There is no greater contradiction than the words “marine mammal”. How can a mammal - like us - be marine - like a fish? How do they do it? What drives their being? And why are they dying? The last question is the one that matters, at least if you happen to be a common seal, whose population is disappearing at an unprecedented rate.

There are two species of seal that you can see in British waters; 20-odd species worldwide. Of the two Brits, the Atlantic grey is bigger, chunkier and more bloated. The not-so-common or harbour is smaller and slimmer, with a prettier, more doggy face and a stepped muzzle.

The two species look different from the air, too. When they haul out of the water, the greys like to line up in loving proximity, shoulder to shoulder to shoulder, while the commons, frequently on the same beach or sandbanks, have a much stronger concept of personal space. Look down, and you will often see a solid phalanx of greys along the waterfront, and behind them the commons making complicated patterns and angles with each other.

In recent times, well-documented epidemics have affected seal populations. One in 1988 reduced seal numbers in the Wash by 50 per cent. A second, 13 years later, reduced numbers by 25 per cent; it caused respiratory failure and killed 90 per cent of infected seals, including those rescued and treated. It affected common seals; the greys were carriers. But that's not the problem today.

What is happening to the not-so-common seals, then? How can we possibly find out, and who will do the finding out? These questions brought me to St Andrews in Scotland, where, facing the heaving North Sea, stands the sea mammal research unit (SMRU) at the Gatty Marine Laboratory, part of the University of St Andrews.

Here is a great meeting of biology brains, each contributing different expertise to this intractable problem. What they are concerned with is less dramatic than the horrors of epidemic, but potentially calamitous. The population graph doesn't show a step: instead, it shows a decline that is both gradual and inexorable. It has been taking place since 1999.

Dave Thompson, a seal biologist at the lab, is involved with aerial surveys and counting. The figures are complex and confusing, but their summing-up is not: a 50 per cent decline in common seals in most of the Scottish population, with a similar but less drastic decline in England.

We are losing common seals at a great rate. The complex process of trying to stop it can't begin until we have found out what started it: and here, right at the start, there are two problems.

The first is that it is very hard to get good scientific information about a creature as elusive as a marine mammal.

The second is that, as more and more data are collected, the more likely it seems that there is no single reason. This is a complex business. If you could say, for example, that the problem was just bastard fishermen shooting seals, then we would have an obvious if difficult solution. And yes, shooting of seals, both legal and illegal, does go on. But the feeling among scientists is that this would not cause a major decline. As populations fall, however, persecution would play a more significant role, as each individual became more important.

Ailsa Hall, deputy director of the SMRU, is investigating disease, which involves capturing and examining live seals. She gathers information, from blood samples and through other techniques, about parasites, pollutants and sickness.

“The seals we see almost always look healthy,” she says. “No lesions, decent weight, good blubber. But we don't see sickly animals, and they could be dying out at sea.” This is not something that you can know.

One way that the scientists keep track of seals is by using an electronic tag. When a seal comes into contact with land, it sends messages to the mobile-phone network and provides details of the seal's movements by text. But if you lose track of a tagged seal, you don't know if the seal is dead or the tag has fallen off.

One possible problem is the algal bloom that comes from fish farms and agricultural run-off. Too many nutrients in the water - too much of a good thing, in short - produces harmful algae.

Then there is the question of diet. There have been a series of catastrophic breeding performances in the seabird colonies of Scotland. There is a desperate shortage of sand eels, and the adults can't catch enough to raise chicks. The reasons for the sand eel decline are elusive. Over-fishing has been a cause in the past, but now it may be that changes in the movements of the fish have been triggered by changes in the sea. In other words, global warming.

Seals are more robust creatures than birds and can take a much greater variety of prey. But these changes may be slowly impacting on the diet of common seals.

The next step, then, is to find out what seals eat by collecting droppings: mammal research frequently comes down to what the researchers cheerfully term “shit surveys”. But information from these is necessarily incomplete: you can gather only what the seals let fall on land. What bears do in the woods, seals do in the sea.

These days orcas - killer whales - are seen more frequently off the coast of Scotland. Maybe they have found a taste for common seals, perhaps compounded by the fact that the seals are orca-naïve - they don't know how to get out of the way. But all this takes place, if indeed it does take place, out of human sight.

Bernie McConnell, a senior research fellow, is involved with the question of what seals actually do: where they go, and how long they do it for. Commons and greys live in different ways: it is a generalisation, but grey seals generally make longer journeys and have a greater range, while commons are more local.

One possibility is that, in changing ecological circumstances, the grey seals are out-competing the commons. The difference in birth strategy separates them completely but when it comes to making a living, there is a clear overlap. The greys may be better at exploiting the change or changes - and if it comes to confrontation, there is only one winner.

Ask too crass a question and responsible research scientists will tell you that such a conclusion cannot be justified at this stage but, on the other hand, it can't be ruled out either. They leave speculation and conclusion-leaping to others.

So let's leap a little. It is now clear that the world is changing beneath our feet, above our heads, across the land, beneath the seas. Changes that may be negligible to us - a degree rise in sea temperature, say - set in motion a succession of events that we can't possibly predict. All we can say for certain is that with one change come many changes, because everything on this planet is ultimately linked.

The possible causes of the common seal decline - algae, ill-health, changing diet, changes in prey population, predation, competition - are all related to change. And although no scientist can say so with full certainty, some or all of these changes are likely to be linked with climate.

Our planet is changing and no one knows where it will all end. That is true for common seals and for humans. Their mystery is, in the end, our mystery.

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