Grass carp/ white amur cannot theoretically breed with common carp. They are not only different species but also different genera. Genetically incompatable. In my opinion the best way to get it out would be to bait an area (same time every day - maybe early evening)with dough balls for several days then fish w/ a dough ball in the baited area. Some expert carp fishermen use softened corn as bait. Bait the area where you saw the fish. If possible do not use any weight on our line except hook (small treble) and dough ball & maybe a slim bobber. If using corn a little differnt terminal tackle is needed. Underline it out into the baited area 3' to 6' ft deep. Sit and wait; something should happen within 1/2 hr. One koi in a pond will not do irreputable damage.


aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management