Edit. I had planned this as a happy Christmas post. I had no idea that as I poked about in the world of bacalao I would find the fishy underside that I talk about at the end. It left me slightly at a loss though, thinking some reality check on the extra-Norwegian world of imitations, might be helpful I have gone ahead and posted.
Across Mexico, chunks of bacalao are sitting in water, being de-salted for dinner at midnight on the 24th. The middle class Christmas dinner reflects the Spanish tradition. A dish of bacalao a la vizcaína (salt cod Biscay-style) is an indispensable part of this meal, along with spaghetti or cannelloni, and either a turkey or a leg of pork stuffed with meat, fruits, and nuts.
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The most upmarket bacalao in the "deli" Europea at about $22 a kilo

Discarded boxes outside Europea

Display of bacalao in Superama, the upmarket branch of Walmart

Norwegian bacalao in Walmart along with the necessary olives
Once the bacalao is de-salted, it is boiled in fresh water, drained, broken into pieces, and the spines are picked out. The onion and garlic are sautéed in olive oil, tomatoes are added, and the sauce simmered for a while. Then to finish, the cod, small peeled potatoes, olives, parsley, pepper, and salt if it is necessary are stirred in and the whole heated gently.
To learn more about bacalao, I consulted Ove Fossa who is President of the Norwegian commission for the Slow Food Ark of Taste to preserve the traditional ways of making the very finest bacalao. Here’s a link to his brochure which is quite fascinating. None of the bacalao in Mexico is of this quality, I fear.
For the grade of bacalao that ends up in Mexico, he recommended the Wikipedia articles.
For bacalao, salted and dried fish: http://en.wikipedia.org/wiki/Dried_and_salted_cod
For stockfish, dried, unsalted fish: http://en.wikipedia.org/wiki/Stockfish
The two articles seem to be fairly accurate.
Both kinds are most often made from cod, generally considered the best quality, but several other species are used as well.
Due to climatic differences, the two are made in different areas. Drying unsalted fish can only be done in cold weather in the north, mainly the Lofoten islands, and Finnmark. Bacalao (klippfisk in Norwegian) is made in the south, near the towns of Ålesund and Kristiansund, where the fish would easily spoil unless salted.
Ove also kindly passed along export figures. Although at this time of year, it’s easy to think that Mexico must be gobbling up the entire Norwegian supply, in fact it’s a fairly small market.
The major buyers of Norwegian bacalao are Brasil (35.9 %), Portugal (30.5 %), the Dominican Republic (6.4 %), Jamaica, Congo, Angola and Italy (each 3-4 %) and then Mexico (2.8 %). The numbers are from 2010, they vary a little from one year to the next.
Norwegian stockfish on the other hand is sold almost exclusively to Italy (57.1 %) and Nigeria (30 %).
The export value of Norwegian bacalao in 2010 was 3.6 billion Norwegian kroner (610 million USD), and stockfish 450 million (76 mill USD). In comparison, the export value of farmed salmon was 30 billion (5,1 billion USD).
There’s a huge market in Mexico, though, for something cheaper perhaps because the rapidly expanding middle class wants to taste the kinds of Christmas dishes that once only the wealthy could afford. Last week Buena Mesa, the food page of one of Mexico’s main newspapers, Reforma, gave hints on how to detect imitation bacalao. Any so-called bacalao without spines was not authentic.
I’d seen lots of imitation bacalao myself in early December in La Viga, the wholesale fish market in Mexico City. It was made of robalo (Centroponus undecimalis), of sierra (in the mackerel family), and, as below, manta ray (aletas means wings).

Bacalao of manta ray in wholesale market at about $3.00 a kilo

Five foot cube of shark "bacalao" from the state of Campeche in the wholesale market

Sharks' fins in the wholesale market
What are restaurants using, one wonders? What controls, if any, are there on the sale of endangered species in the wholesale market? What controls, if any, are there on the naming of bacalao? What is the connection between the rise of shark fin bacalao and the controversy over shark’s fin soup, descrbed here by Fuchsia Dunlop. Clearly as the Reforma article shows, some people are worried about imitations. I would love to know more if any readers have any comments.