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Overfishing and the subsequent collapse of many commercial fisheries has led to an increase in fish farming. The increase in the number of fish farms has adversely affected wild fish populations. Many fish farms are found in coastal regions of the world. In the Scottish lochs, where many of the UK’s fish farms are found, there is a slow exchange rate of water, lochs containing fish farms tend to have unnaturally higher nutrient levels and eutrophic conditions which inevitably lead to more frequent algal blooms.
There has been a dramatic rise in the amount of factory farmed salmon produced in Scotland. There are 340 salmon farms in Scotland, in 1980 the amount of salmon produced was 800 tonnes, in 2000 it was 127,000 tonnes. Salmon are carnivorous, a large proportion of the oceanic catch is caught to feed them - it takes 5 tons of fish caught from the sea to produce one ton of factory-farmed salmon. Inland factory-farmed fish are kept in shallow concrete troughs. The intensive crowding – as many as five fish per square foot – spreads infection and parasites, so factory fish farmers use antibiotics to get more fish fatter faster.
Parasites commonly found on factory farmed fish are also infecting wild populations - wild fish would never come into contact with more than a few lice during their lifetime. Increasing numbers of fish farms has led to increasing numbers of lice in waters which effectively eat fish alive.
Besides antibiotics, growth-promoting drugs and disinfectants, other chemicals used in fish farming include the pigment Canthaxanthin, used to turn the fish's flesh from its natural grey to pink. Canthaxanthin is banned as an additive in food but fed to fish which are bred to be eaten. It is banned in the USA because it is believed to be carcinogenic. According to the executive director of the Marine Aquatic Association, farmed salmon are pale because they are denied their natural carotenoid-rich diet.
As well as altering the natural balance of coastal waters, fish farms attract fish-eating wildlife. So the fish farmers often try to protect their stocks by killing the wildlife, including seals, otters, guillemots, herons, dolphins, porpoises and basking sharks.
On March 4, 1998, a federal law in the US took effect that allows fish farmers in 13 states to kill unlimited numbers of cormorants to protect their profits. The US Fish & Wildlife Service estimates that 92,000 of these birds will be killed by fish farmers each year—about 5 to 10 per cent of the North American population.
Seabird numbers plummet as a result of overfishing while the catch is fed to carnivorous fish and herbivorous livestock as high-protein food.
Northern Hemisphere fish farms are commonly found in the same coastal areas as those polluted by industry, human sewage and agriculture. It is inevitable that fish will take in some of the toxins and concentrate them.
Fish farms also cause their own pollution. One ton of farmed trout produces pollution equal to the untreated sewage of 200-300 people. It has been estimated that the amount of pollution in Scotland due to ammonia output from fish farming is comparable to sewage produced by 9.4 million people. Faeces and food pellets are concentrated around the netted underwater cage but the bulk accumulates beneath the cages. This toxic build-up causes de-oxygenation and can adversely affect local wildlife communities. Eutrophication can occur as the water is enriched with nitrates, phosphates and nitrogenous waste products.
Unfortunately, fish farming is now a global phenomenon for expensive creatures such as prawns and yellow tails. The coastal areas chosen for the farms are usually mangrove swamps, seen as useless areas ripe for exploitation. In fact they provide the most productive and important habitat in the oceans. Ninety per cent of marine fish rely upon the amazing diversity provided by the mangroves, particularly for spawning. Over 2,000 species of fish, crustaceans and plants thrive there.
Mangroves act as buffers, they prevent flooding, stop erosion and are the nursery of ocean life - and they are being ripped up faster than anyone can count. Indonesia, the Philippines, Malaysia, Thailand, Ecuador, Panama - clearance is rampant everywhere. The subtropical regions of the world have lost 70 per cent of all mangrove swamps since 1960, largely to fish farming. The construction of fish farms has led to the decline in wild populations of fish and shell fish in particular. Mangroves are destroyed as more farms are built. Farms rely upon wild larvae to stock them but numbers are dwindling because they are destroying the very habitat from which they originate. After a few years the farms have to be moved, cutting down yet more mangroves. Desolation is left behind.