Not denying that changing your habits and setting an example for your friends has some impact. I'm only trying to frame it in the relation to the problem. Its marginal first order effects are negligible. This concept was hard for me to accept emotionally too, in the space of energy use - whether or not I reduce my usage of electricity and gas has near-zero immediate impact on anything except my wallet. Such is the problem of scale.
The irony is, that the very same thing that's individually almost meaningless, is also the solution once everyone follows it. And there's a point in the middle somewhere, when the idea becomes popular - like vegetarianism today - and suddenly its impact becomes much greater than before. It gets noticed by the market, it starts flowing through feedback loops and begins affecting everything else. One person in a million refusing to eat fish is meaningless. One in a thousand starts to sound like a business opportunity, and business begets more business.
The trick is, how to get from here to there. Basically, how to start and sustain a movement.
I guess my whole point here is: if you care about a social problem then do your individual thing (here: stop eating fish). But don't stop there, try to do more. Try to speed up the change instead of just changing yourself and hoping others will follow. If we know anything about society, we know this: there are two primary motivators for changing behavior - things that impact one's sense of identity/ego, and things that impact one's wallet. Popularizing fish abstinence is an ego-based solution. Taxing the fishing industry is the economics-based one. We should pursue both.