red ear slider biting koi

I need help. We have had a red earred slider in our 3500 gallon koi
pond since it was built 8 months ago. We never noticed a problem with
it. On occasion recently, I even spotted him riding on the large kois
back. I thought it was cute. However, today I watched the little jerk
swim right up to a koi that is 2' and bite the crap out of him. It
obviously hurt the fish judging by the way he took off. The turtle is
only about 2.5". I never would have thought of him attacking a large
fish. We do have smaller fish too, of which I found a dead one with no
tail a couple of days ago. I was mortified, but I think I have found
the culprit now. Heres the issue; how do I catch it. Everytime he is
sunning, I attempt to walk up to the pond and he takes off. I would
prefer not to use a trap with bait because I don't want to damage the
water quality. Has anyone dealt with a bully turtle before? He is a
fast little booger. Any help would be greatly appreciated.