9.1 Introduction

The birds and mammals are in the main the predators in the open seas and coastal regions of the polar regions, and again they are mainly of different species north and south. Birds and mammals are visible on top of the sea ice (Fig. 9.1 ), and they have evolved foraging techniques adapted to the physical nature of the ice and also make use of it as a comparatively safe breeding ground. The emperor penguin in the Antarctic and the polar bear in the Arctic are some of the most iconic marine species on Earth and, although they are as geographically distinct as is possible, sea ice provides a platform for, and dictates the timing of, their entire ways of life. The similarities in the way bird and mammal life histories have adapted to the sea-ice environment the Arctic and Antarctic mean they are dealt with together here, despite the different species involved (Table 9.1 ).

9.2 Seabirds

The diversity of birds frequenting the open waters of the Arctic is low, about 95% of those breeding in the Arctic belonging to four species, the northern fulmar (Fulmarus glacialis), the kittiwake (Rissa tridactyla), Brünnich's guillemot or thick-billed murre (Uria lomvia), and the Dovekie or little Auk (Alle alle; Sage 1986 Pielou 1994 ). These breed on land in colonies which are often huge (Fig. 9.2 ). Arrival at their breeding grounds is synchronized with the break-up of ice so that zooplankton and young fish are available when young are hatched. A colony of Dovekies containing 100 000 pairs transports some 71 t of zooplankton from seas to the colony during the 4 weeks of summer.

The distribution of colonies and of the birds at sea is related to regional abundance and accessibility of prey. Polynyas (see Chapter 7 ), being areas (p. 260 )

of upwelling, are productive and particularly important for overwintering, when they remain open. Fronts (see Chapter 6 ) provide zones of spatially predictable, high productivity and are also marked by concentrations of seabirds. Lancaster Sound (approximately 74°N 85°W) is an area where several million birds congregate in the summer. Of the eight species found there, three, the northern fulmar (Fig. 9.3 ), kittiwake, and Brünnich's guillemot, feed largely on the Arctic cod. Of these three species the kittiwake is a surface feeder, the fulmar has limited diving capability, and the Brünnich's guillemot undertakes wing-propelled dives to depths of 50–75 m for 2–3 min. The cod is sustained by algae growing on the undersurface of the sea ice. This, then, is a simple food web with the flow of energy channelled mainly through one species of fish.

The marginal ice zones (see Chapter 7 ), where melting can create high localized productivity, are important for regional biological activity. Many species of seabirds and mammals use the ice edges as migration routes in the spring where they are dependent on the reliable food supply they offer (Fig. 9.3 ). Availability of prey is reflected, too, in the density of seabirds, which in the marginal ice zone of the Bering Sea is estimated as 500 individuals km⁻² as compared with 0.1 km⁻² in the ice to the north and 10 km⁻² in adjacent open water. Large numbers of seabirds are regularly found in the summer pack ice, feeding on Arctic cod and crustaceans, but only Ross's gull (Rhodostethia rosea), ivory gull (Pagophila eburnea), and the black guillemot (Cepphus grylle) are characteristic of the pack and dependent on ice-associated fauna for the bulk of their food. The two gulls are rarely seen over the open sea although some races of the black guillemot have distributions going into temperate regions. Just south of the ice edge, however, a variety of species is to be found. In the Barents Sea, for example, glaucous gulls (Larus hyperboreus), herring gulls (L. argentatus), (p. 261 )

(p. 262 ) Brünnich's guillemot, and little auks move in from open water to congregate at the ice edge. The ice zone around the coast is thus a barrier to them and breeding colonies are perforce located to the south of the summer ice margin. In the Davis Strait and the Labrador Sea ivory gulls are absent only during the short ice-free period in late summer. During March the largest numbers are at the ice edge near the breeding areas of the hooded seal, where they feed on the afterbirth. In April and May they prey particularly on lantern fish near the ice edge.

Since the great auk (Alca impennis), which nested on offshore skerries, became extinct on 4 June 1844 there has been no penguin-like, flightless, diving seabird in the Arctic. Presumably, the great auk foraged at the ice edge as far away as possible from polar bears. As in the Antarctic, flighted birds, such as the existing auks, use the pack ice as feeding ground but do not breed on it. Access to open water through predictable polynyas seem particularly important for the wintering of birds in high latitudes. Most of the population of Ross's gulls seem to winter around them, for example, at the Velikaya Sibirskaya polynya in the Russian Arctic. Regular wintering populations of Brünnich's guillemot, Dovekies (Alle alle), and long-tailed ducks (Clangula hyemalis) are found in the vicinity of polynyas north of the edge of the pack ice. One of the most astounding examples of the importance of areas of open water in pack ice regions is the recently discovered wintering areas of the spectacled eider (Somateria fischeri), where satellite tracking and subsequent aerial surveys discovered over 300 000 birds in the pack-ice region of the Bering Sea, accounting for the majority of the global population.

In the Antarctic, there are two main groups of seabirds, the procellariforms (albatrosses, petrels, etc.) and the penguins; these are two very different groups that are both very highly adapted to marine conditions (Croxall, in Laws 1984 Knox 1994 Williams 1995 ). The wandering albatross (Diomeda (p. 263 )

exulans; Fig. 9.4 ) navigates and finds prey in immense areas of ocean. It is a large bird, weighing up to 10 kg, with a wing span of up to 3.5 m, which spends most of its adult life at sea and may live to be 70–80 years old. Its flight is seemingly effortless, using updraughts to give it height to glide with minimum movement of its stiffly held, narrow wings. It is mostly at latitudes between 40 and 50°S that albatrosses can find sufficiently strong and constant winds for this mode of flight; however, the closely related southern giant petrel has colonies below 60°S. Using satellite telemetry to track birds equipped with small transmitters has revealed foraging trips from nests on Îles Crozet (45°S 52°W) that may cover between 3600 and 15 000 km, over (p. 264 ) distances of up to 900 km day⁻¹. The recent development of miniature light-level recorders has allowed the movements of albatrosses to be recorded over longer periods and have revealed the regular circumnavigations of the Southern Ocean undertaken by these birds. The flight path and foraging strategy are determined by the wind. When following the wind they fly at a slight angle to the left, which, in the southern hemisphere, leads them away from cyclonic lows and towards high pressure. However, even with these advances in knowledge of where albatrosses travel understanding exactly how they capture squid, their principal prey, remains uncertain, although some species of albatross can dive up to 10 m.

As is obvious to the seafarer, these birds are also opportunistic scavengers. It is this habit that has brought albatrosses into conflict with humans as the areas in which albatrosses overlap with long-line fishing have increased. As its name suggests, long-line fishing involves setting long lines of baited hooks which prove irresistible to the albatrosses, which become hooked and then drown. The resultant declines in the populations of albatross species has made them one of the most threatened bird families on Earth.

A number of petrels species are clearly associated with sea ice, although no flighted birds breed on sea ice and visit it only on transit between areas of open water. The snow petrel (Pagodroma nivea) is even mentioned in maritime guides for sailors as a good indicator for sea-ice conditions, and like the two penguins discussed below this is an obligate ice species (Fig. 9.5 ). The entirely white plumage of the snow petrel allows them to sit on the edges of ice floes waiting for fish and crustaceans to venture from under the floe edge, when the petrel will dive to catch its prey. However, snow petrels will take advantage of any available food source and they have been observed picking at Weddell seal faecal matter around the holes used by the seals to haul out.

(p. 265 )

Snow petrels do not nest near the pack, but instead nest on rocky ledges or cliff faces. Snow petrels have been found nesting more than 180 km from the coast on mountain summits (nunataks) on the Antarctic continent. This is even more remarkable considering that the adults must fly to and from the sea ice to collect food for their chicks.

Ross and ivory gulls in the Arctic use similar strategies to the snow petrel, where their white coloration enables them to feed from the edges of ice floes on Arctic cod, other fish, and amphipods grazing on the underside of ice floes. Just like the snow petrels ivory gulls also nest in snow-free mountain outcrops. It is thought that snow petrels and ivory gulls seek out these nesting sites to have a huge expanse of ice around them to protect them from predation from skuas and foxes/polar bears respectively.

(p. 266 ) Another common petrel in the Antarctic pack ice, especially in marginal ice zones, is the black and white Antarctic petrel. In contrast to the snow petrel this species flies quickly along water edges and dives into the water to pursue prey underwater, swimming using their wings for propulsion. Likewise the Arctic black guillemot and thick-billed murre feed by diving under ice floes to search for crustaceans and fish.

Of the 17 species of penguins there are six—the emperor, king, Adélie, chinstrap, gentoo, and macaroni—that are found in the Southern Ocean (Fig. 9.6 ). For all of these species their characteristic feeding technique is pursuit diving. Miniature depth recorders, logging the number of dives and their depth and duration, have been used to study several species in (p. 267 )

several locations. King penguins (Aptenodytes patagonica) dive to depths of 236–265 m after fish and squid. Chinstrap and Adélie penguins, both krill eaters, dive mostly to shallower depths up to 70 and 170 m respectively. Radiotelemetry, movement sensors, and even miniature cameras have been used to monitor behaviour at sea. This has revealed a range of foraging behaviors related to local distribution of prey including benthic diving in chinstrap penguin, although apparently still feeding on krill. Foraging ranges and chick feeding intervals are different for the various species; gentoo and chinstrap penguins range within about 20–30 km of the nesting site whereas Adélie penguins go further, up to 100 km, although these distances differ from location to location. Of the six penguin species mentioned the king and emperor lay a single egg, whereas the others lay two eggs, although the macaroni penguin always discards the first egg once the second has been laid (reasons for this are the subject of much, unresolved, debate).

The emperor penguin (Williams 1995 Ancel et al. 1997 ) is superbly adapted to existence on sea ice and has had perhaps 40 million years to evolve to this state. The minimum total population is about 195 400, distributed in 42 breeding colonies around the continent. It feeds by pursuit diving for fish, squid, and krill, going to depths of as much as 500 m. Living entirely on the ice when not at sea, it gets round the difficulty of nesting by incubating its single egg balanced on top of its feet and enveloped in a feathered fold of abdominal skin. This needs a flat surface and breeding sites are on level sea ice in sheltered situations (Fig. 9.7 ). Eggs are laid at the end of the the summer and the females then go off to sea to recuperate. Incubation is carried out over winter by the males, so that the chicks eventually fledge by midsummer and adults have sufficient pause to moult and before the onset of the next winter. It was thought initially that young birds drifted (p. 268 )

north on ice floes after fledging; however, recent satellite tracking of young birds immediately after fledging suggests that they may travel well away from the sea ice, even north of 60°S.

The emperor penguin has adaptations for survival through the Antarctic winter when temperatures on the ice may fall to −48°C and winds reach 180 km h⁻¹. First, the bird is large, weighing up to 46 kg, and thus has a small surface area/volume ratio which is further minimized by having flippers and bill about 25% smaller in proportion than other penguins. Insulation is provided by double-layered, high-density feathers and a 2–3-cm-thick layer of subcutaneous fat. The thermal conductivity of fat is a quarter of that of water but the feathers, with the air layer which they entrap, provide more than 80% of the thermal insulation. The feet have a vascular counter-current heat exchange system that reduces heat loss and avoids them becoming frozen to the ice. There is also a nasal heat-recovery system that retains the warmth of the outgoing air to heat the incoming breath, thus avoiding the drawing in of freezing-cold air into the lungs on each breath. These features allow the metabolic rate to remain at normal level down to a critical temperature of −10°C, below which it must be increased if body temperature is to be maintained. A remarkable behavioural characteristic also enables heat and energy reserves to be conserved below this temperature. Unlike other penguins, which in common with other birds have strong territorial instincts, the male emperor does not object to being in close proximity to his fellows and under adverse conditions joins a huddle which may contain as many as 5000 birds, packed 10 to the square metre. The whole huddle moves slowly downwind as birds on the windward side move along the flanks and re-enter it on the lee side. If the emperors are startled and raise their heads, steam can be seen rising from the huddle. It is estimated that by this means the daily loss of body (p. 269 ) weight is cut by 25–50%. At the end of the winter the male emperor, which by then has lost up to 40% of his summer body weight, still retains enough reserves to produce a nutritious secretion which keeps its chick going for its first few days until the female arrives from the sea with food.

Having a life history so intrinsically linked to sea ice means that emperor penguins are inevitably sensitive to changes in both the distribution and duration of sea-ice. Clearly the early break-out of ice before chicks have moulted into their adult plumage can cause substantially losses. Large-scale climatic changes have been attributed to the reduction by almost 50% of an emperor penguin colony in Adélie Land, although there is little evidence of large-scale changes in the population. However, the remote and inhospitable location of many colonies means that regular monitoring numbers is restricted to a relatively small number of the known colonies.

The Adélie penguin, Pygoscelis adeliae (Williams 1995 ), is the most numerous of Antarctic penguins and although it is frequently seen in the pack ice it nests on land during the Antarctic summer. Arrival at the breeding colonies in November often entails a march across extensive sea ice in the expectation that the ice will be replaced by open water during the chick-rearing period in January, when demand for prey is greatest (Fig. 9.8 ). The consequences of a failure of the ice to be replaced by open water was demonstrated in the Ross Sea when a 1000-km-long iceberg prevented the normal pattern of ice break-up and many penguin chicks starved as their parents could not gain access to open water to feed. Once the iceberg moved away the normal pattern of ice formation and break-up resumed and the fortunes of the Adélie penguins were restored.

A tentative estimate of krill consumption by seabirds around South Georgia and the Scotia Arc is 10.9 × 10⁶ t per annum. There is no doubt that, quantitatively, they are an important element, comparable with other predators in the pelagic food web. Their predation must also play an important

(p. 270 ) qualitative role in structuring pelagic communities. As has already been pointed out they have a profound effect locally on terrestrial habitats. Broadly, the ecological impact of seabirds seems similar in Arctic and Antarctic waters. An attempt to make quantitative comparisons around islands at roughly comparable latitudes north and south—in the Bering Sea in the area 55–63°N 169–173°W, and the South Orkneys 61°S 45°W—bears this out. Numbers of birds per unit sea area tended to be higher in the Arctic but since Antarctic birds have a larger mean size the biomasses are similar. Because smaller birds require proportionately more food than larger ones, consumption of pelagic prey is probably greater in the Arctic than in the Antarctic.

9.3 Seals

Seals, being accomplished swimmers amply insulated with dense fur and blubber, are well adapted to polar waters. Nevertheless, they are visibly most associated with their breeding sites on land or ice. Studies in the open sea and precise information on their lives in the pelagic environment is increasing with the use of satellite-linked behavioural recording devices; however, for many species such information remains sparse.

The 25 or more species of marine mammals in the Bering Sea are estimated to consume between 9 × 10⁶ and 10 × 10⁶ t of pelagic and benthic organisms per year. Most seals eat fin-fish but some such as the bearded seal (Erignathus barbatus), prey on benthic invertebrates, and the walrus (Odobenus rosmarus) feeds on bivalve molluscs. The walrus therefore prefers shallow waters and although it may congregate in large numbers on beaches, it characteristically lives in the pack and at ice edges. It mates around midwinter among the ice, copulating under water with one bull serving perhaps 15 females. Walruses also use sea ice for hauling out, often using their distinctive tusks as ice axes, giving them one of their popular names, the ice walkers. They can also break through ice up to 20 cm by ramming the ice with their heads. Their prey is made up mainly from molluscs such as clams, cockles, and welks growing in the benthos, and so they are restricted to hauling out on ice covering shallow (<70 m) coastal regions. However, walruses are also known to eat ringed seals as well. Walruses do not keep open breathing holes, but instead rely on polynyas and open leads between ice floes for their access to the water.

The walrus (Fig. 9.9 ) is widespread and, since a Soviet ban on its hunting, has multiplied considerably. It does not use its tusks for digging, as one might suppose, but supports its head on them while it extracts the soft-shelled clam (Mya truncata) by sucking or hydraulic jetting to excavate a pit up to 30 cm deep. It may find these clams, which have conspicuous siphons, visually but if visibility is poor it furrows the upper few centimetres (p. 271 )

of sediment and identifies its prey by touch. One such furrow was found to be more than 60 m long and yielded 34 clams from depths of over 30 cm. The amount of sediment resuspended by walruses in the Chirikov Basin of the north-east Bering Sea is about 100 million t per year.

Pits left by walruses offer a habitat to many kinds of benthic invertebrate. A remarkable number and variety of agencies disturbing the sediments of Arctic seas can be listed. Another large mammal, excavating pits 1–2 m long and up to 0.5 m deep is the grey whale (Esrichtius robustus), for which major foods are the amphipods Ampelisca and Bybles spp. This animal is estimated to resuspend 172 million t of sediment per year in the Cherikov Basin, three times the amount of sediment deposited by the Yukon River (King 1983 Ainley and DeMaster, in Smith 1990 Ainley et al., in Thomas and Dieckmann 2003 ).

There are seven species of Arctic seal that use sea ice as a habitat (King 1983 Sage 1986 Pielou 1994 ). The spotted (Phoca largha) and ribbon (Phoca fasciata) seals are found in the Pacific/Bering sea region, the hooded (Cystophora cristata) and harp (Phoca groenlandica) seals are found in the north Atlantic/Russian Arctic region, whereas the ringed (Phoca hispida) and bearded (Eringnathus barbatus) seals have a circumpolar distribution. The bearded seal prefers shallow waters free of fast ice, with moving floes and open leads, but it can keep open breathing holes by means of the strong claws on its fore flippers. The ringed seal is the commonest seal in the Arctic Ocean, with a total population of between 3 and 4 million. It is found in open water in the fast ice but rarely on (p. 272 ) floating pack ice or in the open sea. In winter, adults stay under the ice in bays and fjords. The younger ones mostly stay further out at the edge of the fast ice. In summer most lie out on the ice basking in the sun, moulting, and fasting. Like the bearded seal the ringed seal keeps open breathing holes, which may extend through ice 2 m thick, by abrasion with its flipper claws. A low dome of ice, about 4 cm high, marks the hole at the surface and the snow which accumulates round this, if it is sufficiently deep, may be hollowed out and used as a lair. Larger lairs are constructed by pregnant females and the pups are born in them in spring. These lairs, with their adjacent breathing holes giving access to the sea, not only give some protection against polar bears and Arctic foxes but provide shelter from cold and wind. The pup, which has not got the insulating layer of blubber possessed by adults, must derive appreciable warmth from its mother in the confines of the lair.

In the suite of ‘ice-breeding’ seals in the North Atlantic region there are two quite distinct lactation strategies that may well reflect differences in the types of ice habitat used by the different species (Lydersen and Kovacs 1999 ). The grey (which breeds in the ice but is also found in more temperate regions), the harp, and the hooded seals all frequent the outer regions of the pack ice and have a short lactation period; once weaned pups remain relatively inactive while converting blubber into useful body tissues. In contrast, the bearded and ringed seals are found in nearshore fast ice and have a much longer lactation period. During this time the pups enter the water, and even feed for themselves, so that there is a less abrupt transition to nutritional independence.

Several of the Arctic species frequent shallow coastal waters associated with pack ice, and although some appear to remain resident the distribution of others changes in response to the seasonal movements of the pack ice. The harp seal is the most abundant seal in the northern hemisphere and is found in three discrete populations. The largest, of between 4 and 6.5 million animals, is in the north-west Atlantic, and the others are in the Barent Sea and east Greenland. It breeds in the spring on the margins of large ice fields. Pups feed on planktonic crustaceans and adults on shoaling fish such as capelin, herring, and polar cod. The total population is believed to be between 6 and 8 million, a considerable reduction from what it was before commercial exploitation. Like the harp seal the hooded seal moves seasonally north and south with the movement of the ice although it is not distributed quite so extensively. It is solitary at sea, feeding on Greenland halibut, capelin, cod, and squid, for which it can dive to 1000 m. The total population is of the order of half a million animals. In the North Pacific, Chukchi, and Bering Seas, the predominant pelagic seal is the ribbon seal. Other than the periods of breeding on ice in the spring and the summer moult, scarcely anything is known of its movements thereafter. Fish and cephalopods appear to be its main food. (p. 273 ) The total population is now about a quarter of a million (Sage 1986 Ainley and DeMaster, in Smith 1990 ).

In the Antarctic, Weddell, Ross, crabeater, and leopard seals are associated with ice and only the Antarctic fur seal (Arctocepthalus gazella) and Southern elephant seal (Mirounga leonina) are more characteristic of open water (Fig. 9.10 ). The elephant seal is circumpolar in distribution, although divided into three breeding stocks. The fur seal was almost exterminated on most circum-Antarctic islands by the fur trade; however, from near extinction in the nineteenth century it has made a spectacular recovery, with the population now totalling about 3–4 million animals. At South

(p. 274 ) Georgia, home to 95% of its population, its diet is dominated by krill, although fish are also important. Diving behaviour has been monitored by attached instrument packages which show a diurnal pattern with most diving taking place at night when krill are near the surface. The depth of dives is mostly between 20 and 50 m with a maximum of 200 m.

Between May and October, when they are absent from the breeding grounds, female southern elephant seals disperse widely, with some individuals reaching the pack ice while others winter on the Patagonian shelf north of the Falkland Islands. What southern elephant seals do between going to sea after moulting in March to April and returning to shore to breed in September has also been investigated using satellite tracking and time–depth recorders. This has revealed that when at sea, they spend 80–90% of their time underwater, most of it at depths of 200–400 m but sometimes going down to 1500 m. It seems that they feed at the pycnocline, where particulate matter tends to accumulate and attracts squid. The time needed to take breath at the surface is astonishingly short and their muscles must be able to work without oxygen for much of the time when they are submerged. How they navigate, evidently with great precision, when travelling in deep water over the long distances between their breeding and feeding grounds, is not known. Such are the movements of elephant seals that they are now used to carry oceanographic equipment and are actually providing a better understanding of the physical characteristics of the Southern Ocean, as well as of their own foraging strategies.

During the austral winter, some southern elephant seals occur in close association with sea ice, and adult female elephant seals from King George Island fitted with satellite transmitters have revealed that the seals ranged in the sea-ice zone along the Antarctic Peninsula. During winter, some of the female elephant seals also spent several months in heavy pack ice. The availability of the Antarctic silverfish, Pleurogramma antarcticum, may explain why elephant seals are attracted to the pack ice to forage. In contrast to the adults, juvenile elephant seals from King George Island appear to avoid the sea ice and range in deeper, open water (Bornemann et al. 2000 ).

There are four species of ice-breeding seals in the Antarctic: Weddell (Leptonychotes weddelli), crabeater (Lobodon carcinophagus), Ross (Ommatophoca rossii), and leopard (Hydrurga leptonyx) (King 1983 Knox 1994 Laws, in Laws 1984 ) seals. The crabeater is the most abundant seal in the world, although the total population is probably much less than the early estimates of 30 million (Fig. 9.11 ). The Weddell and crabeater seals afford a contrast in use of the ice. The Weddell breeds on the nearshore ice while outside the breeding season they move to the outer limit of the fast ice and the inner zones of the pack ice. Here they are usually seen singly and their mean density has been estimated as 0.14 km⁻². The Weddell seal extends as far north as there is reliable nearshore fast ice in the winter.

(p. 275 )

(p. 276 ) Weddell seals do not feed on krill, but instead mainly feed on squid and fish, in particular the Antarctic silverfish, of which they can get a hundred or more in a single dive. Weddell seals can dive as deep as 700 m and can stay underwater for over an hour. It appears that they have two main diving habits; shallow dives within the top 100 m of the water column and deeper dives between 200 and 400 m (Plötz et al. 2001 ). The shallow dives tend to be the longer and the seals can travel distances in excess of 10 km from the breathing hole on a single shallow dive. Deep dives of short duration probably reflect feeding on benthic fish, whereas long shallow dives go to depths that do not take the seal out of sight of the surface and probably reflect feeding on fish on the underside of the ice. During these under-ice dives seals have been recorded blowing bubbles into fissures in the ice to flush out and capture fish.

It also appears that the seals preferentially haul out during daylight and spend the night in the water, and also that the deepest dives tend to happen during the day. One reason for these diurnal differences in diving behaviour may be related to the different distribution of the Antarctic silverfish at different times of day. The fish move into shallower waters under the ice at night, possibly to feed on krill that are feeding on ice algae on the undersides of the ice, and investigations into the stomach contents of silverfish that have been vomited up by Weddell seals have been shown to be full of krill.

During the winter the Weddell seal remains in the water and uses breathing holes that are kept open by rasping away the ice, using the well-developed incisor and canine teeth. The eventual wearing away of these teeth seems to be a common cause of death. In spring, pupping colonies assemble around breathing holes and predictable tide cracks and most pups are born, on the ice, by mid-November. Pups are suckled for a month or two but enter the water in their second week under their mothers’ care. Mating takes place underwater around midsummer. The Weddell seal is polygynous and a male is able to exert his authority over about 10 the females which frequent one particular breathing hole.

Crabeater seals (Southwell et al. 2004 ) are similar in size to Weddell seals but occupy a different niche in the sea-ice habitat, preferring pack to fast ice. They are more gregarious than Weddell seals and are sometimes found in summer in aggregations of as many as 600 within a radius of 5 km. Counting animals which are spread out over an enormous area of sea ice requires the combined use of aerial photography, ice-breakers, and helicopters to sample strips orientated north–south to penetrate the ice to a maximum distance. The data obtained indicate mean densities of about 4.8 km⁻² in summer and 0.5 km⁻² in winter and spring. Their usual food, somewhat contrary to their name, is krill and their molar teeth have prominent cusps well adapted for filtering out these shrimp-sized organisms and they seem to dive only to the relatively shallow depths where krill are most (p. 277 ) abundant. Pupping takes place on the ice from late September until early November but, because of their inaccessibility in the pack at this time, information about breeding behaviour is sparse. Unlike the Weddell seal and those seals which breed on land, the crabeater is monogynous. Satellite tracking studies, although somewhat limited in number, have shown that the male and female crabeater seal stay together on the ice after the pup has been born and separate once the female and pup enter the water.

A major predator on both of these seals is the killer whale (Orcinus orca). Another reknowned Antarctic predator, often labelled the polar bear of the South, is the leopard seal, which like the crabeater seals tend to live in the marginal ice zones traveling many thousands of kilometres on drifting ice floes. It is regularly recorded on sub-Antarctic islands during the winter. Forty-five per cent of the diet of leopard seals is made up from krill; however, they do also feed on penguins and in particular crabeater seal pups, and they frequently swim at ice floe edges or in waters close to penguin colonies waiting for there prey to enter the water, where the aggressive and agile leopard seals are seen to be superb hunters (Hiruki et al. 1999 ).

Not too much is known about the seasonal distribution of the estimated 200 000 Ross seals. This species is thought to feed predominantly on squid and fish, and it is rarely seen because it hauls out on the thickest ice in fields of pack ice where ships and researchers seldom venture. This species is highly dependent on sea ice for giving birth to its young and for hauling out when it moults. Most recent observations show that they can make up to 100 dives a day to depths between 100 and 300m, although the deepest dive was 762m (Blix and Nordøy, 2007 ).

9.4 Whales

The prerequisite for all whales in ice-covered waters is that there are sufficient areas of open water for the animals to surface and breathe. This is why so few whales are found deep within the pack ice during winter, and why typically whales migrate to sea-ice-covered regions from lower latitudes in summer months. However, where polynyas and areas of open water persist it is possible that whales can survive.

Whales, cetaceans, are of two kinds, the whalebone or baleen whales, Mysticetes, and the toothed whales, Odontocetes. Baleen whales feed by sieving out zooplankton, such as krill and copepods (Fig. 9.12 ), by means of rows of hairy triangular plates carried on each side of the palate. They include the blue (Balaenoptera musculus), fin (Balaenoptera physalus), which are found in both the Arctic and the Antarctic, and the common minke (Balaenoptera acutorostrata) and Antarctic minke (Balaenoptera bonaerensis) that are found in the north and south respectively. The toothed (p. 278 )

whales, which feed on larger prey, include the sperm (Physeter catodon) and killer (O. orca) whales. Many large cetacean species are found in both Arctic and Antarctic seas but, because their life pattern is to feed in polar waters and breed in equatorial waters and there is therefore a 6-month phase difference in breeding between the hemispheres, it seems that there is limited mixing of the stocks across the equator. In the Antarctic there is latitudinal zonation in the distribution of whale species.

The baleen whales feed near the surface, rarely diving to any great depth, and probably finding their prey by both sight and echolocation. The technique of feeding varies; the right whale skims the sea surface, swimming slowly with jaws agape, filtering water as it goes. The blue, fin, and minke whales take gulps of water plus krill then, with mouth closed, the water is forced through the baleen plates by expansion and pushing forward of the tongue with contraction of the ventral grooves which run backwards from the chin. Whales may concentrate krill by encircling a patch then diving to come up vertically beneath it with open mouth, or, by swimming beneath the surface and releasing a trail of air bubbles in which the kill become collected. Balleen whales caught off South Georgia had stomachs full, or nearly full, in 70% of cases as compared with less than 25% for those caught off South Africa. When feeding maximally, up to 4% of body weight is consumed daily.

Sperm whales feed largely on squid, which they evidently find by echolocation and it has been suggested that they can immobilize prey by a projected beam of sound. They can stay below the surface for as long as an hour. The maximum quantity of food found in their stomachs is about 200 kg but in one instance this consisted of one 12-m giant specimen of the squid Architeuthis. Again, daily consumption is about 3% of body weight.

(p. 279 ) Little is known of the feeding of the smaller Odontocetes—killer whales, pilot whales, and dolphins—in the polar regions. Killer whales have been classified according to whether they exhibit different behavioural traits and those that feed on fish tend to be resident in a certain area while those that feed on marine mammals are transients. In high-latitude pack-ice regions orcas often hunt in packs and co-ordinate attacks on large prey such as seals, penguins, and even the large baleen whales. They do not dive deeply but take their victims at the surface. Estimates of food consumption are 4% of body weight for killer whales, up to 6% for pilot whales, and up to 11% for dolphins. As with most animals, the relative food requirement increases as the size of the animal decreases.

It is generally thought that killer whales move out from pack-ice regions during winter, or at most keep to marginal ice zones. However, there have been several reported sightings of killer whales within winter pack ice (Gill and Thiele 1997 ). In the Antarctic on one occasion in August, 60 killer whales were spotted together with 120 minke whales in pools of open water that were cut off from the open sea by 65 km of compacted sea ice. On another occasion a group of 40 killer whales of mixed ages were spotted in leads of water, 400 km south of the ice edge. The presence of a calf within this group would indicate that the whales may have given birth in these sea-ice-covered waters. There have also been sightings of killer whales in Arctic winter sea ice off west Greenland and western Alaska.

In the Antarctic, two types of killer whale have been described, although it is unclear whether this distinction is strictly apropriate: The white form, that feeds on mammals (penguins, seals, and other whales), is found in more open waters and loose pack ice. In contrast the yellow form feeds mainly on fish and found deeper in the pack ice. The ‘yellow’ coloration comes from these whales being covered in thick biofilms of diatoms.

A small toothed whale in the Arctic are the Belugas (Delphinapterus leucas), also called white whales, that migrate over the Arctic often traveling far into the permanent pack ice (Richard et al. 2001 ). Populations of belugas have characteristic migration patterns: for example, Belugas summering along the north coast of Alaska, move great distances well offshore, in deep (>3000 m) water and beneath areas where there is almost complete ice cover to reach their spring feeding areas (Suydam et al. 2001 ). The whales can cover between 50 and 80 km a day through waters with more than a 90% ice cover. Presumably Belugas select their migratory routes in relation to the availability of their prey, such as Arctic cod. They are capable of deep (>300 m) dives during which they are thought to forage on benthic organisms (Martin et al. 2001 ).

Narwhals (Monodon monoceros) are probably among the most unusual looking of the toothed whales. All narwhals have two teeth in their upper jaw. After the first year of a male narwhal's life, its left tooth grows (p. 280 ) outward, spirally. This long, single tooth projects from its upper jaw and can grow to be a maximum of 3 m long. The hollow tusk is usually twisted in a counterclockwise direction. The tusk's function is uncertain, although it is not used to catch the narwhal's prey of fish and squid. It is thought that narwhals can dive to depths greater than 1500 m which makes them the deepest divers of all mammals, and during winter they tend to move from coastal shelf regions where they spend the summer to living over deep Arctic basin waters where they are thought to feed on deep-sea squid (Heide-Jørgensen et al. 2002 ). They are closely associated with the Arctic pack ice throughout the year and utilize leads and polynyas for moving throughout the pack ice in winter. Neither Beluga whales or narwhals have dorsal fins. This enables them to swim close to the underside of ice floes, and this also enables them to break thin ice with their backs.

Whales have a considerable impact on the pelagic ecosystems of polar regions. Before whaling reduced stocks so drastically, baleen whales in the Antarctic took an estimated 190 million t of krill per annum and the sperm whale 10 million t of squid, the corresponding recent figures being 43 and 4.6 million. Possibly because of a greater availability of food for those which have survived, baleen whales have recently shown increased growth rates, earlier maturity, and higher pregnancy rates than formerly. There is now evidence of population increases of baleen whales in many regions; however, the large nature of the ecosystem changes brought about by whaling mean that they may not necessarily reach their pre-exploitation population sizes. Because the large whales are migratory they export the biomass accumulated in polar waters to equatorial regions where they undergo almost total fast. This is an estimated loss of some 18 million t per annum of whale material from the Antarctic, and a corresponding enrichment in energy and nutrients of their breeding grounds (Brown and Lockyer, in Laws 1984 Ainley and DeMaster, in Smith 1990 Knox 1994 Pielou 1994 ).

9.5 Bears and foxes

Although they produce their cubs on land and are closely related to the terrestrial grizzly bear (Ursus arctos), polar bears (Ursus maritimus; Sage 1986 Pielou 1994 ) are essentially sea-ice animals and are strong swimmers (Fig. 9.13 ), aided by well-developed fat layers for buoyancy. Their large forepaws are used as effective paddles to swim with, whereas they use their rear paws as stabilizers or rudders. Apart from occasionally eating berries, seaweeds, or grasses, and a recently developed habit of raiding rubbish dumps, they are dependent on the sea for food, which is mostly seals. They are well adapted to survive low temperatures by virtue of their large size and fur, which has six or seven times the insulating capacity of (p. 281 )

the clothing normally worn by humans at rest in an ambient temperature of 20°C. The fur (made from hollow fibers) is colourless and transparent to short-wave radiation, which is absorbed by their black skin, but opaque to infrared: hence polar bears are not detected by infrared imaging equipment. The thick undercoat of fur is a highly efficient insulator, and so good that after vigorous exercise or in warm weather bears are likely to overheat.

Polar bears are found throughout the ice-covered waters of the Arctic Ocean. The greatest majority of bears roam near pack ice that is thinner or breaks open on a regular basis. Generally bears avoid heavily ridged, rough sea ice and thick multi-year ice, mostly because the densities of seals are low in these ice types. The southern limits of polar bears is basically governed by the southernmost extent of sea ice, and a few have been reported close to the north pole, although generally it is thought that very few stray further than 80°N since the ice generally comproses thick multi-year ice floes.

It is a difficult task to estimate how many polar bears there are, but reliable estimates place the number at about 40 000 in the whole of the Arctic basin, although more cautious researchers would say that it lies somewhere between 20 000 and 40 000. These bears are not part of one large population, but rather divided up into several subpopulations. Bear have been tagged and recaptured since the 1970s, and more lately bears have been equipped with radio transmitters and their positions logged using satellite tracking systems. The results of these studies show that from year to year individual bears remain in the same geographic region.

However, on shorter time scales bears have been shown to travel in excess of 30 km a day for several days in a row. Therefore, within a year many (p. 282 ) polar bears may travel many hundreds, if not thousands, of kilometres. These large roaming distances are of course extended because the bears are travelling on a moving platform of ice that is blown by wind and carried on ocean currents, transporting the ice over many thousands of kilometres. Tracking studies have demonstrated clearly that the bears are not roaming aimlessly but that they know exactly where they want to go. Although much of these distances are completed on top of the ice, because polar bears are such strong swimmers, they can swim distances in excess of 100 km at a time. This results in polar bears sometimes being observed several hundreds of kilometres offshore, probably because the ice floes they were travelling on had melted from underneath them (Fig. 9.14 ).

Polar bears from Svalbard and the Barents Sea that roam huge territories (up to 270 000 km²) accumulate significantly higher concentrations of polychlorinated biphenyls (PCBs) in their fat, blood, and milk compared with bears confined to smaller coastal territories. PCBs are pollutants that used to be used in electrical equipment and are particularly resistant to breakdown when released into the environment. The bears that are covering large distances have to consume significantly greater food reserves than the bears with smaller ranges. The PCBs are contained within the prey, and so since they need to consume more prey they consequentially consume more of the pollutants (Haave et al. 2003 ).

Polar bears largely lead a solitary existence, and bears may have to travel considerable distances in April to find a mate. After mating females start to feed in excessive amounts (mostly on seals) to gain weight as quickly as possible. Typically female bears weigh 150–175 kg, but before they give birth to their cubs they may have laid down fat stores so that weigh up

(p. 283 ) to 450 kg. In late August to October the females move to land where they make maternity dens, and once they enter they have no opportunity to feed again until the following spring. These dens are made in the hard snow of snow drifts or snowbanks, and some dens may be as far as 100 km inland from the coast, although typically they are within 50 km. Maternity dens can also be dug in earth banks in the permafrost. The dens tend to be 2–3 m³ and may have an exit hole 2 m away from the den itself. Cubs (normally twins) are born in November and are nursed by the mothers with fat-rich milk until late February or March. When born the cubs are less than 1 kg in weight, and when they emerge from the den they weigh between 10 and 15 kg. Cubs remain with their mothers up to 2.5 years and so females normally only have cubs once every 2 or 3 years. The mortality rate of cubs can be as high as 40% in the first year, but if the bears make it to adulthood they can be long lived, reaching 30 years of age, because they have no natural enemies (except humans, of course).

Polar bears feed predominantly on ringed seals and bearded seals as well as other species of seal and, occasionally, small whales. They catch seals by lying quietly in wait by their breathing holes and also attack female ringed seals and pups in their lairs. The number and survival of cubs shows great variation, as it is dependent on the level of fat storage in the females prior to giving birth. This in turn is dependent on the availability of ringed seal pups which is linked to fluctuations in key/climatic factors (Stirling and Lunn, in Woodin and Marquiss 1997 ). Polar bears have been hunted by humans, probably causing population declines in some areas, but they are now protected and hunting is managed closely.

The Arctic fox (Alopex lagopus; Sage 1986 Pielou 1994 ), although chiefly an animal of the land, is a scavenger of the leftovers of polar bears and is itself an important predator on pups of the ringed seal, catching them, like the polar bear, in their birth lairs. The Arctic fox does not migrate seasonally, and its fur is the most effective insulator known for any mammal, meaning that its thermoregulation is not challenged even under the most extreme conditions faced in the Arctic winter. From a 3-year study in the Canadian Arctic an average seal-pup predation level of 26% by Arctic foxes was estimated on nearshore ice. Stable isotope analysis of diet of Arctic foxes have shown that the marine component of their diet increases during periods when lemming populations are low. Dalén et al. ( 2005 2006 ) used molecular techniques, among others, to look at Arctic foxes, in particular Scandinavian populations. They have reported large decreases of fox populations despite over 65 years of protection, and that the Scandinavian populations is threatened due to dwindling habitat, a decrease in food availability, and an increase in the numbers of a competing species, the red fox (Vulpes vulpes).