(Pretty much the same thing as net-pen aquaculture) A relatively new method of domesticating/harvesting fish. Fish farming is currently used to raise both Atlantic and Pacific Salmon, and research is being done toward the development of halibut farming.
However, fish farming poses a grave threat not only to commercial fisherman, but to the marine ecosystem as well. For farmed fish are very weak compared to their wild counterparts, and thus have the serious potential to introduce disease and genetic defects into the wild salmon stock.
Wild salmon must adapt to the rigours of their environment - or die. Biologically, this is part of the r-selection strategy employed by species such as wild salmon. When they spawn, such salmon lay huge numbers of eggs, but only a small percentage of these eggs will successfully hatch, and only a small percentage of those that hatch will survive as salmon. Survival of the fittest, in other words.
Farmed salmon, by contrast, must have a much larger survival rate in order to insure profitability on the part of the fish farmers. As a result, many of the weaker fish (those who would have never made it in the wild) are able to survive, thus making the overall salmon population weaker. Furthermore, because farmed salmon are raised in crowded pens, they do not develop the strength that wild salmon develop from their natural environment. These crowded pens also allow for disease to spread and develop much more easily than it will in the salmon habitats, especially given the weaker nature of the fish therein.
As a result of all of this - the lack of natural selection, the less demanding environment, et cetera; farmed salmon are far, far weaker and genetically inferior fish. Anytime a significant number of farmed fish escape from the salmon pens, therefore (which happens on a fairly regular basis); it wreaks havoc on the environment, introducing genetic degradation, and especially disease.
Norway, one of the world's pioneers in fish farming, has already been hit with disasterous consequences because of it. In 1999, the Norwegian government was forced to chemically sterilise (eradicating all life) 15 major river systems in an attempt to curb the damage being done by Gyrodactylus salaris, a parasite originating from farmed salmon stocks. Between 1981 and 1998, Norway had been forced to use this chemical on 24 river systems in attempts to control the parasite.
As stated above, scientists are currently working on the development of halibut farming. While developing a method of successful halibut domestication has been somewhat problematic as of yet; halibut farming, if developed and practised, could potentially be far more devastating than salmon farming has been. Whereas salmon are born in freshwater streams, giving accesiblilty to the controlling of salmon breeding, halibut are born and spend their entire lives at the bottom of the ocean. Therefore, were disease to be introduced to the bottom-fish ecosystem, it would be very, very difficult to do anything about it.