"We've never come across such a phenomenon," marine biologist Ulrich Sommer told news agency DPA. "Many of them have gas bubbles that are making them float. What is usually distributed over several metres of depth has all moved to the surface."
The high concentration of the Aurelia aurita jellyfish were first observed on Monday, and Sommer said a possible connection could be the Kieler Woche regatta that was held over the weekend.
"One can perhaps imagine that the engines of so many boats created a lot of bubbles in the water, which the jellyfish absorbed," Sommer said. "If it got into their gastrointestinal system then the jellyfish would blow up like balloons - and they would probably die from this."
The scientist said he didn't know what else could be responsible for the gassy phenomenon, but said the quantity he's seen matched his observations in other heavy jellyfish years.
Swimmers needn't worry, he said, "The jellyfish are harmless."