Harmful jellyfish have been spotted in increasing numbers along the Norwegian coastline in recent days.
The Norwegian Institute of Marine Research (Havforskningsinstituttet) has received around 250 separate reports of "Perlesnormanet" jellyfish ("barbed wire" jellyfish, Apolemia uvaria) sightings just last week.
"Registrations have poured in," said researcher Tone Falkenhaug, who explained that although the stinging jellyfish are not dangerous for humans, they can prove fatal for fish.
Each jellyfish is actually a colony made up of many smaller individuals, some of which contain venom to protect the colony and stun prey.
This type of jellyfish has been responsible for salmon deaths at breeding facilities in the past, the researchers note. For example, in 1997, 10-12 tonnes of salmon died at facilities in Øygarden and Fedje, Vestland county, in addition to an unspecified number of fish that had to be slaughtered early. In 2001, 600 tonnes of fish were killed as a result of jellyfish damage, most of which were in Trøndelag, central Norway.
Although in the open sea the jellyfish can grow up to lengths of 30 metres, closer to the coast their exposure to waves, turbulence and physical barriers such as fish farming pens means they break up into smaller pieces.
This reduced size actually makes the jellyfish even more dangerous for fish farming, Falkenhaug explained. The smallest fragments can pass through the mesh barrier of fish farming pens, exposing salmon and other farmed fish to harmful stings.
"It is primarily these small pieces that can be problematic. If an individual with stinging cells enters a cage and hits a fish, the fish can get sores and damage its gills," said lead researcher Lars Helge Stien.
The experiences from 1997 showed that the effects on the fish varied from reduced appetite and changed behaviour to death, the researchers said.
Fish that came into contact with the jellyfish suffered skin burns with an increased risk of infection, as well as impaired vision due to eye damage. But one of the biggest risks was damage to the gills, the researchers explained. The salmon that died in previous incidents had fragments of jellyfish in their gills.
"If the gills are severely damaged, this can cause the fish to suffocate, especially if there is also stress and panic in the population. This can also cause further physical damage to the fish if it swims into the net wall or they collide with each other," said Stien.
Close contact with the jellyfish can also cause longer-term consequences during the winter season, the researchers said.
"Cold winter temperatures affect the fish and reduce the immune system, so if a fish gets a wound now, this can give access to opportunistic bacteria, and mortality throughout the winter. Until then, luckily, the sea is relatively warm," said Stien.
The researchers said that there is still a lack of knowledge about the jellyfish species and its potential impact on Norwegian fish farming.
Currently, they said, measures such as lice skirts and bubble curtains can help to keep them out, but this does not solve the problem of small fragments which can pass through these barriers.
The experiences from Øygarden indicated that it was the fish that remained on the surface that were most exposed to being burned by the jellyfish, the researchers said.
However, it is also unclear whether more sightings actually mean a greater danger than previous years, the researchers noted.
The recent surge in people reporting the jellyfish does not necessarily mean increased numbers of the organisms, but may instead be a result of heightened public awareness through media reporting.
"The pattern we see so far this autumn is similar to the pattern from last year. Then the observations of the Perlesnorman picked up strongly in November - and then they dropped off in January," Falkenhaug pointed out.
