I thin mine a couple times a season. My wife loves'em, so I don't mind a few. when I look out and see a dozen or so on the bank... time to thin. I use a homemade trap, basically a rectangle of PVC with a made-up basket of wire mesh underneath. Ramps on each side and a small floating plank in the center to put a can of sardines on. (I'll try to remember to snap a pic) It's really crude but does the job and was cheap.
So anybody that wants some sliders, come and get'em! I usually take them to work and toss them in the big drainage canal beside the station house, or give them to co-workers with kids if they want'em.
Jeff-