Spawning salmon, carp, and pike are too strong for a one-handed grip. Two may work, but if you have a knife, don't touch the fish at all. Just move your hand slowly underwater to within a few inches, approaching from an angle below your target. Then, with the point of the blade facing up, jerk your knife into the body of the fish, as if you were trying to lift it with the blade. It may squirm free, but if it has been injured, it will weaken and surface in a few minutes.