Not all oceans in the world will crash into death of seafood at once, - so the theory is kind of naive.
However, we're on the road downhill, and as has been experienced with overfishing etc, it will take decades to restore.
Then we have an increased pollution factor as well as a warming factor which may change habitats and confuse the ecosystems in the seas (i.e. the North Sea is 3 degrees Celcius warmer now than normally)
My country almost went to a war status with Britain, - it was all about fish. The fish around Iceland was being sucked up at an alarming rate. So, - eventually the fishing grounds became ours, but the damage was enough for us to adapt a very strict quota system that has stayed for some 20 years. The fish is slowly restoring while the European community has basically nuked the North sea, - the Cod (which we fought our Cod wars over, but at home) in the North sea is almost completely gone.
Recently, the world community is upset about recent Icelandic whaling. Yes we did start whaling again.
Everybody is shocked, and the season is over with some 5 or so whales done.
Every ocean fish and mammal is a part of he system. The whales go far for their food, and some species of them are in danger while other are overpopulating at the moment. Typically, for the equilibrum, the whales that were caught were in relatively poor condition - thin. Not enough to eat.
That means too many whales for what they have.
Sort of shows that the downhill is a good guess, but the time given is very questionable. When fish stocks crash, the fishing becomes uneconomical, so it depends, as so many other environmental things, on the MONEY.
