A group of at least 12 feeding fin whales has been spotted by a marine conservation group about 60 miles off the coast of Pembrokeshire.
The Sea Trust says last week's "exciting" discovery could indicate the huge animals are feeding in the nearby Celtic Sea all year round.
The whales were spotted by researchers on a ferry crossing from Cherbourg to Rosslare.
It was the first time the Sea Trust has surveyed this area in 10 years.
The Celtic Sea is an area of the Atlantic covering the waters off Wales, Cornwall and Ireland.
The fin whales were spotted by the observers on their return trip to Ireland in an area of the Celtic Sea midway between Cornwall, Pembrokeshire and Ireland last Thursday.
Hannah Harries, science officer with the Fishguard-based Sea Trust, said she first noticed the whales creating a blow - a large water plume - in the distance.
"I could not believe my eyes at first," she said.
"First we saw the massive blows and then a few minute later a huge fin whale surface just ahead of us.
"I had worked off the Great Barrier Reef and spent many hours watching dolphins in the Irish Sea, but this was my first encounter with the second largest animal that has ever lived on earth.
"It was so exciting and getting better with blows and whales ahead and either side of us, over the next hour."
At least 12 different whales were identified, but there were probably a lot more, Ms Harries added.
The Sea Trust has been carrying out surveys on the Irish Sea on Stena Line crossings between Fishguard and Rosslare and Holyhead to Dublin routes, but the surveys on the French route is a new development.
Fin Whales: Facts
- Second largest animal behind the blue whale
- About 70 to 80 feet along
- Weighs about 80 tonnes
- Work in groups to feed
Director Cliff Benson said fin whales have been spotted about 25 miles in Pembrokeshire in previous autumns, and are also known to feed in large numbers close to the south coast of Ireland in the winter.
In May 2011 a large gathering was seen in the same area that they were spotted last week when large swarms of krill and sand eel were also spotted.
"This could have been a one-off but our subsequent sightings last week points to the possibility that we, together with our Irish Whale and Dolphin Group and Marine Life colleagues, discovered an annual feeding cycle in and around the Celtic Sea," Mr Benson added.