The idea here is that as long as you sustainably catch your fish, then it's all good.
"Sustainable fish stocks are continually declining". In other words, sustainable fishing isn't sustainable itself.
Bycatch, including sharks, has caused massive declines in unfished populations of marine animals. The vaquita is on the verge of extinction, for instance, due to becoming bycatch in shrimp and fish nets and trawlers.
"Sustainable fishing" also doesn't address the amount of garbage thrown into the ocean. 50% of the plastic in the ocean is discarded or lost fishing nets.
Given that you are killing animals that don't want to die, by plastic, by bycatch, and on your plate, the idea that you could do it indefinitely can't be a defense to doing it in the first place. Human slavery is highly sustainable but still is morally indefensible.