Labrador and Newfoundland south to South Carolina, although their occurrence in the extreme southern range is less common.
Habitat and Spawning
River herring are anadromous, meaning that they mature in the ocean and then migrate up coastal rivers to estuarine and freshwater rivers, ponds, and lake habitats to spawn. They spawn on gravel, detritus, or submerged aquatic vegetation. Blueback herring prefer swifter moving waters than alewives do. Nursery areas include freshwater and semi-brackish waters. Little is known about their habitat preference in the marine environment, but adult river herring are most often found at depths less than approximately 330 feet (100 meters) in waters along the continental shelf.
Blueback herring typically spawn from late March through mid-May. However, they spawn in the southern parts of their range as early as December or January, and as late as August in the northern portion of their range. Alewives have been documented spawning as early as February in the southern portion of their range, and as late as August in the northern portion of the range.