In fact they were working for the National Rivers Authority, lifting salmon and sea trout manually over a delapidated Victorian weir at Sleights so that the fish could continue their migration to the spawning grounds in the Esk's shallow headwaters.
The helping hand was being offered on Yorkshire's only significant salmon river because the NRA is currently repairing the weir's fish 'pass' - a series of tanks 12 to 18 inches high up which the salmon and sea trout can leap. The walls of the pass have started to collapse, making its ascent almost impossible.
'The weir was built in 1891 and the fish pass has needed repairing for the last 20 years,' the NRA's area fisheries officer, Chris Firth, said.
'It had deteriorated so badly that many fish could not reach the top of the weir and so were effectively trapped in a pool downstream, sitting ducks for poachers.'
The work, which will cost pounds 80,000 and take another four weeks to complete, involves 'damming off' the pass with copper so that it can be drained and rebuilt.
While the work is in progress fish approaching the weir are stunned by an electric shock and then moved by hand nets into dustbins filled with water. NRA scientists then measure and weigh the fish, and take scale samples to age them, before releasing them upstream of the weir.
Mr Firth said: 'We're using a generator to power an electrode at the end of a glass fibre pole. When a fish comes within a certain distance it is temporarily stunned so we can lift it out. We've moved 700 like this - some as heavy as 15lb - in two weeks.
'It has been a unique opportunity to acquire some invaluable scientific data about the lives and movements of salmon and sea trout.'
Mr Firth believes the work will help restore the Esk to one of Britain's finest salmon rivers: 'It will help a lot more of them to reach their spawning grounds on the moors.'