Just because we farm fish doesn't mean fish farming is producing more protein for the world. Carnivorous fish eat far more in fish-feed, usually made from smaller fish like anchovies and herring, than they produce for the dinner table. It takes three pounds of fish to put one pound of flesh on a farmed salmon, for example. Another concern is that some farmed fish bred for growth are less hardy, like the salmon. If they escape and breed with wild fish, they can weaken the ability of the wild fish to survive.
Fish grown in offshore cages or pens also generate large quantities of waste.
Fish farming has led to a quadrupling of salmon consumption in the United States since 1992, and the price to consumers has been cut by more than half. On the other hand, the farming of herbivorous fish like carp in China and catfish in the United States has relatively few negative consequences for the environment, Brown said. Two years ago, the United Nations estimated that nearly half the world's fish stocks were fully exploited and that if stocks were to have a chance of recovery, one in three of the world's fishing boats would have to stop fishing. Nearly 70 percent of the world's stocks are in need of urgent management, the U.N. said. Yonathan Zohar, a scientist at the University of Maryland's Biotechnology Institute, has estimated that genetically modified farmed fish will become a substitute for wild catches by 2025 as fish stocks worldwide decline.