There has been for some time now a debate between fishing enthusiasts and animal rights activists over whether or not fish feel pain. A recent study by scientists in Scotland has reopened this debate by claiming to have demonstrated that fish in fact do feel pain.
They claim that fish have nociceptors and a part of the brain that responds to them, which is to say that they have a pain pathway. Also, when trout had their lips stung by bees they exhibited a rocking motion that is similar to pain behavior seen in other animal species (see http://news.bbc.co.uk/2/hi/science/nature/2983045.stm for a report on the study.) It has already been known for some time that fish have endogenous opiodes and so it really looks like the preponderance of evidence suggests that fish do feel pain. (see http://www-phil.tamu.edu/~gary/awvar/lecture/pain.html for a table comparing various vertebrates and invertebrates on what we take to be requirements for feeling pain). When you think about it this is what we should expect, seeing as how fish are vertebrates and all. Of course not all fishes are vertebrates and the study I was just talking about used trout so when I talk about fishes I will be talking about fish like trout.
These findings are disputed by some. The standard claim that is made by people who want to deny that fish feel pain is that fish lack the cerebral cortex that allows them to experience the psychological state of being in pain. Pain behavior is not enough, nor is nociception. Pain is a psychological state distinct from the awareness of tissue damage. The problem with this response is that it is not the case that trout do not have any cerebral cortex at all but rather that they have very primitive ones. Their cortex is so simple, in fact, that it does not require a thalamus to relay information to it but rather is directly hooked up to the sensory neurons. Thus we cannot conclude that they do not have pains at all, but only that they have some primitive form of pain.
Also, +notice that the question ‘do fish feel pain’ is an empirical question, not a philosophical question and both parties recognize it depends on the particular brain structures that fish have. This supposes that we can tell, by looking at the brain of the fish, whether or not it experiences pain. Notice also, though that this objection assumes that something is not a pain unless it is felt as painful by the organism that has it, that is unless it is a conscious pain. So, for example, consider a fish like a trout except that its nociceptors are not connected to the brain. This fish will be in the very same states as the one who does have this connection. They will even behave in all the same ways because the brain stem and spinal cord is where most of the action in fish occurs any way. If the higher-order theory turns out to be right, then the way to characterize this situation is one where the latter fish has an (in principle) unconscious pain.
This brings out three important points. 1. It is likely that some fish do have conscious pains and therefore there is reason for thinking that sport fishing is immoral, and that eating fish is as immoral, or moral, as eating other kinds of animals. 2. Fish look like good candidates for helping us to empirically test the higher-order theory of consciousness. And 3. It raises an interesting question for Utilitarians; Do unconscious pains matter? Is it wrong to torture a zombie?