A well-placed shot with a spear will immobilize a halibut fairly quickly and is probably the most efficient means of killing the fish. According to Department of Fish and Game Associate Marine Biologist Ed Roberts, most spear fishermen do not need to dispatch their fish after retrieving them as the actual shot usually does so. To minimize the struggle and ethically kill your halibut, direct your shots to the spine or brain. On those occasions when you may need to dispatch a halibut or other round fish (as opposed to a flat fish), bring it to the boat and strike the fish on the top of the head, in between the eyes, with a blunt instrument like a "fish billy" rather than with a knife. Trying to do so with a knife on a small boat can be dangerous.

If you are a novice, it is probably not a good idea to attempt to struggle with and subdue a large, wounded halibut underwater with one hand while holding a sharp knife in the other. If you are determined to try to kill the fish as quickly as possible while underwater, you might consider tearing out a gill arch with your hands, or severing it with a knife. Be careful doing this, however, because halibut do have sharp gill rakers and teeth that can cause injury to unprotected fingers. Blood vessels in the arches carry copious amounts of blood to and from the gills, so severing these vessels would cause the fish to bleed to death in short order.

Is putting that much blood in the water a good idea? I'll leave that up to you, but remember that the sound waves created by the struggling, wounded fish may attract the attention of other large predatory fish. Remember too that many of these predators have highly developed sensory systems, and these sensations will probably travel farther and quicker through the water than will the blood.