As far as rapidity of death of fish put through mulcher/prehog machines, I would not be too concerned about a lingering death.
I work for a company that makes "professional" mulchers (known in the trade as hoggers) and anything live fed into the machines smaller than a full grown chook would, in my estimation at the very least be unconcious after the first "strike" and gone through the machine in less than 30 seconds, that is , all gone, in small, thumb sized pieces.
These machines are never, in my experience fed with live animals, of any type.
There was a case (according to the lore of these machines [truth unverified] of a person accidently going through one of our machines I believe the remains were hosed out.
the only thing that I have heard that will stop these machines is either steel, like 10mm plate, or a large bulls head.
Have fun, and don't slip in!