Seems I might have confused some people in another thread, when I said EPO=Evening Primrose Oil and then went on to talk about that (it is the GLA in the EPO that you need, so sometimes it is called GLA. You can also get GLA from Borage oil in a more concentrated form.).
Anyway, to clear things up, EPA (not EPO) is something else, namely Eicosapentaenoic acid, one of the essential Omega 3 fatty acids.
EPA is apparently very good for depression/mood swings (among other things). Here is a very interesting article about it:
Depression and pure epa omega 3 fish oil / a natural remedy for depression