A sample of 2,034 individuals (age >55) in Singapore were studied to explore the link between fish consumption and depression. The study was conducted as part of the Singapore Longitudinal Aging Studies (SLAS) project and results were adjusted for all other possible factors such as smoking, exercise, diabetes etc.

Individuals consuming fish three times per week were observed to report lower depressive symptoms, compared with those who did not consume fish. 

The abstract can be accessed here

Wu D., Feng L., Gao Q., Li J.L., Rajendran K.S., Wong J.C., Kua E.H., Ng T.P. Association between fish intake and depressive symptoms among community-living older Chinese adults in Singapore: A cross-sectional study. J. Nutr. Health Aging. 2016;20:404–407.exercise, diabetes etc