The fresh salmon labeled “wild” that you buy off-season may in fact come from a farm, making it more costly than it should be and – perhaps – less healthful than a fish from the ocean.

When we analyzed 23 supposedly wild-salmon filets bought in November, December and March, we found that only 10 were definitely caught in the wild. The rest were raised on salmon farms.

Why should you care whether your salmon comes from a farm or the ocean? Farmed salmon, raised in pens, tend to accumulate more PCBs and dioxins than do wild fish. Studies suggest that farmed salmon may have higher PCBs and dioxin concentrations than many common foods. These industrial chemicals, which can cause cancer and reproductive problems, are fat-soluble and can stay in your body’s fat tissue for years.

Then there’s the cost. In our study, we paid an average of $6.31 per pound for salmon labeled as farmed – all of which was, indeed, farmed – compared with $12.80 per pound for correctly labeled wild salmon. Farmed salmon sold as wild, meanwhile, cost $15.62 per pound.

Although farmed salmon isn’t inherently less healthful – like wild salmon, it’s high in heart-helping omega-3 fatty acids and relatively low in mercury – it’s typically fed concentrated fish meal and fish oil. And all too often, the fish that go into this feed have lived in polluted waters.

Fresh caught wild Copper River Salmon steaks. The best of the best for your salmon eating pleasure. Shallow dof

To increase your chances of getting genuine, fresh wild salmon, buy when the product is most abundant. In the U.S., that’s May through September – the commercial harvest season in Alaska, source of 90 percent of the U.S. supply.

Experts disagree on how to balance the potential risks of eating seafood against the cardiovascular benefits of omega-3, but from a health standpoint, the best salmon choice for most people is wild. And the best time to get it – truthfully labeled – is summer. Otherwise:

Look for canned Alaska salmon. Because the state outlaws salmon farming, Alaska salmon is wild by definition. Your surest bet for a salmon fix, especially in winter, may be canned salmon. It’s fairly cheap, sold year round and generally has “Alaska” stamped on its lid.

Buy farmed Atlantic salmon from Chile, the United States or Canada. Fish from these countries are likely to have lower levels of PCBs and dioxins than salmon from Europe. Of course, stores could lie about farmed salmon’s origins, but the fact that salmon we sampled labeled farmed really was farmed may be a good sign.