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Can Farmed Fish Flourish on a Veggie Diet?

• February 16, 2012 • 4:00 AM

It’s a fish-eat-fish world out there, which is bad news for ailing fisheries providing feedstock for aquaculture. If only some key dinner-table species were vegetarians, smaller fish would be spared.

Seafood reached a tipping point in 2009 when, for the first time, more than 50 percent of fish used for human consumption came from farms.

That might sound like good news for oceans, but farmed fish largely subsist on a steady diet of smaller fish, which are caught from fragile fisheries. It’s not a sustainable equation.

Aaron Watson, a researcher at University of Maryland’s Center for Environmental Science, says there is clear evidence that we are “fishing down the food chain, catching smaller and smaller fish to bring to the table and to bring to market, depleting wild populations and destroying natural habitat.”

Such small species, like sardines, anchovies, herring, and capelin, aren’t thrown whole to their penned cousins; they are ground into ground into fishmeal, or mashed into fish oil. There is a term used in aquaculture: fish-in fish-out, and it’s a rough measure of how many fish it takes to raise each fish that gets sold at the market. And in marine aquaculture it is generally agreed that a lot more fish go in than come out. In 2006, by conservative estimate, 16.5 million tons of fish captured from the wild went to feed farmed fish.

To control costs and extend supplies, fish farmers have tried stretching their resources, replacing increasingly expensive fishmeal with grain or soy protein. It works — to a point. There is a limit to how much one can water-down fish feed and still raise healthy stock.

“Right now most commercially produced diets for fish farms are generally 20 percent to 40 percent fish meal, and up to 30 percent fish oils,” Watson said. The meal provides the proteins, and the oil providing fish with the omega-3 fatty acids essential to their health.

Once a marine fish diet falls below about 20 percent fish meal, he says, “regardless of what protein source is used to replace the fish meal,” the fish start to show “reduced growth rates, reduced food intake, and reduced survival.”

Unlike their leaf nibbling freshwater cousins, the carps and tilapia, some of the best-tasting marine fish, such as tuna, salmon, and sea bass, are carnivores. In many cases, they are apex predators, and they can’t be convinced to eat more veggies.

Nevertheless, Watson decided to give it a shot.

Working with a carnivorous species of cobia in a University of Maryland lab, Watson provided his fish with a carefully planned vegetarian diet, combining proteins from soy, wheat, and corn gluten, with a mix of vitamins and supplements designed to provide all of their essential needs.

“We couldn’t get them to eat it,” he said.

Running down potential factors behind their finicky behavior, Watson considered the possibility that an amino acid deficiency might be involved. But which one?

When Watson added a small volume of taurine — an amino acid found only in meat and fish — to the same plant-based food the fish had rejected earlier, “they ate the feed like crazy, and they grew just as well, if not better, than the same species will grow on commercial diets using fishmeal.” (Watson’s results were published in the February issue of International Aquafeed.)

Without the supplement, Watson says, the vegetarian fish diet would have been nearly devoid of taurine. Although not considered essential for humans, who may nonetheless thrill to an occasional shot of taurine in a swig of Red Bull, predators, such as cats, require taurine in their diets.

In cats, taurine is essential for the development of the eyes – they go blind without it, and it plays a role in the digestive system in other animals as a component of bile acids.

Animals that can’t produce enough taurine to meet their needs can only obtain it by eating meat or fish. “A lot of these top predators and other carnivorous species are like cats,” Watson explains. “Their natural diets are composed solely of being carnivores and killing and eating other animals. A lot of the species we’re working with here have followed that same evolutionary path that cats did, so they also ended up with a taurine requirement.”

Because taurine is not incorporated into animal proteins, including fish proteins, it has generally not been regarded as essential, and Watson says that perception may explain why researchers may have overlooked its importance to fish in the past.

Getting the fish to eat their protein bars leaves one major dietary challenge on the path to vegetarianism: finding a replacement for fish oil as a source of the omega-3 fatty acids.

Professor Allen Place of the University of Maryland’s Institute of Marine and Environmental Technology says there are products made from algae that could fulfill that role, and he has successfully tested an omega-3 source derived from canola on fish in the lab. But he concedes these products currently cost more than fish oil, and getting farmed fish off their pelagic dependency is partly a question of economics. “If one could charge as little as a 10 percent premium for a completely fish oil-free, fish meal-free product, at an industrial level the economies of scale might make it work out,” he suggests.

And so far, it seems the newly converted vegetarian fish might make the grade at the table. Watson says taste testers from University of Maryland Seafood Quality Lab, in a double blind study, could not tell the difference between fish raised on standard fishmeal, and fish raised exclusively on his taurine-enhanced vegetable protein diet.

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David Richardson
David Richardson began his journalism career operating a video news service in Washington, D.C., that covered federal agencies and Congress. His film production work has since ranged from postings at the White House to rural villages of Botswana, documenting community-centered HIV prevention programs. He holds a B.A. degree in government from Dartmouth College. He now writes on science, the environment and policy from Baltimore, Md., where he's had some success growing organic produce in a small backyard garden.

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