Scientists monitored 4,000 elderly people over five years old for age-related macular degeneration (AMD) in the eyes.
They also looked for signs of mental decline by giving them tests for memory, attention, and thinking speed.
Studies have linked regular fish consumption to lower rates of AMD, heart disease, and dementia, as well as increased brain performance.
It was believed that omega-3s are behind all of the health benefits of fish.
These are highly concentrated in the brain and appear to be important for nerve growth and repair.
However, research by the US National Eye Institute showed that dietary supplements alone weren’t enough to keep the brain young or slow down AMD.