In their own habitats, great blue herons are efficient at controlling fish and insect populations, but in your pond they are costly thieves. Although they hunt mainly at dusk and dawn, when the fish are most active, they will hunt any time of the day or night. Koi, although their favourite meal, is not the only pond creature at risk. Herons will also eat turtles, frogs, dragonflies and other fish species from your pond and surrounding area.
Great blue herons are big birds with equally large appetites and are protected by the Migratory Bird Treaty Act of 1918. Do not kill or harm blue herons in an attempt to protect your koi pond. Fines as high as £6,500 can be levied against the offender. Great blue herons are about 2 feet tall with long, tapered bills made to catch fish and other live prey.
Blue herons are built to be skilled fishing birds and they fish in a particular manner that gives homeowners some advantages when making a plan to thwart them. Herons land and walk beside the pond or wade through shallow water up to 2 feet deep and wait patiently to spear a fish that can be as big as 1 foot long. If the fish is too large to eat, the heron may damage or kill it then leave the carcase. Sometimes ponds more than 2 feet deep in the shallow areas can discourage herons.
In warmer states, blue herons do not migrate so they can plunder your pond year-round. During these months, koi loss can be even heavier because your pond will attract herons that have migrated from colder climates. To reduce the number of herons in your yard, set out a heron decoy in May to attract a male heron. Herons are territorial and the male will defend the area to keep potential competition away from his mate. Move the decoy around to add to the illusion.
Reduce the chance for a heron to walk beside, or wade in, your pond. You can build a moat around the pond with gravel. The gravel is uncomfortable for the heron to walk on. Use plants and fences to make a dense border around the pond to discourage herons from walking to the water's edge.
Netting placed over the pond needs to be a couple inches away from the surface of the water or the heron will just push through the holes in the net to get the fish. The net will keep the blue herons from walking in the shallows of your pond. Once the pesky heron has learnt you will not provide any free meals, you can take the netting down.
Commercially available motion detectors that spray water at intruders are available. When herons and other intruders set off the motion detector they are squirted with water and fly or run away. Motion detectors need to be disabled when you are in the garden because they do not distinguish between pond owners and predators.