A "secret slaughter" of seals is being carried out by fish farmers around the coastline of Scotland, campaigners have claimed.
The Seal Protection Action Group told the BBC's Countryfile programme that as many as 5,000 of the mammals are shot in Scotland every year.
The salmon farming industry disputed the figure, and said shooting seals was necessary to protect stocks.
It is currently legal to shoot seals outside their breeding season.
Although seals have become one of Britain's most iconic and best-loved wild animals, their love of fresh salmon has brought them into increasing conflict with a fish farming industry looking to cash in on the growing human appetite for cheap salmon.
More than one million salmon meals eaten in the UK every day, and the industry is one of the biggest employers in the Scottish Highlands.
Common seal numbers have plummeted by a third in Britain over the past seven years, with ecological changes and a shortage of wild fish generally thought to be behind the drop.
But Andy Ottaway, of the Seal Protection Action Group, said he believed the shooting of seals was another major factor behind the animal's decline.
The group was established 30 years ago oppose the mass culling of seals on Orkney.
Mr Ottoway said: "Little did we realise then that the cull would simply be driven underground and continue in secret to this day.
"The seal shooting takes place in very remote locations in sea lochs around Scotland and there are no witnesses, and under the law the industry doesn't even need to release the figures of the numbers they have killed.
"We believe there is a mass slaughter of seals in Scotland - up to 5,000 each year."
Mark Carter, of the Hebridean Trust, said he believed the general decline in seal numbers was particularly noticeable in the areas surrounding fish farms.
"Scientifically we don't know the real reason behind the total decline, but what we do know is that when they are situated near a fish farm then there is a decline and shooting is probably one of the main reasons," he told Countryfile.
"We have got people who have actually witnessed the shooting on fish farms, and we have had several seals washed up with bullet holes in their heads.
"The problem is it is not just adults that find them - my children found one washed up on the beach in front of the house. We did an autopsy and the skull was completely shattered."
It is not just conservationists who are worried. Donald McLean has been taking tourists seal watching in the Sound of Kerrera, off the west coast of Scotland, for 30 years.
The area is traditionally one of the best places in the world to see common seals in their natural habitat.
But Mr McLean said a big increase in the number of local salmon farms - there are currently some 300 in the area - had coincided with a sharp drop in seal sightings.
"When I first started you would come out and see between 60 and 80 seals on average, but now you are down to 10 or 20," he explained.
Mr McLean claimed a large colony of seals which would regularly bask on a rocky outcrop in the Sound disappeared completely when a salmon farm opened up nearby and started shooting them.
But Scott Landsburgh of the Scottish Salmon Producers' Organisation, said a single seal attack could kill several thousand farmed salmon.
He said: "The seals are very aggressive - they attack the nets and can bite through them, and they can also actually use their flippers to steal salmon out of the cages.
"It's not just a few - it is thousands of salmon. Indiscriminate attacks by seals can cause trauma throughout the cages and we can lose 2,000 or 3,000 salmon at a time, not only by attacking them but also just be being in the vicinity."
Mr Landsburgh showed the programme photographs of the aftermath of one recent salmon attack where up to 1,000 salmon were killed by a seal getting into their cage, and said there had been reports of up to a third of fish in a cage being killed from the trauma caused by a seal merely sitting nearby watching them.
"We all like seals, we all want to protect the seals, but our paramount responsibility is the welfare of the salmon," he said.
"Let me put this in perspective - we had 30,000 seal attacks on Scottish salmon farms last year. Our industry has reported to us that 489 seals were shot by the salmon industry."
It is not just salmon farmers who are doing the shooting - seals are also regularly killed by nets men and anglers around Scotland's east coast rivers.
Callan Duck, of the Sea Mammal Research Unit at St Andrews University, said he believed it was legitimate in some circumstances to shoot seals around fish farms.
He said: "If a seal is damaging a salmon cage and releasing salmon into the open water for a ready meal, then probably the only way to solve that problem is to remove the seal as once it has learned to access an easy food source it will keep doing it.
"But to randomly shoot animals at adjacent haul out sites or that happen to be passing the fish farm - no, I don't think [it is justified]."
At least one supermarket chain is considering introducing "seal-friendly" salmon products, and several shoppers who were surprised to learn that seals were still regularly being shot told Countryfile they would be willing to pay more for their salmon if they knew its farming had not caused the death of seals.
Some fish farms are already experimenting with non-lethal methods of protecting their stocks by using nets made from stronger fibres or special acoustic devices to scare away hungry seals.
But it could be that the long-term future of seals could rest in the hands of politicians, with marine bills due to be debated in both the Scottish and UK parliaments in the coming months.
Countryfile was broadcast on BBC1 at 1900 BST on Sunday