One of the primary arguments against fish having a sense of pain is that their brains lack the structural elements, namely the neocortex, necessary for it. Although fish's brains are certainly less complex than ours, they aren't mindlessly floating around in the water. Akin to the fight-or-flight response, fish have chemicals in their bodies that are released when they're in danger [source: Chandroo, Duncan and Moccia]. Fish also possess pain nerve fiber tracts in their bodies that are similar to those that tetrapods (vertebrates with four limbs) have [source: Chandroo, Duncan and Moccia]. Overall, the scientific consensus is that fish have the anatomical requirements to demonstrate neurophysiologic and behavioral reactions to pain as a means of survival [source: Braithwaite and Huntingford].
However, little evidence exists to suggest that the fish also react emotionally to pain like humans. As another way of safeguarding our bodies, our pain response comes with a negative emotional reaction that excites the amygdala in our brains and forms a memory of the unpleasant effect of a particular stimulus. For that reason, most children will touch a hot stove only once. Fish brains do contain an amygdaloid complex, but it produces aggression rather than fear [source: Chandroo, Duncan and Moccia]. Nevertheless, fish are able to form sensory memories. For instance, trout can remember how to avoid getting caught in fishing nets months after the initial experience [source: Donkin]. So while it remains unclear whether fish genuinely suffer from pain, memories of it stick in their brains.