Earlier quoted context omitted.
You're right that at carrying capacity , the limiting factor is food and space. But most of our fisheries are far below that now. Fish can only breed as fast as there are breeding fish available, but we're eating them faster than they are being born. You can view a fishery basically as a giant fish-making factory. If you eat away half your fishery's biomass, then to a very rough first approximation, your factory will…
"your factory will only produce half as many fish." But then why is fish still so cheap and plenty of it avaiable, which was the main point? (but again, I don't know that, I am just refering to the argument)
Related to this: because technological change has made it really cheap and easy to catch fish [2], and up to now, that has more than offset the decreased productivity from damaging our fisheries.
[1] https://en.wikipedia.org/wiki/Fisheries_subsidy [2] http://archimer.ifremer.fr/doc/00222/33368/35784.pdf