The Great Salmon Smolt Run

Posted On: 3 May 2020


If you are walking along the river in our latest sunny weather, keep an eye out for silver flashes in the water. What you could be witnessing are one of the most amazing natural phenomenon of the world – the Great Salmon Smolt Run. These are juvenile salmon that undergo incredible changes before they leave the river and head off into the frozen North Atlantic before making their way back to the Boyne. If the Boyne were the GAA, these fish would be minors heading into the senior league.

The most noticeable feature of these young salmon is their silvery skin. Baby salmon have dark blue-green skin. This helps them to hide in the dark pools beneath overhanging trees along the banks of the Boyne. At this time of the year, their bodies take on the silver sheen of the adult. This allows them to blend in as they glide through the flickering light waves of the open North Atlantic. This will make it harder for seals, birds and larger fish predators to catch them. From about 2000 eggs, only about four will survive to return to the Boyne.

Along with their colour, salmon smolt are one of the few creatures that can change their internal body chemistry. Freshwater fish cannot survive in saltwater as it is toxic for them. The smolt transforms with a process called osmoregulation. In the estuary at Drogheda, they will drink several litres of water per day to regulate how they manage salt levels. Incredibly this causes their kidneys to pump salt out of their blood into the water instead of the other way around. Once they can do this they are good to go.

Smolt in the gravel beds of the Boyne – Photo by Pat O’ Toole

The outer journey of the salmon smolt is just as incredible as their inner change. Previously these young salmon would have spent their lives swimming upriver against the current. Once Mother Nature gives them the signal, they swim downstream and hang a sharp left in the Irish Sea towards the rich deep-sea feeding grounds of the North Atlantic. By June, the Great Smolt Run will be over and they will all be gone to sea. They move fast. Some of them can cover 250km per month. Some will return to the Boyne next Summer, content with their journey as far as the Faroe Islands and the Southern Norwegian Sea. Some may stay out for a few years, exploring the Arctic Sea and become quite large by the time they come back home. Their average journey is about 3000km!

To get back to the same spot where they were born, the salmon use the Earth’s magnetic field for navigation. They also have a fish memory of the Boyne as each river has a unique smell created by the natural chemicals in the water. Once they get back in the freshwater, they reverse their body chemistry, stop drinking and start thinking about romance. Much like most creatures.

As you can imagine, creatures that are sensitive to changes in the water are more vulnerable than most. Salmon populations are actually in decline across the Northern hemisphere. Since the 1970s there has been a 90% drop in numbers. Those who fished the Boyne in the 80s know that there were approximately 10000 salmon running through here. There is nothing like that now.

Climate change is a big factor. The smolt sometimes migrate too early and they are too young. The temperature difference between the Boyne and the Irish Sea can shock them. Predators can pick them off before they even get to the river estuary. Of course, pollution is one of the biggest problems. We just do not look after the rivers enough although it was great to see the latest work being done to keep the spawning gravels on the Boyne free of silt. To try and conserve the fish, the Boyne was made a catch-and-release only river for salmon. Until we change, nothing will change.

It would be terrible to lose these wonderful creatures as they have been coming back to the Boyne since the last Ice Age. Let’s keep an eye out for them.

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