Great blue herons go for them. But perhaps any local predator that eats fish might be possible. Same as with deer and rabbits you have to fence the pond off if it attracts guests. For birds you have to cover the whole thing, water and all or your fish are vulnerable.

Herons may poke pieces, including the eyes out of koi too big for them to take whole.

I have a pond about 40 feet in diamiter over the past 25 plus years, I have tried about everything to keep out the Herons, Kingfishers, snakes, mustrats, and of course dont forget the snapping turtles! An electric fence around the pond works for Herons once, then they just fly into the pond and not walk in. the motion sensor water scarers work during warm months. But the best over all, is the net with small holes or they will put there head through the net and get the small Koi.