How farm-fed seafood can deplete wild fisheries
A study in contrasts: Distant string of long, open rectangular pens hold coho and Chinook salmon being raised in Puget Sound. When antibiotics are delivered to fish in these conventional aquaculture pens, unused drug eventually diffuses down into the water below, where it can affect other wildlife. Drugs left over from treating salmon in round closed-bottom pens (foreground) are filtered out of the water before it is returned to the open environment. Photo credit: David Suzuki Foundation.
As one major oceanic fishery after another has shown signs of stress or collapsed, grocers and fish markets have increasingly turned to farm-fed species to meet demands of the growing global appetite for fish. Today, aquaculture provides between 20 and 25 percent of the 112 million metric tons of fishand shellfishthat humanity downs worldwide.
A decade ago, aquacultures visionaries were touting fish farming as a way to decrease the pressure that human consumption was placing on wild fisheries. And where the farmed fish are plant eaters, this has generally proved valid, observe Rosamond L. Naylor of Stanford Universitys Institute for International Studies and her colleagues in the Oct. 30 Science. Unfortunately, they note, for carnivorous fish such as shrimp and salmon, "the opposite may be true."
Fish dont use 100 percent of what they eat as weight-building nutrition. Thats why aquaculturists must deliver between two and four times the weight of their harvestable fish to the system as feed-fish. "Because of their dependence on wild-caught fish, shrimp and salmon aquaculture deplete rather than augment fisheries resources," the scientists point out.
In many cases, urban fish aficionados dont notice the impact because the wild-caught species are destined to become fish-meal rather than ending up on dinner plates. For instance, the greatly overfished Black Seas largest remaining fishery is the anchovy. Currently, most of the catch is being harvested by the Turkish trawlers (Russia being too poor to fuel most of its traditional fleet here)but not for human consumption. At a meeting in Leavenworth, Wash., earlier this year, Richard Harbison of the Woods Hole (Mass.) Oceanographic Institute noted that the Black Seas anchovies are harvested to provide the fish meal used to feed more desirable farmed species.
This anchovy population, itself on the brink of collapse, at one time appeared near the middle of the regions food chain. As it disappears, so will the fish that used to feed on it, leaving a burgeoning population of only those much farther down the marine chainsuch as small comb jellies (SN: 7/4/98, p. 10), which people wont touch as food.
Today, aquaculture "is becoming the dominant production method for salmon," Naylors team notes. Sating the appetite of these popular fish doesnt come easy. Last year, it took some 1.8 million tons of wild fishground into fish mealjust to produce 644,000 tons of farmed Atlantic salmon. Indeed, Naylors group cites the European aquaculture industry as estimating that harvesting the fish to feed these prized salmon requires 40,000 to 50,000 times the surface area of the salmons cultivationor a region "equivalent to about 90 percent of the primary production of the fishing area of the North Sea." Not surprisingly, European salmon farmers import much of their fishs feed from elsewhere, typically South America.
A report 2 years ago in Environment noted that Thailands shrimp farms owe much of their economic success to overfishing by local trawlers. "Thai trawlers have purposefully fished down the food chain to smaller and smaller fish," explained Biksham Gujja and Andrea Finger-Stich. "This has kept their total production from falling substantially, but it means that 70 percent of their landings are now trash fish, suitable only as animal feed." The growing harvest of such trashy species "has enabled the aquaculture industry to develop with relatively low feed costs," they found.
Other environmental impacts
Fish farming can have other severe environmental impacts as well, the Science report notes. To meet the large and growing demand for their crops, aquaculturists have been developing ever more intensive fish-management practices. They grow lots of fish in a relatively small area. Under the stress of their confinement, the fish become especially susceptible to disease which could slow their growthcutting a farmers profits.
So aquaculture makes great use of antibiotics, drugs that can quickly build up in the sediment. In a paper published earlier this year, Bent Halling-Sørensen and his colleagues at the Royal Danish School of Pharmacy reviewed some of the data on this problem. They note that one such growth-promoting antibioticoxytetracyclinehas been reported tainting the bottom of fish-farming sites at concentrations of up to 4.9 milligrams per kilogram of sampled sediment (on a dry-weight basis).
In fact, they note, "Antibiotic resistance in sediment bacteria are often found in locations with fish farms"and may be playing a growing role in the development of antibiotic resistant germs generally. If true, a growing reliance on farmed fish may actually be eroding the efficacy of life-saving drugs, argues Stuart Levy, the director of the Center for Adaptation Genetics and Drug Resistance at the Tufts Medical School in Boston.
Though farmed salmon are often reared for the later part of their lives in floating pens at sea (see photo), shrimp are intensively reared in coastal areas, usually enclosed ponds. Gujja and Finger-Stich found that these ponds "have a life span of between 5 and 10 years, after which time the exhaustion of the supply of clean fresh- and seawater and the build-up of pond sediments and corrosion forces farmers to relocate."
The pair reported that in 1994 alone, some 20,000 hectares of land were abandoned, and overall, some 150,000 hectares of former shrimp farms now sit as waste zones needing perhaps 2 or 3 decades to recover their fertility. Many of the sites being converted into new shrimp ponds are mangrove swamps, important to the development of marine fisheries and the filtering of impurities out of the water.
In Thailand, half of the land now being shrimp-farmed had been rice paddies, according to a report in World Resources 1998-99, released last May.
The lure of such transitions is purely economic. In her book Rocking the Boat: Conserving Fisheries and Protecting Jobs, Anne Platt McGinn observes that "in a year, an individual shrimp farmer can make up to $10,000 per hectare for intensive production rates of 4 to 5 tons per hectare"or 10 times as much as farming carp might yield. "But these economic returns do not account for ecologicaland economiclosses such as habitat degradation." Platt McGinn cites one study showing that mangroves in Malaysia provide environmental services such as nutrient cycling and water cleaning, that carry an economic value 70 percent higher than the short-term gains shrimp farmers derive from diverting those mangrove areas to aquaculture.
Signs of optimism
Fish farmers are not unaware of the resources they have been claiming, and the bad reputation that many of their activities have acquired. To encourage more environmentally soundbut costlyshrimp-rearing practices, some Asian aquaculturists have begun lobbying for the development of an "ecolabeling" scheme, the World Resources report notes. It would serve like a Good Housekeeping seal of approval, certifying that any shrimp bearing this label had been grown using techniques relatively benign to the environment.
This report also highlights efforts in China, home to almost 60 percent of all aquaculture, to develop a yeast-proteinbased growth meal that could substitute for half of the fishmeal now fed to farmed fish.
In some of the more progressive salmon-rearing operations, fish farmers are raising their Chinook and other species in closed, floating pens so that antibiotics and other wastes can be filtered from the water before its released back into the environment.
A return to the less resource-intensive aquacultural practices typical of Southeast Asia 50 or more years ago would also lessen strains on the environment. Platt McGinn notes that some Vietnamese rice farmers have been able to cut their fertilizer needs by almost 30 percent by raising fish in their paddies. Elsewhere, some farmers are integrating terrestrial and aquaculture operations so that fish wastes are used to enrich their soil.
Paying the price
One reason aquaculture has taken off is the promise of bringing more affordable fish to the dinner table. "But the real cost of cheap food is far greater than we imagined," notes food writer Nicols Fox. If people are asked to pay the true cost of foods, costs that include protection of the environment, the grocery bill would growor most diners would choose to eat differently. For instance, they might refuse to pay $40 to $60 per pound or more for wild-caught tuna or swordfish. Or consumers might eat smaller portions of fish, increasing the share of the dinner plate reserved for grains and veggies. They might also vote with their pocketbooks to buy only fish that has been certified to come from sustainable stocks.
The true price of depleted resources, such as most popular marine fish today (see A grilling analysis), should increase as demand remains flat or increases. Instead, grocers and fish mongers have found ways to offer fish that cost no more today that 5 years ago. In her book Spoiled, Fox argues that consumers should not ignore this apparent economic disconnect. "We need to ask...What is the life history of this fish?" In other words, she says, consumers should demand to know where the fish came from, how it was raised, what resources went into it, and what is a realistic valuation of those resources?
And where the numbers dont add up, she says: Consider eating something else.
Fox, Nicols. 1997. Spoiled: The Dangerous Truth About a Food Chain Gone Haywire. New York: Basic Books.
Gujja, B., and A. Finger-Stich. 1996. What Price Prawn? Shrimp Aquacultures Impact in Asia. Environment 38(September):12.
Platt McGinn, A. 1998. Rocking the Boat: Conserving Fisheries and Protecting Jobs. Worldwatch Institute, Washington DC.
Raloff, J. 1998. Biocontrols may not work for jellies. Science News Online (July 4).
_____. 1998. Drugged waters. Science News 153(March 21):187.
_____. 1998. How low will we go in fishing for dinner? Science News 153(Feb. 7):86.
_____. 1996. Fishing for answers. Science News 150(Oct. 26):268.
_____. 1996. Fishing: Out of control? Science News 149(June 8):367.
World Resources Institute. 1998. Farming Fish: The Aquaculture Boom. In World Resources 1998-99: 158.
Woods Hole Oceanographic Institute
Woods Hole, MA 02543
Rosamond L. Naylor
Institute for International Studies
Stanford, CA 94305-6055
This week's Food for Thought has been prepared by Janet Raloff, senior editor of Science News.
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