After record-breaking mortalities in 2022, the salmon farming industry in Scotland is facing another turbulent year of shockingly high fish deaths and reduced harvest revenues, which is already being part-blamed on blooms of micro jellyfish.

But what exactly are these jellyfish, and are they really to blame? Certainly many are claiming so.  

I started to look into this issue after, I went, with anti-salmon-farm campaigners, to observe a Bakkafrost farm off Mull which, local intelligence had informed, was in the midst of a significant mortality event.

When I interviewed Bakkafrost CEO Regin Jacobsen, he acknowledged the problem at Geasgill farm and two others and blamed the tiny hydrozoans.  

“What we see right now,” he said, “is mainly jellyfish which are coming in huge swarms or blooms. They are coming with the tide. It seems to be that specific sites are more vulnerable than others, and there is also a common factor that if the salmon is standing a very long time, meaning two years in the sea, they get more vulnerable after the second summer. We have three sites - Geasgill is one of them - and these three sites have in common that the fish have been standing there for a second summer now.”  


Watch: Drone footage of Geasgill farm filmed by activists

What are these jellyfish like? 

The type of jellyfish Mr Jacobsen described are “micro”, at less than 1cm diameter, and one of the ways, he said, that they have an impact on the health of salmon is that when the fish breathe, the organisms run through the gills of the fish, stinging and burning them, and effectively reducing their capacity to breathe.   

“Just as lower lung capacity can affect human general health," Jacobsen said,  "impacting on heart health and blood pressure, so the same happens in the fish.  Then if there is somehow a stress event, then there are increased mortalities.”   

Research is being done into the impact of various species of the microjellyfish. Among them is Muggiaea Atlantic, a type of hydrozoan known as a siphonophore. The Fish Health Forum quoted fish veterinarian Chris Matthews, who at an industry event explained how gill disease outbreaks were often linked with the presence of this organism whose range has expanded in recent years and is linked to warmer water. 

Also linked to gill disorders is the micro jellyfish Solmaris corona. But jellyfish don’t just affect the gills, they also can cause skin lesions and are known to carry pathogens, among them the bacteria causing the ulcerative disease tenacibaculosis.  

This year's mortality records show the jellyfish Obelia blamed for problems by several farms.

Are the micro jellyfish affected by warmer seas? 

One of the big questions is whether the rise in micro jellyfish impacts on salmon farms relates to climate change and a warming sea – and whether, therefore, this is only likely to get worse.  This last year has seen what has been described as a marine heatwave, chiefly on surface water and, in the North Atlantic, which was particularly anomalous in the North Sea and Irish Sea in June.   

Regin Jacobsen noted that, over the past ten years, microjellyfish incidents have become more severe and that has coincided this year with the heatwave. “We see that the sea temperature is higher now. This year the temperature looked like it was running one month earlier than last year. We saw already in June this year that we had almost the same temperature in June that we had last year in July. This year in July we had the same temperature as August, and the difference was 2C which is quite significant.”  

Jacobsen, however,  is not so sure that the warm seas are entirely to blame for the blooms. “If you look at the last twenty years,” he says, “we have also seen in the last twenty years similar temperatures – so we are still within the normal deviation. Therefore I don’t think the picture is so clear that we can only blame the temperature of the water.” 

READ MORE: One farm, 2 billion lice. Report shows the extent of Scottish salmon industry infestation 

READ MORE:  A Scottish salmon farm visit. Haunted by mortalities and jellyfish

Are there any other reasons why jellyfish may be impacting salmon farms? 

Possibly the farms themselves are fostering the blooms. The 2013 book Advances in A Aquaculture Hatchery Technology declares: “Ironically, aquaculture may be inadvertently exacerbating the problems with jellyfish blooms.”. 

Increased nutrients around farms, due to excess fish food and waste food, it said, “could create eutrophic conditions that may favour jellyfish over fish”.

Do the fish suffer?

Undoubtedly. There is plenty of research now that shows that fish feel pain and stress. The European Commission said in 2009: “There is now sufficient scientific evidence indicating that fish are sentient beings and that they are subject to pain and suffering notably when they are killed.”

How are salmon farms tackling the issue? 

There is no way of preventing the jellyfish from entering the farms and scientists are recommending general attention to the best possible gill health, so the fish are as healthy as possible if they are impacted.

Bakkafrost also believes a "one summer fish" is the answer. What the company has observed, Mr Jacobsen said, is that most of the problems are happening in fish during their second summer at sea. Hence, one of its key initiatives, he said, is to “produce the fish larger in freshwater in closed systems on land before putting them in the sea”. 

That shorter time in the sea would mean less chance of being impacted by jellyfish incidents. 

Bakkafrost has already been pursuing a similar strategy in the Faroes, raising larger fish on land and reducing the time fish are in the sea.

Is it just the jellyfish that are to blame? 

Jellyfish are also not the only factor impacting on fish gill health. As anti-salmon-farm activist Don Staniford says: "The dead fish we've seen at Geasgill could have been killed by gill diseases, pathogens, viruses, pancreas disease – there is a whole raft of diseases and we only find out when the data is released later in the year." 

"Salmon farming in Scotland is dead in the water," he says. "A deadly cocktail of warming seas, swarms of jellyfish and micro jellies, plagues of lice and infectious diseases is killing off millions of Scottish salmon."

He is right to point out that it is a cocktail of factors. Most salmon farm deaths are, in fact, not officially credited to jellyfish. Rather the reasons noted include viral disease, bacterial disease, heart and skeletal muscle inflammation, pancreatic disease, cardiomyopathy syndrome (severe heart disease), furunculosis (boils), and various other conditions, not to mention those infamous lice.

The struggle against infestation with lice continues, and earlier this year WildFish issued a report which found that last year the Scottish industry breached its guidelines on lice levels, or failed to supply a lice count, for 40% of its count.

By far the most common cause of death given, however,  is “gill health-related”, and in the Fish Health Inspectorate's mortality records kept since 2018, this appears 1265 times, eight times more often than jellyfish, though the two often appear in conjunction, and sometimes also with algae. Lice is mentioned 499 times. 

Inside the phrase "gill health" is a multitude of causes and a whole bigger question about salmon health, welfare, and sustainability.

Jellyfish are just one piece of a jigsaw that this year could see, in 2023, mortalities even worse than the 16.7 million of last year.