Grilse - one-year-old fish - returning to their home rivers this summer have been found to be infected by an ocean-dwelling worm, which causes them to bleed from their rear vents.

According to Scotland on Sunday, cases of "bleeding vent syndrome" have been found on 23 salmon rivers. On some waterways, up to 80% of the fish have been found to have the inflammation as a result of infection, which can stop them spawning. Anglers are being urged to report all cases to fisheries scientists.

Seymour Monro, director of the Atlantic Salmon Trust, a fish conservation charity, said: "Most of Scotland's salmon rivers seem to be encountering this phenomenon. We and others are urgently investigating the origin and implications of the problem."

On the River Lochy, near Fort William, fisheries manager Jon Gibb said there had been a good run of grilse this year, but up to 80% had bleeding vent syndrome.

"The fish have been in cracking condition apart from these badly swollen vents. The problem is, that is where the eggs are and we don't know if this will affect breeding next season. If it does, that could be an enormous problem in the future."

One theory is that the problem is caused by the warming of the world's oceans. As a consequence, the parasitic worm, which can be found in shrimps that salmon eat at sea, thrive.

Jeremy Jones, who works for the Inverness-based Fish Vet Group, said the parasite was being found in much higher numbers this year in the muscle around the rear vent.

"It is causing this intense inflammation. In low numbers the worm doesn't affect the fish, but in large numbers it starts to break down the tissues. What we don't know is whether it will have an effect on spawning."

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