So far this year, scientists along the Norwegian costs have received 110 observations of a jellyfish that can be deadly for farmed fish, according to the Norwegian Institute of Marine Research.

"During the last 20 years, we have only seen this many sightings this year and in 2021," Norwegian Institute of Marine Research jellyfish scientist Tone Falkenhaug said.

This type of jellyfish led to mass deaths of farmed fish in 1997 and 2001. In 1997, the jellyfish killed 12,000 metric tons of salmon, and 600 metric tons of salmon were killed in 2001.

This year, the increase of sightings has mainly taken place in September and October.

Manetforsker Tone Falkenhaug ved Havforskningsinstituttet Photo: Brit Rønningen

Similar to other jellyfish, this species can appear suddenly only to disappear again for several years.

The species is colonial jellyfish, where the individual jellyfish have different tasks. Those that catch food or defend the colony have so-called nettle cells, and it is these cells that can burn and cause damage to farmed fish if they enter a broken netpen.