Here's another reason why you shouldn't eat fish.

Via sharkdivers

Mika says it after all. Yes, at about 1:02 :

No, but really. Unless we know where our fish is coming from, that is, unless we can actually speak to the fisherman who caught our fish and made sure he did it in a traditional way*, we are gonna end up with a huge, empty, lifeless desert of an ocean. The industrial fishing techniques and pirates are literally destroying populations of countless species.

Sharks, Tuna, Cod, Sole, Anchovies... Huge boat-factories track fish banks by radar, destroy the sea bed by scraping and killing everything they find (incuding all kinds of invertebrates). So-called longline fishing bycatches turtles, dolphins.. that are then sold in the black market. Often,  fish that are too young to have had the chance to breed are caught as well. As some of these fish species are top predators, their decrease  causes a disbalance which can lead to lower-in-the-food-chain  species to proliferate. This proliferation in turn exhausts the photosynthetic members of the ecosystem, on whom they feed and then their death causes oxygen depletion. We know what happens after that. Fisheries (farming) of salmon,  tropical shrimp.... are also bad options  since a). they pollute enormously and b).these species are often "fed" with other species that are actually taken from the wild, which brings us back to overfishing. We can make a difference by choosing which fish to eat, or not to eat fish at all. As consumers we have the power. I attempted to explain a very complex situation rather simply, but you should see this documentary or read the book "End of the Line". You can find more information, by species, in a very easy to read way here at Greenpeace.

* Or unless you buy from a source that guarantees a sustainable source.