They don't need to be fed. I kept a few thousand shiners last year without feeding them one bit. In addition, my koi aren't fed from mid-november through april/may. Like most of the fish below (with exception to the "winter fish") they have a greatly reduced metabolism in the cold. Whether or not this kicks in as soon as they cool off from the bait store, I don't know - but one can only assume so.
If your concerned with husbandry you can "feed" them pond water if you'd like. Theres more than enough food in there for them to suffice if you refill it everytime you go out.