Most of mine go back in the lake too, but the ones I keep get put to sleep in a cooler of ice. It slows their metabolism way down and they stop trying to breathe pretty quickly.
Kinda like hunting wild turkeys; as much as I respect and admire those birds, the best way to humanely take 'em is to blow their brains out with a shotgun from fairly close range.
They just don't sell meat that tasty in stores. The pursuit is fun and challenging too.
Jim