The number of jellyfish is increasing globally due to climate change and human impact on the marine environment, and the worrying trend will continue.

Scientists report an increase in comb jellies in Swedish waters along with the arrival in 2018 of an “alien species” called the cling jellyfish, which delivers a painful sting.

But they warn that there is not enough research to understand the full extent of their impact and further consequences at the local level.

To reveal the extent of the increased amount of jellyfish in the local environment, the writers of this article went to Vrångö with Björn Källström, a marine biologist at the University of Gothenburg. We didn’t have to look for long: jellyfish can be seen both in the water and on land.

Marine biologist Björn Källström reports that human activity is responsible for the increase in jellyfish. Photo: Peter Seenan

When our team caught an American comb net, Källström commented: “This is an invasive alien species, which arrived in Swedish waters in 2006. In the summer, people in Sweden report thousands of them.”

In addition to reducing the amount of fish in the sea, jellyfish clog nets.

– When we fish for mackerel, we can’t catch anything, says Andreas Olsson Wijk, local fisherman. “[The jellyfish] fill the net and it is impossible to get anything into the boat.”

Fisherman and restaurateur Andreas Olsson Wijk says jellyfish hinder the catch of mackerel. Photo: Peter Seenan

According to Håkan Karlsten, a local hotel owner who has lived permanently on the island since 1991, “small fishing boats have problems because they are not strong enough to take anything. They have no tools to take them out, or strong engines to counteract the weight of the jellyfish in the nets.”

Professor Lena Granhag, lecturer in maritime studies at Chalmers University, explained that the warming of seawater, caused by climate change, leads to the presence of the American comb net in Sweden and the Baltic Sea.

Excessive levels of nutrients in the water, called eutrophication, also contribute to the growth of the jellyfish population. The main nutrients involved in this process are nitrogen and phosphorus, which are found in agricultural pesticides and fertilizers.

“More nutrients will lead to more blooms,” she said. “When there are a lot of nutrients, algae will bloom. Jellyfish can eat the algae directly, but they actually catch zooplankton – or small shrimp – which in turn have algae on them.”

Källström explained how direct human action is also relevant. Ships can be responsible for introducing other invasive species, such as the clinging jellyfish, which “came in 2018 with a large cargo ship to Swedish waters and managed to survive”.

Overfishing is also “a factor that leads to more jellyfish,” he added, as fish are one of their main predators.

A jellyfish, or ‘manet’ as they are called in Swedish. Photo: Peter Seenan

Increasing jellyfish is a global trend. Along the coast of Haifa in Israel, Jellyfish cause a lot of damage every year, with this increase in recent years. The Society of Ecology and Environmental Sciences in Israel points to climate change as a cause, with overfishing and the agricultural sector part of the reason behind the changing ecosystem.

Jellyfish blooms can also present practical challenges, such as clogging cooling water streams in nuclear power plants. Reactors in Sweden Oskarshamn the nuclear power plant was clogged in 2005 and 2013. It happens regularly at Japaneses nuclear power plant during the summer in a tsunami-stressed energy sector. Nuclear power plants in Scotland faces a similar challenge.

– The problem in Sweden and in many other places is that we have no long-term series of measurements of jellyfish monitoring. It is actually quite difficult to say for any place in Sweden or many places in the world that jellyfish are actually increasing. However, there are several signs of an increase, says Källström.

Jellyfish at Vrångö where locals say they cause problems for fishing. Photo: Peter Seenan

A deeper understanding of jellyfish increases and their link to climate change is dependent on greater financial investment in data collection and marine research.

Initiatives on quantity-based data, such as the reports from The Sea and Water Authority, is a solution to the lack of research on the number of jellyfish and its relationship to climate change. – This is citizen science, people help researchers, said Källström.

Global trends suggest that the impact of jellyfish goes beyond stung tourists and clogged nuclear power plants, to impacting local economies, marine ecosystems and local food production – a worrying indicator of human action and its consequences for our climate that local actors and the international community are failing to address address.

Article by Gothenburg University students Sandra Daniel, Mireia Jimenez Barcelo, Javad Maleki, Peter Seenan and Marina Panicheva