"This is the first study to show, to the best of our knowledge, that a 12-week intervention with omega-3 PUFAs [polyunsaturated fatty acids] prevented transition to full-threshold psychotic disorder and led to sustained symptomatic and functional improvements in young people with an at-risk mental state for 7 years (median)."
So said the quite remarkable findings reported by Paul Amminger and colleagues  (open-access available here) who followed up their previous research study  looking at the effects of a 12-week supplementation program consisting of either 1.2 grams per day of fish oil or placebo. On that previous occasion, said omega-3 PUFA supplement ("700 mg of eicosapentaenoic acid (20:5n3), 480 mg of docosahexaenoic acid (22:6n3), and 7.6 mg of mixed tocopherol (vitamin E)") reduced the risk of progression to psychotic disorder in individuals at ultra-high risk of psychosis for up to a year post-intervention baseline.
The latest results represent quite an impressive post-intervention follow-up to the original Amminger study. Looking at some of the original cohort of participants and drawing on several types of information including screening / questionnaire data and "rate of prescription of antipsychotic medication", the authors were able to quite confidently conclude that "omega-3 PUFAs may offer a viable longer-term prevention strategy with minimal associated risk in young people at ultrahigh risk of psychosis."
Insofar as the precise hows and whys of omega-3 PUFAs potentially affecting psychosis risk, well, we are left in quite a typical position of speculating. "Omega-3 PUFAs provide a range of neurochemical activities via modulation of neurotransmitter (noradrenaline, dopamine and serotonin) reuptake, degradation, synthesis and receptor binding, as well as anti-inflammatory and anti-apoptotic effects, and the enhancement of cell membrane fluidity and neurogenesis." Take yer pick, bearing in mind there may also be additive and interacting effects within this menu of potential modes of action.
If one assumes however that the possible connection between omega-3 PUFAs and various behavioural and psychiatric labels might have some commonalities (see here and see here for example), one might see a few additional ways and means that 'mode of action' might become a little clearer in the future. One factor, cognitive decline linked to cases of psychosis onset, might not however be the prime factor extrapolating from other recent results ...
Music: Felix Jaehn - Ain’t Nobody (Loves Me Better).
 Amminger GP. et al. Longer-term outcome in the prevention of psychotic disorders by the Vienna omega-3 study. Nat Commun. 2015 Aug 11;6:7934.
 Amminger GP. et al. Long-chain omega-3 fatty acids for indicated prevention of psychotic disorders: a randomized, placebo-controlled trial. Arch Gen Psychiatry. 2010 Feb;67(2):146-54.
 Chew EY. et al. Effect of Omega-3 Fatty Acids, Lutein/Zeaxanthin, or Other Nutrient Supplementation on Cognitive Function. JAMA. 2015; 314: 791-801.
Amminger GP, Schäfer MR, Schlögelhofer M, Klier CM, & McGorry PD (2015). Longer-term outcome in the prevention of psychotic disorders by the Vienna omega-3 study. Nature communications, 6 PMID: 26263244