Atlantic salmon
The presence of this species in our waters is a source of pride!
I have taken quite a long time to decide what to write about this month. The seasons have changed, which normally gives me more options as I like to link what I write about to what you might see that month, but for some reason I have been a bit stuck. So, on the way to work I was deep in thought about what I could write about. I passed over the River Avon in Hampshire, an iconic, high-quality, gin-clear chalk stream, which has had limited modification, and it made me think of the salmon I used to watch leap as a child with my grandfather around this time of year.
The Atlantic salmon (Salmo salar) is an anadromous species, a term that comes from the Greek anadromos, meaning ‘running upward’. They start their lives in the headwaters of their natal river before making the epic journey out into the sea, travelling thousands of miles to their feeding ground in the North Atlantic off the coast of Greenland, then return or ‘run up’ the same freshwater rivers where they hatched to spawn (called the salmon run) to start the cycle over again.
Salmon have an interesting life cycle, starting life as a fertilised egg buried in the shallow excavated gravel of the well-oxygenated riverbed, known as redds, where the water moves swiftly over them. The eggs hatch and the salmon at this stage are known as ‘alevin’, a tiny transparent fish that lives in the safety of the gravel. It then grows into a small ‘fry’ before turning into a salmon ‘parr’, maybe 10-12cm long, which has distinctive thumbprints down its flanks with a single red dot between each, feeding on aquatic insects.
After a period of one to six years, the young salmon migrate downstream to the sea as ‘smolts’, becoming silvery in colour ready for the challenges of surviving at sea, where they head to the cold waters of the North Atlantic, feeding on smaller fish and crustaceans. They can stay at sea for one year before they return to the same river as ‘grilse’, or up to 4-5 years when they return as ‘multi-sea-winter fish’.
The salmon that I used to watch leap on the River Test as a child were ‘running’ upriver, back to their natal headwaters. This is quite a spectacle to see and they are at a stage highly prized by anglers. Salmon find their home river by smell, with the timing and duration of the spawning migration depending on several factors – the population, the size and location of the river, water flow, temperature, tidal cycle and when the conditions in the river are favourable for upstream migration – but generally speaking the best time to see them migrating and leaping is from October to November.
After some time back in fresh water, their silvery livery is changed for few dark spots on the back and may have a pinkish flush to the belly. It is not only their colour that changes; the male or cock fish also develops a prominent hook or ‘kype’ on its lower jaw, which differentiates them from the female or hen fish. Once back in fresh water they do not feed but are focused on spawning where they started life as an egg during November and December, before most die (often 90-95%), although a few do return to the sea as ‘kelts’ to feed and return a second time to spawn.
Salmon rivers vary considerably in their ecological and hydrological characteristics in the north and south of the UK, and in the life-cycle strategies adopted by the salmon within them. The varied climate, geology and terrain in the UK means that high diversity can be found within some of the large rivers. The cooler, sparsely vegetated, nutrient-poor rivers in the north, which are prone to sudden increases in flow (‘spates’), result in salmon sometimes taking several years to reach the smolt stage and migrate to sea. However, in the south, rivers flow across gentler terrain and softer rocks, in a warmer, drier climate, resulting in salmon often growing quickly enough to smolt as yearlings.
Atlantic salmon are widely distributed within Europe, ranging from Portugal in the south to Scandinavia in the north, with the UK population contributing significantly to the total European stock. Scottish rivers in particular are a European stronghold for the species. Salmon is a widespread species in the UK and is found in several hundred rivers, many of which have adult runs in excess of 1,000. The latest estimates of the UK spawning population size are, however, about 50% down on the ten-year average.
This is because the species is subject to many pressures, all of which I am sure we are aware of: pollution, the introduction of non-native salmon stocks, physical barriers to migration (e.g. weirs), exploitation from netting and angling, physical degradation of spawning and nursery habitat, and increased marine mortality. However, lots of positive work is being done by individuals and organisations, in and alongside our rivers, to reduce pollution, improve habitat in rivers, and remove physical structures to allow migration.
The salmon is a truly iconic species, and its very presence in our waters is a source of considerable pride, especially as it indicates the good water quality of the river. The UK is home to some of the finest salmon rivers in the world, which provide jobs and income to many remote local economies in Scotland. I just hope that my daughter will continue to see the mesmerising bars of silver leaping in her lifetime and the next generation.
And away!
Megan Lock
Advisory
Image credit: Walter Baxter