LONDON -- An invasion of jellyfish into a cooling water pool at a Scottish nuclear power plant kept its nuclear reactors offline on Wednesday, a phenomenon which may grow more common in future, scientists said.

Two reactors at EDF Energy's Torness nuclear power plant on the Scottish east coast remained shut a day after they were manually stopped due to masses of jellyfish obstructing cooling water filters.

Nuclear power plants draw water from nearby seas or rivers to cool down their reactors, but if the filters which keep out marine animals and seaweed are clogged, the station shuts down to maintain temperature and safety standards.

Britain's Office for Nuclear Regulation said power plants follow a pre-planned programme when these situations occur.

Latest plant availability data from network operator National Grid showed Torness reactor 1 would return to service on July 5 and reactor 2 on July 6, but operator EDF Energy was unable to give a restart date.

Operators often take the opportunity presented by an unplanned stoppage to carry out maintenance work.

"We are working to clear the jellyfish from the waters near the power station. This work, as well as monitoring the area for more jellyfish, is ongoing," a spokesman for Britain's largest nuclear power operator, EDF Energy, said.

Scientists say jellyfish obstructing nuclear plants is a rare occurrence in Britain, though it has happened more often in other countries such as Japan.

"Jellyfish can bloom in really high numbers. It's not particularly common, (EDF Energy) have been a bit unlucky. If you get a bit of calm and warm weather they can turn up inshore in high numbers," said David Conway, a marine biologist at the Marine Biological Association.

Water temperatures off the east coast of Scotland are currently 13 degrees Celsius, one degree above average levels for this time of the year, Britain's Met Office said.

Increasing fishing activity and global warming are giving jellyfish populations a boost, scientists said, potentially making jellyfish invasions at nuclear power plants located near the open sea more common in the future.

"There are suggestions from some science data that over the past few years there has been an increase in swarms of jellyfish. It's possible it's linked to climate change," said Steve Hay, a plankton ecologist who specializes in jellyfish research at the Marine Scotland Science laboratory in Aberdeen.

Overfishing of small fish which feed off jellyfish leaves them less exposed to natural predators and gives them more room to reproduce, the Marine Biological Association said.

(Reporting by Karolin Schaps; editing by Jason Neely)

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