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 website

Acknowledgements

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 White (Marine 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.

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 with annual fluctuations in response to changes in river conditions, nor does it show the process by which 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 be very difficult to come up with a typical impacted stock. Using a theoretical pristine (ie unimpacted) 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 these 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 throughout their lives. Under completely stable conditions the population would be expected to reach an equilibrium 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 state 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 complex, 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 kelts and come back again as previous spawners. Both natural and human impacts on stocks will also vary from 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 and human impacts are likely to change more rapidly than this, so the equilibrium point will be moving.

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 are 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 arbitrary, but do not prevent the model from demonstrating the different impacts that changes to survival can have at different life stages.

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 obstructions or pollution in freshwater.

The model allows the user to change the outcomes of the various stages of the life-cycle of the salmon 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 at each stage - and ultimately in the viability of the stock - when the stock reaches equilibrium after a number of generations. (NB: this is not the immediate effect in the same, or the next, generation.)

The system can be reset to the pristine state with the button situated on the results panel.

Comments or queries

Please send any scientific comments or questions about this model to The Atlantic Salmon Trust and any IT-related queries to Online Integrity.

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 20kg).

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.

An interactive movie illustrating all this in more detail, can be found on the AST website at: www.atlanticsalmontrust.org/salmon-lifecycle