Fish farms threaten world’s supply of fish

But while ocean fisheries are in decline, commercial fish farming – or aquaculture – is booming. Farmers now produce more than one-fourth of all the finfish and shellfish directly consumed by people world-wide.

This rapid rise in aquaculture production “is a mixed blessing, however, for the sustainability of ocean fisheries,” according to a new study by economist Rosamond L. Naylor and biologist Harold A. Mooney.

Writing in the June 29 issue of the journal Nature, Naylor and Mooney point out that, “on balance, global aquaculture still adds to world fish supplies.” But, they warn, the growing demand for farm-raised salmon, shrimp and other commercially valuable species actually threatens the world’s supply of fish.

The “underlying paradox,” write the authors, is that “aquaculture is a possible solution – but also a contributing factor – to the collapse of fisheries stocks around the world.”

Carnivorous fish

“The industry is growing so rapidly that it’s important that people start looking at the future,” says Naylor, a senior research scholar at Stanford’s Institute for International Studies and lead author of the Nature study.

She and Mooney write that “global production of farmed fish and shellfish has more than doubled during the past 15 years. While many people believe such growth relieves pressure on ocean fisheries, the opposite is true for some aquaculture practices.”

Naylor says the greatest potential threat to the environment comes from farms that raise carnivorous species – fish that eat other fish. These include marine shrimp and popular finfish such as salmon, trout and seabass.

Most carnivorous fish are sold to lucrative markets in the United States, Europe and Japan.

To maximize growth and enhance flavour, shrimp and salmon farmers use processed food made from less valuable fish species harvested from the sea – herring, mackerel, anchovy, sardine and other relatively small varieties.

The problem, explains Naylor, is that it takes a lot of mackerel and herring to feed farmed fish. As a result, the average farmer ends up using about three pounds of wild-caught fish to grow a single pound of salmon or shrimp.

About 29 million tons of finfish and shellfish were farmed worldwide in 1997 – a significant contribution to world fish supplies until you subtract the additional 10 million tons of wild fish harvested for feed that year.

In other words, says Naylor, 10 million tons of herring, mackerel and sardine that could have been directly consumed by people or wildlife ended up as processed fishmeal instead.

Another 22 million tons of wild fish also were used for pig and cow feed in 1997.

If that rate of harvest continues, then some populations of herring, mackerel and other fish low in the food chain could virtually disappear from the world’s oceans, according to the Nature study. That, says Naylor and Mooney, would have a direct impact on humans as well as seabirds and marine mammals that depend on these wild fish populations.

“The growing aquaculture industry cannot continue to rely on finite stocks of wild-caught fish, a number of which are already classified as fully exploited, overexploited or depleted,” warn the authors.

“What our study demonstrates is that you don’t get something for nothing,” adds Mooney, the Paul S. Achilles Professor of Environmental Sciences.

“It takes a lot of protein to produce protein,” he explains. “We’re calling upon the aquaculture industry to create a better feed, one that uses fewer fish.”

Mooney says that researchers are trying to develop substitutes for fishmeal, including soybeans and other plants. But fish oils, unlike vegetable oils, contain essential amino acids that carnivorous fish need to survive.

Vegetarian fish

According to the Nature study, the vast majority of animals grown on fish farms thrive on diets consisting primarily of plant food. These include vegetarian finfish – such as carp, catfish, tilapia and milkfish – as well as scallops, oysters and other filter feeders.

These species, which are popular in many cultures around the world, require only minimal amounts of fishmeal in their diet and consequently do very little damage to wild fish stocks – unlike carnivorous shrimp or salmon.

Therefore, according to Naylor and Mooney, species raised on vegetarian diets actually contribute to the world’s supply of fish.

But that could change dramatically, warn the authors, if certain trends continue – especially in Asia, which accounts for roughly 90 percent of global aquaculture production.

Asian farmers produce more than 10 million tons of carp and tilapia every year. But as human populations increase and land becomes scarce, there is less space available to build ponds for carp, tilapia and other popular plant-eating fish. So Asian farmers have decided to grow bigger fish by adding extra amounts of fishmeal to existing ponds.

The result, say the authors, could mean a dramatic increase in the demand for fishmeal, putting even more pressure on herring and other ocean fisheries.

Indirect costs

In addition to depleting wild fish populations, aquaculture also causes indirect harm to the environment, according to the Nature study.

“Many fish farms damage coastal ecosystems through habitat destruction,” says Naylor. She notes that hundreds of thousands of acres of mangrove forests and coastal wetlands throughout Asia have been transformed into milkfish and shrimp ponds. The consequence has been a devastating loss of nursery habitat for indigenous species of finfish and shellfish that people have long depended on in countries such as Indonesia and the Philippines.

Another problem, says Naylor, is that farmed fish sometimes compete with native species and spread exotic diseases. She points out that farmed Atlantic salmon often escape from net pens and invade wild populations. Today, an estimated 40 percent of all salmon caught in the North Atlantic originated in farms. More than 255,000 Atlantic salmon also have escaped into the Pacific Ocean since the early 1980s and are routinely caught by fishing vessels from Washington to Alaska.

But the problem is especially acute in the Atlantic Ocean. “Increasing evidence suggests that farm escapees may hybridize with and alter the genetic makeup of wild populations of Atlantic salmon,” write the Nature authors. “Such genetic alterations could exacerbate the decline in many locally endangered populations of wild Atlantic salmon,” they predict.

Aquaculture also has been linked to the spread of disease. “Since the early 1990s, the Whitespot and Yellowhead viruses have caused catastrophic, multimillion-dollar crop losses in shrimp farms across Asia,” according to the Nature report. The Whitespot virus, which has caused high mortality rates in shrimp farms along the Texas coast, may have been introduced there by the release of untreated waste from nearby plants processing imported Asian tiger shrimp.

Global solutions

The authors say that, while there is no evidence that aquaculture can restore ocean fisheries, the industry does play an important role in feeding a hungry planet. But if it wants to sustain its positive contribution to the world’s supply of fish, major changes must take place in the way the industry does business, the authors conclude.

They recommend that governments offer financial incentives that encourage farmers to use less fishmeal and to raise species that thrive on vegetarian diets.

Agencies like the World Bank also should provide funding to rehabilitate ecosystems degraded by aquaculture and to locate fishponds away from fragile mangroves and other coastal wetlands.

The authors suggest that fish farmers adopt “polyculture systems,” in which different species are raised in the same facility. In Chile, for example, farmers grow salmon and red algae together. Body wastes produced by the salmon help fertilize the algae, which is then sold as a separate cash crop.

According to Naylor, consumers in the United States and other industrialized countries can do their part by ordering fewer shrimp cocktails and more oysters or grilled tilapia.

Naylor and Mooney’s study, “Effect of aquaculture on world fish supplies,” is Nature magazine’s June 29 cover story.