It's not scientific fact with lots of evidence backing it up yet, but it is widely believed that eating fish is good for the brain. Oily fish especially contain amino acids that are thought to have a relationship with intelligence. Fish oil supplements are currently used with kids with ADHD, and pregnant women are advised to eat a couple of portions of oily fisha week to boost their baby's intelligence!
As mncavylover said, you can get the substances in question from vegetarian and vegan sources. Nuts and seeds are excellent sources - can't remember which provide which sources offhand though.
While they may have an effect (as good nutrition inevitably does on all aspects of our bodies and mind), I don't think it's such a big deal as some people make out. Even when I wasn't a veggie, I was never partial to fish and seem to have muddled through the education system regardless!
I don't eat any fish anymore and even before I was a vegetarian I rarely had it. Maybe once every 2 weeks growing up. My IQ is 184. So despite them having studies about it. I doubt if eating fish is going to make you the next Einstein. My fiance never ate fish during her pregnancy, and my daughter is a year and a half old and never had oily fish. She is of above average intelligence for her age as well. Lots of people do lots of studies, and it really doesn't mean too much. So don't get hung up on it.
I know for sure eating fish doesn't get you a higher paying job.
it might or might not have an effect, but since we still can't figure it out, or exactly what the effect is, or even if it exists, i am going to venture a guess that although there might be some effect, it is not huge, and i know for a fact you can get all of the stuff from other foods
Oily fish contains Omega 3 oils that are shown to have a positive effect on brain chemistry and are helpful for people with ADD and bipolor disorder. The body is supposed to be able to manufacturer Omega 3's from Omega 6's, but not everyone is efficient at that and that can cause problems.
The good news is that flax oil contains plenty of Omega 3's. So, if you are considering a veggie diet, get some flax oil to add on your salads.
One of the deficiencies that are associated with a veggie diet is vitamin B12 - apparently red meat is the main source for humans. Even those on fish or fowl only diets can develop B12 deficiencies. Some symptoms of a B12 deficiency are fatigue, lack of energy and dark circles under the eyes. You can get supplements. It's a good idea to add a B-complex vitamin to your diet anyway.