• Cost. You deserve to get what you pay for. Wild salmon usually costs more than farmed. We paid an average of $6.31 a pound for salmon labeled as farmed (all of which was indeed farmed) compared with $12.80 a pound for correctly labeled wild salmon. Farmed salmon sold as wild was even more expensive: $15.62 a pound.

Under federal law, most supermarkets must label fresh and frozen seafood with its country of origin and note whether it’s wild or farm-raised. Fish stores don’t have to disclose that data, but if they do, the law says labels must be accurate. Our tests aimed to see if labels are truthful.

Our first round of shopping took place in the summer of 2005, at the height of salmon season. We bought some salmon labeled farmed and some labeled wild from markets in several states, then had samples tested for synthetic coloring agents fed to farmed salmon to turn their flesh pink-orange. Wild salmon acquire this color from the crustaceans they eat. With­out coloring, farmed salmon would be gray. As you might expect when wild salmon are abundant, these tests found that all 27 salmon were labeled correctly.

Next, we shopped after salmon season and analyzed salmon labeled wild. Of 17 samples bought in November and early December 2005, 10 were the real thing and 7 were farmed. In this small sample, we were more likely to find correctly labeled wild salmon in supermarkets than in fish stores. For our final shopping trip, in March 2006, we focused on stores that had mislabeled before and found that none of six “wild” samples was correctly labeled.

Two salmon we bought weren’t marked wild or farmed, and salespeople said they were organic. However, there’s no federal rule for the use of that term on fish. As it turned out, both fish were farmed.

The average American’s salmon consumption has quintupled in the past 16 years, and with good reason: The fish is high in heart-healthy omega-3 fatty acids and relatively low in mercury. Since the natural supply of wild salmon can’t grow to meet demand (though it’s regulated to avoid a decline), a global salmon-farming industry has developed. The vast majority of farmed salmon in U.S. stores comes from Canada and Chile. (Europeans get most of theirs from Norway and the U.K.)

Although farmed salmon isn’t inherently less healthful, it is what it eats: typically concentrated fish meal and fish oil. And all too often, the fish that go into this feed have lived in polluted waters.

In a study published in the Jan. 9, 2004, issue of Science, researchers reported their analysis of more than 700 samples of farmed and wild salmon obtained from around the world. Farmed had far more dioxins and PCBs than wild. The most-contaminated fish came from Europe; farmed salmon from North and South America were somewhat cleaner but still not as clean as wild salmon.

The good news is that none of our samples harbored malachite green, a potentially carcinogenic fungicide banned in the U.S. but occasionally found in salmon.

Experts disagree on how to balance cancer and reproductive risks against cardiovascular benefits, but from a health standpoint the best choice for most people, especially children and women who may bear children, is wild salmon. Second choice: farmed salmon from the Americas. Here’s the smart way to buy:

Go wild in summer. In the U.S., about 90 percent of wild fresh salmon comes from Alaska, where the commercial harvest starts in May and ends in September. (King salmon is caught nearly all year but is in limited supply from late fall to early spring.) That’s when fresh wild salmon is most abundant and, judging by our tests, most likely to be truthfully labeled.

Look for canned Alaska salmon. Alaska salmon is wild by definition: To protect its wild stock, the state has outlawed salmon farming. But some fresh fish sold to us as Alaska was farmed. Your surest bet for a salmon fix, especially in winter, may be canned Alaska salmon. It’s fairly cheap, is sold year round, and generally has “Alaska” stamped on its lid.

Buy farmed Atlantic salmon from Chile, the U.S., or Canada. It’s likely to have lower levels of PCBs and dioxins than salmon from Europe. (Stores could lie about farmed salmon’s origins, but the fact that tested salmon labeled farmed really was farmed may be a good sign.)

Try other sources of omega-3 fats. We’ve found top-selling fish-oil pills accurately labeled and free of contaminants. Flaxseed, canola, olive, soybean, and walnut oils also provide some omega-3s.

Consider taste. Our expert tasters noted that wild salmon has a stronger flavor and firmer flesh than farmed.