Of the many obstacles that the migrating salmon faces on its way upstream, one of the most significant is a dam, usually built as part of a hydroelectricity scheme. While salmon have a powerful ability to jump, clearing waterfalls up to 12ft (3.6m) high, a dam wall, usually somewhere between 65-165ft (20-50m) high, often far exceeds the fish’s ability. In 1943, an Act of Parliament required the North of Scotland Hydro-Electric Board to preserve fish stocks in the waterways it used. So, when Pitlochry Dam was built, between 1947 and 1952, its design included the first fish ladder of its kind, enabling salmon to pass to and from their spawning grounds above the newly created Loch Faskally. The 340yd (311m) fish ladder is made up of 34 pools, 26ft (8m) long. Each of the pools is 7ft (2m) deep and stands 19in (50cm) higher than the preceding pool, with a 3ft 3in (1m) opening below the water level, so the salmon can swim up from one pool to the next. Three larger pools allow the fish to rest, and a counter records the number of fish migrating up and downstream each year. More than 250,000 salmon have traversed it since it was built. On average, 4,000-5,000 salmon climb the ladder each year, though the last two years have been much lower, with 2,749 in 2020 and 2,637 in 2021. It has become one of the most popular salmon sighting spots in Scotland. The best locations to see them are cold and very clean rivers, typically in remote and mountainous areas, such as the Falls of Shin on the River Shin in Sutherland, Stainforth Force on the Ribble in the Yorkshire Dales, and Cenarth Falls on the River Teifi near Cardigan. Visiting in the early morning or evening on a calm day following rain, from mid-October to mid-November, offers the best chance.
buried approximately 12in (30cm) deep in the redd. To help secure them in times of flood, the eggs stick both to one another and the gravel bed. “Spawning sites can be anywhere along the river, from high in the uplands to 200m from the tidal limit, close to the sea,” says Mark.
“The earliest fish in Scotland typically spawn from October, and this goes on until January, with a wave of spawning coming down a catchment. Eggs are in the gravel until February or March, and then the fish hatch.”
The incubation period varies depending primarily on water temperature. As it nears its end, the egg envelope, or chorion, becomes thinner. The eyes of the salmon can be seen through it, and the wriggling embryos inside stimulate one another into movement and hatching.
The emerging alevins, at 0.4in (1cm) long, are translucent, with few features and a bulging yolk sac on the underside. Burrowing down into the gravel bed, they huddle together and, nourished by the yolk, develop fins, tails and colour. One night, after four to six weeks, they will emerge from the gravel, swim to the surface, and take a first gulp of air, filling their swim bladder.
The alevin has become a fry, now ¾-1¼in (2-3cm) long. The individual fry moves downstream, remaining within half a mile of the redd, to find the microscopic organisms it feeds on, defending any productive feeding grounds vigorously.
Mortality rate at this stage, from predation or starvation, is high. Those that survive their first year grow to 2-3in (5-8cm) and develop distinctive vertical bars along the lateral line, interspersed with red spots. This indicates the transition to the next life stage, called parr. These markings camouflage the fish as it becomes a predator, now feeding on aquatic insects. The growth rate of a parr depends largely on food availability and water temperature, with 16-19°C being optimum. At this stage, some male parr will develop sexually mature gonads and participate in spawning. The ‘precocious parr’, which is tiny by comparison to the adult, waits underneath the mature hen, and when she releases her eggs to mate with a cock, releases his own smaller package of milt, fertilising some of the eggs.
From river to sea
Parr remain in the river for two to four years until they reach over 4in (10cm) in length, and their markings begin to fade. Now begins a period of significant morphological, physiological and behavioural change for the salmon as it becomes a smolt.
“The biggest challenge for them is when they go from being a freshwater to a saltwater fish,” says Mark. “It’s like a caterpillar turning into a butterfly. If you dropped a goldfish into the sea, it would last about a minute before it died, but a salmon’s got to do that as part of its life cycle.” To achieve this, pumps in the gills reverse direction, shifting sodium chloride out of the blood and into the water.
Kidney function changes, and the salmon starts to drink the water it swims in and to produce small quantities of highly concentrated urine. At sea, it will drink several litres per day. These adaptations, called osmoregulation, help the fish guard against dehydration and get rid of excess salt.
Outwardly, the smolt becomes silvery and more streamlined. The ‘upstream’ instinct reverses, and the previously territorial young salmon shoal together to migrate downstream, moving mainly during the night. During this springtime journey, the particular chemical composition of the river water is imprinted on the salmon’s memory to be recalled, sometimes years later.
The estuary is a place of extreme danger for the 5-9in (12-24cm) smolt, where inland and offshore predators, such as seals, porpoises, herons, seabirds and large fish, hunt. To speed their journey, the young salmon ride the ebbing tide out to sea. “The Gulf Stream becomes something called the Slope Current, off the west coast of the Outer Hebrides,” says Mark.
“It has various names, but the most important is ‘smolt highway’. All the fish from the west coast of the UK, France, Spain and Ireland have to get into this smolt highway, which is a belt of moving water and moving food.”
The maturing salmon then head up to the cold northerly waters off south-western Greenland and around the Faroe Islands. Here, they will spend up to four years feeding on a variety of prey, including small fish, such as capelin, sand eels and herring, and crustaceans and squid. Some salmon, known as grilse, will return to breed after only one winter at sea, while others will spend two or more years at sea.
“It’s all life strategy,” explains Mark. “The biggest risk to salmon is feeding. Many animals want to eat them because they’ve got very high oil and nutrient content, so if the salmon can get large enough and build up enough energy to spawn, they’ll come back.”
How the salmon find their way almost 3,000 miles back to the river mouth is not well understood, but it is thought that they may be guided by the Earth’s magnetic field or aided by solar navigation. Near the coast, they are guided by their natal river’s particular chemicals, imprinted in their memory when they
“Rivers and the inhabitants of the watery elements are made for wise men to contemplate and for fools to pass by without consideration"
were smolts. At this point, the smoltification process is somewhat reversed. The salmon stops drinking and produces large quantities of dilute urine: the pump cells in the gills reverse. From silver, the hen salmon turns blue-black and the cock salmon copper, with brown-orange tints. Adults require high water to re-enter the river, so will often wait near the estuary until a time of spate. After this, they lie up in well-oxygenated pools conserving energy until an increase in hormones and river flow stimulates them to move further upstream.
Equipped for the journey
Their homing instinct is precise. The adult salmon return to the exact location they hatched; not simply the same stream, but the same bend or stretch along it. Doing so requires them to clear numerous obstacles, both natural, including waterfalls and changes to the course of the river, and man-made, such as weirs and any hydropower structures.
To negotiate the former, Atlantic salmon have evolved a powerful ability to jump, boosted by the standing wave at the foot of a waterfall. “They can move around in catchments an awful lot once they get into a river,” says Mark.
“Some tracking studies show they can come approximately 40 miles inland and go ‘no, this is not quite right,’ and drop back down again. They move around, making sure that’s where they’re from and locating their natal tributary.”
Back at the breeding grounds, while excavating a redd, the hen releases pheromones, drawing the attention of the cock. Preference seems to be made based on body and kype size. At the breeding peak, this can grow to approximately one fifth of the cock’s body length. If accepted, the male salmon swims alongside the female, and both vibrate as she releases her eggs and he fertilises them with his milt. The hen then covers the redd with gravel from further upstream, creating the beginnings of another redd. She will lay 2,500-7,500 eggs, depending on her size.
After spawning, 90-95 per cent of Atlantic salmon die. The few that survive, usually female, will return to the sea. The carcasses of the salmon will fertilise the water, bringing nutrients, which, in a few months’ time, will benefit their young.
As November draws to a close, and the first light snows fall on the mountaintops, a female, having released all her eggs, allows herself to be pulled downstream. The journey of the Atlantic salmon cycles around again.
• Words: Sarah Ryan
A FISH IN TROUBLE
Wild Atlantic salmon numbers have dropped by 70 per cent in the last 25 years. Through research and advocacy, the Atlantic Salmon Trust is seeking to understand why and to halt the dramatic decline. To find out more, visit https://atlanticsalmontrust.org