Scientists working off the Co Donegal coast have tagged a record 50 basking sharks in three days this week.
"Astounding" is how Dr Simon Berrow of the Irish Whale and Dolphin Group described the numbers, which reflect a fraction of the potential basking shark population in these waters.
"I would normally expect to be lucky if we tagged 50 in a whole year," he said.
The basking shark was hunted off the south and west coasts until 20 years ago. Fishermen nicknamed it the "sunfish" due to its habit of swimming just below the surface.
"It was also known as liop an dá - unwieldy beast with two fins - or more generally liabhán mór, signifying a great leviathan," Dr Berrow said.
"The most evocative Irish name for it is liabhán chor gréine - the great fish of the sun."
Dr Berrow and National Parks and Wildlife Service conservation ranger Emmett Johnston set out earlier this week to tag the shark off Donegal, as part of a project funded by the Heritage Council.
"In two hours last Monday we tagged 23 sharks, and we found 19 the following day - four of which had been tagged the day before," Dr Berrow said.
By Wednesday of this week they had attached their 50th tag, using colourcoded devices fixed with a modified telescopic pole.
Four colours are used to denote the coastal locations of Donegal, Kerry, Cork and "other areas".
Galway-Mayo Institute of Technology and Crossing the Line Films have also funded two satellite tags, which last four months and which the researchers use for specific tracking.
Dr Berrow said recent studies in Britain using satellite data have raised questions about whether basking sharks hibernate in winter. The British results record long-distance movements between Cornwall and Scotland, with one shark travelling 1,878km (1,167 miles) in 77 days across the Celtic Sea and up the western seaboard of Ireland, he said.
"A basking shark tagged in the Isle of Man in 2007 went to Newfoundland, a distance of 9,589km [5,958 miles] in 82 days, which was the first evidence of a transatlantic movement," Dr Berrow said.
Increased marine productivity, which may be linked to climate change, could be one of several reasons for greater numbers of the shark in these waters, he added.
Basking sharks were a by-catch in drift nets for salmon, now outlawed, he said.
The fish were hunted for oil until the mid-1970s off the west coast and Dr Berrow noted that the street lights of Galway and Waterford were lit with basking shark oil in 1742.
"The best-documented basking shark fishery was off Achill Island, Co Mayo. Between 1950 and 1964, 9,000 sharks were killed, with a record 1,808 killed in 1952 alone," Dr Berrow said. The fishery closed in 1975, but Norwegian vessels continued to catch the shark commercially off Co Waterford until 1986, when an estimated 2,000 were killed.
Basking sharks are protected in Britain and Northern Ireland but not in the Republic.
The EU recently placed a moratorium on fishing for the sharks in these waters.
Any sightings of the basking shark or of tagged sharks should be conveyed to the Irish Whale and Dolphin Group.
Further information on the project is available from www.baskingshark.ie.