Carnivorous farmed fish are fed on high levels of fish meal and fish oil. In fact, they require a fish biomass input greater than the fish biomass produced. For the 10 species of fish most commonly farmed, an average of nearly two kilograms of wild fish is required for every one kilogram of fish raised.
Unfortunately, there is an increase in the production trend of carnivorous fish (such as salmon or shrimp) rather than herbivorous or filter feeder fish. Small pelagic fish, such as herring, sardines and anchovies, mainly provide the fish meal and fish oils used for aquaculture feed, increasing pressures on wild fish.
Numbers of popular species such as cod have plummeted; in the Mediterranean, 12 species of shark are commercially extinct. Swordfish in that area, which should grow as thick as a telephone pole, now must be caught as juveniles and eaten when no bigger than a baseball bat. The fish in the seas surrounding Africa and Asia are also in steep decline.