The first was a cross-sectional study involving about 2000 persons 70-74 years old in Norway. Their fish consumption was measured and they took a battery of cognitive tests. The more fish you ate, the better your score on every test, even after adjustment for several things.
The second used data collected as part of a 3-year experiment about something else (the effect of folic acid) with 800 persons aged 50-70. They measured the omega-3 concentrations in the blood of their subjects. Would these predict anything? Their results were more ambiguous:
Higher plasma n–3 PUFA proportions predicted less decline in sensorimotor speed . . . and complex speed . . . over 3 y. Plasma n–3 PUFA proportions did not predict 3-y changes in memory, information-processing speed, or word fluency. The cross-sectional analyses showed no association between plasma n–3 PUFA proportions and performance in any of the 5 cognitive domains.
Cross-sectional correlations between a measure of omega-3 (fish consumption) and cognitive performance are exactly what the first study did find.