There's a reason why fish is referred to as brain food - because it is. Here is some advice from the UK, as reported on the BBC:
Eat oily fish
Essential fatty acids (EFAs) cannot be made by the body and must be obtained through diet. The most effective omega-3 fats occur naturally in oily fish as EPA and DHA. They are good for healthy brain function, the heart, joints and general well being. Oily fish contains EPA and DHA in a ready-made form, which enables the body to use it easily. The main sources of oily fish include salmon, trout, mackerel, herring and sardines. Low DHA levels have been linked to a higher risk of developing Alzheimer's disease and memory loss.
A medical study done in New Zealand concluded:
'Maintaining brain health and getting your brain to perform at its optimal capacity is just as vital as maintaining physical well being and health.'
DHA, is one of the most highly concentrated fats in the brain and known to play a vital role in the structure and functioning of the brain. But as the body cannot effectively make this fatty acid it must be consumed as part of the diet.
Researchers highlighted that as many people fail to eat enough fish and seafood, the brain's performance is potentially compromised.
Professor Stonehouse added: 'These findings contribute to the growing body of research showing that omega-3's play a very important role in brain function throughout the life cycle, even in healthy cognitively intact individuals.'