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On the Shiretoko peninsula in northeastern Japan, winter means two things: ice and eagles. To see why, you need to be there before dawn with a pair of binoculars pointing west. The reward for enduring the region’s subarctic weather at that uncivilised hour? The impressive sight of hundreds of eagles leaving their valley roosts for the ice-covered sea beyond-among them Steller’s sea eagle, one of the world’s largest birds of prey.

This dawn exodus of eagles is probably the greatest wildlife spectacle on offer in Japan. The objective of the sortie is simple: to plunder the rich fishing grounds of the nearby Nemuro channel, the narrow strip of sea that separates Japan from the Soviet Union. But it is not just the fish that attract thousands of Steller’s eagles to Shiretoko in winter. The evidence of recent years suggests that the winter ice floes play an important part in bringing and keeping the birds there. On Shiretoko, eagles and ice go hand in hand.

Since 1980 my midwinter eagle-watching has begun soon after five in the morning when the eastern horizon is beginning to lighten. At the mouth of the Sashirui river, the prime eagle-watching site, pre-dawn temperatures can be as low as – 15 °C. A handful of star-eclipsing shadows signals the start of the exodus, but the early trickle of birds quickly builds to a flood as hundreds leave en masse. Among the first to depart will be some of the 500 or so white-tailed sea eagles that winter on Shiretoko. These are soon outnumbered, though, by up to four times as many Steller’s sea eagles, the real stars of the show.

Named after the 18th century scientist Georg Wilhelm Steller, who took part in Vitus Bering’s ill-fated research voyage across the northern Pacific, the eagle is designated a ‘national monument’ by Japan’s Cultural Agency and classified as a protected bird by its Environmental Agency. Its Japanese name, O-washi, means the great eagle. They are indeed huge. From bill to tail tip, female Steller’s eagles measure over a metre and male Steller’s eagles 88 cm; their wings span two-and-a-half metres. Yet it is not just size that makes Steller’s sea eagle so impressive. Its striking plumage, massive bill and distinctive habits-for instance, most other birds of prey rise well after, not before, dawn-all combine to make Steller’s eagle a powerful magnet for bird-watchers.

From November to March the Shiretoko peninsula is home to the world’s highest concentration of Steller’s sea eagles, and more than 90 per cent of all those wintering in Japan. More than 2000, about a third of the world’s total population, have been recorded there during the February peak. The birds migrate to Shiretoko from their summer breeding grounds on the Bering Sea coast and the Kamchatka peninsula in the eastern Soviet Union, some 2000 kilometres away.

In the early 1980s the world population of Steller’s sea eagles was estimated at only 3000 to 4000. It is now set at over 6000, following a survey carried out jointly by Soviet and Japanese scientists in the winter of 1985/86, which revealed up to 4000 on the Kamchatka peninsula and just over 2000 in Japan itself. Apart from those wintering on Shiretoko and the Soviet island of Kunashir, which faces Shiretoko across the Nemuro channel, the world’s population of Steller’s sea eagles is widely scattered in winter. In conservation terms, Shiretoko is the species’ single most important habitat.

Indeed, it is hard to imagine Shiretoko without its population of wintering sea eagles. Yet in the first half of this century the numbers appear to have been lower. In 1890 the naturalist Henry Seebohm reported regular sightings of the eagle in the region, but by the 1950s it was considered a rare winter visitor there. Perhaps the population had fallen, or perhaps styles of recording changed over time. What is clear is that since the 1950s numbers have surged, and in this humans may have played a part. The theory favoured by most locals is that the eagles are attracted by the fishing fleets, whose winter activities have increased greatly since the advent of strengthened boats that are capable of withstanding ice. The birds take advantage of any fish spilled after the nets are hauled in.

In most years the great exodus on Shiretoko is a daily event. The birds fly out at dawn and begin to return to their roosts as light fades in mid-afternoon. Being short and narrow, the valleys in which they roost provide ideal protection against the peninsula’s winter wind. For the locals the ritual has become as much a part of winter on Shiretoko as snow and ice. Over the past few years, however, this has been changing. Although Steller’s sea eagles have still been coming to Shiretoko in winter, they have been spending much less time there than usual. Their fishing habits, once so predictable, have become sporadic, the dawn exodus less of a daily spectacle.

Why should the eagles have altered their habits in this way? What is it that has made the Shiretoko peninsula less desirable to Japan’s wintering eagles? The key to the problem lies in understanding the Shiretoko ecosystem, and in particular how the habits of the animals that winter there depend on its climate.

In the Ainu language, Shiretoko means ‘end of the earth’. It is easy to see what prompted the name. The peninsula is a mountainous finger of land projecting out of Hokkaido, Japan’s most northerly region. Three volcanos, Rausu, Io and Shiretoko, form the rugged backbone of the peninsula. Hokkaido’s northerly position and separation from Japan proper by the Tsugaru Strait mean that its fauna and flora are more akin to those of Siberia than to the rest of Japan. Its harsh climate contrasts sharply with that of Japan’s southwestern islands, which are cloaked in subtropical forests. Even so, Hokkaido contains some important wildlife, notably a population of brown bears and rare birds such as Blakiston’s fish owl (see Box for further details).

The sea is pivotal to the Shiretoko ecosystem. During the winter it brings migratory mammals and food to the region. The largest marine mammals used to be the whales but stocks were dramatically reduced by Hokkaido’s now defunct whaling industry. Today the largest are the pinnipeds: Steller’s sea lion, types of seal including northern fur, spotted, ribbon, ringed, Kuril and bearded seals. In January and March between 2000 and 5000 sea lions migrate here from the central Kuril islands, moving down the Nemuro channel in search of fish. The winter fishing industry along the Shiretoko peninsula is based on the movement of pollock through the channel. This, combined with harvests of sea urchin, kelp and salmon, each in their season, make the harbour town of Rausu one of the richest coastal communities in Japan.

As well as bringing fish and wildlife, the currents and winds of the Nemuro channel are responsible for another vital component of the Shiretoko ecosystem: sea ice. The Nemuro channel’s current brings ice from the Sea of Okhotsk, but at the same time disperses it so that the channel never becomes entirely ice-locked. The importance of this can be seen along the northwestern shore of Shiretoko, where the winter ice sheets are continuous right up to the coast and where there are few eagles and pinnipeds, and no winter fisheries. Without currents and winds the same would probably be true of the southeastern coast of the Shiretoko peninsula.

The number of eagles on Shiretoko at any one time is strongly influenced by sea ice. Their migration to the peninsula from their summer breeding grounds in the Soviet Union coincides with the advance of ice across the Sea of Okhotsk. As the ice moves towards the peninsula around late December, the number of eagles on the peninsula’s Okhotsk coast increases greatly. The eagles then move across the Shiretoko peninsula and gather on the side facing the Nemuro channel. Drifting ice also provides the eagles with a mobile platform from which to hunt for fish in the Nemuro channel.

Daily eagle counts along the peninsula vary considerably, suggesting that the birds often quit Shiretoko for Kunashir on the opposite side of the channel. The amount of sea ice, the direction of the wind and the direction of the ice drift all appear to influence roosting behaviour. If the wind and currents take the ice out across the channel during the day, the eagles tend to cross the channel to roost on Kunashir.

The rates at which ice floes grow, spread and disappear again are governed by a combination of sea salinity, sea temperature and air temperature. Salinity is affected by advection (heat transfer) from the Pacific. In years of high advection of saline water the ice over much of the Sea of Okhotsk may be thin and incomplete. The sea’s temperature is affected by the movement of currents such as the cold Oyashio, which sweeps down the western side of the northernmost Pacific, and the warm Kuroshio-the North Pacific’s counterpart of the Gulf Stream-some branches of which penetrate the Sea of Okhotsk. Air temperature is dependent on wind direction. The prevailing winter wind is northwesterly and since this travels across Siberia it is very cold. A westerly surge of cold wind promotes extension of sea ice across the Sea of Okhotsk, and a more northerly wind forces the ice towards Hokkaido-and ultimately the Nemuro channel.

Having been resident in Hokkaido for several years in the early and mid-1980s, I took ice on the Sea of Okhotsk to be a long established norm. Records show that its coverage peaked in February 1978. At that time the ice covered virtually the entire sea, marking a line from Kamchatka down the Kuril islands to eastern Hokkaido.

In the winter of 1988/89 it came as a shock to experience warm southeasterly winds that were more typical of summer. By the end of February, for the first time in living memory, no sea ice had formed around Hokkaido. Daily temperatures remained above zero for several days. The same happened in 1990, the fourth winter in succession when little or no sea ice was to be seen either in the Nemuro channel or along the Okhotsk coast. In 1991, despite lower temperatures and heavier snowfalls, the Japanese newspaper Yomiuri reported that the sea ice of the Sea of Okhotsk had reached a ‘record minimum’.

The anomalous weather has brought obvious changes in the behaviour of the eagles. In the early 1980s, when the Nemuro channel contained ice, the numbers and movements of the eagles were clearly related to the whereabouts of the ice. If the current brought ice close to the Shiretoko shoreline, large numbers roosted in Japan. If northwesterly winds shifted the ice across the channel for a few days, the eagles tended to move to the Soviet side of the channel.

But from 1987 to 1990, each time I visited Shiretoko in February the eagles, being without ice to use as a platform, were more dependent on the presence of the fishing fleet. After a first aerial sortie at dawn many eagles would return to their roosts until the fleet was in position in the channel. As there was no ice to lead eagles across from one island to the other, their numbers remained more constant from day to day. The huge number seen on Shiretoko in previous years after the ice had drifted back from Kunashir wasnot reported.

By 1989 people living in Hokkaido became convinced that changes were taking place not only in the distribution of sea ice, but in the entire pattern of winter weather. Many have fastened onto the idea of global warming as an explanation-but not Kazuhiro Osawa of Japan’s Meteorological Satellite Centre. Osawa has studied the development and movement of sea ice in the Sea of Okhotsk, using data from Japan’s geostationary meterological satellite, the Himawari, and from the US’s National Oceanic and Atmospheric Administration (NOAA) satellite.

While Japanese research supports the view that there has been a gradual increase of between 0.3 and 0.5 °C in temperature in the northern hemisphere over the last 100 years, Osawa’s analysis reveals no evidence for a gradual loss of winter ice of the kind you would expect if the loss were a manifestation of global warming. As satellite observations of sea ice only began in 1966, however, they cannot yet provide useful information about less pronounced long-term effects.

The alternative explanation for Shiretoko’s recent weather, and the one favoured by Osawa, is a phenomenon known as La Nina, a cyclical movement of water similar to El Nino. La Nina and El Nino are, in fact, two different phases of what is known as the Southern Oscillation, a cyclical variation in rainfall patterns and wind fields which occurs throughout the tropical Pacific Ocean.

During an El Nino year, a warm southward current has a dramatic effect on the cold waters of the southeastern Pacific Ocean early in the year, especially off Peru. The northwestern Pacific, meanwhile, grows cooler and there tends to be a wider coverage of ice on the Sea of Okhotsk. During La Nina, the other phase of the cycle, conditions are complementary to those of El Nino: the western Pacific Ocean becomes much warmer and ice coverage on the Sea of Okhotsk is reduced.

According to Osawa, the weather of the last few winters reflects this cyclical pattern. It is probably no coincidence, he says, that the winters of 1988/89 and 1989/90 saw the largest La Nina events of this century. Yet Osawa’s measurements suggest that the turning point in the cycle should have been reached in the winter of 1989/90-a prediction at odds with the ‘record minimum’ ice coverage reported earlier this year.

It is, of course, too early to pass final judgment on the oscillation explanation; what happens in Hokkaido over the next few years will be crucial. If Osawa’s theory is borne out, then Shiretoko will once again experience the severity of winter that makes it feel like the ‘end of the earth’ and eagles will once more flock to drifting ice floes. If not, much of the fishing activity may move north into Soviet waters, the importance of Rausu may well decline and the seals and eagles may follow the fortunes of the fleet. Perhaps the spectacular morning exodus of Steller’s sea eagles from Shiretoko will be but a memory.

Mark Brazil is author of A Birdwatcher’s Guide to Japan (1987; Kodansha International & Wild Bird Society of Japan). His latest book is The Birds of Japan (1991; Christopher Helm). He has watched the eagles of Shiretoko for 10 years

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Wildlife at the ‘end of the earth’

Shiretoko’s winters are legendary for being long and severe. As a consequence its ecosystem is less diverse in its breadth of habitats and number of species than other parts of Japan. Even so, it is home to some fascinating wildlife. Japan’s largest population of the brown bear survives here, along with about 10 breeding pairs of the white-tailed eagle (the peninsula is their stronghold in Japan). There are rather fewer of the rare Blakiston’s fish owl. Both birds, like Steller’s sea eagle, are designated as ‘national monuments’ and depend on plentiful supplies of fish.

In the late 19th century a series of unusually severe winters apparently wiped out the region’s sika deer, but slow emigration led to partial repopulation by the 1960s and 1970s. The population continues to increase, though it may eventually be checked by the limited availability of habitat: the harsh climate means that suitable wintering areas are restricted, and some of these are threatened with logging.

In 1964 a large part of Shiretoko was designated a national park. The park, the wildest in Japan, covers an area of more than 37 000 hectares, yet it has never had more than a single ranger-a telling indication of Japan’s lack of commitment to conservation in general. As a result of hunting, three mammals became extinct in the last century, the wolf, the otter and the sea otter. This leaves the red fox, sable, stoat and weasel as Shiretoko’s smaller predators.

Shiretoko’s top predator, the brown bear, hangs on but is under great pressure, largely because of hunting. Despite there being only two documented cases of bears attacking humans other than bear hunters, the risk has led to so called ‘nuisance’ bears being pursued and killed even within the national park. Killing bears outside the open season is permissible in self-defence, which, it is said, is how professional bear hunters maintain their livelihood throughout the year.

A recent study by a research group at the University of Hokkaido reported that more than 270 bears have been killed on the peninsula in the last 12 years. If measures are not taken to protect it, the bear population will be unable to sus-tain itself for much longer; and since the brown bear is widely regarded as a ‘deadly beast’ in Japan, the Japanese government is unlikely to take any. The day is rapidly approaching when the only brown bears on Hokkaido will be those huddled in captivity in the region’s infamous bear farm. Steller’s sea eagle will then be the largest remaining land predator.