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Marine aquaculture


Since the 1970s, aquaculture has been the fastest-growing sector of food production. Some 50 million tonnes of fish and seafood are now produced annually by aquaculture, representing almost half of the world’s consumption of food fish according to the WWF. Yet this putative solution to overfishing carries its own risks and burdens for marine ecosystems.

Risks of aquaculture

In Europe, marine aquaculture began in the 19th century with the cultivation of mussels and oysters. The greatest risk associated with mussel farming has to do with importation from other marine regions, as other non-indigenous, invasive species may be introduced together with imported mussels. The spread of mussel and oyster species themselves can have unpredictable consequences.

An example is the Pacific oyster. This has been cultivated in oyster farms since the mid-1960s in the Netherlands and since the mid-1980s off the German island of Sylt. Mostly in about the last 10 years, the species has spread in the Schleswig-Holstein Wadden Sea. Its larvae notably attach themselves to mussels, turning mussel banks into oyster reefs. This has serious consequences for other wildlife, such as eider ducks, that feed on mussels but not oysters, which they cannot eat due to their extremely hard shells. A species introduced together with the Pacific oyster is the tough, heavily branched Japanese Sargassum seaweed, which can reach three metres in length. Little research has so far been done into the effects this has on other organisms.

Negative impacts of marine fish farming

The intensive production of biomass in marine fish farms causes increased deposition of food residue and fish excrement on the seabed. Sea floor habitats can be changed or destroyed by eutrophication and sludge formation. Other end products of metabolism such as ammonium excreted by fish through their gills can also accumulate in the water. Chemical processes can turn ammonium into ammonia, which is highly poisonous to fish inside and outside of fish farms.

Chemical pollution from fish farms must be critically regarded in general. According to PAN (Pestizid-Aktions-Netzwerk e.V.), up to 300 different chemical agents are used in fish farms to protect fish from diseases and parasites, and also to increase their productivity. Depending on the chemicals, this can have serious effects on the environment and also on consumers. Despite this, not all fish diseases can be controlled. It is known from Norway, for example, that wild salmon found in the vicinity of fish farms are 73 times as likely to be infested with the parasitic salmon louse as salmon far away from such farms. Fish diseases can thus be transferred to and greatly endanger wild stocks.

A further severe risk is posed by farmed fish escaping. Some 500,000 salmon escaped in Norway alone in 2006. These can alter the genetic makeup of wild stocks they mate with. It cannot be ruled out that this will affect migration, a behaviour that salmon have ‘in their genes’.

A far more critical problem with aquaculture, however, is feeding. Most farmed species are predatory fish such as tuna or salmon. That means smaller fish species have to be caught to feed them. Species such as sardine or eel are specifically trawled using small-meshed nets for the purpose. This ‘industrial’ fishing threatens the food sources for piscivores such as seals, porpoise, sea birds and other, large fish species. The FIFO (fish in to fish out) ratio represents the quantity of wild fish needed to produce one kilogramme of farmed fish. With many carnivorous fish species the ratio is in the range of four to five. For tuna it is up to 20.

Towards more eco-friendly aquaculture

Nonetheless, right across the board from feeding through to conditioning, there are nonetheless numerous methods and measures that can be applied in order to make marine aquaculture more sustainable and thus a real alternative to conventional fishing.

There are already land-based fish farming ponds that use efficient filter systems for water exchange so that no excrement, harmful metabolism products and food residues enter the sea. The spread of disease can also be prevented by the use of water recirculation systems. Vegetable-based fish food based on soya or rapeseed could be used to significantly reduce the quantities of fish meal needed and conserve wild stocks. Ecolabels and similar labelling schemes help consumers identify sustainable aquaculture when buying fish.

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