If you can, DO NOT bonk the fish. Just cut the gills, all of them, put the fish on a rope/stringer, tie it to somthing solid, and put it back in the water. It will continue to bleed and get virtually all of the blood out.

The fish might continue to splash around (especially coho), and build up lactic acid in the muscle tissue, but once the fish is in the water, it usually settles down. Before long, it will die of blood loss. I prefer that method than beating it's brains out with a stick, or worse, a rock......

Sorry to go against tradition, but I never bonk my fish. I just bleed'em out.