Salmon Life Cycle
Adult Atlantic salmon predominantly return to spawn in the river in which they spent their juvenile lives, a process known as ‘homing’. The adult salmon find their way back to the spawning grounds using their sense of smell (olfaction). Smell plays an important role throughout the life of salmon, with adults identifying their home river through imprinting to the smell of the river during their migration to sea at the smolt stage.
Spawning usually takes place from November to December although may occur earlier or later depending on factors such as the size of the fish and latitude of the river. Although spawning occurs towards the end of the year when water temperatures are cooler, adult fish can return to freshwater throughout the year and in some instances may spend over a year in the river before spawning. Fish returning earlier in the year tend to be the larger, older salmon that have spent more than one year feeding at sea before returning.
Salmon require clean, well oxygenated rivers and a gravel bed for the female to bury her eggs. The nests excavated in the gravel are known as redds. Spawning between male and female salmon is synchronised by visual and chemical signals. The chemical signals involve the use of pheromones, specific chemicals that are released by the female and detected by the males. These pheromones initially attract the male to the females on the redds and then prepare the male’s reproductive system to successfully fertilise the eggs. Some male parr may also become sexually mature in freshwater and contribute to spawning. The extent of such input varies, but up to ten mature male parr may successfully fertilise the eggs of a single female.
As the sense of smell is vital to successful spawning in Atlantic salmon, any factor in the environment that limits the ability of the fish to detect and respond to chemical signals can have a direct and deleterious effect on the status of stocks. Investigations have shown that river acidification, pesticides, endocrine disruptors and industrial pollutants can all affect reproduction in salmon.
Most Atlantic salmon die after spawning (often in the region of 90-95%). Those that survive may spawn again in a later year. Salmon spawning can be affected by a wide range of environmental and man-made conditions in our rivers and streams, including habitat degradation. For example, deposition of sediment in the gravel used by spawning salmon can reduce spawning success, embryo survival and juvenile salmon production. Investigations have demonstrated that such impacts can be mitigated by the cleaning of spawning gravels, the use of bankside incubators or the establishment of riparian buffer zones to limit sediment input. Longer term solutions require improved catchment management based on understanding the source of sediments and their subsequent dynamics in rivers.
Recent investigations
Understanding the requirements of salmon and the challenges they face at each life stage is vital for managing stocks effectively. Recent investigations have highlighted the following important issues for spawning salmon:
- Some sheep dip chemicals reduce the ability of male salmon to detect female spawning pheromones, resulting in reduced quantities of milt for spawning, and reduce the survival of salmon embryos within the gravel.
- Some herbicides have also been shown to affect the sense of smell in salmon and have a direct effect on the reproductive organs of male fish.
Atlantic salmon bury their eggs in gravel nests known as redds to protect them from predation, light, low temperatures and wash-out during high flows. The burial depth, typically less than 30 cm varies with the size of the female fish and the river bed composition. Salmon eggs are relatively large, and incubation is relatively long, with the resulting embryo (known as an alevin) relying on a yolk sac for a lengthy period before emergence from the gravel and first feeding.
Egg and alevin development are influenced strongly by temperature and this provides the best predictor of when eggs will hatch. Egg burial depth can also influence development, in part by affecting the temperature of the eggs themselves, but also by affecting oxygen content. Burial depth can also be critical in preventing exposure of the eggs during low flows. Other factors also influence development, and therefore the timing of hatching and emergence, such as the composition of the gravel, stream bed conformation and hydraulics, patterns of river discharge, and mechanical shock. Such factors can arise as a result of natural variations, but also through human impact. Egg survival can also be affected by the changing climate (e.g. through more frequent storms risking the wash-out of eggs).
Emergence from the redd occurs when the alevin is no longer reliant on its yolk sac food reserves and when fry need to start feeding. It is generally accepted that the timing of adult spawning has evolved in response to local conditions so that the fry emerge at the best time to take advantage of increasing food supplies. Marked changes in temperature, for example arising from climate change, oxygen levels, or reduced water flow through the redd due to sedimentation or concretion of the gravels during egg and alevin incubation can therefore affect the timing of emergence. This, in turn, can create a mismatch between emergence and optimum environmental conditions for first feeding resulting in increased levels of mortality. An unusually warm period in December 2015 is believed to have contributed to very poor levels of fry abundance observed in many rivers in England and Wales in 2016.
Recent investigations
Understanding the requirements of salmon and the challenges they face at each life stage is vital to manging stocks effectively. Recent investigations have highlighted the following important issues for salmon eggs and alevins:
- In the chalk streams of southern England, the cleaning of spawning gravels and stream channel modifications has been shown to improve the survival of salmon eggs and alevins.
- Small streams, where salmonids often spawn, are particularly vulnerable to growing land-use pressures and environmental change. Their extension and modification for agricultural and forestry drainage has resulted in highly modified flow and temperature regimes. The extensive length of the small stream network exposes them to a wide range of inputs, including nutrients, pesticides, heavy metals, sediment, and emerging contaminants, which along with invasions of non-native species, have implications for salmon populations throughout catchments.
The period between fry emergence and dispersal from the spawning redd and the establishment of defended feeding territories is a critical period in the life cycle of the Atlantic salmon and populations experience high mortality during this time.
The timing of fry emergence in the spring is determined by environmental conditions during the development of the egg and resulting embryo. The main factors are water temperature and dissolved oxygen concentration, both of which can be affected by natural variations, but also by human impacts as well as the effects of a changing climate. In pristine habitats, a combination of adult spawning date and the temperature-dependent rate of egg and alevin development, effectively determine when emergence will occur.
The dispersal of fry from spawning redds occurs principally at night. This is generally accepted to be a predator avoidance tactic, so any disruption to this timing is likely to increase levels of mortality. Fry dispersal is predominantly downstream and may be an important means of maximizing use of available habitat. The majority of fry remain relatively close to the redd site (<1 km), although distances can be affected by flows. Subsequent survival and growth can be influenced strongly by competition, as the first fry to arrive in a resource rich habitat defend these territories, with later arrivals often forced to occupy less-suitable locations.
Recent investigations
Understanding the requirements of salmon and the challenges they face at each life stage is vital to manging stocks effectively. Recent investigations have highlighted the following important issues for salmon fry:
- Statistical modelling used to relate densities of age-0+ salmon to changes in flow conditions in five UK rivers between 1971 and 2015 indicates that salmon densities were reduced in years with extreme high river flows following emergence.
- Low intensity artificial light (light pollution) (about twice that of a full moon) has been shown to delay and disrupt the dispersal of fry, which are smaller in size as a result.
- Small streams, where salmonids often spawn, are particularly vulnerable to growing land-use pressures and environmental change. Their extension and modification for agricultural and forestry drainage has resulted in highly modified flow and temperature regimes. The extensive length of the small stream network exposes them to a wide range of inputs, including nutrients, pesticides, heavy metals, sediment, and emerging contaminants, which along with invasions of non-native species, have implications for salmon populations throughout catchments.
Once feeding territories have been established, salmon parr need to grow as quickly as possible, without getting predated upon, so that they are ready for the next stage of their life-cycle in the marine environment. Growth is primarily controlled by temperature and resource availability, both of which can be affected by natural variations, but also by human impacts as well as the effects of a changing climate.
The rate of parr growth increases with the temperature of the water up to an optimum (~16-19°C). Above this, growth rate is reduced and if the water gets very warm (22-24°C) the fish are likely to become stressed and seek refuge in cooler areas. Growth is also regulated by food availability and habitat quality. Parr will grow fastest in rich habitats (containing lots of suitable resources), where food intake can be maximised at a minimum energetic cost. In practice, habitat quality varies markedly and can be significantly degraded by human impacts and other factors, and parr will expend energy defending territories and seeking refuge from predators to avoid being eaten.
Various factors thus affect survival and the rate at which salmon parr grow and this, in turn, can affect the age at which parr become smolts and migrate to sea. In addition, evidence is mounting that intrinsic factors carried over from parr freshwater life-history stage (i.e. smolt quality) are important determinants of subsequent marine survival rates.
Recent investigations
Understanding the requirements of salmon and the challenges they face at each life stage is vital to manging stocks effectively. Recent investigations have highlighted the following important issues for salmon parr:
- In at least some rivers in England and Wales, relatively large-scale downstream migrations of salmon parr have been identified in autumn, with a proportion of these fish residing in the lower tidal reaches before migrating to sea in spring. Such areas have previously not been regarded as important juvenile salmon habitat.
- The exposure of parr to low, sub-lethal levels of certain contaminants can have legacy effects which make the fish less able to adapt when they migrate to sea as smolts and increase the risk of mortality.
- Salmon parr display seasonal habitat preferences. In the chalk streams of southern England, this includes the use of instream macrophyte cover. Manipulation of instream habitat can be achieved by management of the bank-side canopy, and a detailed knowledge of parr habitat use can be integrated in to river restorations schemes.
The smolt stage is when the juvenile salmon move from freshwater to the feeding grounds in the sea. It is a period of significant change for the fish involving modifications to the body shape and colour, behaviour and physiology which pre-adapt them to a life in the ocean. The juvenile salmon become smolts after they have spent at least a year feeding in freshwater and when they have attained a length of 10 cm and above. River temperature and day length all have a role in controlling the development of parr to smolts. Smolts have evolved to migrate to sea in the spring when the sea temperatures and food availability are best for survival and growth. The timing of the smolt migration is therefore critical and fish entering the sea during this “window of opportunity” have the best chances of surviving and returning as adults. Smolts become adapted to a life in the sea whilst they are still in freshwater and the need to move to survive is thought to be one of the main factors initiating migration.
Smolts generally migrate to sea at night, although fish migrating later in the spring may move during both day and night, often shoaling together during daylight hours to confuse predators. The smolts also move through the river estuary at night and use the ebbing tide to assist their seaward migration. Movement is therefore rapid and, under the cover of darkness, the smolts are less visible to predators. Once in the sea the fish are known as post-smolts and they continue to migrate rapidly to their oceanic feeding grounds.
Smolt migration can be affected by a wide range of environmental and man-made conditions in our rivers and estuaries. For example, changes in flow and temperature regimes associated with climate change, and barriers to migration such as weirs, renewable energy schemes and barrages may delay the movement of smolts into the sea. Artificial light and noise may also delay or inhibit smolt migration. All these factors have the potential to mean that conditions in the sea when the smolts arrive may be less favourable and might affect their subsequent survival.
Recent investigations
Understanding the requirements of the salmon smolts and the challenges they face during their seaward migration is vital for manging stocks effectively. The use of tags and tracking to follow the movements of smolts from freshwater to the marine environment has been as important tool in the research on smolt migration.
Recent investigations have highlighted the following important issues for salmon smolts:
- The conditions that the smolts experience in the freshwater environment can have legacy effects and be important to their subsequent survival in the sea. For instance, some herbicides may delay migration and prevent the smolts from adapting to a life in the sea resulting in poor survival when they move into coastal waters.
- Some herbicides may also affect the sense of smell of smolts reducing their ability to remember their river of origin and return as spawning adults.
- Estuarine barrages delay the seaward migration of smolts and increase the chances of predation.
Further information on recent investigations related to salmon spawning are available here.
Over the past 20-30 years, there has been a marked decline in the abundance of Atlantic salmon across the species’ geographical range. Populations have been declining in most places and, in some cases, stocks have disappeared. Declines have generally been greatest at the southern extent of the range and have occurred despite significant reductions in the numbers of fish removed by fishing. The main reason for the decline has been a marked increase in the natural mortality of salmon at sea. As a result, stocks have seen a sharp fall in the proportion of fish that survive between their seaward migration as smolts and subsequent return to freshwater as adult fish.
On first entering the sea, young salmon become post-smolts and face a range of new challenges. The fish undertake extensive migrations and may stay at sea for a little over one year returning as adult one-sea-winter salmon (or grilse), or for two or more years to return as larger multi-sea-winter salmon. It has typically been assumed that mortality of salmon in the sea is highest in the early stages of marine residency, since predation is likely to be one important factor. Small fish are vulnerable to a larger range of predators than large fish and more predators occur on the continental shelf than in oceanic areas. However, numerous factors can affect the survival of salmon in the sea, although their relative impact and the interactions between them are poorly understood.
The movements of post-smolt and the distribution and habits of salmon while they are at sea are poorly understood. We know that smolts from England and Wales move north with the prevailing currents after leaving rivers and that high concentrations of post-smolts have been identified in the Norwegian Sea; this has been postulated to represent the summer ‘nursery area’. Thereafter, the distribution of the fish likely varies both with the time fish might spend at sea before returning and their region of origin.
Much of our current understanding of the ocean distribution of Atlantic salmon comes from the recapture of tagged fish and this has confirmed that salmon undertake long migrations to oceanic feeding areas. We are thus aware that some salmon from England and Wales can migrate as far as the west coast of Greenland, although this is restricted to older fish that spend more than one-winter at sea before returning home to spawn. Recaptures of different sea-age fish have also been reported from fisheries that previously operated north of the Faroes (winter months) and around the Irish coast (summer months) indicating that fish range quite widely around the North Atlantic.
Of course, the recapture of tagged fish only occurs in areas where there are fisheries and does not provide precise information on the migration routes between the marking site and the point of subsequent recapture. New tagging technologies are starting to shed further light on salmon movements at sea, but none of these are suitable for all purposes and all sizes of fish, particularly smolts.
The wide geographical extent of the changes in salmon marine survival, and the common patterns that have emerged, have long been considered to indicate that broad-scale changes in the marine environment are the main cause of the observed decline. These are thought to be linked to climate change, with stocks being negatively impacted by warming ocean conditions. Evidence from long-term monitoring programmes of marine processes indicates the existence of major fluctuations in parameters such as temperature, salinity and plankton productivity occurring over a similar timescale to the observed declines in many salmon stocks. While precise cause and effect relationships haven’t been determined, it seems highly likely that fluctuations of such fundamental biological significance in the marine environment are likely to be key processes in regulating the survival and abundance of salmon. While changing temperatures can have direct physiological effects on fish, temperature-driven changes in the availability of prey and the distribution of predators are considered to be more likely reasons for the observed changes in salmon survival.
Recent investigations
Understanding the requirements of salmon post-smolts and adults and the challenges they face during the marine phase of their life-cycle is important for manging stocks effectively.
Recent investigations have highlighted the following important issues for salmon at sea:
- Various studies have reported recent declines in the marine growth of salmon, and this has been linked to warming in areas where salmon are located at sea.
- Changes have been reported in the distribution, size and condition of capelin in the North Atlantic compared with 40 years ago. Capelin are an important prey species and a reduction in prey availability and quality will likely have negative consequences for the growth and survival of larger predators such as salmon.
- Investigations have suggested that the thermal conditions experienced by salmon post-smolts during their first summer at sea may be particularly important in regulating the growth and survival of stocks from Southern Europe, including those from the UK.
Adult Atlantic salmon return to their river of origin and can enter freshwater throughout the year. The salmon find their home river using the sense of smell (olfaction). The larger fish that have spent more than one year feeding at sea (multi-sea winter: MSW) tend to return earlier in the year. The smaller fish that have spent only one year at sea (grilse) return later. However, the timing and duration of the spawning migration depends on the population, the size and location of the river, and when the conditions in the river are favourable for upstream migration.
Water flow is the main environmental factor stimulating salmon to enter the river, although other factors such as water temperature, tidal cycle and water quality may all be important. Adequate flows are essential for the entry into the river and adults will often wait just off the coast or in the estuary until river flows are suitable. Once the salmon have entered the river, the spawning migration normally includes a quiescent period where fish reside for long periods within pools or below obstructions, where the water is well oxygenated. During this period the salmon do not feed and yet may be caught by anglers using a variety of baits, lures and artificial flies.
The final spawning migration, when the adults move from the pools to the spawning area, is controlled by river flow and temperature but also by the sexual maturity of the fish. During this period the sense of smell becomes more sensitive and the fish begin to detect the pheromones released by other mature salmon in the river. Small increases in river flow due to rainfall (freshets) are also important in stimulating the upstream movement of the fish to the spawning areas.
Various factors can impact on salmon during their upstream migration prior to spawning, for example, obstacles such as estuarine barrages, tidal lagoons, weirs and hydropower structures, as well as changes to the physical characteristics of the river. Climate change can also pose a threat to upstream migration by altering flows and other river characteristics.
Once spawning has been completed, any surviving fish return to the sea. These fish are known as kelts and they can contribute further to the population by returning a second or, exceptionally, third time to spawn. Seaward migration of the kelts occurs after a period recovering from spawning and is also controlled by river flow and temperature.
The adult spawning migration can be affected by a wide range of environmental and man-made conditions in our rivers. For example, barriers to migration such as weirs and renewable energy schemes may delay or inhibit the movement of the adult salmon. However, suitable river flows and temperature are the most critical factors in allowing the salmon to migrate successfully to the spawning grounds. Any reduction in the river flow due to abstraction and climate change (droughts) may also interfere with the timing and duration of the spawning migration.
Recent investigations
Understanding the requirements of the returning adult salmon and the challenges they face during the freshwater spawning migration is vital for managing stocks effectively. The use of tags and tracking to follow the movements of adults from the estuary freshwater to the spawning areas has been an important tool in the research on salmon migration. Research on the sense of smell and its role in controlling upstream migration and spawning has also improved our ability to conserve stocks of salmon.
Recent investigations have highlighted the following important issues for returning salmon:
- Suitable river flow and temperature are critical to the successful spawning migration of adult salmon.
- Some sheep dip insecticides and herbicides may affect the sense of smell of returning adults reducing their ability to remember their river of origin and detect the pheromones released by other spawning salmon.
- Barriers to migration, such as barrages, weirs and renewable energy schemes may delay or inhibit the upstream migration of spawning adult salmon.
- Climate change will have a significant effect on the migration of spawning salmon, influencing the amount of rainfall and variations in the water temperature.
An interactive salmon population model has been developed which enables you to see how changes at different times during the salmon life-cycle can affect a population. This is supported by a short explanatory film.