The Otter limits

THE sea piles in from the north-west in minty, iceberg colours; throws itself down in sullen breakers, seethes up the empty strand in foam that looks cold enough to burn. The otters' tracks, woven in and out of the edge of the surf and looping up the strand to the dunes, seem, by contrast, posi tively playful.

They set puzzles for me. I try to play Indian scout, weighing the meaning of sudden twists and turns and scuffle-marks (she had a fish, a big one, wriggling, swiping the sand with its tail: but the loops and zig-zags - was a raven harassing her?).

Sometimes I have followed the morning's fresh spoor from the tideline up into the frosty shadows of the dunes, where it becomes tangled in a web of older tracks among the dense clumps of marram. Here, on the crest of the dunes, the otters eat and rest; now and then, I have surprised one and sent it rippling headlong, out across the sand to the sea.

I have thought a lot about the otters, meeting them just often and intimately enough to add a presence to their daily tracks and signs. But I have known frustratingly few facts. So much in books has concerned the otters of inland lakes and rivers - nocturnal animals leading very different lives to those of my seagoing, diurnal familiars. Now I have some facts - a lot of them- and a fresh perspective of animals so special to our wildlife.

Hans Kruuk is a highly-regarded, mammalogist who works with the Institute of Terrestrial Ecology in Scotland. His research on badger clans and territories revealed much about the way those animals organise their lives. Now he has spent five years, with a team to help him, wa\ching otters in Scotland - mainly on the coast of Shetland, where they are still abundant. He kept dossiers on almost 60 individual otters and fitted 19 of them with internal radio transmitters - the ultimate in high-tech surveillance.

His book, Wild Otters: Predation and Populations is first of all a work of science. If you want a formula to calculate the cost in energy to an otter of catching a fish, for a given time in water of a certain temperature, it is: E=l2.l-0.40Tw. On the other hand, if you want to know why an otter doesn't have blubber to keep it warm when diving, it is because that would make it too buoyant and cost it too much energy to dive (seals, which do have blubber dive with very little air in their lungs).

The otter has only its pelt to keep it warm when it plunges - often two or three metres deep in seawater colder than 10 C - to catch a fish on the sea-bottom. Its underfur is so dense that the skin is quite invisible, but the real insulation is in the amount of air it traps. And this, in turn, is what restricts the otter to certain kinds of shoreline - it must have regular baths in fresh water to wash the salt out of its fur and restore its fluffiness.

The most regular otter tracks on my strand take the shortest route between the sea and a little stream that cuts a deep trench through the dunes. Beyond it, a little lake with a reedbed offers the otters a thorough, rinsing swim: I have seen them at it, without knowing their purpose. And well-beaten otter-paths fan out further, to two larger lakes, and a river-channel, where a bath can be combined with fishing for trout, eels and flounder.

Hans Kruuk's Shetland otters have a less generous habitat: they make do with freshwater pools among rocks, and in bog-holes in peat behind the shore. They even make sure there is groundwater trapped in the "basement" of their bolts - a built-in bath that never freezes, even when the islands are blanketed with snow, in Arctic temperatures, as they were this Chnstmas.

Whereas freshwater otters usually lie up above ground say in reed-beds - because their pelts dry better, the Shetland otters spend the nights in their holts. These are big, elaborate excavations, like badger setts, with tunnels Lip to, 20 metres long and chambers lined with seaweed or plastic fertiliser bags for bedding (the otters tuck them under their chin and walk in backwards like a badger).

I now realise that some over-large rabbit holes in the very thinly populated warrens of the dunes at Thallabawn may be the portals of otter headquarters. Alternatively, the females may rear their cubs in holts on small, inaccessible islands a couple of kilometres offshore and use mainland rabbit holes as temporary "hovers". There is much to be discovered, now that I know what to look for.

It took Hans Kruuk and his helpers many months to elucidate the bay in which otters spacer themselves out along the coast - females and their cubs in "home ranges" in the bays, the males roaming more widely and erratically. There is a peak of births in June, a long period of teaching the cubs to fish, and then a final family break-up the following April.

IT is the inherent vulnerability of the otters' existence that Hans Kruuk is at pains to point up - "life at the edge of a precipice" as he puts it. Here they are, strung out thinly along a linear habitat, travelling huge distances daily, back and forth, to get their food. They can't dive too deep, or too far out, yet their catching is so costly in energy that they have to catch a lot in a very short time.

They specialise in bottom, dwelling fish that don't move too fast, but these tend to be thin and slippery and difficult to find among stone's and seaweed (butterfish), rockling and sea scorpions are some of the smaller species). The fish are least active and thus most available - at low tide in the daytime, which explains why I meet otters, apparently so rashly, fishing the surf at high noon.

Dr Kruuk takes a couple of final chapters to consider the modern thr9ats to, otters, among them toxic chemicals such as PCBs and mercury. He found some individual otters apparently fit and well and reproducing with high burdens of PCBs. Mercury may be more ominous, since it accumulates with age.

Shetland's oil terminal at Sullom Voe seems, if anything, to be prime otter habitat: the animals sleep right inside the pump houses. And the wrecking of the giant oil tanker Braer seems to have had little or no impact on the otter population. How benevolent, all the same, my lonely strand can, seem, with its pristine sea and a sky so far innocent of blizzards.