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TERRY GROSS, host:

This is FRESH AIR. I'm Terry Gross.

Eating fish has become kind of complicated, between worrying that there may be toxins in the fish and concerns that the way the fish is farmed or captured may be bad for the water's ecosystem.

My guest, Paul Greenberg, says we're in a wave of psychological denial of staggering scope. With wild fish, we've ignored the fundamental limits the laws of nature place on ecosystems, and we've removed more fish than can be replaced by natural processes.

When wild stocks become over-exploited, we've turned to domestication, but some of the fish we've chosen to farm don't make ecological sense.

In his new book, "Four Fish," Greenberg looks at what's happened to salmon, sea bass, cod and tuna, four fish which he says humanity is trying to master either through the management of a wild system, through domestication and farming of individual species, or through the outright substitution of one species for another.

Greenberg has written about the oceans and fish for the New York Times magazine, book review and opinion page.

Paul Greenberg, welcome to FRESH AIR. Now, you write in your book that in the early 2000s, you started noticing that no matter where you went, there were four varieties of fish that were consistently appearing that had little to do with the waters adjacent to the fish markets where you were. And those four fish were salmon, sea bass, cod and tuna, the four fish you write about in the book. So was that different from the past, that there were four prominent fish?

Mr. PAUL GREENBERG (Author, "Four Fish: The Future of the Last Wild Food"): Well, yeah, fish were different 35-odd years ago, when I was a kid, and I would often visit fish markets no matter where I was.

I grew up in suburban Connecticut, and I loved to go down to the Bon Ton Fish Market there and check out what was on the ice. And by and large, what we would see there would be pretty much entirely wild fish. You know, forty-odd years ago, everything we ate from the sea was wild, pretty much everything. Today, almost half of what we eat is farmed. So it's a huge, huge epochal shift.

The four fish that I talk about that are now appearing very regularly in the market, two of them are farmed, that is the Atlantic salmon and the European sea bass, often also called branzino. And you see those pretty much all the time.

And then the other ones, tuna and codfish, are not farmed yet, or at least not in any large number, but their commercial exploitation has expanded to the point where it's such a huge, global market that they tend to appear everywhere.

GROSS: So let's take a look at salmon. What does salmon represent in the larger picture that you're looking at, how fish have changed and how humans consume and farm fish have changed?

Mr. GREENBERG: Yeah, well, gradually with the way humans have used fish, we've started inland and moved further and further offshore. And salmon represent that first step, with fish that - you know, salmon spawn in freshwater rivers. They're nearby. And we have very close intimate interaction with them right where we live.

So they were one of the first fish that we really hit hard with industrialization. Dams and pollution and all of these different things caused wide-scale extirpation of salmon, particularly Atlantic salmon, throughout their range.

And now what we've seen is, you know, salmon was really the first large-scale domestication project that happened with the fish that we eat. There are many more farmed salmon in the world than wild salmon. And it's a kind of, you know, replacement of a wild-food system with a domestic-food system that has started to be a kind of a model moving forward.

GROSS: So is it mostly the Atlantic that's lost the salmon, still a lot of wild salmon in the Pacific?

Mr. GREENBERG: Yeah, I mean, there are still pretty strong runs of Pacific salmon, particularly in Alaska and the Russian Far East. But in the Atlantic, basically Atlantic salmon are commercially extinct.

And this is a very important thing that consumers need to know. You know, a lot of times when you see salmon in the marketplace, you'll see Scottish salmon, Irish salmon or even Nova salmon. I think, you know, probably you grew up eating Nova lox, for example, right? You know, that was lox from Nova Scotia.

GROSS: Yes.

Mr. GREENBERG: Well today - right? You know?

(Soundbite of laughter)

Mr. GREENBERG: Today, all those fish are farmed. You know, there is almost no wild Atlantic salmon left in the Atlantic. I mean, I've heard the number at something like 500,000 fish total.

So what we've essentially done is we've replaced an extremely productive and, you know, calorie-rich and highly nutritious food system, wild food system, with a domesticated one. And that began in the late 1960s, and it's been probably the driver in changing the way we're taming the sea.

GROSS: Let's talk about how salmon are farmed. First of all, you use the word captive when describing salmon, and I never thought of farmed fish as captives. I always have this image of, like, fish swimming around this kind of like, fenced-off part of the sea.

Mr. GREENBERG: Home on the range, right?

(Soundbite of laughter)

GROSS: Yeah, so, like, okay, they couldn't venture far, but they can, like, swim around. They have their food brought to them, so they don't have to worry about survival. And, you know, of course they're killed for us to eat, but I never thought of them quite as, like, captive.

Mr. GREENBERG: Well, yeah, and in fact, you know, they're often confined in pretty tight spaces. You know, the technology around modern aquaculture or fish farming, that's what aquaculture is often called scientifically, really developed around salmon.

And what they consist of are sort of hoop-shaped cages that are often put up in sort of symmetrical arrays. In Norway, they started doing them in fjords, where they were protected from, you know, wave and wind, and same deal in the Bay of Fundy in Canada. And, you know, the fish are pretty crammed in there.

And what they'll do is they'll usually start them indoors, in tanks when they're still at a very fragile stage. And then what they'll do is they'll transfer them to these large pens. They're - you know, sometimes they're called sea pens or sea cages, where they grow out for a couple of years until they're ready for harvest.

GROSS: Is there room for them to swim, or are they basically just penned in and sitting there?

Mr. GREENBERG: No, no, they can swim. And in fact, for their own health, they have to be able to swim, and they tend to kind of circle around. But you bring up a good point, which is that, you know, the density of salmon is a really big issue in terms of its environmental effects.

Keep in mind that most salmon are grown in wild salmon country, right? So if you put a lot of farmed salmon, confined in cages, in a place where migrating wild salmon still exist, there are going to be deleterious effects on the wild population.

The first that are crammed in real tight, you do get outbreaks of disease. There's been a rolling disease called infectious salmon anemia, which causes bleeding in salmon kidneys. There's a huge problem with a parasite called sea lice, which affixes to - you know, it's a naturally occurring parasite, but when you have fish in extreme densities, then, you know, the sea lice are drawn in to this sort of, you know, aggregation of food.

And there's been some studies that show that sea lice do transfer to the wild populations, and because wild populations of Atlantic salmon are so depressed throughout their range - you know, maybe if there were a lot of wild Atlantic salmon out there, the interaction wouldn't be so bad, but with the numbers being so low as they are right now, anything that knocks them down a peg is a real, real problem.

And, so, you know, the salmon farming industry has a real issue on its hands with how do they continue to produce product for the market without destroying wild runs of existing Atlantic salmon.

GROSS: Now, another concern about farmed salmon is what they're fed and how much they have to be fed. What are the concerns?

Mr. GREENBERG: Well, yeah. This is another big issue about the sort of replacement of a wild food system with a domesticated food system. You know, what do salmon eat? Well, on the farm, anyway, what they eat is other fish. And where do those fish come? The wild.

So, you know, the global catch right now in the world is 90 million tons, which is a lot of fish. You know, it's equivalent to the human weight of China removed from the sea every year.

A third of that is what they call forage fish: herring, anchovies, little things like that. And incidentally, the weight of all of those taken from the sea every year would be the equivalent of the human weight of the United States taken out every year.

Those are harvested every year. They are made into feed pellets. And in the early days of aquaculture, the early days of salmon aquaculture, the feeding was extremely inefficient. There wasn't a great deal of care making sure that the salmon actually ate what they were fed.

So there was a lot of waste, and I think in 2000, the journal Nature published a study that the fish-in, fish-out ratio, in other words the number of pounds of wild fish that was required to make a pound of salmon, could be as much as three pounds of wild fish to make one pound of salmon. So right then and there, you're sort of like, well, that's a pretty screwy equation. You know, why are we coming up with a net marine protein loss?

But to its credit, you know, the salmon industry took this somewhat seriously, or actually very seriously, and because it's also an economic factor for them because, you know, it's expensive buying all that feed.

And over the years, they've instituted a selective breeding program with salmon, mostly in Norway, where they took the 40 original salmon strains in all these different rivers in Norway, and they crossed them and they re-crossed them, and they came up eventually with a salmon that required half the feed of its original wild variant.

So, you know, you could say that was a positive ecological move that the salmon industry did, but unfortunately, the salmon industry keeps growing so that while per-fish efficiency is better, the overall footprint of the salmon industry is just bigger and bigger. So, you know, it's a concern.

GROSS: So one of the many things I don't understand about fish farming is who has the rights to the water? Like, are they farmed in actual rivers or parts of the ocean? And do you need permission from somebody to use it? I mean, these are public waters...

Mr. GREENBERG: Yeah.

GROSS: So how does a private industry or private company get access to public waters to farm fish?

Mr. GREENBERG: Yeah, well, that's a big issue. Generally, the farms are sited not in rivers per se. They're usually in bays and protected areas. States own waters, you know, United States states, own waters out to three miles out. So, you know, it has to go through the state to get, you know, permission.

In the early days of salmon farming, there really wasn't I think an idea of the value of these sites and the possible impacts they could have. I should say that there's not that much salmon farming that goes on in the United States, largely because many states, you know, just don't want aquaculture in their backyard. There's a lot of sort of NIMBY issues where coastal landowners don't want to be exposed to salmon farms.

Canada is another story, and, you know, sites are usually leased through the provincial governments, you know, be they New Brunswick or British Columbia. And, you know, a lot of the public, you know, that grew up with, you know, a wild waterfront, they see these salmon farms come in. They feel, you know, deeply sort of affronted, and they're wondering what say do I have, and why is this salmon farm being put into what is basically public property?

You know, every kind of thing that involves the ocean involves the privatization of a public resource, whether you're fishing for wild fish, you know, and taking it, you know, for your personal sale as a business or whether you're taking a chunk of the ocean and turning it into a sea farm.

You know, that's a privatization, and I think the public needs to know that, you know, they have as much right to the seafront as, you know, a fish farmer or a fisherman.

(Break)

GROSS: So are farmed salmon any more or less healthy than wild salmon?

Mr. GREENBERG: That's a very, very, very big debate. About 10 years ago, the Pew Environment Group commissioned one of the largest studies ever done where they tried to figure out was there any difference between the sort of industrial contaminants and things like that in farmed salmon than in wild salmon?

What they found was that actually, yes, there was. The deal is that wild salmon are actually much more omnivorous than farmed salmon. In the course of a wild salmon's life, they're likely to eat, you know, little crustaceans, sometimes some fish, sometimes, you know, crab here and there, whereas a farmed salmon only eats fish pretty much, plus whatever soy and corn products are put in there to kind of fill out the fishmeal.

And it turns out that it's really important where that fish comes from. In the early days of salmon farming, most of the fishmeal that was used was Northern Hemisphere fishmeal: capelin, herring, mackerel, different things from the Northern Hemisphere.

And by and large, you know, the Northern Hemisphere has higher industrial pollutants than the Southern, and with feed in farmed salmon, it turns out that there were high concentrations of PCBs in the fishmeal that they were being fed. And the contamination in the fishmeal gets passed on into the flesh of salmon.

It turns out PCBs, you know, they're polychlorinate biphenyls, are a very persistent chemical, and they don't wash out of the body very easily. They take many years for the body to be rid of them.

And the same deal is true with salmon. So if a salmon keeps eating fishmeal that's contaminated, it'll get a higher and higher toxicity, and it will eventually pass that on to humans.

So what they found overall, what the Pew study found, was that farmed salmon overall had higher levels of PCBs than wild salmon. Keep in mind that this was, you know, close to a decade ago.

And since then, the salmon industry has started to look at Southern Hemisphere feed sources, mostly Peruvian anchoveta, which are, you know, caught off of Peru. And the Southern Hemisphere fishmeal generally has lower PCBs than the Northern Hemisphere fishmeal.

There hasn't been a large-scale subsequent study since that shift started to occur. So I think the jury is still out. But according to that original Pew study, PCBs were higher in farmed fish than in wild fish.

GROSS: Before we move on to another fish, what are some of the lessons learned from the way we've farmed salmon that you think should apply to the farming of other fish?

Mr. GREENBERG: Well, two things. When you farm a fish in proximity to its wild equivalent, you're taking on considerable risks, and you potentially threaten a viable wild food system. And I truly believe that those things need to be taken into account before any kind of farming is introduced into the open sea.

What's come out of this - there are some really interesting what are called recirculating aquaculture facilities in development. There's a guy named Yonathan Zohar down at the University of Maryland at Baltimore, I think it's called the COMB Lab, who has literally got a fish farm set up in downtown Baltimore where, you know, all the inputs and outputs are controlled.

Even, you know, the waste products are recycled, and it has no negative interaction with the wild. It's energy-intensive, but at the same time, it's something that could conceivably reduce the food miles that, you know, fish have to travel. You know, if you can grow fish in downtown Baltimore and feed it to Baltimoreans, you know, that's probably a pretty good energy equation.

The other thing that came up that I found really interesting, when I was up in New Brunswick, I met a guy named Thierry Chopin, who was doing a project called integrated multi-trophic aquaculture, or IMTA.

And what that does is to kind of turn the whole equation of monoculture of salmon on its head and say, you know, let's not just grow one, single crop. You know, we've learned that with, like, corn and beef and all that kind of stuff, that monoculture is generally a bad environmental choice.

What Thierry is doing with IMTA is that he's growing salmon, mussels, sea cucumbers and edible and industrial-use algae all in a polyculture. Those different extractive creatures, like mussels and sea cucumbers and algae, remove waste from the water.

It's still a pilot project. It's not scaled up to industrial use, but I'd like to see more of that kind of work happen.

GROSS: Let's look at cod. What does cod represent in the big picture that you're writing about?

Mr. GREENBERG: Well, as I said before, the exploitation of the sea starts inshore and moves further and further offshore. And what cod represents is this sort of epic industrial move to the continental shelves, where, you know, beginning around the Middle Ages, huge aggregations of cod were found, first off of Europe. But then people like Mark Kurlansky would posit that that's what brought the Vikings to the New World in the first place were these huge, epically large amounts of cod on the Grand Banks in Canada and then the Georges Bank off of Massachusetts.

So - and they really represent the sort of industrialization of fishing. If all that cod had never been found, I don't think we'd have a fish stick today. And, you know, it's the sort of re-imagining of fish, not as this sort of local, artisanal product but as this mass-scale industrial thing that, you know, fills up our supermarkets and our fast food restaurants.

GROSS: So once cod started being used for fish sticks and all the fish stuff in the fast food restaurants and the frozen food sections, how did that affect the fish, the cod itself?

Mr. GREENBERG: Well, what it did was cause a huge build-up in what's called fishing effort, just bigger and bigger boats, more and more nets in the water, bigger and bigger technology. Because keep in mind, the real big buildup that happened in fishing was post-World War II, you know, when all this new technology, you know, sonar for finding submarines, turned, you know, was easily repurposed to sonar to find cod.

All these polymers, you know, were turned into, you know, huge nets and things that allowed us to just catch many, many more fish. And what we saw was, you know, the destruction of two of the greatest fishing grounds the world has ever seen: the Grand Banks off Canada closed in the late '80s, early '90s, and then large chunks of Georges Bank closed to fishing in 1994.

And we've been sort of waiting ever since, trying to see, is cod going to come back? And if not, what else can we find out there to fill our supermarkets?

GROSS: So is there no more cod left, or is it just a different type of cod that's available now?

Mr. GREENBERG: Well, no. You know, first of all, when talking about the ocean, a lot of people try to reframe things in terms of the land, and they say, you know, cod is extinct, bluefin tuna are extinct.

What we're talking about here is really the loss of abundance, right? There are probably, you know, even on the Grand Banks, there are probably on the order of hundreds of thousands, if not millions, of cod still left. But they are what is called commercially extinct. In other words, the effort it requires to catch those fish isn't really worth the amount of money you can sell those fish at market.

So the Grand Banks is still in pretty bad shape. It's not really showing very good signs of rebuilding. In America, on Georges Bank and in the Gulf of Maine, those are, you know, two different, separate populations of cod, we are seeing some gradual rebuilding.

The Gulf of Maine, according to the fishery service, is 50 percent rebuilt, although, you know, that has some controversy attached to it. We also - there are Pacific species of cod, which are still pretty common, a lot of it caught in Alaska. And there is still a couple of populations of cod that are, you know, commercially viable. There's a big population in Scandinavia called the skrei or wandering cod, and Iceland also has pretty good, healthy cod stocks at this point.

(Break)

GROSS: In your chapter about cod, you write about tilapia, aka St. Peter's fish, which is farmed and is easy to farm. And you describe it as a good example of a fish that works in an industrialized setting. Is tilapia relatively new to American fish markets? I don't remember it from when I was young.

Mr. GREENBERG: Yeah, it's completely new. One fish farmer I talked to said that when he first heard the word tilapia he thought it was a stomach disease.

(Soundbite of laughter)

Mr. GREENBERG: But tilapia, it's a Latin name. What happened with tilapia, it's kind of an interesting story. Some early work was done in Israel. Israel is a very early aquaculturist. But it was also something that people in the Peace Corps really embraced because they found that tilapia, it's one of these things where you can just throw it in a pond and it will eat whatever is in there and it grows and doesn't require too much effort. So it was this kind of like perfect development fish that you could introduce into ponds throughout the developing world. It was a good way of getting protein.

And so what happened throughout, actually coincidentally with the crash of Georges Bank cod and Grand Banks cod, is that tilapia culture started getting serious, particularly in Latin America. And a lot of former Peace Corps volunteers who, you know, became businessmen sort of said well, let's try and turn this fish into something that works for a Western market. And it really started to kind of get going in the late '90s and it, you know, now I think it's the fifth most popular fish in America.

GROSS: So if you eat a lot of tilapia - farmed tilapia - is that considered a healthy fish?

Mr. GREENBERG: Tilapia, because they don't eat a lot of fish meal, they don't have the omega-3 profile that, you know, so many nutritionists say we should be having. That said, as a form of protein, it's better, I think, to eat a low-fat filet of, you know, a sustainably raised fish, than a big chunk of beef or even pork or chicken. It's just leaner. There are some concerns.

Tilapia has something called an omega-6 in it. And frankly, I haven't gone too far into the health aspects on that one but I've gotten a few emails from nutritionists that say that there are some potential ancillary health problems with eating too many omega-6 - having to do with inflammation of tissue and things like that. But overall, you know, I eat tilapia. I think it's a better, you know, in the profile of food that we have to eat out there, I think it's certainly a better choice than beef.

GROSS: So let me sum up what we just learned. You've basically told us that salmon...

(Soundbite of laughter)

GROSS: Salmon are farmed in ways that are probably not environmentally great.

Mr. GREENBERG: Mm-hmm.

GROSS: That they may have PCBs. They did about a decade ago. We're not sure now. So it's a kind of discouraging picture. But tilapia, which are farmed in much more environmentally correct ways with better food and, they're not as healthy as salmon.

(Soundbite of laughter)

GROSS: So, what am I supposed to do when I'm ordering? You know, if I order the salmon then I feel really bad because of the poor ways that most salmon are farmed. If I order tilapia I'm not getting the benefits of omega-3.

Mr. GREENBERG: Yeah. First of all, I think this is the problem with the kind of paring down of fish markets down to these, sort of, four basic fish. You know, the answer might be sort of none of the above. And I'd go back to like if you want to eat something that's healthy and not damaging to the environment, you know, smaller fish like herring, like mackerel, anchovies, sardines, those are all really good nutritious kinds of things that have a good omega-3 profile.

They're not the kinds of things that we are accustomed to eating. You know, like I think Americans in general don't like too fishy a fish, but we might need to kind of readjust to that and kind of start to embrace fish that are smaller and, you know, easier on the environment. As far as, you know, tilapia are concerned, I mean listen, we eat all sorts of unhealthy stuff, right? We shouldn't be eating as much beef, right? We shouldn't be eating as much chicken, probably.

You know, Mark Bittman, who I've been having these endless back and forths about, you know, whether we should farm fish or not, he is saying well, we should just be eating less of everything. We should be eating less meats and more vegetables. Same thing is true of fish. Let's eat wild salmon. But if we're going to eat it, let's eat it sparingly 'cause there's not a lot of it. You know, there's still very healthy runs of Pacific salmon out there, but, you know, not so that the whole world can have a huge chunk of it every day. But a little bit of it every week, it's not a bad thing to do.

(Break)

GROSS: Let's take a look at tuna. What do they represent in the big picture that you're writing about?

Mr. GREENBERG: Tuna are really the last wild fish gold rush that's going on right now. Tuna often live in what are called the high seas, the international waters that are owned by nobody and fished by everybody. Bluefin tuna cross the Atlantic and the Pacific, so do yellowfin, and albacore are quite far-ranging as well. So they're really the wildest of fish that we have out there. And the sushi binge that's happened over the last 20 years is having a serious effect on them. And so I guess they represent fish, you know, whether they should be seafood or wildlife, and I think they're at the heart of that debate right now.

GROSS: What do you mean by the difference between seafood and wildlife?

Mr. GREENBERG: Well, you know, seafood is one of my favorite and most hated words. I mean when you think about it, what a cruel word. And it's consistent language to language. Europeans call seafood, you know, sea fruit, frutti di mare. Russians, I think, say dari-morelle(ph), which, you know, gifts of the sea. So there's this sort of generic thing. Like there's all this stuff down there and we just kind of pull it up and we sort of parse it and figure out what's good, and throw the rest overboard, and we eat it.

But meanwhile, you know, these creatures are wildlife. You know, these are wild animals that have incredible life cycles. You know, bluefin tuna can be 12, 14 feet long, 1500 pounds. They can swim up to 40 miles an hour. They have organs in their head that act as both a sextant and as a compass. You know, they're incredible, incredible animals and yet, you know, we generally think of them as sushi.

So, you know, that I think is something that really needs to be reevaluated at this point. And we have to figure out, you know, what does work as food and what is better left as an animal. And, you know, tuna, particularly Atlantic bluefin tuna, may be the thing that we should think of more as wildlife than food.

GROSS: Now the tuna swim through international waters, so what problems does that pose in terms of regulating the fishing of tuna?

Mr. GREENBERG: The problem it poses is that there's no really hard fast way to regulate them.

(Soundbite of laughter)

Mr. GREENBERG: There are what are called - there's 18 regional fisheries management organizations that are kind of like these United Nations that sit down and kind of hash out, you know, who is going to get what from all the tuna. But a lot of times the negotiations around tuna are built around sort of political compromises that, you know, while there are scientific committees that say you shouldn't take more than this much, a lot of times they'll just sort of say well, you know, you know, Ivory Coast wants more yellowfins and no - but they'll swap us this for that. And you end up with these kinds of quotas that are not really scientifically based.

You know, the most famous thing of that was that two years ago the committee that oversees bluefin tuna, Atlantic bluefin, set a quota at nearly double what scientists on the scientific committee were saying was the threshold for a catch. So you ended up, you know, where the population went down and it's now kind of at a crisis point. And whereas bluefin are kind of the tip of the iceberg, there are all these other tuna species below them that are still in reasonably good shape, but are the kind of next thing on the chopping block should we go through our bluefin.

GROSS: Now a lot of people say we shouldn't even be eating big fish like tuna because that's not healthy. What are the problems?

Mr. GREENBERG: The bigger fish, the bigger tuna, there is a mercury issue that happens. Just like PCBs, mercury does something called bioaccumulation. Mercury contamination levels get more intense the higher you go up the food chain and tuna are, you know, at the top of the food chain so they have the highest mercury levels.

It varies from species to species. But certainly bluefin have a mercury risk. That's the main thing to watch out for. Also, some people say that, you know, again, tuna are a fattier fish again, particularly bluefin. And if the benefit we're looking for in fish is a leaner kind of meat, then maybe bluefin isn't really where we should be going in the first place.

GROSS: I don't know how carefully you've been following the Gulf, but what's your understanding of the fish that are going to be most at risk from the oil spill?

Mr. GREENBERG: Well, the Gulf is a very kind of insidious thing happening. The fish that are going to be affected the most, I think, are going to be the big fish like bluefin tuna, yellowfin tuna, Atlantic swordfish - because they all spawn in the Gulf. And what I think is going to happen is that right now there are plenty of yellowfin tuna and swordfish around to make it seem like everything's fine. But the spill occurred during, you know, a summer when there should be a lot of spawning going on.

And I think what's going to happen is we're going to keep catching yellowfin tuna, bluefin tuna and swordfish throughout the next few years, and in four or five years from now, the fish that should have spawned in 2010 and 2011 just aren't going to be there. There's going to be a huge dropoff. And so I guess my thing I've been thinking about in terms of the Gulf, is we need to be thinking about is keeping as many of these fish in the water right now.

It's kind of going back to salmon. It's kind of the equivalent to what happened to some of the big salmon runs when the big dams were put up in the Pacific Northwest. You know, once those dams got put up, salmon couldn't return to their spawning grounds and those runs oftentimes ended or were severely reduced.

This Gulf spill represents, kind of like, a huge potential, like a biochemical dam in the Gulf. So I really believe the time to start thinking about conserving them is now so that, you know, hopefully four or five years from now the Gulf might clear enough so that these fish can spawn again.

GROSS: What about seafood that is so associated with the Gulf Coast, like shrimp, catfish?

Mr. GREENBERG: Catfish are farmed and they're farmed inland and I don't think they should be a problem. Shrimp's another story. The oil itself, you know, had it been left alone, might not have been - I mean, I'm sure it would be a problem for any kind of thing that lives in the Gulf. But once you spray Corexit, you know, the dispersant on that oil, you break it up into smaller particles to the point where it can get ingested by small creatures like shrimp, like oysters. Some people have said that there is no toxicological problem with these fish and shellfish ingesting oil and hydrocarbons, that they should be able to kind of process that. But nobody knows what the toxicological implication is going to be of Corexit plus oil. You know, what chemical that forms, because Corexit...

GROSS: That's the dispersant.

Mr. GREENBERG: That's the dispersant. Right. Corexit changes the chemical formulation of oil and probably, you know, makes it in a way probably more absorbable by a lot of these creatures. So we could've just really seriously polluted our seafood supply by spraying those dispersants in the Gulf.

GROSS: If you're just joining us, my guest is Paul Greenberg. He's the author of the book "Four Fish: The Future of the Last Wild Food."

I guess, you know, I never really thought of fish as the last wild food until I saw the title of your book.

Mr. GREENBERG: Yeah. You know, and that's what I think is amazing about the ocean. You know, a lot of the book is - there's a fair amount of my going fishing and that's how I first encountered fish. I never bought seafood, I always caught it. And so for me it was always wild. That consumers have become so detached from it that they don't even really think of it as wild is disturbing to me.

A lot of people I know, you know, perfectly intelligent college-educated people, if I mention a fish to them, they can't even really say what it looks like or how big it is or, you know, what it does in the wild. So I kind of think that, moving forward - you know, I'm not saying that we should stop fishing or that we shouldn't have this wild food - quite the contrary. I think it's a beautiful thing to have abundant wild food in our lives.

When you think about what happened on the Great Plains - before American colonists arrived, there was somewhere in the order of probably 60 million bison, and today there are, you know, about 100 million head of cattle. So we basically replaced a functioning wild food system with a domesticated one that has all sorts of environmental repercussions and all sorts of costs associated with it.

Where we stand right now with fish, is, you know, as I said earlier, 50 percent of our seafood is now farmed. We could end up replacing a very good and beautiful and functional wild food system with an expensive, potentially environmentally degrading farmed food system, and I don't want that to happen. I want there to be wild food. I think there has to be some farmed fish as well, but we need to figure out a way to farm it in a way that does not affect wild populations.

GROSS: Well, Paul Greenberg, thank you so much for talking with us.

Mr. GREENBERG: Thanks so much, Terry. It was a lot of fun.

GROSS: Paul Greenberg is the author of the new book "Four Fish: The Future of the Last Wild Food."

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