Although fish rights organisation Equinac has filed a criminal complaint, the Guardia Civil said that it’s not illegal to kill fish by sitting on them, any more than it is illegal to kill fish by smacking them against a rock when you catch them (he gets done under a procedimiento administrativo sancionador, if you understand Spanish law).
The moonfish is not endangered, and as such is not protected by law.
Nor is it a domestic animal.
As such, there is no recourse in law. All the police could do was issue a small fine for not obeying the advice of an official to “leave the poor animal alone”.
I just hope the police are keeping an eye on his freezer, to see whether any human limbs appear in it.