Apolemia "pearl chain" string jellyfish tend to appear in Norway's coastal waters from September to January, with a peak in November.
Photo: Erling Svensen / Havforskningsinstituttet (The Norwegian Institute of Marine Research).
The string jellyfish (Apolemia sp.), a colony species known for forming long, thread-like chains, and which can be deadly for farmed fish, has reappeared in Norwegian waters this autumn, according to the Norwegian Institute of Marine Research (Havforskningsinstituttet), although as yet there have been relatively few sightings compared with previous years.
The species, often referred to as "pearl chain" jellyfish, or Perlesnormanet in Norwegian, is known to possess a strong venom capable of injuring fish. According to the institute, outbreaks in both 2023 and 2024 affected fish farms, with a major bloom in 2023 believed to have caused the deaths of around three million farmed fish. That incident was the first large-scale occurrence of the jellyfish in more than two decades to have serious consequences for the aquaculture industry along the Norwegian coast.
In a news release by the institute, marine researcher Tone Falkenhaug said the jellyfish has so far been observed in Troms and western Finnmark, and pointed out there is a clear seasonal pattern to sightings. "This lasts from September through January, peaking in November. We’ve seen this pattern every year since we first started receiving observations in 2019," Falkenhaug explained.
The institute is continuing to encourage the public to report new sightings through the online platform Dugnad for havet. Since the platform began collecting reports, more than 2,000 observations have been submitted, with the highest number recorded in 2023.
Researchers are still uncertain where the jellyfish originates or why it has become more common in recent years. The Institute's Jellysafe research project is investigating the possible causes, but one leading hypothesis is that the jellyfish arrives with inflows of Atlantic water.
To explore this idea, IMR oceanographer Jon Albretsen and colleagues are using ocean current models to trace the species’ potential movements backward in time. By combining these models with recorded observations, they aim to identify where the jellyfish may have originated.
Preliminary modelling suggests that Apolemia drifts with Atlantic water and enters Norwegian waters mainly through the Faroe-Shetland Channel. “Most of the simulated jellyfish particles match the observations along the Norwegian coast if they begin their journey west of Scotland in February and March of the current year,” Falkenhaug said.
Although researchers have not yet established where the species travels before that point, the findings appear to support the hypothesis that Atlantic inflow is the primary transport route for the jellyfish.
Last year, Norwegian authorities issued warnings following a surge in the numbers of Apolemia jellyfish in the country's coastal waters, leading to extensive fish deaths across multiple fish farming facilities.
At that time, the Norwegian Food Safety Authority (Mattilsynet), which includes oversight of aquaculture in Norway, published guidelines urging fish farmers to heighten their preparedness, implement preventive measures, and be ready to act swiftly to reduce suffering and manage outbreaks.
String jellyfish, also known as "pearl-chain" jellyfish or "Perlesnormanet," can float into net pens, causing severe injuries to salmon, including skin and gill burns. The injuries, combined with stress, can exacerbate other health conditions in the fish, causing further welfare issues.
