A deadly form of jellyfish which has caused £1m in losses to salmon farms off Northern Ireland's coastline has returned to the Irish Sea.

Mauve stinger jellyfish were spotted at the weekend along a popular tourist beach on Portrush's West Strand by an Environment Agency worker.

Scientists have confirmed that the small purple creatures were the same jellyfish that killed more than 150,000 salmon in cages off the north Antrim coast last year.

The Environment Agency said there was no indication so far that there would be a repeat of last year's "catastrophic bloom". Marine conservation officer Joe Breen said the authorities had informed Northern Salmon, the company devastated by the jellyfish invasion.

"A further survey carried out this morning has reported the jellyfish are present at West Strand, East Strand, Portrush and White Rocks. The species have also been observed in the Republic of Ireland, off the County Sligo coast."

Breen advised people to stay clear of the jellyfish, "which are capable of a nasty powerful sting".

In recent years, increasing numbers of mauve stinger jellyfish in the Mediterranean have posed a danger for swimmers. Their presence in the colder northern waters is thought to be due to wind and tidal factors.

A marine biology lecturer said he had heard reports that the jellyfish stretched all the way from Sligo to the County Antrim coast, but it was not yet clear if they were in isolated bunches or a continuous plume.

Dr Jon Houghton of Queen's University Belfast said: "They occur hundreds of miles out to sea so it is very rare to find them inshore. If a prevailing wind comes down, they can be carried in."