As you are obviously aware, since you are asking this question, jelly fish populations have exploded. This is especially true of one of the giant jellies that have appeared in huge numbers in Asian waters and are spreading.
For those who aren't familiar with what happens during a jelly explosion I will point out a couple of things.
Jellies trail long stinging filaments that can be over 100' long and a jelly bloom can sweep the water clean of just about all living things from fish to the smallest organisms. leaving little or nothing in their wake.
This is a new phenomenon and added to the problems from over fishing could wipe out many species.
The most deadly creature in the sea isn't the Great White shark it is the box jelly and the sea wasp. Both are so small that you aren't likely to see them until it is too late (a box jelly's body may only be 1/4" long and clear a sea wasp even smaller looking more like a bit of thread). Both have powerful neurotoxins and no antidote at best the pain can be relieved somewhat but survivors can suffer after affects for many months. It's believed that many drowning victims might very well actually be caused by jelly / sea wasp stings. Even a broken off bit of their filament can still sting.
BTW the only way to deactivate a jelly's filament is to poor vinegar on it. If you are heading to the beach it is a good idea to include vinegar in your first-aid kit.
Getting rid of them is proving to be a worse problem than imaginable since catching them causes the release of thousands or even millions of polyps (the "seed" or egg of jellies). Cutting them up just makes that many more as the pieces can regenerate.
A big problem is that man has eliminated or reduced the populations of most if not all of their natural enemies, like sea turtles.
Another is that like ocean pollution and habitat destruction this problem goes unseen by all but those who make their living directly from the sea or spend a great deal of time on the sea.
Right now science is stymied on a solution to the jelly explosion and no means of culling them has been found so far.
All we really know is that the problem is growing at an alarming rate.