Eating oily fish, like salmon, herring or mackerel, twice a week helps to prevent heart attacks and strokes. Eating oily fish more often than twice a week has not been shown to be any more protective. On the other hand, eating fried fish may increase risk of stroke (Neurology, published online December 22, 2010).
The most popular fried fish are frozen fish sticks, fast-food fish sandwiches and other inexpensive fish products. Most of these are made with farmed tilapia or catfish that are not good sources of omega-3 fatty acids. We don't know whether this is the reason that fried fish are linked with increased risk of strokes, or whether the unhealthful cooking method cancels out the benefits of any fish.
Polyunsaturated fats in fish are classified by their structure into omega-3s and omega-6s. Omega-3s form prostaglandins that help prevent inflammation that causes heart attacks and strokes. Omega-6s have not been shown to prevent heart attacks and strokes. The fish that are rich sources of omega-3s are those that eat plankton or other fish that have eaten plankton. Almost all deep water fish are rich in the healthful omega-3s.
Of the four most commonly farmed fish (Atlantic salmon, trout, tilapia, and catfish) only trout and Atlantic salmon contain relatively high amounts of omega-3s. In contrast, tilapia (the fastest-growing and most widely-farmed fish) and catfish have much lower concentrations of omega-3s, and their fatty acids are the same as chicken (Journal of the American Dietetic Association, July 2008 and December 2008).
Whether or not a farmed fish contains omega-3s depends on what the fish are fed. Catfish and tilapia can thrive on corn. Since corn is cheaper than fish meal, they are usually fed omega-6 -rich corn, instead of omega-3 -rich fish meal, and thus they have insignificant amounts of omega-3 fats in their bodies. However, salmon and most other farmed fish cannot live on just corn, so they must be fed fish meal that is loaded with omega-3s. All salmon (farmed and wild-caught) are high in omega-3s.