More and more of the world's supply of seafood is coming from farms. Although 80% of the w

More and more of the world's supply of seafood is coming from farms.

Although 80% of the world's seafood comes from marine harvests, there is a major shift under way toward aquaculture now.

Nearly 40% of salmon marketed today are raised in captivity, compared with 6% a decade ago. Forty percent of all clams, oysters, and mussels are produced in farm environments, along with 65% of freshwater fish. Between 1990 and 1996, fish-farming production rose from 12.4 million to 23 million tons worldwide, writes Anne Platt McGinn in an article for World Watch magazine.

"The fact that world seafood supplies continue to increase at all is due almost entirely to the phenomenal growth in aquaculture," says McGinn, a research associate at the World Watch Institute. Commercial aquaculture is driven by rising human population at a time when over harvested wild fish stocks are in decline and conventional farm production has leveled off.

Biotechnology is contributing to high-yield aquaculture through transgenics—the transfer of genes from one species to another. Researchers introduce desirable genetic traits into fish, creating hardier stocks. For example, some species of fish have a protein that allows them to live in Arctic waters. By transplanting this "anti-freeze" gene into other species, researchers have created more fish that can survive in extremely cold water, according to Ag-West Biotech, Inc., in Saskatchewan, Canada.

Biotechnologists are attempting to improve a wide range of genetic traits in fish used for aquaculture, developing fish that are larger and faster-growing, more efficient in converting feed into muscle, more tolerant of low oxygen levels in water, and better able to resist disease.

Researchers also are seeking plant-based sources of food as a more efficient alternative to fishmeal. The use of plant protein on fish farms could take some of the pressure off wild fish stocks and address the problem of phosphorous pollution because plants do not contain high phosphorus levels. Wheat, canola, and canola oil are being used as alternative feed for aquaculture, according to Ag-West Biotech, Inc.

While aquaculture produces a reliable source of protein, the industry is rife with environmental problems, asserts McGinn. Perhaps the biggest concern is water pollution: Fish waste and uneaten food accumulates at farm sites and can float directly downstream into water supplies. Farm-related nutrient wastes as well as nitrogen and phosphorus also promote the spread of algal "blooms" that deplete oxygen and kill marine life.

Aquaculture is also an inefficient user of resources, McGinn charges. Fish farms need protein feed, andabout 17% of ocean fish, an over harvested wild resource, becomes food for captive-bred fish. "An estimated five kilograms of oceanic fish reduced into fishmeal are required to raise one kilogram of farmed ocean fish or shrimp, representing a large net protein loss," says McGinn.

Fish fanning does not have to be an inefficient or polluting industry. McCdnn predicts that many consumers will choose sustainably produced fish in the future, just as they prefer dolphin-free tuna today.

All the following statements are the reasons why commercial aquaculture is pushed forward EXCEPT

A．the human population is increasing fast.

B．the wild fish stocks are decreasing due to over fishing.

C．the cost of developing aquaculture is comparatively low.

D．there is no improvement in conventional farm production.