Why tiny jellyfish are such a big threat to salmon farming

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

I became interested in this issue after observing, with anti-salmon farm activists, a bakkafrost farm outside of Mull, which local intelligence reported was experiencing a significant mortality rate.

When I interviewed Regin Jacobsen, CEO of Bakkafrost, he acknowledged the problem at Geasgill Farm and two others, blaming the tiny hydroids.

“What we’re seeing right now,” he said, “is mostly jellyfish swarming in huge schools or blooms.” They come with the tide. It seems that certain locations are more vulnerable than others, and there is also a common factor: if the salmon are in the sea for a very long time, say two years, they become more vulnerable after the second summer. We have three locations – Geasgill is one of them – and what these three locations have in common is that this is the second time the fish have been there.”

Watch: Drone footage of Geasgill Farm filmed by activists

How are these jellyfish?

The jellyfish that Mr. Jacobsen described are actually “micro jellyfish” with a diameter of less than 1 cm. They have an impact on the health of salmon, he said, because the organisms pass through the jellyfish as the fish breathe. The fish’s gills are injured, stinging and burning, and their ability to breathe is effectively restricted.

“Just as reduced lung capacity can affect overall human health by affecting heart health and blood pressure,” Jacobsen said, “the same thing happens in fish.” then there is increased mortality.”

The effects of various micro jellyfish species are currently being researched. Among them is Muggiaea atlantic, a species of hydrozoan known as the siphonophore. The Fish Health Forum quoted fish veterinarian Chris Matthews as explaining at an industry event that gill disease outbreaks are often linked to the presence of this organism, which has been increasing in range in recent years and is associated with warmer water.

The micro jellyfish Solmaris Corona is also associated with gill disorders. But jellyfish don’t just attack the gills, they can also cause skin lesions and are known to transmit pathogens, including the bacteria that cause the stomach ulcer disease tenacibaculosis.

This year’s mortality records show that several farms blame the jellyfish Obelia for the problems.

Are micro jellyfish affected by warmer seas?

One of the big questions is whether the increase in the impact of micro jellyfish on salmon farms is related to climate change and a warming sea – and whether the situation is therefore likely to only get worse. Last year there was what is known as a marine heatwave, which was mainly felt in surface waters and the North Atlantic and was particularly unusual in the North Sea and Irish Sea in June.

Regin Jacobsen noted that micro jellyfish incidents had become more severe over the past decade and that this year coincided with the heat wave. “We see that the sea temperature is now higher. This year it looked like the temperature had risen a month earlier than last year. We already saw in June this year that we had almost the same temperatures in June as in July last year. This year in July we had the same temperatures as in August and the difference was 2°C, which is quite significant.”

However, Jacobsen is not so sure whether the warm seas are solely responsible for the blooms. “If you look at the last twenty years,” he says, “we’ve also seen similar temperatures over the last twenty years – so we’re still within the normal range.” So I don’t think the picture is that clear, that we could only blame the temperature of the water.”

READ MORE: One farm, 2 billion lice. Report reveals extent of attack on Scottish salmon industry

READ MORE: A visit to a Scottish salmon farm. Haunted by mortals and jellyfish

Are there any other reasons jellyfish could affect salmon farms?

The farms themselves may be promoting flowering. According to the 2013 book Advances in A Aquaculture Hatchery Technology, “Ironically, aquaculture may inadvertently exacerbate problems with jellyfish blooms.”

Increased nutrients around farms due to excess fish feed and feed waste could “create eutrophic conditions that could favor jellyfish over fish.”

Are the fish suffering?

Without a doubt. There are now numerous studies showing that fish feel pain and stress. The European Commission stated in 2009: “There is now ample scientific evidence that fish are sentient beings and that they experience pain and suffering, particularly when killed.”

How are salmon farms dealing with the problem?

There is no way to prevent jellyfish from entering farms, and scientists recommend maintaining the best possible gill health in general so that fish are as healthy as possible when infested.

Bakkafrost also believes a “one summer fish” is the answer. The company has observed, Mr Jacobsen said, that most problems with fish occur during their second summer at sea. Therefore, one of his key initiatives is “to increase freshwater fish in closed systems on land before they are released into the sea”.

This shorter time at sea would mean less chance of being affected by jellyfish incidents.

Bakkafrost is already pursuing a similar strategy in the Faroe Islands, breeding larger fish on land and reducing the time the fish spend in the sea.

Are jellyfish really to blame?

Jellyfish aren’t the only factor affecting fish gill health, either. As anti-salmon farm activist Don Staniford says: “The dead fish we saw at Geasgill could have been killed by gill disease, pathogens, viruses or pancreatic disease – there are a whole range of diseases and we’re only finding out if the.” The data will be released later in the year.

“Salmon farming in Scotland is dead,” he says. “A deadly cocktail of warming seas, swarms of jellyfish and microgels, lice infestations and infectious diseases is killing millions of Scottish salmon.”

He rightly points out that it is a cocktail of factors. In fact, most salmon farm deaths are not officially attributed to jellyfish. Rather, the reasons mentioned include viral diseases, bacterial diseases, inflammation of the heart and skeletal muscles, diseases of the pancreas, cardiomyopathy syndrome (severe heart disease), furunculosis (furunculosis) and various other diseases, not to mention the infamous lice.

The fight against lice infestations continues and earlier this year WildFish published a report stating that the Scottish industry had breached its own lice count guidelines or failed to report lice counts on 40% of lice levels over the past year .

However, by far the most commonly cited cause of death is “gill health” and in the Fish Health Inspectorate’s mortality records kept since 2018 this occurs 1265 times, eight times more than jellyfish, although both are commonly found in conjunction, sometimes with algae. Lice are mentioned 499 times.

Behind the term ‘gill health’ there are a multitude of causes and a much larger question about the health, welfare and sustainability of salmon.

Jellyfish are just one piece of a puzzle that this year, in 2023, could see an even higher death rate than last year’s 16.7 million.