The following questions were asked on the California Department of Fish & Wildlife website:
QUESTION: I have a question about safely bringing large halibut onboard. Because the Pacific halibut caught in Alaska are often over 100 pounds, deckhands use pistols or small shotguns to kill the fish before bringing them on board. This is to prevent the fish from causing damage or hurting anyone once on the deck. Would this method be legal to use in California ocean waters with large fish? Of course, the fish would already be “landed” by first being gaffed. Is it even legal to carry a pistol while fishing on a private boat near shore?
ANSWER: Sport fishermen may take halibut by hand, hook and line, spear fishing, spear, harpoon or bow and arrow (California Code of Regulations Title 14, sections 28.65, 28.90 and 28.95.) Firearms are not a legal method of take for halibut, so a gun may not be used to assist in taking or landing the fish.
In some areas, it may be legal to carry a pistol on a private boat but there are closures that prohibit the possession of any firearm on a boat along portions of the Monterey and San Luis Obispo county coastlines within the California Sea Otter Game Refuge. If considering carrying a pistol on your boat, you need to research local laws and ordinances within the jurisdictions you will be transiting on your fishing trip.