Fish possess chemicals in their brains which reduce pain. If we were to believe that fish do not experience pain, why would these chemicals be present?
Researchers at Liverpool University (Sneddon, 2004) reported that opiate receptors were found in fish as well as enkephalin like substances in various brain areas of goldfish, catfish, African lungfish and rainbow trout.
Russian scientist (Chervova) applied short electric shocks to trout, cod, carp and sturgeon. The fish reacted by a jerk of the tail. Various natural analgesics given to the fish caused a reduction in the reaction. The scientist stated that it was safe to say that fish have a similar chemical pain damping system to that which higher vertebrates possess.