In countries like the U.K., Canada, Norway, and Chile, salmon and trout farming are one of the fastest-growing forms of agriculture. Salmon farming is not increasing in the United States because of heavy competition from other countries, and higher environmental standards for fish farms in the US. Salmon farming, like other food producing operations such as beef, wheat or tomatoes can impact the environment.

However, the difference between shore farming and fish farming is that shore farming takes place on private land, while fish farming often takes place on the public waters. Organic wastes from fish cages can have a significant effect on water quality and the population structure of organisms, beyond the boundaries of the fish pens, increasing the occurrence of toxic algal blooms. Scotland, as well as Chile and China, has had serious toxic algae blooms. Algal blooms can cause the death of huge numbers of wild fish and other species, and great harm to wild fisheries. However, even a month of fallow time can return the area to pristine condition.

Like other agriculture production, aquaculture must stand up to a rigorous evaluation of any environmental impact. Salmon aquaculture has come under increasing scrutiny from environmental nongovernmental organizations (ENGO's). In Canada, salmon farming sites occupy a small portion of the coastal zone areas where they are located. The total area occupied by Canadian salmon farms in British Columbia and the Bay of Fundy in New Brunswick is about 8,900 acres (36 km²) which is less than 0.01% of the coastal area where these sites are located. Still, even though salmon farms occupy only a small percentage of the public waters, scientists have found a significant degradation of the areas where they exist, with lowered oxygen levels, replacement of native seaweeds with invasive seaweeds, increased algal blooms, reduction of wild species, and loss of nursery habitat for wild fish.

Wild Pacific and Atlantic salmon stocks have seen significant declines over the last several decades, before salmon farming operations started. These declines were caused by a combination of factors including climate change, overfishing and freshwater habitat destruction. However, rivers with fish farms have experienced accelerated decline of wild stocks caused by spread of diseases such as infectious salmon anemia, and parasites such as sea lice from farmed to wild salmon.

Concerns have been raised on the East coast that wild Atlantic salmon may interbreed with and catch disease from salmon that escape from farms. Canadian salmon farmers have significantly reduced the escape of their salmon. The evidence shows that the escape of farmed salmon on Canada's west coast poses low risk to Pacific salmon. However, young wild salmon swimming down river to the ocean are free of sea lice parasites before they swim past the salmon farms, and laden with sea lice after they pass the farms. Most die from these sea lice.

Many farmed fish species are carnivorous, meaning that other wild fish species must be harvested to maintain the fish farm. For example, herring are used to make salmon feed. Since herring are the backbone of the North Atlantic food chain, increased fishing pressure on their numbers is a serious threat to all other fish species which depend on herring for food. It is argued that fish farms, far from removing the pressure on wild fish stocks, increase it. Others argue that it takes less fish (in the form of the fishmeal component of an aquaculture diet) to produce a unit of table fish through aquaculture than through the natural food web. Fisheries that are based on species lower on the trophic web (such as many species used for fishmeal) are also more resistant to overfishing than typical table fish fisheries.

The fish farm industry is trying to decrease its reliance on fish for fish feed. The vast majority of aquaculture production on the global scale (species such as carps, catfish and tilapia) occurs with the use of feeds with very little or no fishmeal. A portion of the fish meal used in fish feeds for highly carnivorous species comes from the trimmings and discards of commercial species. More studies are being done concerning shifts in feed composition using poultry and vegetable oils as substitutes for fish protein & oil. However this use of land based feed ingredients results in a decrease of the Omega 3 fish oils in the farmed fish (although in some cases a 'washing out' of the terrestrial oils can be achieved with a short period of feeding with marine oils prior to harvest). The current relectance to further reduce fishmeal and marine oils in the commercial diets of species such as the salmonids and shrimps is not based so much on technical difficulties as consumer resistance to the taste and health qualities of vegetarian fish. In the long term, alternative sources of long chain Omega 3 fatty acids (the most difficult ingredient to source from non fish sources) may be developed from zooplankton or microalgal origins.

Other problems with aquaculture include the potential for increasing the spread of unwanted invasive species, as farmed species are often not native to the area in which they are farmed. When these species escape, they can compete with native species and damage ecosystems. Another problem is the spread of introduced parasites, pests, and diseases.

Cite: Wikipedia