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Last month, we told you about companies that are growing salmon on dry land. That's an effective — but expensive — way to reduce water pollution caused by fish farms. After all, marine aquaculture provides about half of the seafood we eat.

So a Canadian researcher named Thierry Chopin is pushing to develop a less expensive technology that could be used to clean up the many fish farms that are already operating in coastal waters. His approach involves creating a whole ecosystem around a fish farm, so the waste generated by the salmon gets taken up by other valuable seafood commodities, like shellfish and kelp.

We caught up with Chopin, a marine biologist at the University of New Brunswick, in St. George, New Brunswick, a town on Canada's Bay of Fundy. The bay is famous for its huge tides — which are 30 feet here at St. George — and for its salmon farms. Chopin has been working with a company called Cooke Aquaculture to reduce the fish waste that washes into the bay.

We take a skiff out to the fish farm. The salmon have already been harvested, but there are rafts made of black PVC piping, sticking out of the water like catwalks. They are home to cultivated seaweeds and mussels — species that thrive on fish waste.

"What we are doing is nothing more than recycling the nutrients," Chopin explains. "Instead of looking at them as waste, we look at them as nutrients for the next species."

The seaweed soaks up excess nitrogen and phosphorus from the fish waste, while the mussels thrive on the small organic particles. "For the bigger particles that settle more to the bottom, you need also invertebrates. So we are developing now sea urchins, sea cucumbers, maybe sea worms," Chopin says.

The idea here is not only to reduce the impact of fish farming but to produce more foods and ingredients that the salmon company can sell.

Chopin hasn't done a formal study as yet, but he thinks this system will reduce water pollution from the fish farm by 10 to 50 percent.

He notes that this integrated approach to aquaculture is nothing new — it's been practiced in Asia for centuries. He's simply bringing it to modern, industrial aquaculture. He calls it Integrated Multi-Trophic Aquaculture.

To give me a closer look, Chopin and Lenny Totten from Cooke Aquaculture pull up a string of seaweed growing on a rope underwater so that we can taste the new product. One variety is called sugar kelp.

"You see, it's crispy a little, but it's a sweet taste, and there's no aftertaste," Choping says. "And it's sugary." Then we nibble on a frond from another species, golden kelp. It's chewier and bland. It's sold to a company in Monaco, which extracts natural compounds from it to be used in cosmetics.

The mussels they grow here are a much easier sell.

Totten says his company was at first a bit reluctant to try this venture, but the results have been good. "You can see the difference just in the water around farms," he says. "The water's a lot clearer."

Chris Mann, who has spent years working on marine conservation issues at the Pew Charitable Trusts, says this integrated growing approach is worthwhile, but hardly a panacea. It addresses the mostly localized problem of water pollution, but it doesn't address other problems with aquaculture: the spread of fish parasites, the escape of caged salmon or — worst of all — the need to harvest wild fish to feed the salmon. That's a big problem for inland aquaculture as well.

"The [fish] feeds have tended to use a large amount of wild-caught ingredients, wild fish," Mann says. "That's putting pressure on already strained wild stocks of fish."

The fish used to feed farmed salmon — species like anchovies and sardines — are also critical food sources for wild salmon, tuna, swordfish and so on. So you'd think that a marine conservationist would be dead set against fish farms. But if your goal is to conserve the world's resources and reduce the carbon footprint of protein, the answer isn't so simple.

"In a lot of ways farmed seafood, across the spectrum, is a better, less harmful source of protein than most of our terrestrial livestock," Mann says.

Eating less animal protein is one way to get at this problem. But for a world that has a taste for it, Chopin's approach at least makes a dent in the environmental impact of farmed fish. And he's trying to address at least a small piece of the fish-meal problem by developing a protein supplement for fish, made from seaweed.

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