If you go HERE it will tell you that if you eat fish at least once a week it could slow the age related mental decline.
My Granny did eat fish at the very least once a week for most of her life. She was still diagnosed with dementia before she was 70 years old and it did progress into Alzhiemers.
She even caught her own fish, cleaned it, boned it, and cooked it, while she lived in Minnesota. I watched her do it a few times.
All these studies done and half of them I don't believe sometimes.
Oh well