Secret is to cut them off with scissors and not too far, they won't bleed then.

Punch. Round hole less blood( black crap). You are not drowning them I gather ? Fresh water makes them swell big time . And loose flavour. Salt water ice slurry bang . There gone.

 mate just clamps down with his thumb, half-cuts /half rips it off. I tend to be more careful, use shears to cut the bottom half of. And I don't reckon it makes one iota of difference to taste, no matter how you do it,. The important part, as LJ says, is to not expose them to too much fresh water for too long. if you have to use fresh water, make it very cold and quick. Salt is best. This year, I have just been dumping ice on top of the catch in the bucket. Let them cool down while I have a coffee and some breakfast. That done, go out and put the water in the big boiler, heated by the Mongolian burner, AKA the Saturn rocket.  Once that is underway, add water to the bucket, melting the ice so they are in very cold water for not much more than the  five minutes it takes to boil. Just chucking them into fresh water for 15 minutes is not the best way to go. Chilled salt water is the best but we can't all manage it.