Cranes, along with herons and kingfishers, are a common pest of garden ponds and a serious threat to any koi found there. The birds can stand completely still for up to 20 minutes. That makes the koi get used to the crane's presence, which in turn makes the koi easy pickings. While there are no ways to completely crane-proof your pond, there are several things you can try to discourage the birds.
Measure the depth of the koi pond using a tape measure. It should be at least 3 feet deep at every point. Cranes cannot wade in water that deep, which means they can't pull off their standing trick. If your pond is too shallow, hire a contractor or koi pond installer to deepen it where necessary.
Install a deck around the entire edge of the pond. Make the deck stand at least 13 inches above the surface of the water, and jut out over the surface at least 6 inches. The cranes will not be able to stand on the deck and reach the fish. The material of the deck is not relevant.
Install a pond net across the surface of the pond. Pond owners traditionally use these nets to keep out leaves in the fall, but they can also effectively keep out cranes. Place one large rock every 12 inches along the border of the net to anchor it in place.
Install a motion-activated sprinkler system along the edges of the koi pond. The sprinkler system will sense when something the size of a crane tries to land in the water, spray the bird, and frighten it away. The motion-activated sprinkler, also known as a Scarecrow, can be screwed into your garden hose. Install more than one sprinkler if possible if you need it to cover a space larger than 30 feet.