A study published August 2010 in the Journal Pediatrics looked at a large cross-sectional study of Japanese teenagers and describes that a higher intake of fish, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) is independently associated with a lower prevalence of depressive symptoms in boys but not in girls.
Now whilst it is generally felt that the role of fatty acids is vital in terms of brain health and function there is a scarcity of substantive epidemiological evidence to support direct benefits in terms of inhibiting depression.
This group looked at 3067 boys and 3450 girls aged between 12-15, all of whom completed relevant dietary and cooking related questionnaires.
Fish included canned tuna, dried fish, small fish with bones, oily fish (e.g., sardines, mackerel, Pacific saury, amberjack or yellow tail tuna, Pacific herring, eel, and tuna), and other fish (e.g., salmon, trout, whitefish, freshwater fish, and skipjack).
The study found that the prevalence of depressive symptoms was 22.5% for boys and 31.2% for girls. Depressed boys were less likely to be young, exercise, have fathers with a high educational level, live with fathers and mothers, and have a large number of siblings. They also had a higher mean body mass index and a lower mean value of intake of fish, EPA, DHA, EPA plus DHA, vegetables, and folate.
Depressed girls were less likely to exercise regularly, have parents with a high education level, and live with fathers. They also had a lower mean vegetable and folate intake.
There are considerable limitations to studies of this kind even though there are large numbers, including the methodology as well as social behaviour patterns that may hide depression in the girls. One can safely conclude that there is a link between fatty acids and behaviour but can we say that these specific forms account for the differences in outcome – probably not in terms of public health policy, but certainly worth bearing in mind when faced with a teenage boy suffering from mood swings and depressive behaviour that does not eat fish!
 Murakami K, Miyake Y, Sasaki S, Tanaka K, Arakawa M. Fish and n-3 Polyunsaturated Fatty Acid Intake and Depressive Symptoms: Ryukyus Child Health Study. Pediatrics. 2010 Aug 16. View Abstract