I have had this pond for 15 yrs. and have been waging war against various koi predators the entire time. Blue Herons have been the major problem. I have used a mesh covering which does work except it is horrible for the esthetics of the pond. Over time it would catch leaves and other debris and fall into the pond, also covering the water lilies. I have used an oscillating sprinkler head with a motion sensor, but you have to keep it on and change the battery frequently. The best solution was staking fishing line around the pond. That worked well as fish eating birds with long legs walk into the pond and the fishing line freaks them out and they fly away. However, after all this and my koi were growing large a mink (yes a mink I have pictures) came through and wiped all of them out. There is no defense as they swim into the pond and just feast away. I have resolved to just stock the pond with small koi when there is a sale and or fancy goldfish.