There are two main types of net-pen mariculture. One type involves rearing fish in enclosed nets from small juveniles to adults.
Fish are fed, given drugs to control disease, and harvested. The second type involves fattening ocean-caught fish in enclosed nets
by supplying large amounts of feed for several weeks to months before harvest. The use of net pens has increased dramatically in recent years as a means of rearing large numbers of salmon, tunas, and other commercially valuable fishes. Salmon net-pen operations can be found extensively in Scotland, Ireland, Norway, Chile, Canada and the US while net-pen rearing of tuna, sea bass and sea bream has boomed in the waters off Spain, Greece, and Italy. In Asia, sea bass and grouper netpen rearing is common. This is a capital-intensive industry with far-reaching consequences for the environment and for the economics of competitive capture-based fisheries in rural regions (FAO 2004).
There are several potentially serious environmental consequences of net-pen rearing. Waste from cages damages bottomdwelling plants and animals, reducing biodiversity. Concentrating large amounts of fish in small areas can lead to disease outbreaks. Viral and bacterial diseases, as well as parasites, can then be transmitted to wild fish. Data from Ireland, Scotland, Norway and Canada suggests that net-pen rearing is linked to outbreaks of deadly sea lice in wild salmon (Naylor and Burke 2005). To control outbreaks, some farmers use antibiotics and parasiticides. Along with uneaten feed and fish faeces, these chemicals foul local waters. As a result of pharmaceutical discharge, resistant strains of infectious organisms emerge. Net-pen operators often kill or aggressively deter predators such as herons, seals, sea lions and killer whales through tactics such as sirens. Salmon farms are believed to interrupt the migration of wild salmon and the movement of killer whales in Europe and North America (World Fisheries Trust 2002). In these regions, interbreeding with escaped fish can also reduce fitness of wild fish.
Toward best practices
Net-pen farming can be modified to better protect the environment as well as improve economic output. Farmers can apply a “systems approach” by raising ecologically complementary species together, a practice known as polyculture. One example is to breed algae, filter feeding bivalves and fin fishes together to reduce feeding waste and nutrient loading and increase the overall accumulation of edible protein (Box 1).
Evaluating ocean currents and water circulation – which effect how coastal zones can assimilate organic matter from farms –
can cut down accumulating organic wastes. Training farm workers in feeding practices can also reduce feed losses. (Soto and
Norambuena 2004). Farmers can reduce the density of fishes in pens, which, along with water re-circulation systems or rotation of cultured stocks, could reduce disease outbreaks. Technological advances in vaccination and breeding for
immunity can be applied to reduce use of pharmaceuticals. Finally, designing flexibility and diversity into the types of fish produced can act as insurance against environmental changes in the coastal zone from climateinduced alterations in sea temperatures and other environmental factors.
|Box 1: Two ways of reducing waste
Many mariculture operations create undigested feed and faeces, which can pollute waters and, over time, destroy habitat and disrupt marine food chains. One organism’s waste, however, is another organism’s food. This is the idea behind polyculture, or integrated aquaculture, which involves growing two or more “crops” together for a balanced ecosystem approach. Long practised in freshwater fish farms, polyculture can also be applied to marine operations. In Sungo Bay in Northern China, nitrogen excreted by the two billion scallops produced there each year is used to cultivate kelp. A Canadian pilot project in the Bay of Fundy shows
that kelps and mussels grow faster when cultivated near salmon. Researchers also found that kelps, mussels and salmon can be grown together safely and economically, and are working on policies that would allow for commercial-scale operations. Greener feeds Paradoxically, raising some farmed fish species requires more fish biomass than those species produce. This is due to the high levels of fish meal and fish oil in aquaculture feeds. Production of a single kilogram of carnivorous marine fish such as cod or seabass typically uses two to five kilograms of processed wild-caught fish. Efforts are underway to develop substitutes for fishderived feeds using oilseeds, soybeans, meat by-products and microbial proteins. Chinese researchers are creating a
protein supplement based on yeast. However, feed substitutes have yet to match the nutritional value of fish meal and oil. This has led some environmentalists to conclude that plant-eaters such as carp are one of the few sustainable aquaculture crops.
Sources: AquaNet 2003, Ecological Society of America 2001, World Fisheries Trust 2002, World Resources Institute 2001