Global production of farmed fish, shrimp, clams, and oysters more than doubled in weight and value during the 1990s while landings of wild-caught fish remained level. Many people look to this growth in aquaculture to relieve pressure on ocean fish stocks, most of which are now fished at or beyond capacity, and to allow wild populations to recover.
Production of farmed fish and shellfish does increase world fish supplies. Yet by using increasing amounts of wild caught fish to feed farmed shrimp and salmon, and even to fortify the feed of herbivorous fish such as carp, some sectors of the aquaculture industry are actually increasing the pressure on ocean fish populations.
The available scientific evidence indicates that some types of aquaculture are on a destructive path that poses a threat not only to wild fish stocks but also to the industry’s own long-term potential.
One of the most disturbing trends is the rapid expansion and intensification of shrimp and salmon farming and culture of other high-value carnivorous marine fish such as cod, seabass, and tuna. Production of a single kilogram of these species typically uses two to five kilograms of wild-caught fish processed into fish meal and fish oil for feed.
Besides this direct impact on wild fish stocks, some aquaculture as currently practiced degrades the marine environment and diminishes the ecological life support services it provides to fish, marine mammals, and seabirds, as well as humans. These impacts include
- Destruction of hundreds of thousands of hectares of mangrove forests and coastal wetlands for construction of aquaculture facilities
- Use of wild-caught rather than hatchery-reared finfish or shellfish fry to stock captive operations, a practice that often leads to a high rate of discarded bycatch of other species
- Heavy fishing pressure on small ocean fish such as anchovies for use as fish meal, which can deplete food for wild fish such as cod, as well as seals and seabirds
- Transport of fish diseases into new waters and escapes of non-native fish that may hybridize or compete with native wild fish As aquaculture production continues to expand and intensify, both its reliance and its impact on ocean fisheries are likely to increase. The balance between farmed and wild-caught fish, as well as the total supply of fish available for human consumption, will depend on future trends in aquaculture practices.
If the goal of aquaculture is to produce more fish for consumers than can be produced naturally, then it will become increasingly counterproductive to farm carnivores that must be fed large amounts of wild-caught fish that form the foundation of the ocean food chain. Indeed, non-carnivorous species such as marine mollusks and carps account for most of the current net gain in world fish supplies from aquaculture.
Without clear recognition of its dependence on natural ecosystems, the aquaculture industry is unlikely to develop to its full potential or continue to supplement ocean fisheries. We recommend the adoption of four priority goals for aquaculture:
- Encourage farming of species lower on the food web – that is, fish with herbivorous or omnivorous diets or filter feeders such as oysters
- Improve feed management and efficiency in industrial aquaculture systems and develop substitutes for fishderived feed ingredients
- Develop integrated fish farming systems that use multiple species to reduce costs and wastes while increasing productivity
- Promote environmentally sound aquaculture practices and resource management Governments have a key role to play in developing regulations to protect coastal ecosystems and in reexamining subsidies to unsustainable marine fisheries. Development agencies are strategically placed to help in developing and implementing sustainable production practices and in financing otherwise economically and socially unattainable reforms in developing countries. If public and private interests act jointly to reduce the environmental costs generated by fish farming, present unsustainable trends can be reversed and aquaculture can make an increasingly positive contribution to global fish supplies.
AQUACULTURE IS A DIVERSE ACTIVITY:
Three-fourths of global aquaculture production by weight involves finfish and shellfish; the other fourth is seaweed. Worldwide, more than 220 species of finfish and shellfish are farmed. The range of species includes giant clams that obtain most of their nourishment from symbiotic algae, mussels that filter plankton from the water, carps that largely graze on plants, and salmon that prey on smaller fish (Figure
1). Typically, the farmed species are enclosed in a secure system such as a pond or floating pen in which they can be raised under suitable conditions, sheltered from predators and competitors, and sometimes fed and medicated with antibiotics and other drugs. As the intensity of an aquaculture operation increases, fish are confined at higher densities, supplied with all nutritional requirements, and managed more heavily. The more intensive the operation, of course, the larger the volume of wastes generated and the greater the possibilities for the spread of disease.
From one aquaculture operation to another, the intensity of culture practices and their impacts on marine ecosystems vary widely (Figure 2). Clams, oysters, and other mollusks are generally farmed along coastlines, with wildcaught or hatchery-reared seed grown on the sea floor or on suspended nets, ropes, or other structures.
The animals feed entirely on ambient supplies of plankton and organic particles in the water. Finfish may be farmed in ponds, tanks, or cages. Most marine fish and species such as salmon that migrate between fresh and salt water are reared in floating net cages near shore, and all their nutrition is supplied by formulated feeds.
Carp, catfish, and other freshwater finfish are usually grown in ponds, often integrated within agricultural settings. Crustacean farming is dominated by shrimp, which are grown in coastal ponds. Farming of both shrimp and freshwater finfish varies greatly from one operation to another in intensity and in reliance on formulated feeds.
In the past decade, two distinct sectors have emerged within this diverse industry. The first includes commercial farms that rely on intensive and semiintensive methods to produce commodities for regional or global markets. The second encompasses family and cooperative farms that rely on less intensive practices to produce low-value species for household subsistence or local markets.
The line between these sectors is growing more blurred, however. In China and other parts of Asia, for example, many small-scale farming operations are intensifying as land and water resources become increasingly scarce and valuable.
Asia produces roughly 90 percent of global aquaculture output, and China alone contributes more than two thirds of the total. Although Europe, North America, and Japan together produce just over one-tenth of the global total, these regions consume the bulk of farmed seafood that is traded internationally.
Various species of carp dominate the tonnage of farmed fish produced worldwide, and carp production for local or regional use by relatively low income households has increased dramatically in Asia (mainly China). In contrast, increased volumes of salmon, shrimp, and other high-value species are marketed mainly in industrialized countries. Farmed output and markets for other lower value species such as tilapia and milkfish have increased in both developing and industrialized countries.
Most farmed mollusks are still consumed locally and regionally in China and in other developing countries. However, production of certain species for global markets has increased in several developed countries. These species include the Pacific cupped oyster, blue mussel, New Zealand mussel, and Yesso scallop.
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