Overfishing usually results in jellyfish blooms - if there are no fish to consume the available zooplankton then jellyfish will. There's stacks and stacks of papers written on that phenomenon.
> So people just need to start harvesting jellyfish and eating them instead of fish.
Why to eat stupid jellyfishes when all that you need to enjoy a 100% nutritive and delicious diet is to start eating other people?. Hoomans are plenty, easy to find, fatty, err sorry... enormous, yummy, delicious and had everything that our body needs.
Can we talk seriously again now? Only the 0,0001% of the jellyfish species are edible and they are basically a water globe devoid of nutrients.