Jellyfish aren't really my thing. I'm not a biologist and on the now rare occasions when I'm in the sea I don't see them. I suppose they're interesting - they're certainly weird - and I'm willing to be concerned that their numbers are increasing as those of ordinary fish (those with backbones) are falling. So I read a recent New Scientist article with interest.
The article documents the growth in numbers and the problems they cause then looks at ways of controlling the numbers. (The increased numbers are due to human activities such as overfishing, run-off of agricultural chemicals and global warming.) Those ways include swarms of specialised robots (!) and manipulating their fertility with hormones but nothing seems to work well. And, given how little we know about them, a killing spree seems a bad idea.
And jellyfish, like everything else, are food for something. Probably the main predators include those very fish we've been removing by overfishing! So the right answer to the jellyfish problem is to restore the health of the oceans by stopping overfishing, over-use of agricultural chemicals and global warming; and possibly by releasing lots of little fish.
The Jellyfish lesson, then, is to show restraint and restore the balance of nature. A true ecological lesson.