A massive influx of jellyfish has led to the partial shutdown of a nuclear reactor in northern France.

Two of Gravelines nuclear power station’s six units were shut down on Wednesday and Thursday because of the presence of jellyfish in the seawater filtration system, which is used for cooling, Électricité de France (EDF) said.

A third unit has been shut down since 10 August due to the same issue, while a fourth has been closed for an inspection.

Gravelines, one of the country’s biggest nuclear power stations located between Dunkirk and Calais in the Hauts-de-France region, was also closed last August because of jellyfish in its filters.

EDF said that units three, four and six were now shut.

Unit three had only been reconnected to the grid on 18 August after the first wave of jellyfish earlier this month, the company said, while units one and two were also operating at reduced power as a preventative measure.

Gravelines’ six units each produce 900 megawatts of power. The station is cooled from a canal connected to the North Sea.

Several species of jellyfish are native and can be seen around the coast when the waters are warm.

Jellyfish are at their most active in the warmer months. Hot weather increases plankton levels at the water’s surface, drawing jellyfish up, experts say.

Measures have been implemented to try to prevent jellyfish infiltration, including a partnership with fishermen who capture and release them further offshore, according to French outlet Ici.

There have been similar issues at other nuclear stations, including at the Torness plant in Scotland in 2011 and Sweden’s Oskarshamn plant in 2013, but such cases are rare.