Are your fish disappearing from the pond without a trace? The 7 suspects and how to unmask them

In spring, you counted twelve koi. By June, only seven remain. No carcass, no scales on the bank, nothing. When fish disappear without a trace, there is almost always a predator leaving with its entire prey, and the grey heron is by far the most common culprit. But it is not the only one, and some disappearances have nothing to do with wildlife. Here's how to investigate and, above all, how to put an end to it.

Before blaming a predator: false disappearances

Three situations can create the illusion of a theft even though the fish have never left the pond, or barely.

• The fish are hiding. After an attack, even an unsuccessful one, fish remain hidden at the bottom or beneath the water lilies for several days. In spring and autumn, the cool water also makes them much more discreet. Count them at feeding time, in mild weather.
• A dead fish has disappeared. In summer, a small carcass sinks, decomposes within a few days and may be eaten by other fish, snails or larvae. A low-level die-off can therefore go unnoticed.
• A fish jumped out of the water or was sucked into an inadequately protected skimmer. Orfe are prolific jumpers, especially when the water quality is poor. Check the surrounding area, the skimmer basket and the filter.

The 7 suspects and their clues

— The grey heron

The number one suspect. It arrives at dawn or dusk, settles a few metres away, and walks into the shallow water. It swallows its prey whole, up to 30 or 40 cm long, and a single bird can empty a garden pond within a few weeks. Clues: no tracks, fish disappearing by average size, sometimes an injured fish with a V-shaped mark on its back, and very skittish fish.

— The cat

It fishes from the bank, especially in ponds with low banks or raised ponds. It often leaves remains or fish on the lawn. It mainly targets fish that come to the surface.

— The kingfisher

Magnificent, but dangerous to small fish less than 8 to 10 cm long. It dives from a perch overhanging the water. If only fry and small goldfish disappear, it is the culprit.

— The great cormorant

Especially on ponds and large pools. It dives and hunts underwater, in groups. Losses are rapid and massive, and survivors often bear beak marks.

— The otter and the mink

The otter is gradually recolonising many regions. It hunts at night and often leaves remains on the bank, along with webbed footprints in the mud and characteristic droppings. The American mink leaves similar, smaller tracks.

— The raccoon

Found in several regions of Europe, it fishes with its front paws, overturns plant pots and disturbs stones along the edge. Clues: nighttime chaos on the banks and small hand-shaped footprints.

— Crows, magpies and gulls

Less well known, but very real. Crows and magpies learn to seize small fish that venture into areas less than 10 cm deep, and gulls attack at the surface near the coast. They target small fish and very shallow ponds.

The best investigative tool costs less than 50 euros: a small trail or security camera with motion detection, aimed at the pond for three or four nights. You will know exactly what you are dealing with and can choose the right protection.

Protections that really work

1. The net: effective against all birds and most mammals. Stretched 20 to 30 cm above the water and firmly secured at the edges, it leaves no chance. Its only drawback is aesthetic, which is why fine, dark mesh is preferable, almost invisible from a few metres away.
2. Perimeter wire: one or two nylon wires stretched at heights of 15 and 30 cm all around the pond. The heron, which enters on foot, bumps into them and gives up. Discreet and highly effective against herons, it is useless against kingfishers.
3. Electric fencing: a low-intensity wire around the pond discourages herons, cats, raccoons and otters without hurting them.
4. Steep banks and a deep zone: a heron does not fish in more than 40 cm of water. A pond without a shallow beach, with a zone deeper than one metre, considerably hinders it.
5. Deep hiding places: large-diameter PVC pipes, caves, dense plants. The fish take refuge there at the first sign of danger.
6. Deterrents (decoys, reflectors, sensor-activated sprays): useful as a supplement, provided they are moved frequently. Birds quickly get used to anything that does not move.

Let us remember that the heron, kingfisher, otter, and cormorant are protected species in France and Belgium: we deter them, we do not harm them.

Also read

How can I stop the heron from eating my fish?

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Frequently asked questions

— Is a fake heron effective?

Sometimes, and only for a few weeks. Since the heron is territorial, a motionless decoy can deter it at first. Move it every two or three days and never rely on it alone.

— How can I protect my fish from crows?

Remove or protect areas less than 10 cm deep, where they can catch fish, and add floating plants. Netting remains the definitive solution if losses continue.

— What time does the heron come to fish?

Especially at dawn and dusk, and more often in spring, when it feeds its young. It may return every day as long as it finds fish.

— Is a deep pond enough to protect koi?

It greatly limits the heron, which fishes on foot, but not the cormorant, otter, or cat. Steep banks and hiding places greatly strengthen the protection.

In summary

• Fish disappearing without a trace: think heron first.
• Check for false disappearances: hidden fish, a decomposed carcass, jumpers, or the skimmer.
• A motion-activated camera can identify the culprit within a few nights.
• Netting, perimeter wire, or electric fencing: the only truly reliable protections.

Last updated: September 28, 2026.