Background
River herring is a term used to refer collectively to alewife and blueback herring. The two species are commonly referenced together as they have similar life histories and look very similar¹.  They are native to the Atlantic coast of North America and co-occur in rivers throughout much of their range¹. The native range of alewife extends from Labrador in northern Canada to South Carolina, while that of blueback herring extends from the Gulf of St. Lawrence to Florida². Both species are anadromous, returning to their native freshwater to spawn after 3-6 years in the ocean³. Alewife use stream corridors to access ponds for spawning, although they will also use quiet reaches of rivers, whereas blueback herring prefer flowing waters for spawning^3,4. Alewife spawn at cooler temperatures than blueback herring, and as a result, begin spawning 3-4 weeks earlier. Despite these differences, there is much overlap in spawning timing of the two species². River herring are iteroparous, which means that they return to freshwater to spawn multiple times⁴. In New England, spawning may extend from late March to early June⁵. Adult river herring migrate to the ocean after spawning, while juveniles migrate from nursery habitats in Fall of the same year⁶. 

River herring are an important source of food for many species of birds and fish⁶.  Historically, river herring supported thriving fisheries⁷.  Although early dam construction, habitat degradation, and fishing resulted in large declines, a stable fishery existed through the 1960s⁷.  Recent declines have been precipitous and prompted a petition to consider the two species for listing under the Endangered Species Act (ESA), but the petition was denied and the species are unlisted⁸.  Recent declines may be due to overharvest in marine fisheries and higher levels of natural mortality due to rebounding striped bass populations^8,9.

Climate Impacts
Anadromous species, like alewife and blueback herring, use a wide range of habitats throughout their life cycle, making predictions about responses to climate change particularly complicated.  Each life stage is likely to be impacted in a unique way⁸.  However, initiation of spawning is known to be cued by temperature².  Many studies have found long-term warming trends for North American rivers over the past century¹⁰.  Spring water temperatures in several New England streams have begun reaching suitable spawning temperatures about twelve days earlier than in the 1970s, implying that runs of river herring are occurring earlier¹⁰.

Warming in the marine environment has also been observed and will likely increase¹¹.  Such warming is expected to shift distributions of many fish species poleward.  A similar shift has already been observed for alewife, and may continue, given predictions for future warming¹¹.

The recent petition to list river herring under the ESA prompted a modeling study to isolate the impacts of climate change on river herring populations⁸.  This study determined that overall, climate change is likely to impact river herring populations negatively in the Northwest Atlantic Ocean, from Cape Hatteras to the Gulf of Maine, with reductions in suitable habitat.  However, the authors of the study were careful to note that these projections isolate the effects of ocean warming and that these effects will not occur independently of other stressors, including biotic interactions and the effects of dams.