New jellyfish attacks leave scientists baffled

Jellyfish attacks were responsible for the deaths of at least three million salmon and trout last year, says the Norwegian Institute of Marine Research, and tackling the problem is perplexing marine scientists.

It has already been reported that 62 million fish died from various causes in Norwegian fish farms in 2023 – six million more than a year earlier.

But the impact of jellyfish known in Norway as Perlesnormanets (“pearl Norman” or “string” jellyfish, Apolemia uvaria) has now become clear. They were responsible for much of that increase, Lars Helge Stien, Animal Welfare Manager at the Institute of Marine Research said.

String jellyfish look like a long thread, but they are actually a colony of several jellyfish. They do not occur naturally in Norwegian waters, but tends to drift in with the ocean currents.

“These threads break up when they hit the net wall in the cages. When they hit the fish, they shoot out a spike that injects nettle poison,” Stien says.

When attacked, some of the fish die instantly while others experience fainting or abnormal behaviour, which can cause them to suffer wounds and injuries, he added.

“So we have both an acute mortality rate, but perhaps also a longer-term mortality rate”, Stien added.

As in previous years there is generally mortality from issues such as diseases, lice treatments and other factors, but much of the increase last year was a result of the pearlnormanet jellyfish attacks.  It is difficult to avoid them, the Institute believes, but it is looking closely at the problem.

The last previous major jellyfish attack was in in the 1990s, and the authorities are hoping it will be another two decades before there is another one.

“We obviously want to keep the mortality rate as low as possible,” said Jon Arne Grøttum, Director of Aquaculture at the industry organisation Seafood Norway.