Salmon and Sea Trout Facts

The Atlantic Salmon Trust aims to be the definitive resource for information about salmon and sea trout, their remarkable lives and the threats that today are jeopardising their very survival.

Salmon parr (I) can normally be distinguished from young brown/sea trout (II) by the more streamlined shape, deeply forked tail, longer pectoral fin, lack of orange on adipose fin, smaller mouth, sharper snout, only 1-4 spots on gill cover (often one large spot), well defined parr marks.

Salmon Smolts
When the salmon parr begin to migrate to the sea, usually in March, April and May, they gradually become more elongated and the fins darken. A layer of guanine crystals is laid down in the skin. rendering the body more silvery in colour and obscuring the spots and finger-marks, except on the gill-covers. They then become Smolts.
Salmon Smolts

How do you tell the difference between Salmon and Sea Trout?

Salmon (I) can be distinguished from large sea trout (II) by a more streamlined shape, concave tail, slimmer tail wrist, upper jaw reaching no further than rear of the eye, few if any black spots below lateral line, 10-15 (usually 11-13) scales counted obliquely forward from adipose fin to lateral line – trout have 13-16.


Salmon Sea Trout
General appearance Slender and streamlined More round and thickset
Head Pointed More Round
Position of the Eye Maxilla (bony plate usually alongside mouth) does not extend beyond rear rear of eye Maxilla extends beyond eye
Colour Relatively few spots Often heavily spotted
Scale count (number from adipose fin to lateral line) 10-13 13-16
Fork of tail Usually forked Usually square or convex
Wrist of tail Slender Broader
Handling Easy to pick up by tail


What should fishermen look out for particularly in connection with any salmon or sea trout they catch?

Biologists who work with salmon and sea trout often mark the fish that pass through their hands in any of the ways depicted below. Capture of a marked salmon should be reported at once to the appropriate fishery authority. Frequently the address is on the tag or mark. Usually the information required is: date, place and method of capture; length, weight and sex of fish and a sample of scales taken from between the dorsal and anal fins at above the lateral line.


  • Tagging should be carried out only by trained and authorised personnel.
  • Fins that have been clipped do, with the exception of the adipose, regenerate.
  • The adipose fin should not be removed, as clipping is internationally recognised as an indication that the fish has been micro-tagged.
  • If adipose clipped fish are killed, the head (where the micro-tag is located) should if possible be sent to the nearest fisheries laboratory.

What does anadromous mean?

The Atlantic salmon and sea trout is referred to as being anadromous because of its habit of migrating from the sea into fresh waters to spawn. This is the exact opposite of the common eel which leaves fresh waters to spawn in the Sargasso Sea, and is therefore called catadromous.

What is osmoregulation?

Osmoregulation is the control of the levels of water and mineral salts in the blood. All fish which migrate from fresh water to salt water during their life cycle must go through this process.

The salmon is an excellent osmoregulator. However, like virtually all osmoregulators, the salmon is never in true equilibrium with its surroundings. In the ocean, the salmon is bathed in a fluid that is roughly three times as concentrated as its body fluids, meaning that it will tend to lose water to its surroundings all of the time. And, because the composition of its body fluids is so different from the ocean water, the salmon will be faced with all manner of gradients that are driving exchanges that will continuously tend to drive its body fluids’ concentration and composition beyond homeostatic limits. In particular, the very high concentration of NaCl (sodium chloride) in the ocean water relative to its concentration in the salmon’s body fluids will result in a constant diffusion of NaCl into the salmon’s body. Unless dealt with effectively, this NaCl influx could kill the salmon in a short time. In sum, a salmon in the ocean is faced with the simultaneous problems of dehydration (much like a terrestrial animal) and salt loading.
However, in fresh water, the problem is basically reversed. Here, the salmon is bathed in a medium that is nearly devoid of ions, especially NaCl, and much more dilute than its body fluids. Therefore, the problems a salmon must deal with in fresh water environments are salt loss and water loading.

How Does The Salmon Solve Its Osmoregulatory Problems?
Fortunately, the salmon has some remarkable adaptations, both behavioral and physiological, that allow it to thrive in both fresh and salt water habitats. To offset the dehydrating effects of salt water, the salmon drinks copiously (several litres per day). But in fresh water (where water loading is the problem) the salmon doesn’t drink at all. The only water it consumes is that which necessarily goes down its gullet when it feeds. Of course, when an ocean-dwelling salmon drinks, it takes in a lot of NaCl, which exacerbates the salt-loading problem. Kidney function also differs between the two habitats. In fresh water, the salmon’s kidneys produce large volumes of dilute urine (to cope with all of the water that’s diffusing into the salmon’s body fluids), while in the ocean environment, the kidneys’ urine production rates drop dramatically and the urine is as concentrated as the kidneys can make it. The result of this is that the salmon is using relatively little water to get rid of all of the excess ions it can.

Time course of the salmon’s acclimation responses
The behavioral (drinking or not drinking) and physiological changes a salmon must make when moving from fresh water to salt water — and vice versa — are essential, but cannot be accomplished immediately. Thus, when a young salmon on its seaward journey first reaches the saline water at the mouth of its home stream, it remains there for a period of several days to weeks, gradually moving into saltier water as it acclimates. During this time, it begins drinking the water it’s swimming in, its kidneys start producing a concentrated, low-volume urine, and the NaCl pumps in its gills literally reverse the direction that they move NaCl (so that they’re now pumping NaCl out of the blood and into the surrounding water.
Likewise, when an adult salmon is ready to spawn and reaches the mouth of its home stream, it once again remains in the brackish ( i.e. less concentrated than full-strength sea water) water zone of the stream’s mouth until it is able to reverse the changes it made as a juvenile invading the ocean for the first time.

Can anything be done to counter the problems caused by the growth of salmon farming?

Codes of best environmental practice are being developed and improved. These include use of the principle of integrated sea lice control, through co-ordinated fallowing within sea lochs and bays to break the cycle of sea louse survival, followed by co-ordinated treatment to prevent cross infestation. Legislation to enforce these codes is essential. The Scottish Executive set up the ‘Tripartite Working Group’ in 1999 which involves fish farming and wild fish intersts. A sub-group – Restoration – chaired by the Trust’s Director has the remit to conserve and improve wild fish stocks. The Trust’s Biologist spends much of his time acting as the sub-group’s coordinator of these activities on the west coast of Scotland.

What do parr feed on when they are in fresh water?

The larvae of aquatic insects and other aquatic invertebrates together with terrestrial insects which fall into the water.

Is the growth of salmon farming significant for wild salmon and sea trout stocks?

Farmed salmon production in the North Atlantic area has increased dramatically, particularly in Norway, but also on the west coasts of Ireland and the Scottish Highlands.

A number of problems have resulted, which include:

  • High concentrations of sea lice, which multiply in the confined conditions of sea rearing cages. As explained earlier, migrating sea trout and salmon smolts can be very vulnerable to attack by these lice. In some rivers, wild stocks have virtually collapsed.
  • Escapes of farmed fish, which are known to be able to interbreed with wild fish. Since stocks in individual rivers are locally adapted to optimise their survival, this interbreeding has been shown to reduce the fitness of wild stocks for their local environment.
  • Pollution of the water environment, by uneaten food, fish faeces, or medications used to treat farmed salmon in their cages.
  • The risk of the spread of disease or parasitic infestation, such as Infectious Salmon Anaemia and Gyrodactylus salaris.

Do all Atlantic salmon migrate to sea?

No. Although most Atlantic salmon spend part of their lives at sea there are some which are non-migratory. In several lakes in eastern North America there is a form known as a land-locked salmon, Salmo salar sebago (Girard), though their access to sea is not barred. The fish is popularly called Ouananiche (Lake St. John) or Sebago salmon (Nova Scotia, Quebec, New Brunswick, Newfoundland and the New England States). In Lake Vänern in Sweden there is a non-migratory form of Atlantic salmon called “blanklax”. Land-locked Atlantic salmon also occur in Lake Ladoga in Russia and in Norway in Lake Byglandsfjord. There are also land-locked Atlantic salmon in South Island, New Zealand.

What is a grilse?

A grilse is an Atlantic salmon which has spent only one winter at sea before returning to the river. Salmon grilse are often indistinguishable from multi sea winter (MSW) salmon except by scale reading. They are smaller on average (2-3lb in May, 5-7lb in July) but when they enter rivers in September often attain 8-10lb and in October 12-15lb.

How big can a salmon grow?

Atlantic salmon can grow to a very large size and the biggest, which have reached up to around 70lbs (32kg), are usually caught in Norway and Russia. However, some very large fish have been recorded in Scottish rivers. It is generally accepted that the largest one caught on rod and line in the UK was taken by Miss Georgina Ballantyne in the River Tay in 1922: it weighed 64lbs (29kg). There is an 1891 report of a huge salmon of 70lbs, also caught in the River Tay, but on this occasion in a net.

What is the largest salmon ever recorded?

The largest recorded Atlantic salmon, a male caught in Norway’s Tana River, weighed 35.89 kg. and was over 150 cm. in length.

How have salmon stocks changed over the years?

All around the North Atlantic, stocks have been in general decline over a number of years. Some stock components, such as early-running or ‘spring’ fish, have suffered particularly badly. Actual stock levels are difficult to estimate, except on rivers with reliable counting facilities, but catch figures can be used to give an indication, particularly of trends.