By the start of the new millennium salmon had almost vanished from the River Carron, in Wester Ross.
It is one of the 84 per cent of rivers across the country where any fish hooked must be returned alive to the water.
But populations dropped so low that annual catches had plunged to around five in 2000 and 2001.
And the crash had occurred almost overnight, leaving river owners and anglers puzzled and deeply concerned.
Biologist and salmon expert Bob Kindness has been working on the Carron since 1995, painstakingly charting the movements and fortunes of its fish.
“Salmon numbers in the river plummeted almost overnight and there seemed no obvious explanation,” he said.
“The habitat was good, there were no obstacles for fish passage.
“But there was almost nothing in the river.
“I couldn’t find fish.
“They were very close to extinction.
“But there had been a series of big winter spates over a five-year period, causing damage to the riverbed.
“Salmon lay their eggs in gravel, and that had been repeatedly torn up and washed away.
“The problem was that there were no young fish and so none to spawn the next generation.
“That’s when we decided to try a special restocking programme.”
The technique involves captive breeding from healthy native fish caught in the river, raising the young in tanks and then releasing them at three or four months old.
For the first few years between 5,000 and 10,00 fry were released into the Carron but this was upped to 150,000 in 2001.
Catches in 2004 jumped to around 140 fish.
Fast forward 20 years and the results of cutting-edge DNA analysis suggest the efforts have been worthwhile and the method could be used to help the species survive in rivers across Scotland and elsewhere.
The River Carron Conservation Association (RCCA), a group of estates and farms with interests in the river, funded the research, which was carried out by scientists at the University of the Highlands and Islands (UHI).
Kindness has been collecting genetic samples from catches on the river for a decade by clipping a small piece of tail fin from each salmon captured.
Researchers from the UHI’s Rivers and Lochs Institute at Inverness College have now analysed DNA from three years of salmon catches.
The results were then compared with profiles from salmon used in the stocking programme to find out whether domestically reared juvenile fish were surviving the journey to sea and back to spawn in the Carron as adults.
The findings show that this type of stocking can not only contribute significantly to the fishery but can be done in a way that is sympathetic to the natural ecosystem while avoiding risks to the health and genetic integrity of the natural salmon population.
But experts say further research is needed to create a fuller picture of the impacts of restocking.
Mr Kindness is enthusiastic about the conservation value of captive breeding, which has an incredibly high success rate - with almost 99 per cent of eggs harvested going on to hatch.
“It’s a numbers game,” he said.
“All rivers are different but they all need to have as healthy a smolt [the stage when young salmon are ready to journey to sea] run as possible.
“The numbers must be sufficient in order to guarantee some will make it back to breed."
He believes that the timing of release of the young fish also has a part to play in their prospects for survival – ideally when predators are at their fewest.
And he disagrees with the suggestion that captive-bred fish are either not as healthy or more vulnerable than their natural-born cousins.
“There those who say wild-reared fish are more likely to make it back to their birth rivers, but evidence shows the returned stocked fish are just as healthy and fit.
“And I don’t think domestication is an issue when it comes to the survival of stocked fish.
“I’ve watched fish for more years than I care to remember and I believe captive-bred fish are not at a disadvantage.
“Even though they are raised in tanks, they retain all their natural instincts – avoiding predators, catching food – and will put them into action when they’re released.”
He acknowledges that increasingly extreme weather, including droughts and floods, caused by global warming will continue to impact wild salmon but restocking could help stave off local extinctions.
“Climate change is definitely one of the issues, and that’s not going to go away,” he said.
“And it’s happening a lot faster than nature can counteract.
“We’re resigned to the fact that we will lose naturally spawned eggs every year.
“What stocking does is safeguard eggs when they would usually be at their most vulnerable – when embedded in the gravel – and protect very young fish from being eaten.
“The bottom line is that we need to provide more fish and I don’t believe nature can do that on its own.
“But we need to act before it’s too late.
““There is a possibility to turn around the fortunes of Scotland’s wild salmon.”
Official figures from Marine Scotland Science suggest the Carron’s salmon population has revived, with catches increasing from a five-year average of 10.6 in 2001 to 187.2 in 2020.
In addition to the stocking programme, the Riparian owners of the Carron have worked to improve the biodiversity of the glen through which the river runs.
Some 370,000 trees have been planted on the Glencarron Estate alone.
“We now have a biodiversity corridor of native trees which runs the full length of the river.
This should, in time, slow flood run-off and give the fish a better chance to breed successfully.” said Shaun Macdonald, chair of the RCCA.