UFO Slice

The caudal component of fish brain may have been left intact following incomplete decapitation. Either the brain stem is operating alone and the motility is due to ganglionic reflexes, or some portion of the motor cortex is also in tact.
The good news (if you dislike animal suffering, as I do) is that even if some portion of the motor cortex is intact, this fish is in all likelihood not experiencing reality at all at this point. It is existing as a non-sentient organism (like an oyster or jellyfish). It may be capable of using enough oxygen to power its organ systems for now. However, without intact gills, it is likely already dying of hypoxia. If not, it will die due to lack of nutrients.