Tay District Salmon Fisheries Board

Protecting and Improving The Tay System


The mysterious migration

Once they enter sea not a great deal is known about the salmon's migration. It does appear from attempts at netting that the smolts move away from the coast very quickly and recent experimental fishing for smolts at sea has shown they migrate north pretty quick. No-one knows exactly what happens to Tay smolts but oceanographers from the Marine Laboratory in Aberdeen have speculated that they most likely move well out into the North Sea before taking advantage of northerly flowing currents over towards Norway. it is then thought they would use these currents to move out into the Norwegian Sea.

An abundant larder

Once out in the ocean the salmon make for rich feeding areas in the northern ocean, into areas mixing of currents causes nutrients to be drawn up from the deep ocean into the productive surface layers. The salmon seem to spend much of their time, by day at least, very close to the surface, but they can also be found deeper down, especially at night. At sea the salmon appears to be a pretty voracious predator and eats small crustaceans and other fish which it encounters. Consequently they grow fast relative to most sea fish, growing from a few ounces to three pounds in 12 months, to eight pounds in 18 months and about 12 pounds in 24 months.

Grilse and salmon

After some time at sea the process of sexual maturation commences. Once this physiological process has kicked in the fish then commence their long return leg. However, the length of time it takes for this process to start differs between fish. Some fish will actually start to mature after only as little as a year at sea. On their return from the sea such fish are relatively small and are known as grilse. However, some fish do not mature for another year. By convention such fish are known as 2 sea winter salmon. But there are some fish which remain for longer and are known as 3 sea winter salmon. Occasionally some spend a fourth winter at sea. By and large these bigger salmon are not fish which have survived to spawn several times, but like grilse are also returning to spawn for the very first time.

What determines whether a fish returns as a grilse or an older salmon?

Multi sea salmon
The Tay is famous for its MSW salmon
Experiments on fish farms have shown that the speed at which salmon have grown influences the age at maturity. If food is abundant and the fish are growing fast, more fish will be inclined to mature early as grilse.

However, what also seems to be very important is genetics. Some strains of salmon have been found to produce a greater or lesser proportion of grilse than others. In the past for example, the Tay was famed for producing a high proportion of large 3 sea winter salmon while the Tweed, Dee or Spey did not. At sea it appears that fish which return as grilse do not travel as far as fish which mature later; a pretty natural conclusion! Grilse do not appear to travel too far out into the Atlantic but those which return as older salmon have been found to travel as far as the western coast of Greenland. Fish which will return as grilse do not get that far.

The home leg

Once the process of maturation has started, the salmon head back from the ocean. They are able to navigate back to their home localities and once the coast is reached they can detect the river of their birth through its odour. During this process it seems that the great appetite which salmon obviously have at sea wanes, as it is suppressed by hormonal changes. By the time the salmon is near to freshwater some have already stopped feeding altogether.