It will probably be multiple issues that come about from fish farming world wide that make our oceans crash.
1) Habitat destruction, mostly in tropical areas of the world. Breeding grounds for warm water fish, mangroves and shallow estuaries are being bulldozed in and made into holding ponds at such an alarming rate that less than 10 % will remain. Think about the size of the entire coast of North, South America from Columbia East and then South to 500 miles south of the Amazon river estuary being destroyed. Shrimp farms can only be used for 18 months at best and then abandoned. 80% of shrimp sold world wide may come from farming.
2) The worlds forage fish are being used to feed farm fish. It may be as high as 75% of the worlds forage fish feed farms. That does not leave enough bio mass in the worlds oceans to feed native fish populations. Just look at the Poggy industry of the mid alantic states. There is little being done to curb the entire loss of part of the food chain. As bad as this is, in countries of the 3rd world, 2nd world and some of the 1st world their entire fishing fleets are now just fishing for forage fish.
3) Disease, every fish farm operation creates disease in one form or another. One can not put that much living mass into such a small area and have a healthy situation period. In tropical areas of SE Asia and S. America Native shrimp populations are all but gone.
We are all aware that our oceans are in trouble. Yes we win a battle here and there but we are dealing with a world wide crash. This is not a slow crash as we would expect. Even those of us in our 50's and 60's may live to see it. With the earth getting warmer for what ever reason and the greed of the new world order don't expect much in the way of a good life for our kids by the time they are 50 or 60. If the oceans crash the major population of the earth goes with it.
There are answers to fixing the problem but this long winded enough for now.