Map depicting 1) rivers that historically supported wild anadromous Atlantic salmon and 2) the range of distinct population segments native to the US and overlapping into southernmost Canada. Observed differences in life history strategies and genetic structure were the basis for delineating the range of each US DPS and the southern limit of the boundary population occupying the St. Croix River/Bay of Fundy. Canada's Department of Fisheries and Oceans is responsible for delineating Canada's unique populations. Map created by Esther Cushing.
About the Atlantic Salmon
The Atlantic salmon (Salmo salar) is a fish of many identities. Some Atlantic salmon are anadromous and require unobstructed access between freshwater, estuarine, and marine environments. Others are “landlocked” and never venture to sea. Each variety continues to hold tremendous value whether from a recreational, ecological, or economic perspective.
Over the past several decades, commercial culture of anadromous Atlantic salmon expanded globally. Domesticated forms are now widely available, arriving fresh to markets and restaurants from commercial aquaculture sites off the coasts of Maine, Canada, Norway and even Alaska and Chile. These “farm-raised” salmon are grown in captivity and only ever see the inside of hatcheries and marine net pens. Spending every life phase (from egg to adult) in a human-created environment, natural selection for survival in the wild has been replaced by domestication.
Conversely, the wild anadromous stocks that once inspired the title “King of Fish” in the U.S. are now scarce. From a purely aesthetic standpoint, a truly wild individual is a link in an unbroken chain of ancestors that all completed their entire life cycle under natural conditions. Under contemporary standards, however, a more practical definition of wild is an individual that spends its entire life cycle in its natural environment and whose parents were naturally spawned and reared.
Today, conservation hatcheries intervene in certain phases of the natural life history of Atlantic salmon to help slow/reverse the decline of wild anadromous populations and create/enhance recreational fishing opportunities for landlocked salmon. Individuals that are produced under various state and federal conservation hatchery programs are released (stocked) into their natural environment instead of being held captive through adulthood like commercially farmed individuals.
Anadromy: A clever life history strategy
Anadromous, or “sea-run”, fishes are called such because adults return from sea to "run” up their natal river to spawn. Atlantic salmon adults that have returned from sea to spawn do so in flowing stretches in late autumn. Fertilized eggs are buried under 5-8 inches of gravel in redds (an area of disturbed gravel containing one or more nests). Due to the Northeast's cold winters, eggs incubate slowly and do not hatch until March or April. Sac fry, or alevin, remain hidden in redds until they deplete their yolk sac reserves (mid-May).
They emerge as fry and begin feeding on invertebrates, including small aquatic insects. Now at greater risk from predators, fry develop lateral "parr" markings as a form of camouflage and thus enter the parr life stage. Parr typically remain in freshwater riffle areas until they undergo a physiological transformation (smoltification) to develop a tolerance for saltwater. As smolts, they emigrate from their natal rivers from late April to early June. Smolts reach Newfoundland and Labrador by mid-summer as postsmolts. They spend their first winter at sea south of Greenland, taking advantage of the North Atlantic’s productivity and entering adulthood at sea. A small percentage of adults return to spawn after one sea winter; however, the majority of adults spend a second year at sea, feeding in the waters located to the southwest of coastal Greenlandand returning to Maine after their second winter to spawn in November. Only a small number of adults return the following year as three sea winter fish to complete their life cycle. Unlike their selmparous counterparts, Pacific salmon that spawn only once and die thereafter, Atlantic salmon are iteroparous (capable of spawning more than once).
|Although the above life stage sequence is typical of Atlantic salmon in the US, variation in run timing and other strategies are exhibited as well. For example, some Atlantic salmon parr use estuaries or lakes instead of rivers as rearing habitat. And some male parr do not undergo smoltification, opting instead to stay in the riverine environment. These “precocious parr” are smaller and less able to compete with big males that have returned from sea to breed with females; however by staying in freshwater, they avoid the risks associated with migrating and living in the marine environment. This flexibility, or life history plasticity, enables populations to use a wide range of environmental resources. Variability in life history strategies is an important “bet-hedging” strategy that allows some segments of a population to persist through unfavorable environmental conditions.|
Population-level processes such as life history plasticity, philopatry and straying have resulted in the formation of unique anadromous Atlantic salmon populations along the continuum of their range. For example, Fey et al. (2006) described four distinct populations segments (DPS) whose freshwater range occurred within the US, based on unique genetic, life history and zoogeographic characteristics (this document can be found in the NEST website E-Library under key documents).
And as we have seen, anadromy and these processes have numerous advantages, most notably allowing Atlantic salmon to utilize a variety of habitat types/food resources and put their eggs in more than one basket so to speak. Unfortunately, the high value placed on anadromous Atlantic salmon (and other sea-run fishes) and their need for unobstructed access between freshwater, estuarine, and marine environments makes them particularly vulnerable to overexploitation, habitat fragmentation and degradation, and a myriad of other byproducts of our society.
Even though Atlantic salmon exhibit a variety of strategies that enable them to occupy a range of habitats and cope with fluctuating conditions, local and regional extinctions are increasingly common. Despite intensive efforts throughout New England to recover the Atlantic salmon to its prominence as King of Fish, the altered structure of biological communities, continued poor fish passage at dams, and poor marine survival are just some of the obstacles hindering the recovery Atlantic salmon. This suggests that present rates and forms of environmental changes are exceeding the ability of Atlantic salmon to adapt. Impassable dams, for example, not only prevent adults from reaching spawning grounds but also cause a cascade of changes in the ecological linkages to which salmon are inextricably tied.
Taking a look at the history of wild anadromous Atlantic salmon in the US can help us paint a better picture of their current status and trends, as well as their future sustainability and importance relative to domesticated varieties.
Anadromous Atlantic salmon: Past, Present & Future
Epitomized by the annual presentation of the “Presidential Salmon” to the U.S. president from 1912 to 1992, wild forms of anadromous Atlantic salmon were historically a fundamental element of New England cultures, economies and ecosystems. With a pan-North Atlantic range including the coastal rivers of Northeastern North America, Iceland, Northwestern Europe and the North Atlantic Ocean, Atlantic salmon once provided valuable and productive recreational, commercial and subsistence fisheries. Atlantic salmon co-evolved with 11 other socially and ecologically important sea-run species (such as river herring and shad) and together dominated Maine’s river ecosystems.
In the US, anadromous Atlantic salmon once ranged from the Housatonic River north to the St. Croix River on the US/Canadian border. Native to nearly every major river in this range, adult Atlantic salmon returning to U.S. waters to spawn were estimated around 500,000 annually.
The impacts of over-fishing, water quality degradation, and barriers to migration caused by waste disposal and hydropower development were seen as early as the 1800s with the extinction of the southernmost Long Island Sound DPS. The Central New England DPS went extinct shortly thereafter. By the mid-1970s, only a few hundred adults were returning to Maine.
In 2000, the U.S. Fish and Wildlife Service and the National Marine Fisheries Service jointly listed the Gulf of Maine (GOM) DPS as endangered. At the time of the listing, the Androscoggin River, the portion of the Kennebec River above the site of the former Edwards Dam, and the stretch of the Penobscot River above the site of the former Bangor Dam were not included. This postponement was due to an inadequacy of available genetic data to evaluate the salmon populations present in these larger systems relative to smaller coastal rivers (e.g. the Dennys). At the time, it was thought that populations occupying large rivers might be subject to different natural selection pressures than populations occupying small coastal rivers, and that this might result in unique life history adaptations and genetic structuring. In 2004, a biological review team reconvened to reexamine this “large river hypothesis”, and in 2006 published a status review of Atlantic salmon in the US that rejected this hypothesis in light of new genetic and life-history data.
Fortunately, Atlantic salmon show a remarkable degree of resilience, a trait that fosters optimism in the face of recent declines. Recent studies suggest that Maine populations still possess substantial levels of genetic variability, and a larger pool of available genes equates with a greater likelihood that some individuals will persist through unfavorable times. Thus, opportunities for recovery lie in providing a diversity of high quality habitat and restoring the ecological links Atlantic salmon need to persist. Restoration projects that take an ecosystem perspective will lay the foundation for sustaining Atlantic salmon into the future.