nature

The Salmon Leap & life cycle of the salmon

salmon

When the glaciers melted after the last Ice Age in the UK, the Highland landscape was formed, creating valleys and lochs. Erosion by debris, boulders and gravel led to the formation of the River Shin and the spectacular waterfalls.

The River Shin is one of the great salmon rivers in Scotland and is one of the rivers, including that of the Cassley and Carron that run into the River Oykel.

The surrounding area consists of many hills and a large number of streams and burns bring water down to form the river. The water appears slightly brown due to the peaty soil.

The water in the River Shin is fresh water and is superbly clean, the perfect conditions for the Atlantic salmon and other fish such as trout that require a constant flow of fresh water.

The Falls of Shin are a natural feature of the River Shin, next to the visitor centre. Visitors each year can watch Atlantic Salmon battle to return to their place of birth to spawn the next salmon generation.

Salmon Life Cycle

The life cycle of the salmo salar, the wild Atlantic salmon is extraordinary with the fish enduring great extremes throughout their lives.

It is in late autumn that the cycle begins with the cock fish (male parent) fertilising the thousands of eggs which the female has laid in a “REDD” or hollow of fine gravel at the top of the river near its source. The parent fish, now called ‘kelts’, cover up the eggs and drift back down- stream and back out to sea.

The eggs, or OVA remain in the gravel for about five months, before hatching out into alevins. Having survived on the yolk sacs of their eggs, the baby fish (now called fry) start their lives by moving into the river, feeding on the tiny organisms that live in the river. With the constant threat of predatory fish and birds as well as flooding, about a quarter of the original hatch will not survive.

The fry that have survived grow to become three or four inch long parr and then continue to live in the river for two to three years, feeding on the tiny organisms that live in the river, before becoming smolts and going out to sea in spring. Some smolts stay close to the shore, living on the many sprats and sand eel and then return to the river as grilse after about a year, weighing several pounds. Other smolts will travel further from the river, travelling thousands of miles across the Atlantic to the feeding grounds of south east Greenland and Iceland.

Having spent two or more winters at sea the adult salmon, of which only a small percentage will have survived, start their return journey home to the River Shin to spawn for the first time. The urge for the salmon to return to the precise place of its birth is instinctive and irresistible. During this journey they will encounter otters, seals, sea birds, wild mink and predatory birds. Another major threat posed to them is man, as many are caught by the deep sea fishing nets or killed by pollution. If they surpass all these threats they now face their final challenge.

It is during the months of February and September that salmon can be seen hurling themselves up the Falls of Shin in a bid to reach their home. Before they attempt to jump, the fish listen to the water to judge how much is actually falling over the falls. This is a key factor that affects their ability to survive the jump, if there is enough water in the river and the temperature of the water is right, then is there enough oxygen in the water for the salmon to survive.

The salmon require tremendous power to leap and they develop what scientists call “burst speed”. This requires using anaerobic muscles rather than aerobic which is used for swimming, which can contract quickly and generate intense bursts of power, lasting only a few seconds and propelling themselves forward at incredible speeds, which can be up to eight metres per second.

If the leap fails, it can take hours if not longer for the salmon to recover to attempt it again and in fact many fish die in the process. The River Shin is only a small river, about five miles long, but the drop from Loch Shin to the Kyle of Sutherland is 270 feet. These brave fish fight against great adversity to return home to their place of birth.

There has been a dramatic decrease in the number of baby salmon since the Hydro Electric dam at Lairg was built. The dam has unfortunately affected the ability of salmon to spawn on the river.

An Act of Parliament has ensured that sufficient water is now released over the dam and into the river, up to 70 million gallons in mid summer to allow the fish to pass up and down the river freely.

To ensure there are sufficient stocks of fish, each year the Kyle of Sutherland District Fishery Board strips the eggs from the brood hen fish and develops them at the hatchery near Ardgay as part of stock conservation. They then release 500,000 in late April. However, only about one per cent of the original ova will survive to become young or adult salmon.