Otters: Scientists on trail of the elusive and charismatic mammal

Water analysis and a new technique called eDNA will help us work out how many otters live in Ireland

Once, as a young zoology student, I spent many winter days – often stretching into the gloaming hours – searching for otters in Mayo’s Burrishoole catchment. Beneath the shadow of the Nephin Beg mountain range, I walked the liminal spaces where land and water meet, around the peripheries of freshwater Lough Feeagh and brackish Lough Furnace and along the hill streams that pour into them.

I quickly worked out where they were; their cigar-shaped droppings, deposited at junctions between water and field, confirmed their presence. But after four months, the only otter I saw was cold and dead, a roadkill victim. For a mammal so charismatic and large (an Irish name for them is “madra uisce”, the water dog), otters somehow find a way to keep away.

Their crepuscular habits may explain why they seem elusive, but they leave a trail of tell-tale signs of their comings and goings. Deep imprints of their large webbed feet, with five teardrop-shaped toes, may remain in the mud after they slide into the water. If you’re lucky, you might hear their distinctive calls – a high whistling and a sharp bark-like sound – before they disappear out of sight.

But it’s their tarry droppings that scientists have historically relied on to build a picture of their population. These deposits – known as spraints, after the French word to “squeeze out” – have an unforgettable “ottery” scent which some have described as the smell you’d get from mushing a fish into a bowl of lavender. For me, it’s like fresh hay sprinkled with jasmine tea.


Otters will spraint at vantage points near water. In Burrishoole all those years ago, I found spraints on grassy tidal islets, the mouths of streams, prominent rocks and small peaty mounds along the water’s edge. I remember one soft grassy tussock beside Burrishoole Bridge where the river cuts a deep channel, and the dense waters of Lough Furnace are flowing south through the seven-arched stone bridge towards Clew Bay and into the Atlantic.

Years later, I’d check that mound and find spraints each time. It’s no surprise; salmon, eels and other migratory fish have to navigate these narrow waters and, with hungry otters around (they need to eat about 15 per cent of their body weight every day), it’s a sniper’s alley for fish. For the lucky otter, it’s a feast.

Where a spraint is found, you know an otter was around. But these faecal deposits, full of skeletal remains of their seasonal diets, tell a deeper story. In Burrishoole, I discovered the bones of eels, salmon, stickleback, sea scorpions, water beetles and butterfish, as well as crab claws in a few samples. Otters produce a jelly-like goo from their intestine, which they secrete before they spraint; it eases the passage of the multiple, sharp bones.

Researchers can extract DNA from spraints to find out the sex of the otter, whether individuals are related, and what other, non-boney prey they have eaten. Dr Sam Browett, a researcher at South East Technological University in Waterford, studies otters around Mayo’s internationally important limestone marl lake, Lough Carra. He wants to know each individual otter in the area and determine what they’re eating. The high-fat content from fish oils in the spraints can frustrate the DNA extraction process, he says, but the fresher the sample, the better.

Although they hunt rats, birds, woodmice and rabbits – and, curiously enough, eat blackberries – otters have always seemed to me ill at ease on land. They lollop along a rocky coastline, beach or field, hurriedly going about their business, but theirs is a watery world. Once they slip into a stream or lake, leaving a V-shape ripple in their wake, they transform into aquatic athletes which can stay underwater for four minutes and easily outpace salmon or trout.

How many otters are in Ireland? We don’t know. The last survey, from 2011, gave a “favourable” assessment of the population, but since that time our landscape has been transformed and reshaped by extensive infrastructure building, flood-relief works and dairy intensification. They may be able to withstand this change; we just don’t know.

And so, since we’re legally obliged under EU law to find out how otters are fairing, it’s encouraging that the National Parks and Wildlife Service (NPWS) announced a new national otter survey last month.

Scientists will rely on something other than field signs, such as spraints, to answer this question. For the first time, they will literally test the waters for otters, using a new genetic technique that relies on a rather unsavoury fact: our rivers, lakes and streams are full of otters’ sloughed skin cells, faeces, urine, saliva, blood and hairs. These contain valuable DNA – think of water as a kind of census of biodiversity information – which scientists can then test to confirm their presence.

This method will be led by Dr Allan McDevitt, a freshwater and marine biologist at the Atlantic Technological University in Galway, who has successfully used it to survey otters in England. He will test 16 rivers in Galway and Waterford, and the process is simple: a couple of litres of river water are scooped up and put through a fine filter to capture the bits of bodily material. The DNA is extracted and scientists use an otter-specific test to identify otter DNA.

It’s the first time this technique, known as “eDNA”, has been used on otters in Ireland; if it proves successful, it may revolutionise how scientists rapidly monitor and assess species.

Meanwhile, if you spot an otter or the traces of one, let the NPWS know; they want your help.