In the normally level-headed world of angling, reports of giant salmon and the toppling of a record set by Georgina Ballantine, a Scotswoman, in 1922 are rarely greeted with anything other than scepticism and outright derision.

But an internet chat room that claimed a salmon weighing up to 100lb had been caught on the River Ness brought excitement to one of the oldest country pursuits in Britain yesterday. “The man has just caught the Loch Ness monster,” an angler on one message board wrote.

In a deep pool just 150 yards from the northern entrance to Loch Ness, a number of anglers watched on Saturday afternoon as another hooked a salmon that, for a few tantalising hours, looked as though it might be the biggest caught in Britain.

At least four witnesses were there when, held underwater, the male salmon was measured at nearly 5ft (1.5m) in length and the best part of 50 inches in girth. The anglers posed for photographs using their mobile telephones and it was released into the river. Crucially, it appeared that no scales had been on hand to weigh it.

“It was way bigger than anything I have ever seen, it was massive,” Grant Sutherland, the head gillie at the Dochfour estate, which owns the stretch of river where the fish was caught, said.

Dozens of internet postings followed the first online news of the catch at 5.35pm on Saturday. “There has just been a huge cock salmon caught on Dochfour beat of the River Ness this afternoon,” came the first report. “This fish seemingly measures 2 inches longer then the current British record of 64lb.” But while the Dochfour estate said yesterday that it had sent photographs, a sample of the scales of the fish and the measurements for analysis at the Fisheries Research Services in Scotland, questions were mounting.

None of the anglers present when the fish was caught could be reached for comment and photographs posted on the internet were later withdrawn.

Mr Sutherland refused to speculate on the weight of the salmon, but he did admit that at least one of his measurements — the alleged 50-inch girth — may have been wrong.

Based on measurements and scale samples, it should be possible to determine the age of the fish and to estimate its weight. The British Record Fish Committee, the body responsible for verifying records, said that the fish could not enter the record books because it did not meet certain criteria, including being weighed on scales that can later be checked.

Off the scales

— The world’s longest goldfish measures 47.4cm (18.7in)

— The heaviest freshwater fish is a 646lb Mekong giant catfish, in Thailand

— The smallest is the female Paedocypris fish at 7.9mm The sailfish is the fastest, at 110kph (68mph)

— The butterfly fish has the longest name. In Hawaiian it is “lauwiliwilinukunukuoioi (long-snouted fish shaped like a wiliwili leaf)

Sources: national-aquarium.co.uk, Guinness Book of World Records, Australian Museum Fish Site, Natural History Museum

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