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The two important omega-3 fatty acids found in fish are called eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). They are listed under the "Fatty Acids - Total Polyunsaturated" heading. DHA is listed as 22:6 n-3 and EPA is listed as 20:5 n-3. Add them together to calculate how much EPA and DHA you are getting from the food.
Here's an example. If you search for salmon, you will be presented with a list of different types. If you choose a three ounce portion of wild, raw coho salmon, you will be presented with a list of all the nutrients. The 22:6 n-3 is equal to 0.558 grams of DHA and 20:5 n-3 is equal to 0.365 grams of EPA. This gives you a total of 0.923 grams (923 mg) of omega-3 fatty acids.