More details of the background to the modeller, of how salmon stocks work and of the assumptions
underlying the model can be found on the
The AST and the Institute of Fisheries Management wish to acknowledge the late John Gregory, for his
encouragement and help with the development of the modeller. It was largely inspired by the
description of salmon stock dynamics in the 2012 IFM publication: Code of Good Practice for
Freshwater Fisheries Management - (https://ifm.org.uk/wp-content/uploads/2016/02/IFM-Final.pdf )
We are also grateful to Ted Potter for devising the basic model and then helping to refine it and
take it forward, to Simon Walter (Online Integrity) for developing it as a web tool, and to Jonathan
Institute, Ireland), Ian Davidson, (Natural Resources Wales), Ronald Campbell (Tweed Foundation) and
Simon McKelvey (Cromarty Firth Fishery Board) for their advice on key aspects of the model.
Q & A
What is the modeller based on?
The modeller is an illustrative one, based on a pristine but reasonably typical river stock, but with
current low levels of marine survival. It does not attempt to show the full complexities of a stock
annual fluctuations in response to changes in river conditions, nor does it show the process by
the stock will reach equilibrium. It is not designed to represent an actual river stock.
Why is the modeller based on a pristine stock and why cannot losses be reduced for some stages?
All stocks in Britain and Ireland are impacted to varying extents by human activities, and it would
very difficult to come up with a typical impacted stock. Using a theoretical pristine (ie
stock as the base line for the modeller makes it possible to illustrate the effects of impacts at
different life stages. It is then possible to show the effect of reducing these losses caused by
impacts by reversing the process.
Why does the modeller show the change to the equilibrium stock numbers rather than the immediate
effect of new impacts?
The number of salmon that return to a river is affected by the conditions that the fish face
their lives. Under completely stable conditions the population would be expected to reach an
state where the numbers at each life stage will, on average, be the same from one generation to the
next. A change to a new set of conditions would cause the population to move to a new equilibrium
after several generations. In real life, numbers of even a stable stock will fluctuate, sometimes
dramatically, from year to year, but the modeller removes this year-to year variation to show how a
typical stock is affected on average by changes in survival at different stages. This is why, for
example changing parr numbers will also change egg numbers: what is shown are the changes to average
numbers once the stock has reached a new equilibrium.
Is the modeller applicable across the range of the Atlantic salmon?
No, it is designed to represent an average salmon population in the UK and Ireland. It would be quite
different in many respects for more northern stocks, for example, which tend to spend longer in
freshwater. However, the same, key limitation points would exist!
What happens in a real salmon population?
The modeller represents a highly simplified salmon population. A real population will be more
with juveniles remaining in freshwater for variable lengths of time, producing 1, 2 and 3 year-old
smolts. Adults may also spend 1, 2, 3 or even 4 years at sea and some fish will return to sea as
and come back again as previous spawners. Both natural and human impacts on stocks will also vary
year to year. In addition, both natural conditions and human impacts are always changing, so the
equilibrium point will be moving.
How long does it take to reach equilibrium in the real world?
This is highly variable and will depend on the extent of the impact and the nature of the stock. If a
stock was, for example, suffering from poor marine survival for a number of years, it would not be
unusual for a recovery to take at least 10 years to manifest itself. In reality natural conditions
human impacts are likely to change more rapidly than this, so the equilibrium point will be
What type of factors are most likely to block or delay a population from reaching equilibrium?
Factors such as marine survival, over-exploitation, habitat degradation can all exercise a major
influence on the overall stability of a salmon population.
What assumptions about average survival between life stages is the modeller based on?
The survival percentages underlying the modeller are typical of many UK and Irish salmon stocks, but
not based on any particular river.
Why are there limits on the changes you can make at some life stages?
These are necessary to prevent the modeller generating unrealistic outcomes. The limits are
but do not prevent the model from demonstrating the different impacts that changes to survival can
at different life stages.
How it Works
No two salmon stocks are the same, and this simplified demonstration of salmon stock dynamics
reflects the biology and life history of an average stock in the British Isles. The stock can
comprise ( by altering the option under the heading Composition of stock) either grilse
only or a
mixture of grilse and multi-sea-winter (MSW) salmon; in this mixed stock, 50% of the fish mature at
the end of the first year at sea and the remainder remain at sea for another year. Actual stocks
have many different proportions of grilse and salmon, and these can change over the years.
Using the model
The starting state of the model represents a pristine stock with no adverse impacts such as
or pollution in freshwater.
The model allows the user to change the outcomes of the various stages of the life-cycle of the
by clicking on the various buttons to:
change the carrying capacity of the river for fry and parr;
impose pressures (i.e. reducing the survival) at different life stages;
change survival rates at sea; and
introduce exploitation of returning adults by nets and/or rods.
The effect of these changes (green is positive, red is negative) is reflected in the numbers of fish
each stage - and ultimately in the viability of the stock - when the stock reaches equilibrium after
number of generations. (NB: this is not the immediate effect in the same, or the next,
The system can be reset to the pristine state with the button situated on the results panel.
This modeller is designed to provide anglers, managers and everyone interested in salmon with a
clearer understanding of how salmon populations work: i.e. the relationship between the number of
spawning fish, the number of juveniles they are likely to produce, and how many of these are likely
to survive, and, in their turn, spawn. The modeller also illustrates the effect that pressures at
different life stages can have on salmon numbers.
It is important to note that the modeller is an illustrative one.
It is not designed to represent an
actual river stock, and it cannot be used as a management tool to show the short-term impact on
salmon numbers of measures affecting an actual stock.
The modeller is based on a pristine (ie not impacted by human activity) but
reasonably typical river
stock with current conditions at sea. It shows the effect that impacts on the stock will have on
numbers at different life stages once the stock has reached a new equilibrium after
a number of
generations - this is not the immediate effect in the same, or the next, generation.
Details of how to use the modeller can be found at How it Works
and answers to questions about the
model, including explanations of why it is based on changes to the equilibrium of a pristine stock,
are in Q & A. More details of the background to the
modeller, of how salmon stocks work and of the
assumptions underlying the model can be found on the AST
The Salmon Life Cycle
The Atlantic salmon is an anadromous species, which means that the fish spawn in freshwater but
migrate to sea for part of their life to grow and mature.
Adult salmon lay their eggs in fresh water, in the autumn or winter, by burying their eggs in gravel
redds. After the eggs hatch, the embryonic alevins remain in the gravel, drawing nourishment from
their yolk sacs. When their yolk reserve is almost exhausted the young fish (fry) emerge from the
gravel, disperse and begin to feed, growing into parr.
The juveniles (parr) set up territories in suitable fast-flowing water, which they defend against
competitors; this tends to impose a limit on the population size, often referred to as the carrying
capacity of the stream. Once they attain a size of 10 to 20 cm - usually after one to three years -
the parr undergo morphological, physiological and behavioural changes to become smolts in
preparation for migrating to sea in the spring.
The smolts emigrating from European rivers move northwards towards the Northern Norwegian Sea;
thereafter their migration routes are poorly understood, although the fish destined to return after
two or more years at sea feed primarily along the coast of West Greenland – the bulk of the European
salmon at west Greenland are from Scotland and Ireland.
Salmon return to freshwater after one to three (or occasionally more) years at sea; those that
return after one year are referred to as grilse or one-sea-winter (1SW) fish. They are generally 3lb
to 7lb in weight, while the older fish are called multi-sea-winter (MSW) and weigh 8lb to 20lb.
Those fish that stay at sea for 4 years or more can reach much larger sizes, 30lb to 45lb (15kg to
Salmon of different sea-ages tend to return at different times of year and often spawn in different
parts of a river. The age structure of the stock is therefore an important part of its diversity
which may support both optimum production throughout the system and provide fishing opportunities
over a large part of the year.
The model can illustrate a salmon stock, comprising either grilse only or a mixture of grilse and
multi-sea-winter (MSW) salmon.
In the mixed stock, 50% of the fish mature at the end of the first year at sea and the remainder
remain at sea for another year.
Adult salmon lay their eggs in fresh water, in the autumn or winter, by burying them in gravel redds.
Each female will lay about 600 eggs per pound of her body weight. After hatching, the embryonic
alevins remain in the gravel, drawing nourishment from their yolk sacs. When their yolk reserve is
almost exhausted the young fish (fry) emerge from the gravel, disperse and begin to feed. The number
in the box represents the number of eggs laid. In a clean, unimpacted river with plenty of gravel,
at least 90% can be expected to hatch.
Carrying capacity for eggs and fry
Salmon eggs require clean gravel to survive, and production in a river may be affected by lack of
spawning gravel or habitat for the fry when they first emerge from the gravel.
Impacts on eggs and fry
Egg survival is most commonly affected by sediment limiting the supply of oxygenated water to the
eggs. However, they may also be affected by such factors as pollution, acidification and high water
As the juveniles grow, they disperse more widely; the number in the model is for the number after
their first summer, when they are referred to as parr. Parr set up territories in suitable
fast-flowing water, which they defend against competitors; this tends to impose a limit on the
population size, often referred to as the carrying capacity of the stream.
Carrying capacity for parr to smolts
Juvenile salmon need specific conditions within a river to survive. As the young fish grow, they
compete for space and the weaker are pushed out, to starve or be eaten by predators. However many
eggs hatch successfully, the number of parr that survive to become smolts will be limited by the
river's carrying capacity. Opening access to more streams or creating more habitat for parr will
increase the carrying-capacity and thus the production of smolts.
Impacts on parr to smolts
Carrying capacity constraints mean that natural losses of parr are substantial. The greatest
additional impact at the parr stage is likely to be water quality. Predators may also reduce parr
numbers, although this should only be a concern where predation levels are unnaturally high.
Once the parr attain a size of 10 to 20 cm - usually after one to three years - they undergo
morphological, physiological and behavioural changes to become silvery smolts and prepare for
migrating to sea in the spring. The number in the model is the number of smolts at the start of this
migration which will usually be around 1% or less of of the total number of eggs laid. Salmon
smolts migrate to sea between late March and early June in the British Isles.
Impacts on emigrating smolts
Losses during their downstream migration can be high and directly affect the number of returning
adults. In rivers in their natural state smolts form shoals and move rapidly, usually at night,to
minimise predation. Anything that delays them, such as barriers, exposes them to increased losses
from predators such a fish-eating birds and other fish. Hydropower installations may pose a
particular problem through losses and damage from turbines. Delays also increase stress on smolts,
which can adversely affect their physiological adaption to salt water, and so their chances of
survival at sea.
Salmon at sea
The smolts that leave European rivers move northwards as post-smolts towards the northern Norwegian
Sea; thereafter their migration routes are poorly understood, although some of the fish destined to
return after two or more years at sea will migrate as far as the coast of west Greenland. The bulk
of the European salmon at west Greenland are from Scotland and Ireland.
The grilse and salmon return rates are the percentages of the total numbers of emigrating smolts
(see previous box) that return to the river after one year or longer respectively. For the mixed
stock (grilse plus salmon) this means that because only half the smolts return as grilse, the return
rate for grilse appears lower than for the grilse only model.
Impact on marine survival
Salmon at sea are thought to have been adversely impacted in recent years by changing environmental
conditions and changes in the density and variety of plankton species in the surface layers of the
ocean. Concerns have also been expressed regarding the possible impact of by-catch of salmon, taken
in pelagic fisheries for mackerel and herring.
Returning Grilse and Salmon
Salmon return to freshwater after one to three (or occasionally more) years at sea; those that return
after one year are referred to as grilse or one-sea-winter (1SW) fish; they are generally 3lb to 7lb
in weight. Salmon returning after more than one winter at sea are called multi-sea-winter (MSW), and
are often referred to as ‘salmon'; The majority of MSW salmon return as two sea-winter (2SW) fish,
but a small number return as 3SW or even 4SW salmon and can reach sizes of 30lb to 45lb (15kg to
20kg). In the past the numbers of 3 and 4SW fish were significantly higher The longer time spent at
sea means that more fish die, so the total number of fish returning under the mixed stock scenario
is lower than with the grilse only one.
Impacts on returning grilse and salmon
Salmon of different sea-ages, returning at different times of year, often spawn in different parts
of a river. The age structure of the stock is therefore an important part of its diversity which may
support both optimum production throughout the system and provide fishing opportunities over a large
part of the year. On their return to their native estuaries and rivers salmon may be impacted by net
fisheries in coastal waters and estuaries, and angling also causes losses, even if the great
majority of fish are returned. Low flows may prevent fish entering rivers, leading to losses from
predation and other causes, and their migration upstream may be impeded by barriers or poor water
quality, particularly in low flow conditions, causing further losses.
Even without human impacts, not all returning salmon and grilse survive to spawn (the modeller
assumes a loss rate of 5%), but those that do reach spawning gravels then start the whole cycle
again. The eggs produced by each female may be fertilised by several males, including precocious
parr; this helps ensure genetic diversity.
The conservation limit for a salmon river stock represents the minimum number of spawners needed to
ensure that, over time, that stock will thrive. If numbers consistently fall below the conservation
limit, there is an increasing risk that the stock will collapse. Because salmon numbers fluctuate
widely from year to year, reflecting changing conditions in rivers (low flows, floods, etc.) and at
sea, failing to meet a conservation limit from time to time is not a cause for real concern, but it
is important to ensure that in most years salmon numbers in a river exceed the conservation