Temps de lecture :4 minutes
Can continued expansion of aquaculture increase the global fish catch enough to feed the world’s growing need for fish protein? Certainly, some growth in world aquaculture can be expected, but just how much is not clear. One analysis projects that global production could nearly double by 2020 to 70 million metric tons (OECD 2001:112-113). Several factors are pushing this growth in both intensive aquaculture and in small-scale, farm-based efforts. Global demand for fish is rising even as many ocean stocks are declining, and aquaculture techniques and technology continue to improve. In addition, small-scale aquaculture offers farmers a ready source of both subsistence food and cash, and these benefits are likely to promote expansion beyond its traditional stronghold in Asia (FAO 1997:24-25).
However, there are also serious constraints on aquaculture’s growth. For one, fish farming requires both land and water—two resources already in short supply in many areas. In Thailand both these resources have been diverted in recent years to fuel the growth of the aquaculture industry. For example, nearly half the land now used for shrimp ponds in Thailand was formerly used for rice paddies; in addition, water diversion for shrimp ponds has lowered groundwater levels noticeably in some coastal areas. In China, the concern over loss of arable land has led to restrictions on any further conversion of farmland to aquaculture ponds (Holmes 1996:35-36).
More serious still are the environmental impacts of aquaculture operations, especially the intensive production systems and large-scale facilities used to raise high-value shrimp, salmon, and other premium species. Shrimp farming has taken an especially heavy toll on coastal habitats, with mangrove swamps in Africa and Southeast Asia being cleared at an alarming rate to make room for shrimp ponds (Gujja and Finger-Stich 1996:12-15, 33-39; Iwama 1991:192-216). In just 6 years, from 1987 to 1993, Thailand lost more than 17 percent of its mangrove forests to shrimp ponds (Holmes 1996:36). Destruction of mangroves leaves coastal areas exposed to erosion and flooding, and has altered natural drainage patterns, increased salt intrusion, and removed a critical habitat for many aquatic species (Iwama 1991:177-216). […]
Intensive aquaculture operations can also lead to water shortages and pollution. Raising 1 ton of shrimp in a farm requires 50-60 thousand litres of water (Anonymous 1997:109). When that water is flushed from the ponds into surrounding coastal or river waters in exchange for fresh supplies, its heavy concentrations of fish feces, uneaten food, and other organic debris can lead to oxygen depletion and contribute to harmful algal blooms. In Thailand alone, shrimp ponds discharge some 1.3 billion m³ of effluent into coastal waters each year (Holmes 1996:34-35). In Scotland, producing a ton of farmed salmon results in the release of about 100 kg of nitrogenous compounds, like ammonia, into nearby waters (Roth 2000:38). Nutrient pollution from aquaculture, in turn, can cause declines in aquaculture productivity by promoting outbreaks of disease among the fish (Naylor et al. 2000:8).
Paradoxically, some aquaculture production also puts more pressure on ocean fish stocks, rather than relieving pressure. As noted previously, carnivorous species like salmon and shrimp depend on high-protein feed formulated from fishmeal—a blend of sardines, anchovies, pilchard, and other low-value fish. But it is also becoming more common, especially in Asia, to boost the weights of herbivorous and omnivorous fish by giving them feed that contains as much as 15 percent fish meal and fish oil. There are growing concerns that the addition of extra fish meal and oil could place significant pressure on the pelagic fisheries and marine ecosystems that supply it (Naylor et al. 2000:4, 8). By some estimates, as much as 33 percent of fishmeal is used for aquaculture feeds, and it takes roughly 2 kg of fishmeal to produce a kg of farmed fish or shrimp. The result is a net loss of fish protein (Naylor et al. 2000:4-5).
The Food and Agriculture Organization of the United Nations (FAO) asserts that some progress has been made in reducing the environmental impacts of aquaculture. For example, several countries where salmon are farmed have instituted controls on production to ensure that pollution is kept within acceptable limits (FAO 1997:22). In some cases, new technology has also helped. In Puget Sound, on the west coast of the United States, one salmon farmer is using a giant, floating, semienclosed tub to raise his fish rather than the usual porous pens made of netting. The tub prevents fish wastes from polluting surrounding waters and also keeps fish from escaping and intermingling with wild salmon, which would contaminate the gene pool of the native fish (Christensen 1997:27-29). Integrating the production of fish and other marine products, like seaweed and mussels that grow well in wastewater from intensive farms, can also help reduce the nutrient and particulate loads. In Chile, some salmon are farmed with a red alga that removes nitrogen and phosphorous wastes from the cages. The effluent can also be used to produce a seaweed crop, offsetting the costs of creating the integrated farming system (Naylor 2001:9).
Even in the problematic shrimp-farming industry, there are some initial signs of progress. In South Asia, a major shrimp producer has instituted a temporary ban on new ponds until the government adopts an acceptable social and environmental policy (FAO 1997:22). In some locales in Thailand, farmers are voluntarily coordinating the flushing and filling of ponds to reduce the spread of diseases. In addition, some shrimp farmers are advocating an “ecolabeling” scheme that would certify shrimp grown by producers using more benign farming practices (Christensen 1997:29).
Progress in aquaculture research can also be expected to help in the transition to low-impact, high-productivity fish farming. For example, Chinese researchers are developing a protein supplement based on yeast that can substitute for more than half the fishmeal in aquaculture feed preparations. Further, work on fish breeding has already produced a strain of tilapia that grows 60 percent faster and with higher survival rates than native tilapia (Holmes 1996:34-35).
In the end, aquaculture’s contribution to the global food supply will likely turn on how well these and other innovations can help fish farms more closely mimic natural ecosystems, with better recycling of nutrients and less waste generation (Folke and Kautsky 1992:5-24). That will mean fewer inputs and impacts, without eroding aquaculture’s profitability and versatility.
Greg MOCK, Robin WHITE and Amy WAGENER
Editor : Wendy Vanasselt