More serious still is concern over 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 [14] [15]. In just 6 years, from 1987 to 1993, Thailand lost more than 17 percent of its mangrove forests to shrimp ponds [16]. Destruction of mangroves has left these coastal areas exposed to erosion and flooding, has altered natural drainage patterns, has increased salt intrusion, and has removed a critical habitat for many aquatic species [17].

Intensive aquaculture operations can also lead to water pollution, which is also a major concern. When flushed into nearby coastal or river waters, 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 cubic meters of effluent into coastal waters each year [18].

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. Some 10 to 15 percent of all fishmeal goes to aquaculture feeds, and it takes roughly 2 kilograms of fishmeal to produce a kilogram of farmed fish or shrimp. The result is a net loss of fish protein [19] [20].

The Food and Agriculture Organization of the United Nations 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 [21]. 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 [22].

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 [23]. In addition, some shrimp farmers are advocating an "ecolabeling" scheme that would certify shrimp grown by producers using more benign farming practices [24].

Progress in aquaculture research can also be expected to help in the transition to low-impact, high-productivity fish farming in the future. 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 [25].

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 [26]. That will mean fewer inputs and impacts, without eroding aquaculture's profitability and versatility.

References and notes

14. Op. cit. 8, pp. 12-15, 33-39.

15. Op. cit. 1, pp. 192-216.

16. Op. cit. 11, p. 36.

17. Op. cit. 1, pp. 177-216.

18. Op. cit. 11, pp. 34-35.

19. Op. cit. 2, p. 22.

20. Op. cit. 11, pp. 34-35.

21. Op. cit. 2, p. 22.

22. Jon Christensen, "Cultivating the World's Demand for Seafood," New York Times (March 1, 1997), pp. 27-29.

23. Op. cit. 2, p. 22.

24. Op. cit. 22, p. 29.

25. Op. cit. 11, pp. 34-35.

26. Carl Folke and Nils Kautsky, "Aquaculture with Its Environment: Prospects for Sustainability," Ocean and Coastal Management, Vol. 17, No. 1 (1992), pp. 5-24.