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AUDIE CORNISH, HOST:

From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish.

ROBERT SIEGEL, HOST:

And I'm Robert Siegel. About half of the seafood we eat comes not from the open sea, but from marine farms. And as global demand for protein grows, people will want more seafood. Many wild fish populations are already overfished, so aquaculture is bound to expand. But that comes at a price to the environment. For example, fish waste can pollute coastal waters where fish farms are based.

NPR's Richard Harris traveled to New Brunswick, Canada, to look at a project that is designed to reduce the environmental impact.

RICHARD HARRIS, BYLINE: The town of St. George is on the east coast of Canada, along the shore of the Bay of Fundy. The bay is famous for its huge tides, which are 30 feet here at St. George. And it's also known for its salmon farms.

(LAUGHTER)

HARRIS: Lenny Totten, from fish-farming giant Cooke Aquaculture, has agreed to take me and Professor Thierry Chopin out to view an unusual twist on the typical salmon farm.

LARRY TOTTEN: It cleared off. It was really foggy a minute ago.

HARRIS: Oh, really?

TOTTEN: Oh, yes.

THIERRY CHOPIN: OK, so I have two life jackets.

HARRIS: We walk down a steep gangplank - the tide is going out - hop into a motorboat and head around the breakwater. Chopin is taking us to an experiment he's been running for more than a decade, a collaboration between the University of New Brunswick and Cooke Aquaculture. They grow seaweed and mussels interspersed with their fish.

CHOPIN: Here we are already at Crow Island. OK? So the salmon have been harvested already. But here what you see is three seaweed rafts, three seaweed rafts. Then there are mussel rafts and again three seaweed rafts.

HARRIS: The rafts are made of black PVC pipes, sticking out of the water like catwalks. They are anchored between the buoys where the salmon grow. The kelp and mussels absorb some of the salmon excrement that would otherwise pollute the Bay of Fundy.

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

HARRIS: The seaweed soaks up excess nitrogen and phosphorus. The mussels thrive on the small organic particles from the fish waste.

CHOPIN: And then 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. That was the next phase we are developing.

HARRIS: What proportion of the nutrients that the fish produce get soaked up by the other elements of your system here?

CHOPIN: Well, it's still something we are debating, as a matter of fact. It's more than 10 percent, it's between 10 and 50 percent, but exactly how much, I don't know yet.

HARRIS: 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.

(SOUNDBITE OF WATER)

HARRIS: To give me a closer look, Chopin steps out of the boat and onto one of the seaweed rafts. It sinks lower in the water.

I see why you wore rubber boots.

(LAUGHTER)

HARRIS: Chopin and Lenny Totten pull up a rope that's covered with a seaweed called sugar kelp. Not exactly an everyday item at the grocery store but it is edible.

CHOPIN: You see it's crispy a little. It's very sugary.

HARRIS: And it has a very mild taste, actually.

CHOPIN: Yeah.

HARRIS: Yeah.

CHOPIN: So that's the other one. This one is golden kelp. It's another species. You can try that.

HARRIS: A little chewier.

CHOPIN: Yeah, that's a species we use for cosmetics.

HARRIS: They have a deal with a company in Monaco, which extracts natural compounds from the kelp to make cosmetics ingredients. And Chopin is also experimenting with using seaweed as a protein supplement for the salmon, to reduce the amount of fishmeal they need to eat as they fatten up.

So if every salmon farm in the world adopted this practice, would there be a big enough market for the seaweed that you could produce?

CHOPIN: Well, that's the thing is we need to develop the market, especially in the Western world. In Asia, people know what to do with seaweed.

HARRIS: At the next raft, concentric circles of the black pipe, Thierry Chopin shows off a product that's much easier to sell than seaweed. Think moules marinier, mussels minus the white wine and the garlic broth.

CHOPIN: OK, so in here - here we have the mussels. So you can see the mussels attached to each other.

HARRIS: They will be sent through a machine to scrub them, strip off the barnacles, and prepare them for seafood counters.

Lenny Totten from the aquaculture company is holding up this hefty string of purple shells with an obvious sense of pride. He's been on the water 18 years but mussels and seaweed are recently acquired tastes for him.

Did it take a lot of arm-twisting on Thierry's part to get you guys to try this out?

TOTTEN: I think so, at first.

(LAUGHTER)

HARRIS: Why was that?

TOTTEN: Just old school - salmon - a lot to learn. So I think it's all good. And I mean, we can see the difference just in the water just around farms. The water is a lot clearer.

HARRIS: What do your neighbors think of it? Anyone interested in following your lead?

TOTTEN: I don't know.

(LAUGHTER)

TOTTEN: I hope not.

(LAUGHTER)

TOTTEN: Not yet. Let us get going, first.

HARRIS: It is a risky proposition. First, learning how to grow new seafood crops, then trying to create markets for them. One advantage, though, is they fetch a premium price for their salmon because they market it as having lower environmental impact than other farm-raised salmon.

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.

CHRIS MANN: That is mostly addressing the nutrient output of aquaculture, which can be an important impact locally. But it doesn't really do anything about the feed problem.

HARRIS: The feed problem is even more worrisome to Mann. You see, salmon are carnivores, so fish farms buy many tons of fishmeal to feed the salmon.

MANN: The feeds have tended to use a large amount of wild-caught ingredients, wild fish. That's putting pressure on already strained wild stocks of fish.

HARRIS: Fish like anchovies and sardines, which are also critical food 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, it's not so simple. That's because, if you think in terms of what's best for the global environment, including the carbon footprint of growing food, fish are a comparatively good source of animal protein.

Some conservation groups have backed off a bit on their opposition to fish farming because the alternatives are worse.

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

HARRIS: Everything we do has an impact. It's a matter of choices. Eating less animal protein is one way to go. But for a world that has a taste for it, Thierry Chopin's approach at least makes a dent in the environmental impact. And if he can use seaweed to substitute for a fraction of the fishmeal, that can help, too.

CHOPIN: It's a slow progression but a sure progression, I would say, of changing gradually aquaculture practices. And it's not the only solution but it's one of them.

HARRIS: Chopin 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.

Richard Harris, NPR News.

(SOUNDBITE OF MUSIC)

CORNISH: This NPR News.

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