Over the years many studies have reported conflicting results. Some of the confusion is due to the fact that some studies have looked at omega-3 as a stand-alone therapy; others have tested it in combination with antidepressants, and different studies have used different fish oil formulations and dosages. In a study with 432 patients diagnosed with at least moderate depression, patients took either fish-oil capsules or a placebo every day for eight weeks. Among anxiety-free patients, symptoms improved significantly more with fish-oil than with the placebo. Taking anti-depressants, or not taking them, did not affect the results. On another topic, a new study shows that women who take fish oil pills appear to have a lower rate of breast cancer.

See http://www.raysahelian.com/fishoils.html for details. I, personally, notice only a mild effect on mood from fish oil supplementation compared to the ones below.