THE prolonged heatwave could bring swarms of rare freshwater jellyfish, plus a host of other effects, an expert has warned.

Rivers have dried up – including the upper River Teme where Environment Agency staff had to rescue fish (pictured above). And they’ve been busy at other rivers and lakes across the country too.

Top fishery scientist and ecologist Ian Wellby has revealed the higher water temperatures can produce unusual natural occurrences as well as help  survival of tiny young fish.

Ian, of independent consultancy BlueRoof Ltd, explained: “Thanks to the wet spring, water levels in most stillwaters are reasonable but the temperatures will be higher and this can produce unusual occurrences.

“One could well be the freshwater jellyfish, of which I have only heard two reports in 30 years and seen one blurred photo. But I wouldn’t be surprised if they show up somewhere this year.”

A freshwater jellyfish - they could crop up now in the UK.

A freshwater jellyfish – they could crop up now in the UK.

The freshwater jellyfish – Latin name Craspedacusta sowerbii – originates in the Yangtze basin of China but has now spread worldwide.

The jellyfish can pop up almost anywhere but normally survive just a few days, often in big swarms.

They are thought to arrive on the feet of migrating water fowl, and survive for a while where water temperatures are high enough.

The small species, which grow to about an inch in diameter, do have stinging tentacles that can kill small fish and can cause irritations on human skin. They mostly eat small invertebrates like daphnia.

Boom in fish fry during heatwave

Ian Wellby explained some of the other more likely consequences of the hot and largely dry weather.

“It isn’t that unusual for fish to spawn twice in a year but the high temperature means this will be more likely everywhere – and some fish might even spawn three times,” said Ian.

“The heat also encourages more small insect life which fish fry feed on and this helps the survival of greater numbers.

“Of course there is the downside of the threat of deoxygenated water and river water levels are bound to get low.

“The usual suspects like the Teme and small rivers in the Peak District are already drying up, but hopefully some rain will come to replenish others before a serious situation is reached.

“Anglers need to be extra careful when handling sensitive fish like pike and barbel, minimise their time out of the water and give them time for full recovery before release.

“Also anglers need the commonsense to avoid doing things like unhooking a fish on a mat that has been in the sun all day,” he concluded.

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