"It's a terrible problem. They're like aliens," Noriyuki Kani of the fisheries federation in Toyama, northwest of Tokyo, told Reuters ahead of the conference.

There are no official figures on the size of the problem, but Kani says the financial losses are obvious.

"If your nets are full of jellyfish, of course there is no space for fish," he said.

Cutting up and disposing of the giants can turn a three-hour fishing trip into a 10-hour marathon, while valuable fish are poisoned or crushed under the weight of the unwanted catch...

Despite their size, the invertebrates aren't toxic enough to cause serious harm to humans, but fishermen often wear goggles and protective clothing to avoid stings when dealing with them.