Caithness Field Club

The Atlantic Salmon (Salmon salar)
by Bob Walker

When the Romans came to Britain the salmon was present in the Thames in large numbers, impressed by the sight of this large silver fish jumping they called it Salmo the Leaper. What evidence we have suggests that the various species of Salmo evolved some 500,000 years ago, probably in fresh water. Its strange life style results from its need to spawn in fresh water and the fact that its growth rate demands a far greater food reserve than can be found in freshwater. The brown trout also goes to sea to feed in some cases, to become the sea trout, both are the same species. Again the rich feeding allows for greater growth rates in the sea trout.

The following description of the life cycle of the Atlantic salmon is numbered with reference to the diagram at the end of this article.

The hen salmon lays her eggs in November, December and January in gravel beds of well oxygenated and fast flowing streams, either feeder streams or the head waters of the parent river. She cuts a groove in the gravel using powerful beats of her tail, this groove is her spawning redd. She will make several of these over a period of days and will lay eggs in them all. A fish will produce about 800 eggs for each pound of her body weight. When she is ready the male or cock fish moves up alongside her and begins to quiver, these movements stimulate the hen to release eggs, as she does so the cock releases his milt containing the sperm over them and the eggs are fertilised. The fertilised eggs fall into the groove and the hen covers them with fresh gravel swept in by her tail. Many eggs are lost at this time. In the Pacific salmon which breeds in huge shoals, the eggs are washed up on the river banks in their millions, forming drifts. At this point all of the adults die and it is believed that the dead adults and eggs are essential for enriching the river. However in the case of the Atlantic salmon some adults survive to return to sea, but the majority die.

The eggs under the gravel slowly develop over a period of between three and five months, the actual time depends on water temperature, protected from predators and strong currents.

Eventually the eggs hatch into alevins which feed on their yolk sac for about a month before becoming fry.

The fry has to fend for itself and will grow into a parr fairly quickly depending on the available food supply. Obviously the more it eats the faster it grows.

The parr with its distinctive finger marks on its sides, which act as camouflage, feed on tiny crustaceans and larvae. At this stage the main predator is the trout. If you ever visit Loch More when the trout from the river are running up into the loch to reach their spawning grounds in the feeder streams, you might see the ghillies trapping them. They take them to other lochs for stocking, but the real aim of the exercise is to reduce the numbers of this predator. Depending on how well it feeds, the salmon remains as a parr for between 16 months and 5 years.

It now becomes physiologically adapted to live in salt water and develops its silvery coat. It is now a smolt of some 6 inches in length and it goes to sea. Here they move to the area west of Greenland where they feed on the hugh shoals of shrimp like creatures called capelin.

After a year they will be up to thirty times their weight as a smolt. They may return to the river to spawn as grilse, fish between 3 and 10 pounds in weight. If they stay longer they are maiden fish. A two year maiden may weigh 10 to 15 pounds and a four year one can weigh 30 to 40 pounds.

There are records of salmon over 70 pounds caught in British waters last century, but the definitive record belongs to a 64 pound cock fish caught by Miss G.W. Ballantine in 1922 on the Tay. There are also modern stories of pieces of salmon found on the shore, usually a head or a tail which "experts" claim could have been well above the record when whole.

How the fish navigate to find their own river of birth is a mystery, but fine it they do, possibly chemicals in the river act as scent trails, possibly even pheromones released by their own clan of salmon parr in the river.

Salmon face human threats to their existence. Pollution, especially acid rain is badly damaging some rivers, especially in Scandinavia. Drainage for forestry planting can silt up the redds. Forestry can also lower water levels in streams in the winter allowing the redds to become frozen. Tagging allowed man to find the feeding grounds off Greenland and overfishing has had a big impact. Fish returning have to run the gauntlet of miles of monofilament drift nets, stake nets and river mouth netting before they face the angler, who in all honesty has little impact on the salmon numbers. This is on top of the seal's and otter's predations. The stake nets in Thurso bay were not deployed this year following a disastrous season in 1994. Many fish escape the nets with damaged skin and are prone to fungal diseases.

Another source of disease is believed to be escaped farm fish which can enter rivers, the upsurge in salmon cages around our coast not only concerns the salmon fisheries but other conservation bodies. The waste falling to the sea bed below the fish cages can do much damage, as can the addition of drugs and other chemicals to combat salmon diseases and parasites. Work on cleaner fish which would live beside the salmon and feed on their parasites might be a blessing to all concerned.

Recent research has shown some adult males who never leave the river, staying at about half a pound in weight. These males can sneak in to fertilise some of the eggs when a female spawns, the long term effects of this mutant gene on the salmon is hard to assess. With the pressures against the salmon at sea mounting it could well be that this stay at home gene could increase as a result of natural selection.

Experiments where adult salmon are put into fresh water lakes and lochs for anglers to catch have had some success, but it is expensive and the fish will not survive or spawn in a body of water with no in-flowing or outgoing streams.

by Geoff Leet

Mr Ken Dixon of Lamigoe, Skerra, drew the attention of the Field Club to this inscribed stone near Torrisdale. The picture, 60" x 18", is East facing on one of a pair of free-standing rocks near the bend in the burn.

Mr Dixon believes that the stone was recorded on the Ancient Monument Inventory in 1907. The map reference is NC 664617.

About 300 yards due South of the stone the burn runs West of a massive outcrop on which are inscribed names and 1875, Torrisdale. This raises the possibility that the salmon is also of recent date.