River Herring History and Biology

What are river herring?

River herring are silver in color and laterally compressed. They grow to be approximately 10 inches long, however, repeat spawners can be up to 14 inches.  Illustrations by Diane Rome Peebles.

River herring are silver in color and laterally compressed. They grow to be approximately 10 inches long, however, repeat spawners can be up to 14 inches. Illustrations by Diane Rome Peebles.

River Herring include two species of fish, the Blueback Herring (Alosa aestivalis) and Alewife (Alosa psuedoharenous), native to the eastern seaboard. The two species are not easily distinguishable and have historically been grouped together in population monitoring. However, Alewives range from Maine to the northern tip of South Carolina, whereas Blueback Herring extend well into Florida.

These fish are anadromous, meaning they live most of their lives in the ocean and return to freshwater rivers and lakes to spawn (lay eggs). Unlike Salmon, which typically die in the streams where they spawn, herring return to the ocean and can make several trips upstream throughout their lifetime.

Fueled by a diet of plankton, these slender swimmers travel in large schools. Their migration is triggered by water temperature - when the Mystic River reaches 51 degrees F, typically in mid-April, the herring begin their annual migration to freshwater spawning grounds. An estimated 589,000 river herring swam 7 miles up the Mystic River to the Upper Mystic Lake in 2018. The average female produces 60,000-100,000 eggs each time she spawns. Alewives seek out upstream ponds and lakes to spawn, whereas Bluebacks prefer the main stem sections of rivers and streams. Spent fish return to the ocean soon after spawning, and up to 25% will return within their lifetime. Young (fry) wait until the summer to make their seaward migration, when they have grown to approximately 32-152 mm.

River herring play a crucial role in the freshwater, estuarine, and marine ecosystems they inhabit. They feed a multi-ecosystem food web, connecting resources in the ocean to predators upstream. For example, they are prey for seals, and other marine mammals; seabirds, cormorants, ospreys, herons, and eagles; tuna, cod, trout, and several species of bass; mink, fox, raccoon, skunk, weasel, and turtles. These small fish are also used as bait for lobster fisheries in Maine. As zooplankton eaters, they turn an inaccessible resource into the food for a large number of species humans consume, enjoy and profit from.

© Shervin Azad Arya


Over the course of the last century, river herring have experienced a precipitous decline in population. In New England, river herring are historically significant because of their role in the regional fisheries. Cured herring were a critical part of the local and international economy as early as the 1500’s. In 1908 alone, 13,311,000 lbs of smoked herring were produced. However, physical obstructions to migration, overfishing, trawl-fishing methods, poor water quality, and inadequate spawning habitat have decimated the population. In 2006, NOAA National Marine Fisheries Service listed river herring as a species of concern. Read more.

Locally, however, we have seen an increase in the herring migration. In 2010, the Upper Mystic Dam was rebuilt to include a passable fish ladder. With data collected by citizen scientists through the Mystic River Watershed Association's Herring Monitoring Program, the organization has documented an increase from an estimated 198,932 river herring in 2012 to an estimated 589,000 river herring in 2018. It’s important that this data collection continue to serve as a benchmark for this species, which plays a critical role in maintaining healthy riverine, coastal and pelagic ecosystems.

What can you do?

One fish, two fish! It's easy - help count the number of river herring by watching short videos recorded at the Upper Mystic Lake Dam in Medford, MA. Every video watched provides important data on the local population of this migratory fish.