For another perspective on omega-3 blood levels, here's an interesting study in northern Quebec Inuits.
Traditionally, Inuits consumed large quantities of omega-3-rich seal, fish, caribou, and whale, even eating the fat. However, like the rest of the world, modern Inuits have increased consumption of store-bought foods, largely processed carbohydrates. Along with this trend has emerged more heart disease, diabetes, and depression.
A group from Laval University and University of Guelph, both in Canada, examined the relationship of plasma EPA + DHA levels and measures of psychological distress. This group had previously shown that Inuits older than 50 years had twice the plasma omega-3 levels (11.5%) compared to those younger than 50 years (6.5%), reflecting the shift away from the traditional diet.
Psychological distress was measured with The Psychological Distress Index Santé-Québec Survey (PDISQS-14): the higher the score, the greater the psychological distress. (In the graphs, tertile 1 is least distressed; tertile 5 is most distressed. Sorry about the small chart graphic--click on the graphic to make it bigger.)
From Lucas M et al 2009 (http://www.nutrasource.ca/NDI/Assets/Articles/Plasma%20omega-3%20and%20psychological%20distress%20among%20Nunavik%20Inuit.pdf)
"Our main finding was that women in the second and third tertiles of EPA+DHA concentrations in plasma PLs [phospholipids] had a 3 times lower risk of having a high-level PD [psychological distress] score than women in the lowest tertile."
While the relationship is stronger for women, you can see that, the higher the EPA + DHA plasma level, the lower the likelihood of psychological distress. Interestingly, the tertile with the greatest distress and lowest EPA + DHA levels had a plasma level of 7.0-7.5%--far higher than average Americans.
(Plasma levels of EPA + DHA were used in this study, which tend to reflect more recent omega-3 intake than the more stable and slower-to-change RBC Omega-3 Index that we use. Plasma levels also tend to run about 10-20% lower than RBC levels.)
Of course, there's more to psychological distress than omega-3 blood levels. After all, eating fish or taking fish oil capsules won't make money worries go away or heal an unhappy marriage. But it is one variable that can be easily and safely remedied.