Is that an otter?

No, it's a rock. Is that an otter? No, it's a fish jumping. Hmmn. We - veteran wildlife chronicler Laurie Campbell and myself - are seated above a weir on the River Whiteadder, Berwickshire, and scanning the water for the slightest sign of one of Scotland's most beloved creatures. Frankly, it is the wrong time of day - otters typically come out at dawn and dusk, and it is now late morning - but there are worse things to do than sit here in the sun on the off-chance. Anyway, there is no such thing as wasted time when you are observing nature, as far as Campbell is concerned.

"Look, see the kingfisher?" he says suddenly, pointing into the shadows below an overhanging elder. Alas no; it eludes my gaze. This does not bode well: if I can't see a bird dressed like a Rio carnival reveller, then otter-spotting is probably a lost cause. But not to worry - there are many other things to see. Our attention is soon captured by a stunner of a damsel fly, which Campbell immediately identifies as a banded demoiselle, a great big flamboyant thing with a shimmering iridescent blue body. Campbell is excited because this species is rare in Scotland; this is the first time he has seen one in Berwickshire. Our otter-spotting trip is turning out like many of Campbell's expeditions - he goes out looking for one thing and ends spotting something else entirely.

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Our hope of seeing otters is not without foundation since they have been spotted on the Tweed at this time of day, which Campbell believes reflects their growing confidence that humans will not hurt them. This area has been a good otter-spotting location for Campbell in the past, but usually he goes to much greater lengths than this. Today, we are sitting out like picnickers on the grass; other times he has come in the very early morning and spent hours hiding in the base of a dense hawthorn tree, wearing camouflage gloves and a face net. This place is good because salmon migrating up river get stuck in the pool at the bottom and it becomes a "killing ground" for otters; he has seen some dramatic life and death battles here.

"Once I saw an otter struggling with a salmon in front of the hawthorn bush," he says. "It must have been the best part of a 20lb salmon. The river was very full - they rolled and rolled around in it and the water brought them right in front of me, but eventually the otter lost its hold and the salmon got away." Otters typically drag big prey out of the river to eat it, but they only consume the part of the salmon around the gills, leaving the rest for scavengers like foxes. Campbell once found a fresh salmon halfway across a field, so took it home for his tea. He got 9lbs of flesh from it.

Campbell, 56, has caught otters on film scores of times over more than 20 years and has built up an incomparable archive on the Scottish otter as it returns to rivers and lochs. That enchanting photographic history now features in a ravishing new book, Otters: A Return To The River, a collaboration between Campbell and writer Anna Levin, published on Thursday. It is a celebration of the resurgence of Scottish otters following their dramatic decline in the mid-20th century and shows how far they have come since the filmmaker Hugh Miles produced a BBC film on wild otters in the early 1980s (an accompanying book, Track Of The Wild Otters, sold more than 100,000 copies). Dubbed the "impossible task" because of the animal's rarity, Miles shot exclusively in Shetland; otters on the Tweed were unheard of then.

The story of the otter's decline is a cautionary tale about the risks of unfettered exploitation and thoughtless development. Otter hunting with hounds, for their pelts, sport and to protect fish stocks, was one factor in their dwindling numbers but the main cause was pollution. Organo-chlorines were introduced as pesticides in the 1950s and soon got into the food chain, poisoning some animals outright and interfering with physiological functions in others. As apex predators, otters were getting a huge dose. Add rising road traffic deaths and drainage of key habitats, and by the late 1970s, otters were hardly to be found in Britain, with only Scotland's west coast and islands still a stronghold.

Then efforts to clean up waterways and improve fish stocks, protect otter habitats, build otter highways in culverts beneath busy roads and ban otter hunting, slowly started to produce results. Otters are now present once more in every county in England and all across Scotland; they have been spotted in the heart of Edinburgh on the Water of Leith and in Glasgow on the Kelvin, the White Cart and the Clyde.

It is hard to know how many there are and they are certainly not free of threats - road deaths are a worry and chemicals in waterways could be affecting their reproductive health - but perhaps as many as 8000 otters now live in Scotland, a healthy number, though not as high as it might be. With their grace and cat-like beauty, they are the darling of British wildlife.

Books and film have of course played their part, particularly Tarka The Otter (1927), set in Devon, and the million-selling Ring Of Bright Water (1960), by Gavin Maxwell, whose birth centenary is in July. It charts the true story of an otter Maxwell brought from Iraq to the west coast of Scotland, and was made into a film with Virginia McKenna and Bill Travers.

Otters are mustelids, part of the weasel family, along with pine martens, stoats, polecats and badgers, and eat a varied diet of fish but also frogs, insects and, on an opportunist basis, birds and small mammals. Campbell explains how they have developed clever, if gruesome, ways of enjoying a particular delicacy, toads' legs. Species like the common toad have poisonous skin and are therefore avoided by many predators, but the otter "peels" their hind legs, leaving the skin behind.

The hopeful otter spotter needs to know how to identify clues to the animal's presence. These include muddy slides on riverbanks, flattened areas of grass near water which they roll on to dry their fur, and otter spraints (faeces) which in coastal areas are often full of little bones from crustaceans, while inland have a darker tarry appearance. They are typically located on large rocks and tree stumps to mark the otter's territorial boundaries.

In the early 1980s, Campbell became Scotland's first full-time professional wildlife photographer and he has won 26 awards in the BBC Wildlife Photographer Of The Year competition, but he has never felt the need to stray beyond Scotland, and finds rich pickings right on his doorstep in the Borders. He was brought up in Berwick and has been exploring the nearby countryside all his life.

"I'm one of those people who doesn't like change very much; I like a sense of belonging," he says. Today, the pools of great crested newts he used to visit as a boy are gone, but otters are a welcome new neighbour. "When they started coming back, that was ground-breaking. These were rivers I had known all my life." His delight in their presence is palpable.

In the Borders, the rivers are exceptionally clean and well stocked with fish. The Whiteadder (pronounced "Whit-uddr") meanders through agricultural land and small woods, forming a wildlife corridor that provides a focus for Campbell's work - not that otters only ever stay next to water. The animals can often be seen quite a distance from rivers and sea. Their holts are frequently situated on hillsides in dense undergrowth (Campbell has watched them going to and fro between den and water) and they are also perfectly capable of striking out across country. To cross from one river to another, rather than follow the water to the point of confluence, they will head off through fields, woods and, more dangerously, over roads, to take advantage of the most direct route (Campbell realised this during two recent cold winters when he followed their tracks in the snow).

Campbell often bivouacs outside to catch early morning images of otters (he was out until 3am the night before we meet, though that was to watch badgers) and has seen a range of behaviour, including juveniles playing, cubs calling for their mothers, and a rare sighting of a suckling mother drifting downstream on her back, revealing her swollen abdomen.

It would have made our day to see a female otter with her cubs or a dog otter wrestling a salmon. But I do not really expect to see the animal - and then I do. It crosses the A1 just a few metres in front of my car near Grantshouse, north of Berwick, with traffic coming fast in both directions. It is so quick dashing across the two carriageways that I cannot see its face, but there is no mistaking its size and gait. The sight is both uplifting and heart-wrenching, its road crossing strategy seeming to involve bolting headlong on to the tarmac. No wonder so many otter sightings are of carcasses heaped on verges.

It is a reminder of the precariousness of this animal's existence. As well as the threat otters face from traffic, there are other dangers looming. While many anglers and ghillies delight in seeing them, there have, remarkably, been calls for culls from other sections of the angling community. Campbell worries they are still not up to strength and will not necessarily become a permanent fixture unless continuing threats towards them are properly managed.

There are still strong grounds for optimism, however. The otter remains greatly loved and fiercely protected, and perhaps its elusiveness is one of the reasons why. But the work of Laurie Campbell displays the otters of Scotland's rivers as they have never been seen before. n

Otters: A Return To The River is published by Birlinn, priced £14.99, on Thursday. It is also available as an e-book.