Another Life: Surveying territorial toilet stations for signs of our otters

Two big spring tides, one higher than the other, have left separate skirts of seaweed at the bottom of the boreen. Safely beyond this slippery alliteration, I mount a grassy bank beside a deep pool, a brackish mix of stream and sea at the ragged hem of the strand. One tuft of grass is bigger and greener than all the rest and topped, as I hoped, with the otter’s dropping that we have learned to call spraint. A little blurred by last night’s rain, it is dark, prickly with well-chewed fish bones, and smells pleasantly enough of musk.

Otters have used this territorial toilet station for the 30-odd years since I discovered it, and surely many generations before, the fishy droppings (sticklebacks, eels, the odd trout or flounder) nourishing the grass with extra nitrogen. I have seen the animals, thrillingly, from time to time, suspended in a green wave or humping across the strand to the dunes – one, memorably, being chased by a raven for its catch. Their tracks in the sand are delicate and unmistakable, each pawprint spread lopsidedly like the petals of a flower. But now that I don’t go roaming so far or so often, the regular droppings on this one mound of grass have to be assurance enough.

I first learned about otters, their spraint and tracks from a young English couple who knocked at the door in the spring of 1981. They were the Chapmans, Peter and Linda, assiduous field workers for the UK's philanthropic Vincent Wildlife Trust. They had spent 15 months camping in a Dormobile beside the rivers and estuaries of Ireland, mapping otter signs at some 2,400 sites spaced out along the waterways (all this, as they ruefully confided, without actually seeing an otter).

They were bravely crawling about under bridges even in the troubled North, checking for spraint on boulders or tracks in the mud. And in this pioneering survey of the island's Lutra lutra they found them at 90 per cent of the places where they should be, from the Liffey in Dublin suburbs to the heights of the Kerry mountains. This made Ireland the otters' most secure refuge in Europe. The Chapmans' survey became the baseline for the welfare of our otter population. But when the National Parks and Wildlife Service made follow-up surveys this past decade the field signs, or their absence, at some 850 sites in the Republic suggested a dramatic decline: first to 70 per cent and then to 63 per cent by 2012.

There had been 75 service staff in the field, mostly county wildlife wardens, using "standard otter survey" methods well tried in Europe for more than 30 years. Yet the national results were completely out of line with those from Northern Ireland: those showed a healthy increase in otter numbers over the same period. Was there some big flaw in the methods or modelling on either side of the Border? A critical review was commissioned from Quercus, the ecological research centre at Queen's University Belfast.

Its report has painted quite a different picture. Far from declining, the National Parks and Wildlife Service can now say happily, otters should be shown as present at up to 93 per cent of suitable sites right across the country and even on offshore islands. Their “poor” conservation status, as declared to the European Commission, can now be changed to “favourable” or “good”, with an updated population of 15,000-16,000 adult otters, one of the largest in Europe.

What went wrong in the Republic has warranted a paper in the European Journal of Wildlife Research, entitled Detecting Detectability: Identifying and Correcting Bias in Binary Wildlife Surveys Demonstrates Their Potential Impact on Conservation Assessments. Analysing otter incidence at 1,229 sites across Ireland, it challenges use of the "standard otter survey" as vulnerable to "bias and error".

The Northern survey was carried out by a single team including an experienced expert. The Republic’s survey – covering, of course, a vastly larger area – used multiple teams of uneven training and persistence. (The paper urges testing individuals with the United States’ CyberTracker Certification.) Other important variables were the number of bridges in a surveyed stretch of river (otters often use their footings as sprainting stations) and whether high recent rainfall and swollen rivers could have swept the signs away.

How this sorts into statistics I leave to science, enjoying, instead, the work of Michael Longley, whose poems about this landscape count each otter with affection. He braved this month's spring tides to record new poems for a BBC programme in a cottage at the far side of the duach. In one he describes his two-year-old granddaughter watching her first otter in the lake. The pleasure was half expected by her mother "Because, when you were expecting her, / You last watched an otter from this spot, / Your body a holt for otter and child."

Look soon for his fine new collection, The Stairwell, from Jonathan Cape.