I don't know if they are the type responsible for that, but I remember seeing these guys in the Yellow Sea off the west coast of South Korea.
The tourist spots are starting to see what commercial fisherman have been dealing with for years.
Jellyfish swarms have been increasingly depleting fish stocks off the coast of Japan and South Korea for some time now. There was also a freak case of jellyfish completely wiping out an Irish salmon farm last year. In that case, the swarm supposedly covered over 10 square miles and contained (literally) billions of jellyfish.
I don't know if they are the type responsible for that, but I remember seeing these guys in the Yellow Sea off the west coast of South Korea.