Otter, Mink or Water Vole? Identifying Riverbank Mammals from Tracks and Signs

The animal is usually the last thing you find. A riverbank at dawn gives you a ripple and a vanishing back; a reedbed gives you a rustle and nothing more. But otters, mink and water voles leave a great deal behind them — droppings on a boulder, a nibbled stem, a hole in the bank, a webbed print in silt. Learn half a dozen signs and you can name a riverbank resident without a single clear sighting. That matters, because one of these three is a native species still clawing its way back, one is a native species in serious trouble, and one is an invasive predator that should not be here at all.

Spraints, scats and the smell test

Start with droppings, because they are the most conspicuous thing a riverbank mammal leaves and the fastest route to an identification. Otter droppings are called spraints, and otters deposit them deliberately, in public. Look on flat-topped boulders in midstream, ledges under bridges, grass tussocks on the bank, and the tops of old holts. They return to the same spots repeatedly, so you often find a latrine — a scatter of droppings at different ages, some fresh and black and tarry, others faded grey and crumbling.

Fresh otter spraint smells far better than it has any right to: sweetish, faintly fishy, often described as resembling jasmine tea or damp hay. Break one open with a stick and you will usually find fish bones and scales, sometimes fragments of crayfish shell. Mink scats look superficially similar — dark, twisted, roughly the same finger-length — but the smell is the giveaway. Acrid, musky, frankly unpleasant. The contents lean towards fur, feathers and small mammal bones rather than fish.

Placement helps too. Mink drop scats more casually: on a stone mid-channel, at a den entrance, along a path, sometimes in small clusters but rarely in the big, faithfully reused latrines otters maintain.

One common false positive: fox scat. It is twisted with tapered, pointed ends, usually sits on a path or a prominent tussock, and contains mammal hair, fruit pips and beetle cases. If it tapers to a point at both ends, it is probably not an otter.

Footprints: size, webbing and gait

Soft silt, wet mud and thin snow after rain are your best surfaces. Look along the water's edge and on any exposed mudbank between the water and the vegetation.

• Otter: large — roughly 6–9 cm across — with five toes and clear webbing between them, plus a broad heel pad. Otters bound when travelling, so the prints arrive in loose groups rather than an even line, and a tail drag is sometimes visible in wet mud or snow.
• Mink: noticeably smaller, roughly 3–5 cm, with five toes and much less obvious webbing. The toes splay widely.
• Water vole: small, around 2–3 cm, and star-shaped because of the prominent pads under the toes. There are four toes on the front foot and five on the hind. Clear prints are rare, so treat them as a bonus rather than a primary clue.

Photograph any print with something for scale — a coin or a pen — and check the width before you check the length. Width separates otter from mink more reliably than anything else.

Holes in the bank: read the surroundings, not just the hole

Every burrow in a riverbank looks like every other burrow until you look at what is around it. An otter holt is a substantial entrance, often dinner-plate sized or wider, usually tucked under the root plate of a large tree, beneath a fallen log or among boulders. There may be a worn slide or a muddy path running down to the water, and the entrance is often deliberately concealed.

Water vole burrows are much smaller — roughly 4–8 cm across — and typically appear in clusters low in the bank, at or just above the waterline, often under overhanging vegetation. The tell is the tidiness: closely cropped grass around the entrances, a flattened runway running along the bank, and neat piles of cut stems nearby.

Brown rat burrows can look similar to water vole holes, but they tend to be bigger, messier, and surrounded by scattered debris rather than a grazed lawn. Rat droppings are tapered and pointed; water vole droppings are blunt and barrel-shaped. Mink dens are opportunistic — under roots, in drains, in log piles, sometimes in old rabbit burrows — and often sit in more disturbed ground than either of the others.

Water voles: follow the food

If you only learn one water vole sign, learn the feeding station. Water voles cut stems and leaves and leave them in small neat piles where they feed. The stems are typically bitten off at a distinctive 45-degree angle — the difference between a pencil sharpened with a knife and one sharpened in a sharpener. Lengths are usually just a few centimetres.

Alongside that, look for:

• Runways: flattened tunnels pushed through bank vegetation at the water's edge.
• Grazed lawns: short, cropped grass around burrow entrances.
• Droppings: blunt-ended, cylindrical, greenish-brown when fresh, tic-tac shaped, left in small piles on runways and stones.

Water voles have declined severely across Britain, so a record of feeding stations or burrows is genuinely useful. Send it to your local biological records centre rather than keeping it to yourself.

A short field routine

1. Stand back from the water and scan for raised objects — boulders, bridge ledges, tussocks — before you move.
2. Check those objects for droppings. Note how many and how fresh.
3. Smell a fresh one from a distance, or lift a fragment on a stick. Sweet points to otter; acrid points to mink.
4. Examine the ground around any latrine for prints, and measure the width.
5. Walk the bank slowly, watching the waterline for burrows, runways and cut stems.
6. Photograph anything you are unsure of, with a scale in the frame.
7. Leave it as you found it. Do not open up a holt, dig at a burrow or clear vegetation to get a better look.

Timing, fieldcraft and a word on the law

Signs are easiest to read after rain, when mud and silt hold detail, and in late winter and early spring when bankside vegetation has died back and the waterline is bare. Early morning is quieter, but mid-afternoon works perfectly well for tracks. Keep dogs out of the water at known holt and burrow sites, and stay off the immediate bank edge — collapses are easy and the damage lasts.

Both otters and water voles have legal protection across the UK, though the detail differs between nations, and disturbing an otter in its place of shelter or damaging a water vole burrow can be an offence. Looking and recording is fine. If you want to do anything beyond that — surveying, trapping, bank works, even photographing with artificial light at a holt — check with the relevant conservation body first: Natural England, Natural Resources Wales, NatureScot or DAERA in Northern Ireland.

And if you find mink signs, record them. Mink are not protected, they take water voles and ground-nesting birds, and records help the people working to keep their numbers down. A scat on a stone is a small thing, but it is evidence, and evidence is what conservation runs on.

Photo: Vinícius Vieira ft / Pexels