In the United Kingdom there are a number of species that pose a direct threat to Koi stock. As a Koi keeper, knowing about these predators and how you might protect Koi against them can literally be the difference between life and death for your stock. Exactly what species pose a danger will depend on a number of factors, including what precautions you may already have in place, in which area of the country you live and your proximity to important features in the local landscape, such as rivers and lakes. One predator that doesn't seem to have a restriction on its whereabouts is the Grey Heron.
Heron
Without a doubt, the most potent threat to Koi in the UK is the Heron and it also happens to be one of the hardest predators to protect against, but there are some tried and tested ways that you can attempt do so. Heron do not specifically target Koi of course, but when there is a Heron about then all pond-fish are in danger and so the information here is relevant for all pond-keepers with any kind of ornamental fish stock. Like all species Herons are here because quite frankly, they're incredibly good at what they do. The Heron is adapt at hunting aquatic prey and they are a common sight in the British Countryside, where you'll see them all year round standing in shallow water waiting out their prey. It's unsurprising that a garden pond and a stock of ornamental fish is a great feeding opportunity for a Heron. Whilst you might think that hunting in a garden or a busy city would increase the chance of the Heron's hunt being disturbed, the captivity of the fish makes the hunt a much quicker process and it's thought that because of its ease, young Herons are often the ones hunting in ponds because of the almost guaranteed catch. In the UK the only type of Heron you'll see is the, although you'll find other members of the Heron family in Egrets and Bitterns.
Measures that you can take to protect your Koi against Grey Heron include:
Use deep water to your advantage
Do not have shallow areas or steps around the edge of your Koi pond. Heron thrive in shallow water, it's their perfect platform for hunting.
Use plant life as cover:
Floating plants and lilies, for example, can give Koi an excellent place to hide from a Heron's gaze. This tactic is not only good for the ecosystem of the pond, but also beneficial should your pond be visited by a Heron. This will also provide some cover should a Heron be flying overhead. Bear in mind the impact of the seasons on plant life and the cover that it will provide.
Pond Netting as a physical barrier
Placed in a way as to cover the pond, Netting can provide a physical barrier between a prying Heron and the Koi stock. In addition to Heron, this method will secure the pond against even the most determined predator. The application of the Pond Net is key and needs to be high enough from the water's surface. Of course with Pond Netting there is an obvious aesthetic trade off. It's for the pond-keeper to decide whether a Net is the answer or if it's an eye-sore too far that's also a barrier to them enjoying their pond fully.
Decoy Herons
There's mixed reviews over the use of decoys such as fake plastic Herons. There is the thought that placing a decoy close to your pond will deter a real Heron stopping by and some fish-keepers swear by them. It's thought that Herons will not land if another Heron is already fishing in that spot and there may be some logic to this, as how often in the wild do you see Heron hunting in the same immediate area as another? Not very often.
Other fish-keepers that have tried and tested a decoy Heron see little point, as sometimes the real Heron will stop by anyway - ignoring the other plastic bird altogether. A quick image search online will show you plenty of examples of this caught on camera by pond-owners.