Salon.com has a very good article about Chilean salmon for Walmart. The article is actually an excerpt from the book "The Wall-Mart effect" by Charles Fishman.

[..] The salmon is a "farm-raised product of Chile," according to the sign, and it's fresh. It managed to get from southern Chile to a small town seventy miles outside Philadelphia - more than five thousand miles - without even being frozen. The salmon fillets are priced at $4.84 a pound. Almost any American over thirty is old enough to remember a time when you could hardly buy a quarter of a pound of salmon for $5.00. Any American over forty can recall an era when salmon was a delicacy. A half pound of smoked salmon, the kind you'd put on a bagel, might have cost $16.00 or $20.00. But there it is, in the Wal-Mart display case -- pink, oily, and alluring -- salmon fillets for $4.84 a pound. That's not a special; it's the everyday low price, and available in most supercenters from one end of the country to the other. It's a couple of dollars a pound cheaper than farm-raised salmon at a typical supermarket. It's less than half the price of the farm-raised salmon sold by Whole Foods.

Salmon for $4.84 a pound is a grocery-store showstopper. If prices contain information, if prices are not just a way of judging whether something is expensive or affordable but contain all kinds of other signals about supply, demand, prestige, and even the conditions under which products are made (bad freeze in Florida, expensive orange juice; hurricane on the Gulf Coast, expensive gasoline), then salmon for $4.84 a pound is a new, unintended Wal-Mart effect. It is a price so low that it inspires not happiness but wariness. If you were so inclined, you couldn't mail a pound of salmon back to Chile for $4.84. It's a price so low, it doesn't seem to make sense if you think about it for even a moment. Salmon at $4.84 a pound is a deal that looks a lot like a gallon jar of Vlasic dill pickles for $2.97 -- it's a deal too good to be true, if not for us as the customers, then for someone, somewhere. What exactly did Wal-Mart have to do to get salmon so cheaply?

"Five years ago," says Rodrigo Pizarro, "salmon wasn't on the list of exports. Chile didn't have any salmon twelve years ago." Pizarro is an economist who heads Terram, a Chilean foundation dedicated to promoting sustainable development in Chile. Understanding the impact of salmon farming is one of Pizarro's most urgent projects. When he says that twelve years ago Chile didn't have any salmon, he's not exaggerating for effect. He means it literally.

Not only is the Atlantic salmon not native to Chile -- the Chilean coastline, of course, runs along the Pacific -- but as Pizarro puts it, "Atlantic salmon is an exotic species in the whole Southern Hemisphere." The Atlantic salmon doesn't appear naturally anywhere south of the equator. Farming salmon in Chile is a bit like farming penguins in the Rocky Mountains. Now, however, not only are there far more Atlantic salmon in Chile than people, there are ten times as many, maybe even one hundred times as many. More salmon are harvested in Chile now than anywhere else in the world, including Norway. Even as the price has drifted down, the value of Chile's salmon exports has risen nearly 70 percent in five years. Chile wants to increase the amount of salmon it exports by 50 percent again by 2010.

I could copy all 3 long pages of the artcile...

But the article indicates that it is not really so that "without Walmart the [Chilean salmon] business would not have been started":

If you look at the growth of three things between 1990 and 2005, the graphs are near perfect shadows of one another: farmed-salmon production in the world, farmed-salmon production in Chile, and Wal-Mart's grocery business. They all start low on the scale, and go almost vertical after a few years. Wal-Mart did not create the farmed-salmon business; Wal-Mart did not plant the salmon farms in southern Chile. But the dramatic growth of domesticated salmon drove down prices for salmon and fed Wal-Mart's ability to deliver salmon to the fish counter; and the dramatic growth of Wal-Mart's grocery business created a huge opportunity, and a huge appetite, for salmon that has fed the salmon-farming industry.

What I am concerned most is the ecological effects. James Lovelock, the author of the controvercial Gaia hypothesis, had said that the most crucial part of Gaia's mechanism are the oceans/seas, since the water covers most of the survace, contains half of the biomass, and provides key chemical feedbacks. Especially arable regions of continental shelves can be vulnerable. Look what is happening in Chile now:

"Have you ever seen a hog farm?" asks Gerry Leape, vice president of marine conservation for the National Environmental Trust, a Washington-based environmental nonprofit group. "These fish [salmon] are the hogs of the sea. They live in the same sort of conditions, it's just in water. They pack them really closely together, they use a lot of prophylactic antibiotics, not to treat disease, but to prevent it. There's lots of concentrated fish waste, it creates dead zones in the ocean around the pens." [..]

"The density of fish depends on the nation, but they grow tens of thousands of fish per net, 1 million or 1.5 million per farm. Then they all go poo. There is a huge amount of waste going into the ocean. People say, oh, that's natural, all fish go poo in the ocean. But not in that kind of concentration. It just smothers the seabed." One million salmon produce the same sewage, says Leape, as sixty-five thousand people.

The ocean pens suffer from another source of pollution -- excess feed. Any food that isn't consumed settles to the ocean floor, adding to the layer of feces. The waste itself contains residues of antibiotics and other chemicals used to keep the fish healthy during the two years it takes them to grow to harvestable size.