Another way to humanely subdue and dispatch a halibut is to bleed them. It's easy to cut the gills by punching a short knife through the "seam" in the middle of the cheek.
As Kirk said, get them on the game clip ASAP!
I'm sorry for your loss. I know it's a heart-breaker. Been there, done that.
As halibut get in the larger size brackets, they have remarkable strength and determination that should not be underestimated.