Unless you’re squeamish, this may be one of the best methods to quickly dispatch a dying fish.
For larger specimens, a blow needs to be made rendering the fish unconscious at the least, and causing outright brain destruction at best. It is essential that the fish is correctly dispatched before it can regain consciousness, and the first strike will need to be decisive. Too weak a blow, and you will only cause pain and more suffering.
If you are in any way unsure of how to dispatch a larger fish in one blow, then this method is not for you. An active, wriggling fish can be a hard target, and a last minute twitch can be the difference between success and failure.
Concussion and head trauma can be used after the successful administration of anaesthetic, which can make the task considerably easier, and give some leeway in the event of a failed first hit.
Smaller fish (the majority of or aquarium residents) can be decapitated, as long as the brain is subsequently destroyed. This will involve pithing, which requires that a sharp object is placed into the brain cavity ensuring its destruction. If you’re not familiar with techniques to pith a fish, seek guidance beforehand.
Do not decapitate without pithing. Although death will follow, the detached head of a fish may still be both alive and aware for several minutes after.
From the Practical Fishkeeping article.
I'm sorry it has come this
