the reason the hatcheries kill spawn is they have to check for internal hemoraging desease this can only be checked by killing the fish , if they are used for spawning and not checked the desease could spread to all the offspring from that years spawning efforts , as far as any excess fish that were not used for spawning they are released, almost always it is the bucks, one good healthy buck can fertilize quite a few hens , so the hatchery needs are abuot 6hens to 1 buck. by the way the steelhead that are not caught or used at hatcheries will spawn naturally , go back out and depending on available food ,gain weight and return about 1-2 yrs later , assuming they get lucky and don't get waylaid along the way