There is an argument going on in the omega-3 supplement circles as to the question of flax seed oil vs fish oil: which is better?
To combat omega-3 fatty acid deficiencies many health professionals are turning to the use of fish oils since these oils are high in the omega-3 fatty acids DHA and EPA. When we consume flax seed oil which is very high in the basic essential fatty acid alpha-linolenc (LNA) acid the body must convert the LNA into EPA and DHA. When you consume fish, and fish oil, you side step this process.
However fish oils are much more expensive, per dose, than even the highest quality flax seed oil. Fish oils are also much higher in lipid peroxides which are counter productive to the benefits of essential fatty acids. Fish oils also carry the risk of bringing along food-chain-contaminants such as mercury, which may be present in the fish from which the oil is extracted.
Besides the relative health benefits of flax seed oil versus fish oil, flax seed oil, properly preserved (in your freezer), is much easier to take than fish oil is. You can simply add flax seed oil to your food. It tastes delicious if it is still fresh (if your flax seed oil does not taste good it has become rancid and it should be thrown out right away or used as a substrate from which to produce a very nice natural paint medium).
Also, instead of oil, you can use ground flax seeds (around about a quarter of a cup of seeds ground up in a coffee grinder would provide a daily dose). You can sprinkle ground flax on a salad, or add it to a sauce (after cooking the sauce). Ground flax seed needs to be eaten right away to prevent rancidity.
One more point in favor of flax seeds as your essential fatty acid source is in the fact that, not everyone wants to down a couple handfuls of pills every day for the rest of their lives. When you use flax seed oil as your omega-3 source you do not need to down 10 or 15 fish oil pills a day. That many pills make up the standard minimum dose for fish oil supplementation.
A supplement such as flax seed oil, on the other hand, that is a natural addition to your dinner (or lunch or smoothie) has a much better long-term prognosis as a permanently employed and enjoyed supplement. It is for all of these reasons; flax seed oil rather than fish oil is preferable as the source of omega-3 fatty acids.
Besides flax seed oil, properly extracted, and unrefined pumpkin seed oil, hemp seed oil, walnut oil can provide fairly significant quantities of omega-3 oils to the diet though these oils are not quite as rich in omega-3 as flax seed oil is.
And, then, of course, on the other hand, if you are a fish lover, deep sea, wild-harvested cold water fatty fish can also provide a more healthy source of omega-3 fatty acids if you can afford it. But the question of food chain toxicity still exists. One precautionary measure you may take to prevent your fish from poisoning you is to stick to small fish like sardines and herrings. These fish are much lower on the food chain, and therefor much safer to eat.
To the question of flax seed oil vs fish oil, which is better, my vote is on flax seed oil. In the long term this is true, but only if you consume sufficient amounts and you are careful to avoid trans and damaged oils and you are careful to limit your omega-6 oil intake. In the short term fish oil is good, and if you are not able to control the other oils in your diet yet, fish oil is, in fact, indispensible.
References:
Murray, T. N.D. and Jade Beutler, R.R.T., R.C.P.,(1996). Understanding Fats & Oils, Progressive Health Publishing, Encinitas, CA.