There's no mercury to worry about, and it does contain omega-3 fats...but not the best kind.
Troll the medical literature, and you'll come up with study after study showing that fish and fish oil are good for us, especially for our hearts but maybe also for our moods and immune systems. Various epidemiologic investigations have found that people who eat fish regularly are less likely to have heart attacks, suffer strokes, or die from sudden cardiac arrest. The definition of "regularly" varies, but it usually means at least a couple of times a week, although eating fish even once a month has been shown to make a difference.
Fish, and especially fish oil, have also been the subject of dozens of randomized clinical trials, most involving people with existing heart conditions. In large amounts (several grams a day), fish oil has been shown to nudge various cardiac risk factors ("good" HDL cholesterol, triglycerides, blood pressure) in the right direction. Smaller amounts (a gram a day) work against irregular heart rhythms — particularly atrial fibrillation, the chaotic beating of the heart's two upper chambers. Fish oil is believed to block the sodium channels in the membranes of heart muscle cells, making them less likely to "misfire" and contribute to irregular beating.
The evidence doesn't line up perfectly. A review published in 2006 in the British Medical Journal came to the conclusion that fish oil doesn't have much benefit after all. Results from studies of fish oil supplements for people with implantable cardioverter-defibrillators have been mixed.
Getting fish oil into your diet can be difficult. Eating fish will certainly do it — if you feast on salmon, trout, mackerel, and other oily species. A three-ounce serving of those fish supplies about a gram's worth. But you'd need to eat more than a pound of farmed catfish to get that much fish oil. Or 12 ounces of light tuna canned in water. Fish oil supplements — which usually come in the form of 1-gram capsules — may be more practical. But the capsules are large, and side effects are a problem, ranging from merely unpleasant (stomach upset, fishy aftertaste) to downright unhealthy (higher LDL levels, albeit at doses of several grams a day).
There's also mercury contamination to think about. Mercury accumulates in the food chain, so some of the heaviest concentrations are found in long-lived predatory species that are also some of the most desirable from the standpoint of fish-oil consumption. Fortunately, the supplements haven't been implicated as a source of mercury exposure. Some brands claim to be made from sardines, anchovies, and other fish at the lower (and presumably less contaminated) end of the food chain.
The third omega-3
Flaxseed oil is being heavily promoted as an alternative to fish oil. The health benefits of fish oil are believed to derive principally from two omega-3 fats, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Flaxseed oil contains a third, plant-based omega-3, alpha-linolenic acid (ALA). Other foods (especially walnuts) and oils (canola and soybean, for example) contain ALA. But at about 7 grams per tablespoon, flaxseed oil is by far the richest source.
ALA hasn't been studied nearly as much as fish oil. And while some experts see promise in the few results so far, others caution that there's not nearly enough evidence to go on.
The main problem with ALA is that to have the good effects attributed to omega-3s, it must be converted by a limited supply of enzymes into EPA and DHA. As a result, only a small fraction of it has omega-3's effects — 10%–15%, maybe less. The remaining 85%–90% gets burned up as energy or metabolized in other ways. So in terms of omega-3 "power," a tablespoon of flaxseed oil is worth about 700 milligrams (mg) of EPA and DHA. That's still more than the 300 mg of EPA and DHA in many 1-gram fish oil capsules, but far less than what the 7 grams listed on the label might imply.
A serving is six capsules
A better comparison might be between the fish oil and flaxseed oil capsules. The first thing to notice about flaxseed oil is the serving size. For the Spectrum brand, it's six capsules! According to the label, that six-capsule serving supplies 3.4 grams of ALA, which converts to roughly 340 mg of omega-3s. A single fish oil capsule will give you about that much omega-3.
Another problem with flaxseed oil is the omega-6 content (about two grams per tablespoon). Omega-6 metabolism competes with ALA for some of the same enzymes, so ALA might not reach its full omega-3 potential if there's a lot of omega-6 around.
The bottom line
Flaxseed oil will give your diet a nice little omega-3 boost in the form of alpha-linolenic acid. You might try adding some to your salad dressing. But it's a backup, not a substitute, for the omega-3s in fish and fish oil because of the conversion factor. If you're in need of omega-3s but are concerned about mercury, fish oil capsules might be a good choice. Some brands are made from fish with little mercury content. But don't give up on eating fish. Salmon, pollock, and catfish are all low in mercury. And canned light tuna is lower in mercury than albacore ("white") tuna.