The jellyfish, which are usually found in slightly warmer waters than red jellyfish, can give unpleasant stings, although they are not as harmful as their red cousins.
“There has not been a bloom like this in more than ten years,” Lene Friis Möller, who studies jellyfish at the University of Gothenburg, told The Local on Monday.
“Blue jellyfish are not new – they have existed for a long time – but they come around in cycles so many people in this region will not have seen anything like this before.”
There is no national monitoring of jellyfish numbers in Sweden, so Friis Möller says it is “hard to say for sure” exactly how many more jellyfish are floating around the west coast, but she says it is clear from her own sampling and the work of other academics that the blue creatures are on the rise in the area, due to a complex pattern of factors including changing currents, temperatures and food availability.
“What is quite rare is for so many blue, red and moon – the most common type of jellyfish – to be in the waters at the same time and it is good for the ecosystem that they have returned.”
She added said that while swimmers should be sure to keep a close lookout for the stinging creatures, they should not be too concerned about getting hurt.
“If you get stung by a blue jellyfish or a red one, you should rinse yourself with a lot of water. Very very few people get a bad reaction, they are not so dangerous. Moon jellyfish can't hurt you at all.”
Last week Norway’s Institute of Marine Research also spoke out about the jellyfish invasion, which is additionally affecting waters around southern and eastern Norway, close to the border with Sweden.
“I can’t remember receiving so many reported sightings ever before,” Jan Helge Fosså, a marine biologist at the institute told Norway's Aftenposten newspaper.
Fosså said that he expected the jellyfish to follow the currents further north, but was unsure of why numbers had reached such high levels.
Other experts noted that while stinging jellyfish can be unpleasant for swimmers, they are important for some sea life.
“The only wildlife we know that depend on stinging jellyfish are tortoise and blowfish. Without jellyfish, they would probably be extinct,” zoologist Petter Bøckmann told Aftenposten.