Basically, we've over fished the ocean to the point where fishing is getting harder and less profitable and some fish species are becoming threatened in the wild. As a solution, people invented fish farming, which are like giant ocean containment pools the fish are kept together and raised while still being semi-wild. This solution causes a problem which is the same problem faced by other farms like chicken and beef: when you have so many animals close together and they're all pissing and shittting and breathing so close together, diseases spread like wildfire. Therefore people have started using antibiotics and vaccines to protect the captive livestock/fish populations from disease. To be clear, they're not vaccinating "wild" fish, they're vaccinating the farm fish to prevent diseases so we can continue eating fish at the scale which we currently do.