Worthwhile read:
"Farming fish has been practiced for thousands of years, but not in the manner now underway on many temperate coasts worldwide today. Traditionally, fish that eat vegetable matter were used, such as carp or tilapia. For thousands of years Chinese fish farms have cycled waste from vegetable crops through their fish and then used the waste from the fish to fertilize the next vegetable crop. This sustainable, closed loop system created protein. In the late 1970's however, a Norwegain hydro company, Norsk Hydro initiated the first corporate effort to farm salmon.
Salmon are carnivores. No one has successfully farmed a carnivore. A terrestrial equivalent would feed chickens to dogs and eat the dog. The underlying equation in farming carnivores is a net loss in protein, and would not be profitable if full price is paid for the feed. Salmon farming takes two - five pounds of wild fish to produce one pound of farm salmon. This represents a net global protein loss as most of the fish used to make pellets are high quality food fit for human consumption. In 1999, 189,000 tons of Chilean whiting was sold to the make fish farm pellets for $12.9 million, when it could have produced $102.9 million if sold for human consumption.
Salmon farming is not sustainable. It starves one ocean of fish, and pollutes another with the same fish. Its profit margin is so slight it can not afford to deal with its own waste. Its product is of questionable food quality being high in PCBs, low in omega oils and dyed pink. It is favoured politically because it produces salmon without a river, leaving the resource rich watersheds of British Columbia open for exploitation. It is a classic example of destruction of the commons to promote the privately owned."