Background
American shad are an anadromous (a species of fish that moves from the sea to a river for spawning) species of fish that undergoes extensive ocean and freshwater migrations¹.  Historically, shad may have migrated up to 620 mi/1,000 km upstream in coastal rivers to spawn¹.  Shad range along the Atlantic coast from northern Florida to the St. Lawrence River in Canada, and are most abundant between North Carolina and Connecticut¹.  Populations once occurred in nearly 140 rivers extending from the Pinware River in Labrador to the St. John’s River in Florida².  Current populations are greatly reduced from historical levels, resulting in the closure of some fisheries².

In the Connecticut River, shad spawn in May and June³.  Surviving adults leave the river by mid-August and migrate to summer feeding grounds in the Gulf of Maine³.  In November, when water temperatures drop below 12°C/53.6°F, fish migrate south to Florida until February, when they begin migrating back towards the Connecticut River³.

Climate Impacts
Shad start spawning, leave rivers, and make ocean migrations based on temperature^3,4.  Fish from the Connecticut River move south to Florida by following the 12-13°C (between 53-56°F) isotherm, which is a line of temperature that changes with the seasons³.  Introduced shad in the Columbia River are migrating more than five weeks earlier than they did in 1949 following a long-term warming trend (related to multiple factors including construction of impoundments)^4,5, which suggests that changes in migration timing are likely due to climate change⁴.

Many aspects of shad life-history follow latitudinal gradients that are likely to be linked to climatic factors.  South of Cape Hatteras, shad are semelparous (they die after spawning) while north of Cape Hatteras, they are iteroparous (many individuals will survive spawning, return to the ocean, and spawn again in subsequent years)⁶.  In addition, the degree of iteroparity increases moving north from Cape Hatteras.  Spawning occurs earlier in southern latitudes, timed to correspond with optimal temperatures for survival of eggs and larvae⁴.  Additionally, the proportion of shad that are mature by age five increases from north to south, with 25% of New England fish compared to 70% of South Carolina fish maturing at age five⁷.  This all implies that climate has the potential to have multiple direct effects on shad life-history, complicating predictions about response to climate.  In addition to these direct effects, higher temperatures favor colonization of new habitats by invasive zebra mussels⁴.  Colonization of the Hudson River by zebra mussels has been associated with slower growth and reduced abundance of American shad⁴.   

Studies in the Connecticut River have shown that high river flows and low temperatures during spring spawning result in high mortality of eggs and larvae³.  These two factors were correlated with year-class strength in the period from 1966 to 1973³.  Based on this information, increased temperatures might be expected to favor shad abundance in the Connecticut River.  However, how temperature might interact with flow is unclear.  Projections for the Northeast suggest a longer period of summer low flows⁸, which could be favorable for spawning shad.  However, other hydrological changes are expected, including earlier timing of peak spring flows⁹ and increased frequency of extreme storm events⁸.