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Habitat: Riverine and Marine

River herring need a variety of habitats for spawning, rearing, and transitioning to and from saltwater. As adults, river herring reside in marine waters most of the year and move to freshwater rivers to spawn. The substrate preferred for spawning varies greatly and can include gravel, detritus, and submerged aquatic vegetation. Blueback herring prefer swifter moving waters compared to alewives. Nursery areas include freshwater and semi-brackish waters. Little is known about their habitat preference in the marine environment.  

River herring have seasonal spawning migrations that are cued by water temperature. Depending upon temperature, 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. The river herring migration in Canada extends from late April through early July, with the peak occurring in late May and early June. Blueback herring generally make their spawning runs about 2 weeks later than alewives do. River herring conform to a metapopulation paradigm (e.g., a group of spatially separated populations of the same species which interact at some level) with adults frequently returning to their natal rivers for spawning but with some limited straying occurring between rivers.

General Habitat Threats


River herring require safe, timely and effective passage to spawning and rearing habitat in freshwater systems as well as migration to the sea for growth to maturity. Dams and other barriers to upstream and downstream passage (e.g., culverts) can block or impede access to habitats necessary for spawning and rearing; can cause direct and indirect mortality from injuries incurred while passing over dams, through downstream passage facilities, or through hydropower turbines; and can degrade habitat features necessary to support essential river herring life history functions. Human-made barriers that block or impede access to rivers throughout the entire historical range of river herring have resulted in significant losses of historical spawning habitat for river herring and declines in abundance of both blueback and alewife populations. While estimates of habitat loss over the entire range of river herring are not available, estimates from studies in Maine show that less than 5% of lake spawning habitat and 20% of river habitat remains accessible for river herring.  

Dams are also known to impact river herring through various mechanisms, such as habitat alteration, fish passage delays, and entrainment and impingement. River herring can undergo indirect mortality from injuries such as scale loss, lacerations, bruising, eye or fin damage, or internal hemorrhaging when passing through turbines, over spillways, and through bypasses. NMFS has developed a primer outlining the critical information needs for planning and installing fish passage.

          John Catena, of NOAA, supervises the crew as a fish ladder is being installed and fitted into the cut in the dam. Massachusetts, Byfield, Essex County. November 21, 2000.
Picture credit: Essex County Greenbelt Association, Cindy Mom

Climate Change

Impacts from global climate change induced by human activities are likely to become more apparent in future years (Intergovernmental Panel on Climate Change (IPCC)). Climate variability rather than climate change is expected to have more of an impact on river herring from 2024-2030. Impacts of climate change on river herring have been investigated in marine habitats and are ongoing in freshwater habitats. Many observed changes in river herring biology related to environmental conditions have been noted, but few detailed analyses are available to distinguish climate change from climate variability. 

Water Quality (Chemical)

Nutrient enrichment from non-point sources of pollution has become a major cumulative problem for many coastal waters; especially in the mid-Atlantic and southeast regions of the U.S.. Nutrient loading results from the individual activities of coastal development, marinas and recreational boating, sewage treatment and disposal, industrial wastewater and solid waste disposal, ocean disposal, agriculture, and aquaculture. Excess nutrients from land based activities accumulate in the soil, pollute the atmosphere, pollute ground water, or move into streams and coastal waters. Nutrient inputs are known to have a direct effect on water quality. For example, nutrient enrichment can stimulate growth of phytoplankton that consumes oxygen when they decay, which can lead to low dissolved oxygen that may result in fish kills; this condition is known as eutrophication.  

In addition to the direct cumulative effects incurred by development activities, inshore and coastal habitats are also threatened by persistent increases in certain chemical discharges. The effects associated with known contaminants (e.g., polychlorinated biphenyls (PCBs), hydrocarbons, non-industrial waste) on river herring life history is largely unknown. Recent studies have highlighted the possible effects of contaminants, such as deicing road salt and copper, on anadromous species’ egg development.

Water Quantity (Physical and Thermal)

The physical characteristics of streams (e.g., stream width, depth, and current velocity; substrate; and temperature) can be altered by water withdrawals. River herring can experience thermal stress, direct mortality, or indirect mortality when water is not released during times of low river flows and water temperatures are higher than normal. Water flow disruption can also result in less freshwater input to estuaries, which are important nursery areas for river herring and other anadromous species.


Dredging can negatively affect alosine populations by producing suspended sediments and migrating alosines are known to avoid waters of high sediment load. Fish may also avoid areas that are being dredged because of suspended sediment in the water column.  Filter-feeding fishes, such as alosines, can be negatively impacted by suspended sediments on gill tissues. Suspended sediments can clog gills that provide oxygen, resulting in lethal and sub-lethal effects to fish).

Nursery areas along the shorelines of the rivers in North Carolina have been affected by dredging and filling, as well as by erection of bulkheads; however, the degree of impact has not been measured. In some areas, juvenile alosines were unable to enter channelized sections of a stream due to high water velocities caused by dredging.

TEWG Habitat Subgroup

The Technical Expert Working Group (TEWG) Habitat Subgroup has considered issues specific to the impacts from various factors affecting river herring habitat including,, but not limited to, connectivity (e.g., fish passage), water quality/quantity, and appropriate habitat characteristics. Additional information on these discussions, including identified research needs to inform the topic and ongoing efforts can be found at: