I am excited by Wal-Mart's plans for their wild-caught and frozen fish to be 100% sustainable.  

However, this skirts the issue of farmed Chilean salmon, which is delivered fresh.  There's a long section in the book "The Wal-Mart Effect", available from Salon at http://www.salon.com/tech/books/2006/01/23/walmart_effect/index.html, that goes into the details - but briefly, 65% or farmed salmon sold in the US comes from Chile.  1/3 or so is sold by Wal-Mart, so Wal-Mart is able to control conditions in Chile under which salmon is raised.  So far, low cost has been the overweening factor, although that might be changing.  Farmed salmon is now Chile's second largest export.  Conditions for factory workers, and the ecological impact of salmon farming, are not high enough on anybody's list.  So Wal-Mart has some room left to go where fish and sustainability are concerned.