Killer Whales Taking over the World.

[Lost]

The problem is due to ships. Jellyfish get carried across biomes when they get sucked into the water ballasts of ships and drained halfway across the world. This is how they end up outside of their own ecosystem. This causes problems in natural predation, since the animals in the new biome probably don't eat jellyfish, or that particular breed.

Or y'know, we could just let the Japanese eat them...

Actually, the Chinese cuisine comes through for that. They harvest Cannonball Jellyfish to be eaten.