There’s something eating my fish

John Bryan looks at the most common predators of valuable fish stocks, and what can be done about them.

We quite often get a phone call or email along the lines of “There’s something eating my fish! What is it? And what can I do about it?” which can lead to a difficult conversation about the rights and wrongs of wildlife legislation. The systematic predation of a lake or pond can leave people fighting for their livelihood and finding half eaten, dead or dying fish day after day must be heart breaking. While the range of likely culprits is quite small, unfortunately most of them are protected under law and require special permission to control directly.

Predatory birds will be visible flying around the site before you see any real impact on the fish. Grey herons are UK-native birds that naturally hunt on rivers and inland waterways. They are usually solitary, stealth hunters and stand motionless in the shallows waiting for fish. Fish weighing around 200g are ideal prey, as two roughly equal a heron’s daily food requirement. Cormorants, in contrast, are naturally costal birds with a resident UK population of 9,000 pairs, jumping to 40,000 pairs during winter migration. These diving birds will take small fish, but will also injure and kill larger fish by pecking them through the side.

01_European_mink_is_curious

A voracious predator, the American mink will take a wide range of prey and eats about one third of its bodyweight every day. Although much smaller than the otter it is often mistaken for one, especially when in the water.

 

Mammalian predators will take bigger fish but won’t arrive overnight in large numbers. They have to move over land but can travel significant distances and once established will breed. Mainly night-time hunters, they are harder to spot so the first signs will be often be partly consumed remains left on the bank. You must not assume that you’re dealing with a mink – the population of otters is growing and spreading so the chance of encountering one is increasing. Otters are protected under both the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2010. It is an offence to deliberately capture, injure or kill them, to damage, destroy or obstruct their breeding or resting places, or to disturb them in their breeding or resting places.

Dealing with protected species

Removing or controlling protected species is not impossible, but it is complex and carefully controlled, so preventative measures are essential.

Fish refuges have been known to significantly reduce the impact from all the diving predators. These are essentially mesh cages, sometimes with floats attached as markers and anchored to keep them in place. The mesh size is crucial as it must allow the fish though but keep the predators out, so is ineffective for large fish. A maximum mesh size of 85mm is recommended to prevent otters becoming entangled.

Herons like to wade into water and don’t like steep-sided lakes, and vegetation in the water margins makes fish harder to spot. Herons are slow to take off so don’t like heavily vegetated banks where they can’t see potential predators. Mammals can be excluded by perimeter mesh fences, which are particularly effective if supplemented by electric fencing. Scaring cormorants is proven technique, but requires people to be available at first light when cormorants prefer to feed. It can be useful opportunity to accurately record how many birds are visiting the site and how often – useful evidence if applying for a control licence. There are many guides and suggested approaches available on the internet, including “Otters and Stillwater Fisheries”, available from the Natural England website.

Although there is some similarity, when seen side by side otter tracks (on the left) are clearly bigger. If the print can’t be covered by a £2 coin then it’s not a mink track.

Although there is some similarity, when seen side by side otter tracks (on the left) are clearly bigger. If the print can’t be covered by a £2 coin then it’s not a mink track.

Those who are desperate can apply for a special licence to control. It will be limited for use only in certain defined circumstances and usually covers a specific season (for example, September through to Spring), and expires within a year. The licensing page on the Natural England website currently refers largely to birds, but the process is open to mammals as well so don’t be put off. There is not a specific form for fish-eating mammals but the instruction is to complete and submit the current form even though it refers to birds.

The application must meet three fundamental tests:

  • Serious damage is being or is likely to be caused.
  • Non-lethal anti-predation measures have been tried and found to be ineffective or are impracticable.
  • It is reasonable to consider that direct control of fish-eating predators will reduce, or prevent from increasing, the level of damage.

Controlling Mink

One predator you can do something about is the American mink. They are active 24 hours a day, but mostly at dusk and dawn, and will eat almost anything; fish, small mammals, rabbits, ducks, ground nesting birds, eggs and poultry. Mink are widespread with males travelling up to 20km a day in early spring continuously searching for mates. A female can have up to 10 kits, which will be forced out of the home territory from August and are quick to colonise new areas – sometimes up to 100km away.

A simple design – the mink raft is a sandwich of plywood and expanded polystyrene to create a stable and durable floating platform. Note the tunnel securely fixed in place. Holes drilled through the roof of the tunnel and into the raft base can be used to take dowel or metal excluder rods when trapping.

A simple design – the mink raft is a sandwich of plywood and expanded polystyrene to create a stable and durable floating platform. Note the tunnel securely fixed in place. Holes drilled through the roof of the tunnel and into the raft base can be used to take dowel or metal excluder rods when trapping.

A typical territory is 1km to 6km of riverbank and surroundings, throughout which a mink will devastate the wildlife, eating approximately one third of its body weight every day. Studies by the Wildlife Conservation Research Unit (WildCru) at Oxford University have suggested that when otters and mink coexist the large otters are the dominant species and cause a shift in the mink behaviour. Sometimes the mink become more active in daylight to avoid the otters, but in other cases they just switch their diet away from rivers and take more land-based prey. Otters moving in will not necessarily solve a mink problem, and may make it worse.

The clearest signs of mink, apart from their discarded kills, are their tracks. Otters and mink are often mistaken for each other but the otter is about twice the size of a mink. If you find tracks that you think might be mink or otter there is a simple way to check – the £2 coin test. A mink print can be covered by a £2 coin, the print of an otter cannot. Droppings are also very different; Mink scat is long and thin, usually pointed at the end, contains traces of fur and has a strong unpleasant smell. Otter spraints are rounder, and usually just contain fish scales and small bones.

On a busy river bank clear signs can be hard to see and this is where a fantastic idea from the Game and Wildlife Conservation Trust comes in. A mink raft is a floating platform specifically designed to record mink activity and sometimes to deploy a trap. A sandwich of plywood and expanded polystyrene creates a floating platform that can be secured a little way from the bank. A tray of clay fits into the centre of the raft, sticking through to the water below so that the clay keeps moist. A tunnel placed over the trap completes the raft. This provides the perfect and controlled micro-environment to attract and record mink or otter presence. Mink are inquisitive and attracted to anything a bit unusual, and to tunnels in particular. Once you’re sure that you’re dealing with mink the same tunnel can be used to house a suitable trap. More on mink rafts and plans for making one are available from the Game and Wildlife Conservation Trust website.

The mink raft must be secured to prevent it drifting away, but allowing enough slack for rising or falling water levels. A covering of vegetation helps to naturalise the raft.

The mink raft must be secured to prevent it drifting away, but allowing enough slack for rising or falling water levels. A covering of vegetation helps to naturalise the raft.

When trapping on the riverbank, areas of shallow water and the ‘beaches’ formed at river bends are favourite spots. You can use a self-contained trap life the WCS tube trap, a cage trap, or build a tunnel. In either case you should position it with the entrance facing the water, secure it well, and cover with foliage or stones, leaving just the opening visible.

Approved spring traps in natural tunnels are very effective and these can be dug into the bank if none have occurred naturally. You can maintain permanent artificial tunnels by digging a short length of wide plastic pipe horizontally into the bank. In no time at all this will naturalise and make a prefect trapping location when required, but don’t forget to apply suitable stops to exclude non-target species.

Bait is not always necessary, but when it is used keep it fresh. Strong smelling fish or meat works well, as do the mink gland lures that can be bought from the USA. Don’t rely entirely on smell; a visual attractor such as a few small feathers around the entrance to the tunnel will help get your set noticed.

It might not be traditional vermin control but if something is already eating your fish there are still a few things that you can do. If nothing is eating them yet make the most of your chance to prevent it.

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