An Atlantic salmon’s life is a round trip. It begins in a gravel nest in a river, carries the fish out across the North Atlantic, and ends back in that same river, where it breeds. Only a small share of each generation completes the loop, and the losses pile up at a few predictable bottlenecks rather than falling evenly. Unlike Pacific salmon, an Atlantic salmon does not always die after spawning, and a few return to breed again.
Eggs Buried in River Gravel
In autumn the female digs a nest in the riverbed gravel and buries her eggs there after males fertilize them. The eggs need gravel free of fine silt and rich in oxygen.1Limnologica. Incubation, hatching and survival of eggs of Atlantic salmon (Salmo salar) in spawning redds influenced by groundwater Whether they develop depends on groundwater seeping in, oxygen and water temperature working together.2Limnologica. Incubation, hatching and survival of eggs of Atlantic salmon (Salmo salar) in spawning redds influenced by groundwater
Silt is the common enemy. Fine sediment from farming, logging or construction fills the gaps between stones, and clogged gravel slows the water flowing past the eggs. Less flow means less oxygen delivered and more waste building up around the embryos. That is why salmon choose riffles, where water keeps moving, over still pools.
Oxygen in the gravel is highest just after the nest is dug and tends to fall over the winter of incubation.3Hydrological Processes. A field‐based assessment of oxygen supply to incubating Atlantic salmon (Salmo salar) embryos The flow of water through the gravel slows over the same period, reaching its lowest around hatching.4Hydrological Processes. A field‐based assessment of oxygen supply to incubating Atlantic salmon (Salmo salar) embryos So the steady delivery of oxygen, not just how much is dissolved, best predicts whether embryos live.5Hydrological Processes. A field‐based assessment of oxygen supply to incubating Atlantic salmon (Salmo salar) embryos6Hydrological Processes. A field‐based assessment of oxygen supply to incubating Atlantic salmon (Salmo salar) embryos
The eggs sit buried for months through the cold season. Cold water slows development, so the timing of hatching follows temperature. Newly hatched young stay in the gravel, living off a yolk sac, and they emerge into open water in spring once the yolk is used up.
Life in the River as Young Salmon
The first weeks out of the gravel are the deadliest. In one wild stream, about 79% of young salmon died in the period just after leaving the nest.7PubMed. Competitive interactions shape offspring performance in relation to seasonal timing of emergence in Atlantic salmon Survival depended on how crowded the stream was and on when the fish emerged.8PubMed. Competitive interactions shape offspring performance in relation to seasonal timing of emergence in Atlantic salmon The earliest emergers did best and grew largest, even though they met the harshest cold.9PubMed. Competitive interactions shape offspring performance in relation to seasonal timing of emergence in Atlantic salmon
That loss is normal. A female lays thousands of eggs to offset heavy losses, and only a few spots in a stream can feed a young fish. A population stays stable if, on average, about two offspring from each breeding pair return to breed.
Survivors become parr, young river salmon with dark bars on their sides. They usually stay in the river one to four years, depending on how warm and productive it is. Each one defends a small feeding territory in the current and eats insect larvae drifting past. Larger fish hold the best spots and smaller ones get pushed out. In winter they shelter among stones on the riverbed. They share streams with brown trout, which tend to out-compete them, though often not in fast water.10Fisheries Research. Habitat requirements of Atlantic salmon and brown trout in rivers and streams11Regulated Rivers: Research & Management. Contrasting temperatures, waterflows, and light: seasonal habitat selection by young Atlantic salmon and brown trout in a boreonemoral river
Some male parr mature while still small in the river and sneak fertilizations at nests. In one wild population, these early-maturing males fathered about 60% of young.12Freshwater Biology. Impact of precocious male parr on the effective size of a wild population of Atlantic salmon They greatly boost genetic diversity among the fathers in the population.13Freshwater Biology. Impact of precocious male parr on the effective size of a wild population of Atlantic salmon
Turning Into a Smolt to Survive Salt Water
Before heading to sea, a young river salmon turns into a smolt, a slimmer, silvery fish that loses its dark bars. The silver colouring helps hide it in open water. Then the fish begins drifting downstream with the spring current, usually at age two or three.
The bigger change is inside. The sea is saltier than a fish’s blood, so water drains out of the body, and a freshwater fish would dehydrate there. A salmon in salt water drinks heavily and absorbs fluid through its gut.14PubMed Central. Late transfer to seawater improves intestinal development of osmoregulatory capacity in Atlantic salmon smolts The gut rebuilds itself ahead of time to absorb more water.15PubMed Central. Late transfer to seawater improves intestinal development of osmoregulatory capacity in Atlantic salmon smolts The gills also switch to pumping out excess salt, reversing the job they did in the river. All of this must be ready before the fish reaches the estuary, and the changes are cued by the cold of winter followed by lengthening days.16Fishes. Integration of Gill and Intestinal Osmoregulatory Functions to Assess the Smoltification Window in Atlantic Salmon
Smolts are ready for sea for only a few weeks in spring. Fish that miss that window lose their salt tolerance again, which is why the spring run downstream is so tightly timed. In fish farms that speed up the light cues, some smolts never fully adapt and become stunted after moving to sea.17Aquaculture. Characterization of the smoltification process and post-sea transfer stunts of Atlantic salmon (salmo salar) in full-scale intensive production
The Dangerous Run to the Sea
Smolts often move at night and ride the current. Every hour spent in the estuary is an hour exposed to birds, seals and larger fish, and delays can push them past their smolt window. Smolts that move fast through the estuary are more likely to reach the sea.18PubMed. Interaction between migration behaviour and estuarine mortality in cultivated Atlantic salmon Salmo salar smolts Predators gather at choke points, such as cod feeding on smolts at an underwater ridge between estuary and fjord.19PubMed. Interaction between migration behaviour and estuarine mortality in cultivated Atlantic salmon Salmo salar smolts Survival falls as smolts move from fresh water toward the ocean.20Marine and Coastal Fisheries. Linking Behavior, Physiology, and Survival of Atlantic Salmon Smolts During Estuary Migration In rivers where smolts linger and swim back upstream, more die in the inner estuary.21PubMed. Estuarine survival and migratory behaviour of Atlantic salmon Salmo salar smolts
Hydroelectric dams are the hardest human barriers for smolts, because the small fish swim weakly.22PubMed Central. Challenges in downstream dam passage and the effect of dam removal on Atlantic salmon ( Salmo salar ) smolt migrations Dams can kill smolts directly or slow them down.23PubMed Central. Challenges in downstream dam passage and the effect of dam removal on Atlantic salmon ( Salmo salar ) smolt migrations Turbines can injure or kill fish passing through them, and slow reservoirs above a dam confuse fish that rely on current to find their way. Predators wait in the path of stunned or slowed fish. Bypass channels and spill over the dam help some fish avoid the turbines.
Longer delays at dams were linked to lower survival in a Maine river.24Canadian Journal of Fisheries and Aquatic Sciences. Effects of dam-induced delays on system-wide survival of Atlantic salmon smolts during high-flow, high-survival years in the Penobscot River, Maine, USA Getting past a dam isn’t the whole story, though: at one dam over 96% of fish passed, yet survival just below it was far lower than at another.25USGS. Spatial variation in landlocked Atlantic Salmon smolt survival associated with dam passage, avian predation, and stocking location
Growing Up in the North Atlantic
Young salmon in the estuary eat mostly insects.26Journal of Fish Biology. Geographical differences in marine feeding of Atlantic salmon post‐smolts in Norwegian fjords Further out they switch to crustaceans and the larvae of sand eels, herring and cod-family fish.27Journal of Fish Biology. Geographical differences in marine feeding of Atlantic salmon post‐smolts in Norwegian fjords How much they eat varies a lot by place and year.28Journal of Fish Biology. Geographical differences in marine feeding of Atlantic salmon post‐smolts in Norwegian fjords
Salmon from as far apart as North America and Spain feed in waters around Greenland.29ICES Journal of Marine Science. Stable isotopes suggest the location of marine feeding grounds of South European Atlantic salmon in Greenland Off West Greenland their main prey is capelin and small shrimp-like crustaceans.30Marine Ecology Progress Series. Changing trophic structure and energy dynamics in the Northwest Atlantic: implications for Atlantic salmon feeding at West Greenland They grow quickly, from a smolt of a few inches to a fish of several kilograms.
Not all salmon stay at sea for the same time. Some return after one winter as smaller fish, called grilse. Others stay two or more winters and come back much larger, and bigger females carry more eggs. The mix of ages varies by river.
Young salmon survive less well at sea in years with warmer surface water and less plankton growth.31PubMed. Spatial synchrony in the response of a long range migratory species (Salmo salar) to climate change in the North Atlantic Ocean Populations from distant rivers therefore rise and fall together, because they share the same ocean feeding areas.32PubMed. Spatial synchrony in the response of a long range migratory species (Salmo salar) to climate change in the North Atlantic Ocean River protection can’t offset a poor year at sea, which is why counts can drop even in well-kept rivers.
Finding the Way Home and Spawning Again
Salmon appear to record the magnetic signature of where they entered the sea and use it to steer home, a pattern shown in Pacific salmon.33PubMed Central. Geomagnetic imprinting predicts spatio-temporal variation in homing migration of pink and sockeye salmon Near the coast, smell takes over. Young salmon memorize their river’s scent, mainly while becoming smolts.34Biology Bulletin Reviews. Mechanisms of homing in salmonids Returning adults follow that remembered scent upstream to their birth stream.35Biology Bulletin Reviews. Mechanisms of homing in salmonids
The trip upstream is hard. Adults usually stop eating once they enter fresh water and live on stored fat for months. They leap falls and fight currents to reach the gravel, and before spawning males develop a hooked jaw and a redder colour.
Spawning takes place in autumn, and the spent salmon drift back to sea. They have little left, since they lose about 40% of their body weight in the river.36PubMed Central. DNA and scale reading to identify repeat spawning in Atlantic salmon: Unique insights into patterns of iteroparity Few make it: in one Norwegian river only about 5% of returning adults were repeat spawners.37PubMed Central. DNA and scale reading to identify repeat spawning in Atlantic salmon: Unique insights into patterns of iteroparity Most repeat spawners spent a full year at sea recovering and came back about 2.5 kg heavier.38PubMed Central. DNA and scale reading to identify repeat spawning in Atlantic salmon: Unique insights into patterns of iteroparity
Those rare fish matter. Larger repeat spawners carry many more eggs, and they spread a population’s breeding over more years.
Sea Lice, Disease and Farmed Escapees
Sea lice are small parasites that feed on salmon skin and multiply around open net-pen farms. Farms sit in the fjords smolts must cross, and thousands of caged fish breed huge numbers of lice larvae. Smolts are small, so a few lice can be deadly. In trials where some smolts were protected with anti-lice medicine, 12% to 29% fewer of the unprotected fish came back as adults.39NINA Report. Impacts of salmon lice emanating from salmon farms on wild Atlantic salmon and sea trout The effect showed up in some places and years and not others.40NINA Report. Impacts of salmon lice emanating from salmon farms on wild Atlantic salmon and sea trout In Norway, the chance of a poor angling catch rose about 47% when farms exceeded 0.1 adult female lice per farmed fish.41Journal of Applied Ecology. Negative association of sea lice from fish farms on recreational fishing catches of Atlantic salmon Lice can harm smolts even where there are no farms.42Aquaculture Environment Interactions. Effect of sea lice on the survival of wild Atlantic salmon smolt in areas with and without open net-pen salmonid aquaculture in Newfoundland, Canada Fallowing farms during the smolt run is one fix.
Farmed fish that escape bring a different problem. Offspring of farmed and wild salmon grow faster but survive worse, and they push wild young out.43PubMed Central. Fitness reduction and potential extinction of wild populations of Atlantic salmon, Salmo salar, as a result of interactions with escaped farm salmon. Farm salmon are bred for fast growth in cages, and traits suited to cages hurt survival in rivers and the sea. The mixed offspring look healthy, but fewer live to breed. Repeated escapes can therefore drag down a wild population, possibly toward extinction.44PubMed Central. Fitness reduction and potential extinction of wild populations of Atlantic salmon, Salmo salar, as a result of interactions with escaped farm salmon. In most Norwegian rivers studied, escapees had made little genetic mark, though about 3 in 10 populations had changed.45PubMed Central. Three Decades of Farmed Escapees in the Wild: A Spatio-Temporal Analysis of Atlantic Salmon Population Genetic Structure throughout Norway Wild salmon carrying more farm ancestry go to sea and mature younger.46PubMed Central. Introgression from farmed escapees affects the full life cycle of wild Atlantic salmon
The parasite Gyrodactylus salaris has spread to 53 Norwegian rivers.47PubMed. The battle against the introduced pathogenic monogenean Gyrodactylus salaris in Norwegian Atlantic salmon rivers and fish farms In those rivers an average of 86% of young salmon die.48PubMed. The battle against the introduced pathogenic monogenean Gyrodactylus salaris in Norwegian Atlantic salmon rivers and fish farms
Warming Rivers
Young salmon grew fastest at 14 °C, slower at 18 °C, and slowest at 22 °C.49PubMed Central. Effects of Water Temperature on Growth, Hematological Measurements and Stress-Related Gene Expression of Atlantic Salmon (Salmo salar) Parr Reared in a Recirculating Aquaculture System Above 14 °C they turned food into growth less efficiently.50PubMed Central. Effects of Water Temperature on Growth, Hematological Measurements and Stress-Related Gene Expression of Atlantic Salmon (Salmo salar) Parr Reared in a Recirculating Aquaculture System At about 69 °F (20.5 °C), water is warm enough to stress young salmon and stunt growth.51Encyclopedia of Puget Sound. The Life Cycle and Survival of the Atlantic Salmon
Heat waves add another strain. Young salmon switch on protective cell responses during a single day of heat and seem to cope.52FACETS. Physiological responses to a short-term, environmentally realistic, acute heat stress in Atlantic salmon, <i>Salmo salar</i> But one hot spell may leave them less able to handle the next.53FACETS. Physiological responses to a short-term, environmentally realistic, acute heat stress in Atlantic salmon, <i>Salmo salar</i>
Fish look for relief where they can. They seek cold pockets where groundwater or tributaries enter, and shade from bank trees keeps streams cooler. Low summer flows make rivers heat up faster, so planting trees along banks is a practical fix.
Why Salmon Matter to Rivers
Spawning salmon add nitrogen and phosphorus to rivers through their waste, eggs and bodies.54Freshwater Biology. Increases in benthic community production and metabolism in response to marine‐derived nutrients from spawning Atlantic salmon (Salmo salar) The more nutrients delivered, the more the river’s base of algae and insects grows.55Freshwater Biology. Increases in benthic community production and metabolism in response to marine‐derived nutrients from spawning Atlantic salmon (Salmo salar) Salmon also raised the fat content of the stream’s algae film, insects and young salmon.56Ecosphere. Effects of spawning Atlantic salmon (Salmo salar) on total lipid content and fatty acid composition of river food webs Digging nests stirs up the riverbed, adding to the effect.57PubMed. Marine-derived nutrients, bioturbation, and ecosystem metabolism: reconsidering the role of salmon in streams
That creates a feedback loop. Fewer returning adults means a poorer river, and a poorer river feeds fewer young salmon. This can keep a depleted run from recovering.
Can Hatcheries and Dam Removal Bring Salmon Back
Dam removal works fast. After a dam came out, spawning habitat downstream improved from low to medium quality within a year or two.58Journal of Great Lakes Research. Effect of dam removal on habitat use by spawning Atlantic salmon On Maine’s Penobscot River, salmon passed former dam sites as easily as open river.59Marine and Coastal Fisheries. Upstream Movements of Atlantic Salmon in the Lower Penobscot River, Maine Following Two Dam Removals and Fish Passage Modifications At a remaining dam, though, about one-third of fish took over a week to get through a fish lift.60Marine and Coastal Fisheries. Upstream Movements of Atlantic Salmon in the Lower Penobscot River, Maine Following Two Dam Removals and Fish Passage Modifications
Hatcheries have a weaker record. Hatchery-bred salmon released into the wild had only about 36% of the lifetime offspring of wild fish.61PubMed Central. Captive-bred Atlantic salmon released into the wild have fewer offspring than wild-bred fish and decrease population productivity Years with more hatchery fish had lower total population productivity.62PubMed Central. Captive-bred Atlantic salmon released into the wild have fewer offspring than wild-bred fish and decrease population productivity Even one generation in a hatchery cut breeding success to about half that of wild fish.63PubMed Central. Reduced fitness of Atlantic salmon released in the wild after one generation of captive breeding
Hatchery life rewards traits that fail in the wild. Hatchery females may spawn poorly, and their young may survive less well. Fish released as fry did better than those released as smolts.
Readers can help by supporting groups that restore river banks and remove old dams, and by following local fishing rules, including catch and release. Where possible, choose farmed salmon from closed or well-regulated farms.