This is weird.
I believe it is intended to prevent the use of firearms to take fish from shallow streams ( example salmon or steelhead trout spawning upstream from large lakes) by shooting the fish with rifles or shotguns. The Fish Cops can thus arrest/ticket/confiscate firearms found in possession by residents near fishing areas. No poaching of fish need have happened yet, or witnessed, just the possession is a presumption of actual guilt by the fish cops of these crimes.