IF you take all the adult fish and don't leave any to breed then there's no fish at all!

 This is the basis of Southeast Asia's fishing Crisis. The other problem with this is that if you overfish it also causes more Jellyfish everywhere too because little fish and adult fish eat the tentacles of Jellyfish and they die otherwise.

However, if you over fish the Jellyfish become so prevalent that their tentacles sting any baby fish remaining  and they all die and then over fishing becomes a worse and worse and worse problem like now.  

The other problem is it screws up the whole ecosystem in the ocean to where upper end things like Sharks and orcas and whales often have nothing left to eat.

So, over fishing tends to kill just about everything in the ocean except jellyfish.