This site talks about the benefits of fish over flax, though actually it talks about seal oil which isn't available. http://www.omega3sealoil.com
On one page, it quotes the book "Protein Power" in regards to the human body's capability to convert ALA to EPA/DHA.
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... but taking flax seed oil is kind of like buying crude oil and running it through your home distillery to make gasoline for you car. If that's the only way you can get gasoline, then that's what you have to do. If you can buy the gasoline already distilled, however, it is much more efficient to do that and avoid the hassle of the home distillation process.
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It discusses the good and the bad of fish oil (because the site touts seal oil). One bad element is that you can't trust the amount of Omega-3 the way you can in flax:
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The fatty acid content of fish oil depends upon the species, season, latitude and thus the temperature, fresh or salt water, and finally its gender. It can be noted that the female with its roe, is a much richer source of fatty acids than the male. Cold water fish are the richest sources of DHA and EPA. Some species contain more than others, and the content of an individual species varies somewhat from fish to fish. The DHA content of farmed vs. wild fish can be considerably different depending upon the feed given to commercially raised fish. The source of fish is also important. Antarctica has cleaner waters than many other regions used for fish catches. For example, anchovies from Antarctica are very small fish, high in Omega 3 fatty acid content and very low on the food chain.
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Also, fish oil comes from the by-product of fish processing waste - often fish subject to heat, spoiling, etc. Peroxide levels tend to be high (molecular distillation probably gets rid of the peroxide, though).
My own analysis is that flax oil provides about 1/3 the amount of EPA/DHA after conversion from ALA as that found in fish oil (assuming optimum conversion rates of 10% EPA and 5% DHA which, if we are eating clean and watching our saturated fats and w6:w3 ratios and supplementing correctly, hopefully isn't a bad assumption). Flax, on the other hand, has 1/4 the amount of saturated fat as in fish oil. So it seems that you're substituting one good for one bad. Personally, I'm going to stick to the previous advice and mix the two oils in my diet.