With their pretty villages and miles of waterways, the Norfolk Broads in Britain attract a million tourists each year, but there are fears the smell of dead fish could be getting up visitors’ noses.

While recent temperatures topping 20˚C (68˚F) have attracted the first holidaymakers of the season to the area, the pleasant weather could have the unfortunate side effect of increasing a build up of toxic algae, which depletes oxygen levels in the water and kills fish.

Environment Agency (EA) workers have recently been forced to relocate more than half a million fish and are carrying out daily monitoring after anglers spotted carcasses floating in some waterways, including Hickling Broad near Potter Heigham.

If warm weather continues through the spring, this could increase the bloom, meaning the figure could reach more than one million, a spokesman said.

Anglers have warned the rising numbers of dead perch and bream could put off visitors to the National Park – previously re-branded as Britain’s Magical Waterland – and have a knock-on effect on biodiversity.

Bosses at the Broads Authority, which manages the area, have said they are working hard with the EA to find a solution, adding that the problem should not put off visitors as it was confined to small areas.

Kelvin Allen, eastern region chairman of the Angling Trust, said the algae had been a problem for several years but it had been particularly prevalent this year.

He said: “Our members first noticed fish behaving strangely around Christmas – they were moving from their normal locations and weren’t interested in food.

Kelvin Allen of the Angling Trust at Herbert Woods, Potter Heigham, is warning that an algae build-up is killing fish and putting tourists off visiting the Norfolk Broads. Photos: PA/Environment Agency

“Obviously, this is of concern to anglers but it also has further-reaching implications. If you take into account the birds that feed on the fish and the wider food chain, this could have serious consequences to the environment.”

Anglers first noticed fish behaving strangely around Christmas

The algae bloom has been attributed to a mixture of environmental conditions, including salinity and water temperature.

It is not known what has caused these conditions to alter so dramatically, but Allen suggested dredging and land drainage.

He said there needed to be a coordinated response to bring together groups such as anglers, farmers and tourism businesses to find a solution.

“The Broads are the UK’s largest inland fishery and 18 per cent of visitors take part in angling at some point during their visit,” he added.

“People come here to enjoy nature and the outdoors. If we don’t protect the natural environment, we could lose the very thing that makes it special.

“Apart from anything else, as temperatures start to rise in summer, the smell of dead fish could become quite unpleasant.”

A Broad Authority spokesman said: “The fish issue is confined to a tiny part of the waters and once whatever water conditions are causing it have resolved, numbers will naturally recover.”

The EA rescued and relocated 230,000 fish in March after they were found shoaling in a shallow haven away from the bloom near Potter Heigham.

A further 410,000 have recently been moved from Somerton, roughly six miles away.

Once transported a sufficient distance from the bloom, the fish are able to swim freely into surrounding rivers.The last similar occurrence in Hickling Broad was in 2012 when 35,000 fish were rescued and relocated.

An EA spokesman said: “Our teams have taken action to rescue and relocate more than half a million fish in the Norfolk Broads ,which were in danger of suffocating due to the effects of a Prymnesium algae bloom.”

Dan Hoare, from the Broads Authority, said it was working with specialist laboratories in France and the UK to develop better testing techniques to find the best approach to tackling the issue.

He added: “We understand the public concern and the impact this is having and are doing everything we can to support and facilitate the Environment Agency, as the organisation responsible for fish welfare and events which affect it, in monitoring and resolving the situation.”