Ah Wikipedia >< Consumers should be aware of fish oil 'capsules' and oxidation of said capsules via themselves or the processing of the fish oil. The oxidation of the fish oil renders it useless for benefits and basically flushing money down the drain; thus reducing wanted efficacy. Basically the lipids undergo peroxidation (because, hey, highly unsaturated) and the EPA and DHA levels are therefore dramatically decreased. In fact, it may even be harmful to consume (free radicals, aldehydes ect, fat which can actually increase your chances of blocking arteries).
It is highly dependent on source and quality. One study, I believe, only 11% met the guidelines with low oxidation. I would recommend against just going to the local store and picking up a supplement off the shelf expecting it to actually contain the DHA and EPA levels you need. Many of the studies in support of utilization omega-3 fatty acids are using it in its pure form. Rather than some preserved soft gel or capsule you get over the counter.
Vulnerability is a feast. Don't let them see, don't let them see.