Omega-3 fats are "essential fatty acids" or EFAs, meaning they cannot be made by the body. They must be obtained from food, therefore making outside sources of these fats "essential."

While omega-3s are found in both animal and plant foods, they are much more concentrated in animal sources. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are the two major omega-3 fatty acids in fish. The greatest amounts of EPA and DHA are found in oily, dark-fleshed fish that live in deep, cold waters such as sardines, bluefish, and salmon.

Alpha-linolenic acid (ALA) is the other essential fatty acid found most abundantly in canola oil (11%). It is also found in flaxseed and walnuts and omega-3 enhanced eggs.

References

Bourre JM. Dietary omega-3 fatty acids for women.Biomed Pharmacother. 2007 Jan 2. Siddiqui RA, Shaikh SR, Sech LA, Yount HR, Stillwell W, Zaloga GP. Omega 3-fatty acids: health benefits and cellular mechanisms of action. Mini Rev Med Chem. 2004 Oct;4(8):859-71.