Although it might be easier to relate to mammals, there’s also scientific evidence that fish feel pain. When they’re pulled out of the deeper parts of the ocean, their eyes pop out, their bladders burst, and they slowly suffocate to death.
“In summary, most recent interpretations of the results of many studies lead to believe that fish have the structures necessary and the capacity to experience fear and pain and can thus suffer” – Scientific Panel for Animal Health and Welfare, commissioned by the European Food Safety Authority, June 2004 (p. 156)
Research has shown that fish recognise individuals, are social, and even look out for each other. In order to maximise profits, it’s becoming common practice to breed fish in filthy, polluted tanks where they have no space to swim.