Homeopathy

I'll just point out the obvious here (ie: I am not disagreeing or agreeing with you.)
A link between a high fish diet and and reduced Alzheimer's onset means one thing, that there is a possible causal link between a high fish diet and and reduced Alzheimer's onset.
It does not mean there is a causal link between fish oil and reduced Alzheimer's onset.

Firstly- did you mean to write casual then causal?

Secondly, and anyway, the causality is enhanced for the reason below:

that study is particularly interesting, because it accounts for many other factors by selecting people from Western norway who have a very small gene pool, and activity and environment, etc. and attempts to justify any other differences that might arise, as well as being a very thorough study in itself.

Here's a quick summary I wrote some time ago in a thesis on AD- (65%- decent)

Two diets have been seen to have positive effects in relation to both dementia and AD. These are fish diets and the Mediterranean diet. The fish and seafood diet was studied by Nurk et all (2007) with 2031 subjects from Western Norway, who were subject to cognitive testing. Fish and seafood are high in the omega 3 polyunsaturated fatty acid docohexaemoic acid, which has been shown to reduce the amyloid burden. This study showed that it increased cognitive ability: diets containing over 10g of fish per day showed significantly higher test scores than those on less than 10g of fish per day, with the maximum effect being noted at 75g/ day, with a positive correlation. The study also showed a lower incidence rate of AD amongst the fish eaters.