By the start of the new millennium, salmon had almost disappeared from the Carron River at Wester Ross.
It is one of the 84 percent of the country’s rivers where any hooked fish must be returned alive to the water.
But populations have fallen so low that annual catches have dropped to around five in 2000 and 2001.
And the accident had happened almost overnight, leaving river owners and anglers bewildered and deeply concerned.
Biologist and salmon specialist Bob Kindness has worked on the Carron since 1995, painstakingly retracing the movements and fortunes of its fish.
“The number of salmon in the river dropped almost overnight and there didn’t seem to be an obvious explanation,” he said.
“The habitat was good, there were no obstacles for the passage of fish.
“But there was hardly anything in the river.
“I couldn’t find any fish.
“They were very close to extinction.
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“But there had been a series of significant winter floods over a period of five years, causing damage to the riverbed.
“The salmon laid their eggs in the gravel, and the gravel had been torn and washed away several times.
“The problem was that there were no young fish and therefore none to spawn the next generation.
“That’s when we decided to try a special replenishment program.
The technique involves breeding in captivity from healthy native fish caught in the river, raising the young in tanks and then releasing them when they are three or four months old.
During the first years, between 5,000 and 10,000 fry were released into the Carron, but this number rose to 150,000 in 2001.
Catches in 2004 jumped to around 140 fish.
Fast forward 20 years and the results of a cutting edge DNA analysis suggest that the effort was worth it and that the method could be used to help the species survive in rivers in Scotland and beyond.
The River Carron Conservation Association (RCCA), a group of estates and farms with interests in the river, funded the research, which was conducted by scientists from the University of the Highlands and Islands (UHI).
Kindness has been collecting genetic samples of river catch for a decade by cutting off a small piece of tail fin from each salmon caught.
Researchers at UHI’s Rivers and Lochs Institute at Inverness College have now analyzed DNA from three years of salmon catches.
The results were then compared to the profiles of salmon used in the stocking program to determine whether the home-reared juveniles survived the trip to sea and returned to spawn in the Carron as adults.
The results show that this type of seeding can not only contribute significantly to fishing, but can be done in a way that respects the natural ecosystem while avoiding risks to the health and genetic integrity of the population. natural salmon.
But experts say more research is needed to create a more complete picture of the impacts of replenishment.
Mr. Kindness is excited about the conservation value of captive breeding, which has an incredibly high success rate – with almost 99% of harvested eggs hatching.
“It’s a numbers game,” he said.
“All rivers are different, but they all need such healthy smolts. [the stage when young salmon are ready to journey to sea] run as possible.
“The numbers have to be enough to guarantee that some will come back and breed. “
He believes that the timing of the release of young fish also has a role to play in their prospects for survival, ideally when predators are at their lowest.
And he doesn’t agree with the idea that captive-bred fish aren’t as healthy or more vulnerable than their born cousins.
“There those who say wild-raised fish are more likely to return to their birth rivers, but evidence shows that stocked fish returned are just as healthy and fit.
“And I don’t think domestication is a problem when it comes to the survival of stocked fish.
“I’ve been watching fish for more years than I can remember and I think captive-bred fish are not at a disadvantage.
“Even though they are raised in ponds, they retain all of their natural instincts – to avoid predators, to grab food – and will put them into action when released.”
It recognizes that increasingly extreme weather conditions, including droughts and floods, caused by global warming will continue to impact wild salmon, but restocking could help prevent local extinctions.
“Climate change is definitely one of the problems, and it’s not going to go away,” he said.
“And it’s happening much faster than nature can counteract.
“We are resigned to the fact that we will lose naturally laid eggs each year.
“What stocking does is protect the eggs when they would usually be the most vulnerable – when they are embedded in the gravel – and protect very young fish from consumption.
“At the end of the day, we have to provide more fish and I don’t think nature can do this on its own.
“But we need to act before it’s too late.
““ It is possible to turn the situation around wild Scottish salmon. “
Official figures from Marine Scotland Science suggest the Carron salmon population has recovered, with catches dropping from a five-year average of 10.6 in 2001 to 187.2 in 2020.
In addition to the restocking program, residents of Carron have worked to improve the biodiversity of the valley crossed by the river.
Some 370,000 trees have been planted on the Glencarron estate alone.
“We now have a biodiversity corridor of native trees that stretches the length of the river.
This should ultimately slow the flood runoff and give the fish a better chance to reproduce successfully. Said Shaun Macdonald, President of the RCCA.