Sightings this week of a pod of fin whales off the Mayo coastline have been described as "very significant" by the Irish Whale and Dolphin Group.
Up to half of the marine mammals - the world's second-largest creature after blue whales - were recorded feeding between three and five miles off Clare Island and Achillbeg over the past few days. Both fin and humpback whales have also been sighted off the south coast this week.
Mr Pádraig Whooley of the Irish Whale and Dolphin Group said that up to 15 fin whales and some 500 common dolphins had been seen about 19 miles south-east of Galley Head. However, this had become a pattern at this time of year which they had established through their work on the south coast, "whereas the Mayo sightings are very exciting", he added.
"It demonstrates the need for far more research in other parts of the coastline to determine more precise levels of activity of these mammals."
The reports from Mayo were made by Mr Johnny King, owner of the Western Kingfisher angling charter vessel from Cleggan, Co Galway. Mr King was working with Bord Iascaigh Mhara (BIM) on tagging bluefin tuna about five miles south-east of the Bills Rocks west of Clare Island when he noticed significant activity by gannets feeding on herring shoals.
The herring had attracted up to 20 grey seals, and his vessel then came upon about six fin whales feeding on the fish. The whales moved west of Clare Island and were still in the area when he left.
Mr Whooley said the peak period for fin and humpback whales in southern waters was October-November, with initial sightings in June. "The continental shelf is closest to our land mass off the Mayo coast, so it makes sense that there should be intense whale activity there," he said.