Forestpisces
Fish Herder
I firmly beleive that as grusome as it is a swift whack on the head or decapitation is the most humane way, freezing does not kill the fish only shocks them which lead to a slow death, much like a human falling through ice and clove oil has been proven to burn the gills of fish untill they eventually simply cannot function much like humans in gas chambers...so all in all quick painless and guaranteed to work is a rolling pin.