Otter Behaviour: What We Saw Filming Otters at Night on Mull

Otters are meant to be hard to find. So when Dan Harris set himself the challenge of filming one at night on the Isle of Mull, spending 24 hours following them, the first surprise was that his day began with an otter hunting in the shallows right beside where he'd woken up.

By the end of a long night in a packraft on a still loch, he'd found something better: a mother and two cubs learning to hunt in the dark. You can watch the full episode here, and this is what that footage shows about how otters actually behave.

An otter at breakfast

The otters on Mull are Eurasian otters (Lutra lutra), the same species found on British rivers, rather than sea otters, which live only in the North Pacific. In most of the UK, otters are shy and mainly active after dark. River and loch otters tend to be crepuscular or nocturnal, meaning most active around dusk and dawn or through the night. On Scotland's west coast, the rules look different. Coastal otters feed almost entirely in the sea and are mainly active during the day, which is why Dan could watch one hunting at first light, diving between the kelp for small crabs and fish and surfacing to eat them.

It comes down to food. Inshore waters are so rich in fish and crustaceans that coastal otters don't need to roam far, with home ranges of around 4 to 5km of coastline, compared with 20km or more for river otters. In other words, otter behaviour depends as much on where they live as on the time of day. An otter needs to eat around 1 to 1.5kg of prey a day, and on this coast, there's plenty of it.

Why the coast is so good for otters

To see what the otters were feeding on, Dan went snorkelling in the loch. He didn't see an otter underwater, but what he did find explained a lot: thousands of sand eels in the shallows, spiny starfish and green shore crabs scattered across the seabed, and moon, compass and barrel jellyfish drifting through the open water. The sheer amount of food is a big part of why this stretch of coast supports so many otters. Because otters need clean water and plenty of prey, their presence is often taken as a sign of a healthy waterway. Pollution and habitat loss are among the threats the species still faces, which is why clean water and conservation efforts matter.

A detour: the red deer calf in the fence

Back on the road, an otter dashed across in front of the car. Dan stopped and scanned the roadside with a thermal imaging camera to see where it had gone, and found something else entirely: a red deer calf with a hind leg badly tangled in a wire fence.

He left the camera recording, put a towel over the calf's head to keep it calm, and cut through the wire. The calf bolted back to its mother on the other side of the fence. It's a good reminder of how much thermal imaging picks up that the eye misses, and of how dangerous roads and fences can be for wildlife. Road traffic accidents are the single biggest source of otter deaths in Scotland, other than natural causes.

If you find an animal caught in a fence, it's usually safest to call for help rather than step in yourself. In Scotland, the Scottish SPCA helpline is 03000 999 999.

Night on the loch

For the night shoot, Dan took a packraft out onto the loch. The main rule was to stay as quiet as possible. It was a calm night, which is good for visibility but means sound carries a long way over still water, and otters have far better hearing and sense of smell than we do.

The first few hours turned up almost nothing. At one point a head appeared in the water, but it belonged to a grey seal, one of several hauled out on nearby rocks. It was pitch black, and without infrared and thermal cameras Dan couldn't see a thing.

Then, at 4am, a heat signature appeared on the shore. First it looked like a single otter. Then it was three: a mother and two cubs.

What the footage shows about otter behaviour

The cubs were young. Their fur had only recently become waterproof, which happens at around two to three months, and the family was entering the water for what looked like the first time. Mother otters teach their cubs to swim and then to hunt, and here she was leading them along the shore as they learned to find crabs and fish in the shallows.

Otter cubs usually stay with their mother for around a year, though sources give a range of roughly 8 to 15 months. Dan's own figure from the day was 12 to 13 months. Coastal cubs have a lot to learn, from hunting in salt water to looking after their coat.

That last part matters more than it sounds. Otters have no blubber, so they depend entirely on their fur to stay warm. Salt left on the fur interferes with the oils that help it repel water, which is why coastal otters need regular access to fresh water to rinse it out. As Dan explained on the shoot, otters on Mull travel inland to wash in fresh water. Without that, the fur would lose its insulation and the otter could quickly get dangerously cold.

Even in complete darkness, the family could forage along the shore, running along the water's edge and hunting by the senses otters rely on: sharp hearing, a strong sense of smell, and sensitive whiskers.

Otter behaviour at a glance

What happened on Mull fits a wider pattern in how Eurasian otters live:

• Activity: river and loch otters are mainly nocturnal, while coastal otters are mainly active by day. The time of day an otter feeds depends on its habitat and food.
• Territorial behaviour: otters are solitary rather than living in social groups, and use scent marking, mainly droppings called spraints left in prominent places, to defend their range and communicate.
• Feeding: otters hunt in the water, and as Dan saw on Mull, often surface to eat what they catch. They need around 1 to 1.5kg of prey a day.
• Fur care: they have no blubber, so grooming and regular fresh water matter, especially on the coast.
• Family life: a female raises her cubs alone, teaching them to swim and then to hunt, over around a year.
• Communication and play: otters are mostly quiet while hunting and feeding. Studies of captive Eurasian otters found that mothers and cubs keep in touch with a high-pitched whistle that carries over long distances, and otters also play, including sliding down banks into the water.
• Senses: sharp hearing, a strong sense of smell, sensitive whiskers, and the ability to close their ears and nose underwater.

Watching otters without disturbing them

The biggest lesson from the shoot is restraint. Keep your distance, stay quiet, and let the otters carry on with what they're doing. Never approach cubs, and stay well away from holts.

The law backs this up. Otters are a European protected species, protected in Scotland under the Conservation (Natural Habitats, &c.) Regulations 1994 and across the UK under the Wildlife and Countryside Act 1981. It's an offence to deliberately or recklessly disturb one in a holt or other place of shelter. Shelters are protected whether or not an otter is in them. In Scotland, NatureScot advises that you may need a licence to fit camera traps near holts, or to photograph otters at or near a holt or while they're rearing young.

If you do see otters on Mull, the Mull Otter Group welcomes sighting reports, and says it never shares information on where otters are located. That's worth following when you share your own sightings too. If you come across an otter cub calling for its mother, watch from a distance, and if she hasn't returned within an hour or two, or before dark, the Scottish SPCA helpline (03000 999 999) can advise.

The kit behind the footage

The nighttime footage was filmed using the Nightfox Whisker, Nova and the Thermal Master DV2, which let Dan watch and film the otters without any visible light. If you want the fuller species detail, including diet, breeding and legal protection, our key facts about otters piece covers the rest. The same series has taken Dan to a badger sett and a starling roost, and each has its own behind-the-footage write-up.

Next in the series: Dan heads out after dark in search of whales, using a thermal imaging camera to scan the Scottish lochs.

FAQs

Are otters nocturnal? It depends where they live. River and loch otters are mainly nocturnal and most active at dusk and dawn, while coastal otters in Scotland are mainly active during the day, as the footage from Mull shows.

How long do otter cubs stay with their mother? Usually around a year, though sources give a range of roughly 8 to 15 months. During that time the mother teaches them to swim and hunt.

Are otters territorial? Yes. Eurasian otters are solitary and defend a territory, using scent marking, mainly droppings called spraints, to mark their range and communicate. They only tend to come together to mate, apart from a mother with her cubs.

Why do coastal otters need fresh water? Otters have no blubber and rely on their fur for insulation. Salt left on the fur interferes with the oils that keep it waterproof, so coastal otters need regular access to fresh water to rinse it out.

Is it legal to film otters? Watching and filming otters from a distance is fine, but otters are a European protected species. It's an offence to deliberately or recklessly disturb one in a holt, and in Scotland you may need a licence to photograph otters at or near a holt, or to fit camera traps near one.

What should I do if I find an otter cub on its own? Watch from a distance. If the mother doesn't return within an hour or two, or before dark, the Scottish SPCA helpline on 03000 999 999 can advise.