Fish oil is famously high in polyunsaturated fatty acids that are essential to brain function and cardiovascular health. While there is an established link between people with depression and low levels of these omega-3 fatty acids, scientists are still trying to definitively prove that the reverse is also true. Some encouraging studies found that omega-3s have boosted the mood-elevating power of prescription antidepressants; in another large study, omega-3 supplements improved the symptoms of those diagnosed with depression but who do not suffer from anxiety (although this research was funded by the makers of the supplements). Try sashimi of wild salmon, mackerel and arctic char, all of which have high levels omega-3s and low levels of mercury.
Added perk: Because sashimi is low in calories and saturated fats, incorporating it into your diet will ensure you're still smiling when you step on the scale.