The farming of multiple species in a single pond — polyculture — was practiced for centuries before the advent of industrial-scale aquaculture.
Even today, four of the most widely cultivated fish species are sometimes produced together in the same ponds in China: silver carp (a phytoplankton filter feeder), grass carp (a herbivore that grazes aquatic plants), common carp (an omnivorous bottom feeder that eats detritus), and bighead carp (a zooplankton filter feeder).
This type of system efficiently uses food and water resources from all levels of the pond ecosystem, thereby reducing costs and wastes while increasing productivity.
Integrated systems can also be used for high-value fish, such as salmon and shrimp, in order to reduce waste outputs, diversify products, and increase productivity.
Some studies show that seaweed and mussels grow well in wastewater from intensive and semi-intensive aquaculture systems, and as a result, reduce nutrient and particulate loads to the environment.
In Chile, for example, salmon can be farmed along with a type of red alga that removes large amounts of dissolved nitrogen and phosphorous wastes from salmon cages.
The effluent output from salmon farming is thus used to nourish a seaweed crop, and the added revenue from the sale of the seaweed more than pays for the extra infrastructure needed for the integrated system.
If government policies required fish farms to internalize the environmental costs of waste discharges — that is, by making sewage treatment mandatory — then integrated systems that reduce the waste stream would be even more profitable. Some caveats apply: Human health considerations now limit the marketability of mollusks raised in the waste stream from intensive fish farming areas, and such concerns must be addressed in order to make these types of integrated systems economically viable.
Promoting Sustainable Aquaculture:
Long-term growth of the aquaculture industry depends on both ecologically sound practices and sustainable resource management. Governments can encourage such practices by stringently regulating the creation of new farming facilities in mangroves and other coastal wetlands, establishing fines to minimize escapes of fish from aquaculture pens, enforcing strict disease control measures for the movement of stock, and mandating effluent treatment and inpond recirculation of wastewater.
Many aquaculture operations have adopted such practices even in the absence of strict government policies, especially with the heightening of environmental concerns in recent years. In poor countries, however, such policies are often neither politically enforceable nor economically and socially feasible.
Despite significant improvements in the industry, many ecologically sound technologies remain on the shelf and underused in the field. This is an arena where external funding agencies such as development banks can play a strategic role by encouraging the development and financing the implementation of sustainable aquaculture technologies, the rehabilitation of ecosystems degraded by aquaculture, and the protection of coastal ecosystems.
Whether aquaculture depletes or enhances net fish supplies in the future will depend to a large extent on how markets for resources are managed. The absence of regulations or price disincentives on coastal pollution by fish farms, for example, limits mollusk farming and slows the adoption of non-polluting technologies by other marine aquaculture systems. Furthermore, government subsidies to the ocean fisheries sector often prevent farmed fish from undercutting the market for wild-caught fish, at least until ocean fisheries are fully depleted.
Whether farmed fish can replace or provide market alternatives for ocean catches will depend significantly on the economics and policies of fisheries in various nations.
High fixed costs of fishing fleets, labor considerations, and continued subsidies to the ocean fisheries sector — subsidies that currently approach 20 to 25 percent of gross fisheries revenue globally — may prevent increased aquaculture production from lowering catches of wild fish in the short term.
In the case of salmon, for instance, increased farm production did not result in reduced capture levels despite 30 to 50 percent declines in the international prices for four of the five main species of wild salmon (chinook, coho, pink, and chum) during the 1990s.
Salmon catches worldwide actually rose by 27 percent between 1988 and 1997. Similarly, despite rapid growth in alternative farmed fish such as tilapia, wild capture of hake and haddock has remained relatively stable during the past decade.
Finally, perhaps the largest unknown for both the private and public sectors is the future availability of freshwater sites for aquaculture production. Increasing scarcity of freshwater resources could severely limit the farming of herbivorous fish such as carps and tilapia. This constraint on the future growth of freshwater systems makes it even more urgent to develop marine aquaculture systems that are both ecologically and socially sound.
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