Fish oil contains a high concentration of the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are essential for a multitude of biological functions.
Fish do not actually produce omega-3 fatty acids, but rather accumulate them from their diet. Prey fish, like herring and sardines, accumulate omega-3s by consuming microalgae (like spirulina and chlorella). Predatory fish, like mackerel, lake trout, albacore tuna and salmon are at the top of the food chain and accumulate omega-3 fats by conuming prey fish.
Because of this, these species can accumulate toxic substances such as mercury, dioxins and chlordane. For this reason, many people have turned to fish oil supplements to safely acheive adequate omega-3 fatty acids in the diet.
Make sure you choose a high quality fish oil like Carlson's which is regularly tested using AOAC international protocols for potency and purity by and independent FDA Registered laboratory.
ECO-NOTE: Despite its recent popularity and known health benefits, for the safety of our oceans and the wildlife in Antartica avoid krill oil.