Background
Atlantic salmon are large silver fish with dark spots along the upper portion of their body. During spawning, they display colors of reddish-green to grayish-brown. They are migratory anadromous fish, spending a large portion of their lives in saltwater but migrating to freshwater to spawn. In the northwest Atlantic Ocean, the historical range (prior to European settlement) of Atlantic salmon extended from southern Greenland to as far south as the Housatonic River in Connecticut ⁷. Atlantic salmon have a relatively complex life cycle where juveniles, also known as parr, typically spend their first 1 to 3 years in freshwater streams and lakes before migrating to marine habitats. The transition period from freshwater to marine habitats is known as “smoltification”. Adults spend 2 to 3 years in the ocean feeding primarily on fish and crustaceans before returning to natal streams to spawn ^5,13. Atlantic salmon prefer to spawn in a gravel substrate with some degree of water circulation to keep eggs oxygenated. Eggs hatch in March and April, and parr feed on larval insects in habitats with vegetation and structural cover ⁵. The survival and growth of juveniles is dependent on predation, disease, competition, water temperature, and food supply ^3,5. Atlantic salmon have the ability to spawn multiple times within their life cycle, unlike Pacific salmon, which spawn once and die.

Salmon and other anadromous fish are important sources of marine-derived nutrients; fish act as energy conduits to freshwater habitats as they migrate from the ocean, up river, and are distributed across the terrestrial landscape by predators such as raptors, bears and mink. These marine nutrients enhance the growth of plants, wildlife, and fish — including juvenile salmon — that occupy streams and the surrounding areas where salmon are found ^2,14,15.

Prior to European settlement, nearly every major river north of the Hudson contained an abundance of Atlantic salmon ⁵. More recently, however, New England salmon populations have declined significantly as a result of overfishing, poor water quality, and dam construction ^8,15,19. These factors drastically reduced Atlantic salmon populations as early as the 1800s. By the end of the 19th century, three of the five largest salmon populations in New England (Connecticut River, Merrimack River, and Androscoggin River) were eliminated ⁴. By the year 2000, the Gulf of Maine population was listed as endangered ¹³. State and federal agencies have invested in stocking programs, but overall these efforts have resulted in few returning adults ¹⁷. This is likely due to continued overfishing and the persistence of dams in New England waterways. Salmon aquaculture has recently increased to meet market demands; however, farmed salmon threatens wild salmon populations through disease and parasite outbreaks as wells as genetic, habitat, and food web impacts.

Climate Impacts
Climate change will likely impact Atlantic salmon in a number of ways. Salmon are a cold-water-adapted species, and their daily activities and survival are dependent on temperature. The maximum critical temperature for Atlantic salmon survival (20° C/68° F, depending on life stage) will likely be exceeded in many areas with climate change ¹⁸. Projected increases in temperature associated with climate change have been shown to alter the proportion of residents (those individuals that remain within the system year-round) and migrants (those that leave the system seasonally) in a similar species (masu salmon), which may impact Atlantic salmon distribution and population dynamics ²⁰. Warming ocean temperatures have been associated with declines in Atlantic salmon survival ^6,9. In addition, warming temperatures can disrupt food webs and alter species interactions through changes in predation and competition ^1,12. Increasing sea surface temperatures have also been associated with declining body condition and fat content (or energy storage) in spawning adults ¹⁶. Thus, heat stress will likely impact growth, survival, and reproduction.

Atlantic salmon are also dependent on stable and reliable stream flow patterns. Changes in the timing, magnitude, and duration of stream and river flows from altered precipitation and temperature regimes are expected to further stress anadromous salmon populations by decreasing recruitment, survival, and productivity ^10,21. The presence of dams and other man-made barriers can alter river and stream temperature and flow in addition to restricting migration and connectivity between salmon spawning, nursery, and ocean habitats ²².

The combination of climate change and existing stressors can have disproportionate effects on Atlantic salmon populations. Because Atlantic salmon populations have been driven down to such low numbers, their overall genetic diversity has declined, making them less able to adapt and less resilient to future shifts and stressors. Overall, Atlantic salmon have an increased vulnerability to climate change due to their complex life cycle, reproductive strategy, use of both marine and freshwater habitats, and stress due to other human activities ¹⁸.