Protect Koi Carp In Your Pond

There is no human so protective of anything in the world than the Koi-keeping fish enthusiast of his prize Koi carp. A Koi keeper will have nurtured his precious pisces through bitter winters and stifling summers, in conditions that would make an average swimming pool look like a sceptic tank. This would have meant no mean expense, and so when it comes to keeping away predators, a further capital outlay ‘to preserve the investment’ has got to be included in the costs.

The most likely visitor with murder on the menu is the heron.

Any open pool is simply like an open door to a burglar. If there are any ponds, lakes or long established pools in your area, herons probably have them sussed and as they do their routine round of them, even in urban areas, it will be inevitable that your bit of water will be added to the itinerary of at least one heron very soon.

The pressures on them to find new feeding grounds are increasing all the time as their numbers slowly increase – the national population has risen 5% since 1998.

When you spot a heron for the first time in your garden you get a peculiar feeling of mixed blessings. Standing over 90cm tall, it is the epitome of elegance, so first and foremost, you cannot help feel that it is in fact a privilege to have something so graceful visit your garden.

As the bird notices your presence, as it spreads its wings ready for flight it is a surprise to see it 2 metre wing span. Next comes the bubbling fury that comes with having caught an intruder and thief in the act, followed by a feeling of desperation to see what damage has been done.

The heron, Aredea cinerea can survive on a varied diet of fish, mice, frogs, eels and even small rabbits. It can stand motionless for hours in or near water waiting for fish to pass within stabbing distance of its extending neck and beak.

When it strikes, the fish not always held, but often stabbed and may escape with a gaping hole in its side.

Many times I have found fish that have proved too big to manage despite being stabbed and lifted from the pool, only left to flounder and suffocate on the lawn. Even if the heron is totally unsuccessful, the stress that the visit will have caused will make the fish immediately more timid, making them very much more prone to disease at a time when their resistance to disease may be low anyway.

Even if it is just one fish that may have suffered there will be a knock on chemical effect on the rest of the fish in the environment. Many people think that all the fish have been cleaned out when they first look in, but very often they emerge from impossible hiding places some time later.

Check ammonia levels in the pool for the next couple of days. This is secreted by the fish in excess during periods of stress.


Many people are at first persuaded to adopt their own plastic or concrete version of a heron, this public enemy number one, in the belief they are so territorial, its presence may discourage a real one from landing.

Well, at certain times of year these foibles are shelved in favour of other priorities- feeding and mating.

Through the winter and spring the heron is looking for new territory and may often be looking for a mate, so a plastic heron at these times may actually work as a lure.

When it is time to produce offspring they gather together in huge flocks called heronries that often consist of one hundred or more nests, not a loner then. 

When you actually see a heron in your garden, it is very often as a result of change in your routine. Throwing back the curtains first thing in the morning or arriving home early from work, the wily bird will have been caught unawares. Normally it would have confidently assessed your routine and work your absences into its daily itinerary.

Your only sign of something untoward would be the paranoid fish (diminishing in number) and perhaps a dislodged planting basket or two. 

The next plan of discouragement generally takes the form of some sort physical obstruction, at first manifesting itself as a thin invisible piece of fishing line at the main port of entry.

This however quickly leads to wiring the whole perimeter and then crisscrossing it this way and that, until something like a model facsimile of a French power station is achieved, which is second only to the last resort, a net.

The trip wire theory is based on the fact that the bird approaches its feeding station from a little distance away from the pool. It will never land directly in water. As it lifts its leg backwards at the ‘elbow’ to move it forwards, anything like a wire set at roughly 12 to 15cm above the ground will impede it, besides which just the touch of something on its sensitive legs will send it skywards.

But, if there is a way round the bird will eventually figure it out. There follows a series of cartoon-like capers as you get more crafty and the bird gets ever more devious. 

Technology and ingenuity offer a number of variations on the theme of trip wires that can be invisible, electrified, infrared and ultrasonic, which can hum, emit terrifying bangs, ear piercing shrieks or chatter and squirt a spray of water.

They all work to a certain extent – until the bird gets used to it or finds a way round it.

If the pool were totally dedicated to koi, without the fringe of plants that would offer cover and a step into the pool, with a depth, length and width that any self respecting koi could seek a casual refuge in, with perhaps the modern trend of a pergola to shade it from the sun, then this should be too much of a problem for it.

In the hybrid koi/natural or formal water garden, fish-hides under paving slabs on bricks or large bore drainage pipes could be organised as boltholes. You could train your fish to only rise for food at a specific time and to be wary of the heron form as it stalks the surround – for this a plastic heron could be put to use.

In this sort of pool the fish are not just in danger from herons, particularly smaller fish. Cats, mink, even badgers can be a problem, if not wholly successful at catching fish, can still turn the pool into a mire of confusion.

Docile fish have been easy prey to the likes of magpies and jays, and for those of you down near the coast there are is a potential problem from any cheeky sea birds. One visitor that often helps to spread the bad reputation of the heron is the duck.

Now I love ducks, but I cant vouch for them if you have a pool and two or more mallards call by for a sojourn, gasping for a belly full of best goldfish after a long journey.

Things will never be the same again and there aint no crisscrossing trip wiring going to keep them away from a good party. Here comes trouble.

Deterring Herons Summary

  • Change you routines regularly and go down to the pond regularly. Nevertheless feed the fish at routine times when you are always there, so they grow to learn that every figure next to the pool is going to feed them.
  • Give them something to hide in or under.
  • If you catch a heron at it, shout and yell and try to frighten them as much as possible.
  • If you have a pool net, net it straight away, just so that it is immediately discouraged if it makes a return visit. 
  • Also don’t restock straight away especially if you haven’t got a net. Wait for the bird to strike it off its itinerary.
  • Think about trip wires or sensor devices. Fishing wrapped round canes and running round the pool edge set about 12 to15cm from the ground is the first plan of defence then…. Perhaps you could set up an armed guard with Super Squirters.


Shishi Odoshi. Simple and made from bamboo it worked for the deer in Japanese gardens and could easily work for you against herons and makes a perfect water feature.

Available at most water garden retailers 


This device consists of a trip wire that triggers such a mighty bang that bird is never likely to return. A whole round of toy gun cap pellets provide the noise supplemented by the appearance of demonic glaring eyes.

Even if there are no birds to frighten off it should get a quiet Sunday afternoon barbeque going with a swing.


This is an invention by Wilf Starsmore and is effectively an electric fence suspended around the pool. There is enough current to deter but not to maim. It can be powered by battery or mains and does the job – no messing for a running cost of approximately £4.00 per year.

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