Cooperative Fish and Wildlife Research Units Program:
Education, Research and Technical Assistance for Managing Our Natural Resources


Range wide, river-specific dam impacts on Blueback Herring, Alewife and American Eel.

August 2023 - December 2025


Participating Agencies

We propose to develop and parameterize Alewife, Blueback Herring and American Eel river-specific models across their entire ranges. These models will be populated with estimates of region-specific life-history parameters, estimates of habitat (e.g., acres) and the dam landscape. Once developed, these models will be freely available to hydro practitioners for the use in planning for species conservation and recovery relative to hydropower at both local and regional scales. The challenges for developing the habitat and population model components vary greatly among these three species.

Alewife and blueback herring (collectively “river herring”) range from South Carolina to Newfoundland . These two species are anadromous – growing in the marine environment and returning to fresh water as presumptive spawners (alewife returning first in the spring, blueback herring timing overlaps, but is later). The two species are similar in appearance and life history, but critical distinctions (from each other and from American shad) prevent a direct reapplication of the GIS work used to model American shad. Blueback herring do not usually swim far upstream to spawn (historic runs in the Connecticut River not withstanding). Blueback Herring prefer to spawn in fast currents or over hard substrate .

In contrast, adult alewife prefer to spawn in slow moving water such as lakes or flowages. In the northern part of their range, they are iteroparous, meaning that adults return to the ocean after spawning. Both species’ juveniles spend 2-7 months in freshwater, and at 23-100 mm total length they migrate to the ocean. As anadromous species, blueback herring and alewife spawning is contained in isolated freshwater systems. These species are, therefore, amenable to river specific population modeling.

A similar approach with the catadromous American Eel represents a fundamental challenge as the growth phase of this species is in fresh water, and spawning occurs in the ocean. The population is believed to be panmictic, a single population across the entire range of the species. After beginning life in the Sargasso Sea, American Eel larvae are dispersed by ocean currents across the eastern coast of North America . They eventually transition to a translucent “glass” stage near coastal waters and begin actively swimming upriver, when many encounter dams in pursuit of freshwater lakes, rivers, and streams. Because these barriers delay movement and limit American Eel establishment in headwater reaches , many dam structures have been retrofitted with juvenile eel ladders to offer upstream passage opportunities . Once in fresh water, eels may reside in these systems for more than 20 years before beginning a transoceanic spawning migration to return to the Sargasso Sea, during which they must pass the same dams they ascended as juveniles. These fish represent a challenge in the variety of habitats they can inhabit during their growth phase, as well as the integrated nature of range-wide river contributions to a single population.