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December Star Species: the Atlantic Salmon

Every month we ask one of our Forest Rangers to tell us all about an animal, plant or fungus which gets their vote as a UK star species.

This month, Martin Summers, Forest Ranger at Deerpark in Cornwall, leads us on a story of adventure, survival and homecoming with the wild Atlantic salmon:

“One of the most exciting moments here at Deerpark over the last few years was the discovery that Atlantic salmon are swimming upstream in the West Looe River to spawning grounds very close to our location. They are not coming in great numbers but if you are very lucky you might catch sight of this amazing phenomenon during December.

A life journey begins

In the winter, high upstream in freshwater rivers where the water is flowing fast, you might spot areas of the river bed that look as if they have been disturbed. The gravel may appear to be a lighter shade and perhaps slightly raised. These are redds, the spawning grounds of salmon. You can see them in the river just above Deerpark.

The female salmon swishes her tail in the gravel to create a dip into which she lays her eggs. The male fertilises the eggs and the female swishes a gravel cover back over the fertilised eggs. The female may then return to the sea but the male, job done, dies. We have found a few dead salmon in the river and console ourselves with the fact that it means new life has begun.

A carefree(ish) childhood

The eggs hatch in the spring and tiny alevins are born. They remain buried in the redds for a few weeks, feeding off the yolk sac. Eventually they run out of nutrients and the young salmon, now known as fry, emerge from the gravel beds in search of food. In their first year they grow quickly, feeding on organisms within the water.

After a year the salmon, now called parr (keep up), are feeding on small insects too and they enjoy up to 4 years in their river homes growing and hopefully thriving.

Now a new phase begins with, naturally, a new name, our smolts embark on their first journey. Like students off to university, they leave their comfortable homes and set off in groups, or shoals, for the big world; in this case the English Channel, followed by the great Atlantic Ocean. Unlike students, they won’t be back at Christmas with their washing. But they will be back one Christmas.

Life beneath the ocean wave

One of nature’s miracles now occurs; our freshwater friends have to adapt to a saltwater environment. The balancing act of maintaining body fluids and ions is called osmoregulation. The salmon have to move from the river, where sodium is low, to the sea where it is high. Its body needs to adapt quickly. As it moves downstream it drinks a lot and decreases urine production. When it gets to the sea the molecular pumps on its gills go into reverse. In the river they were pumping sodium in. Now, in the sea they pump sodium out because, of course, the sea is salt water. Without this switch-over the salmon would die very quickly from dehydration and salt loading.

Once out at sea, our salmon, which is finally called an adult salmon, spends a year or two battling the elements, hunting, fighting and surviving. Those that come back after one year are known as grilse. More than one year and they have the rather disappointing and unwieldy name, multi-sea-winter salmon.

The Salmon Run - back home to spawn

The next miracle occurs once the salmon are ready to reproduce. Having spent years at sea the salmon somehow navigate their way back to the estuary, swim against the flow of the river and head for the very place they were born. The pumps in their gills switch back to freshwater mode as they leave the sea.

For Pacific salmon this journey can be thousands of miles. Our Deerpark Atlantic salmon may have visited Greenland or the Faroe Islands. Trawling at sea may be a problem for them if their location is known.  

The latter part of the journey is famous for the images of salmon leaping up waterfalls, one of the most bizarre spectacles in the natural world. Our plucky salmon are on a mission and nothing will stop them!

Once back to their spawning grounds, eggs are laid, fertilized and covered, and time is up for the adult fish…but the cycle of life has begun again.

The salmon at Deerpark are few and far between but that scarcity makes a sighting all the more pleasurable. Now is the best time of year to look out for the adult salmon making their way back to the spawning grounds. We may or may not be lucky, but we will certainly do our best to witness this wonder of nature - without disturbing them of course”.