Need a predator to control koi population

[fishb0ne]

My dad has a 5000 pond. It is about a foot deep at one end going all the way up to above one's waist at the other. At this end the water never freezes all the way and this is where the koi spend the winter. He's got a waterpump with filter, which feeds a waterfall at the deep end.

The problem is overpopulation. Every spring the koi reproduce. We found a couple of people that offered to take some off our hands but we also thought we should introduce some sort of predatory fish that will grow no bigger than 4 inches [so he won't start harassing the bigger koi] that will hunt and eat some of the offspring.

The question is, what kind of fish should we look for? Could this fish spend the winter in the pond? It would be preferred but if not, it's not a problem since we can accomodate it indoors in our 75g aquarium. What should we feed said fish once he manages to feed off the smaller koi?

[piranha]

Don't see why you couldn't sell the offspring. Koi are such a popular fish for ponds. Why not try your local aquarium store if you can't sell them. I'm sure they wouldn't refuse them.