Antarctic Marine Mammals

The Southern Ocean is the oceanic region surrounding the continent of Antarctica. Its southern boundary is defined by the narrow coastal continental shelf of Antarctica itself. To the north the boundary is defined by an oceanic frontal feature known as the Antarctic convergence or southern polar frontal zone. This zone marks the boundary between cold southern polar waters and northern temperate waters. The ocean temperature can change by as much as 10°C across the front, which may be only a few miles across. The polar front is an important physical feature that determines marine mammal distributions. It defines the normal southern extent of the distributions of most tropical and temperate marine mammals (Fig. 1).

A second feature that is important to marine mammals in the Antarctic is the annual sea ice. The seasonal changes in sea ice cover can lead to up to 50% of the Southern Ocean being covered in ice during late winter, but by late summer this can have contracted to 10% of the winter maximum. These large seasonal fluctuations in the sea ice have profound implications for the ecology of the Southern Ocean, including that of marine mammals. Many marine mammals, including most cetaceans, migrate north across the polar front in winter.

I. Antarctic Species

This section deals with true Antarctic species defined as those species whose populations rely on the Southern Ocean as a habitat, i.e., critical to a part of their life history, either through the provision of habitat for breeding or through the provision of the major source of food. Species that inhabit the sub-Antarctic. which is generally seen as including the islands that circle Antarctica in the region of the polar front or the polar frontal zone itself, are not included.

The Southern Ocean accounts for about 10% of the world’s oceans but it probably supports >50% of the world’s marine mammal biomass, including six species of pinnipeds, eight species of baleen whales, and at least seven species of odontocete whales. Therefore, in terms of the diversity of species, the Antarctic is host to only one-fifth of the world’s pinniped and a little less than one-fifth of the world’s cetacean species. This low diversity may be attributed partly to the lack of land masses to cause isolation and speciation and also because, although large in its total area, the Southern Ocean does not have the diversity of habitats and prey species seen in other ocean basins.

Among the pinnipeds, there is one species from the family Otariidae (eared seals, which include fur seals and sea lions) and there are five species from the family Phocidae (earless or “true”‘ seals), but all of these come from a single subfamily, the Monachinae (see Table I). This list is as notable as much by its absences as it is for those that are present. For example, there is no representative of the phocid subfamily Phocinae, which contains a diverse collection of species of Northern Hemisphere seals. There are also no representatives of the subfamily Otari-inae. which includes all of the sea lions, and there is only one representative of the diverse Southern Hemisphere subfamily Arctocephalinae. which includes the southern fur seals.

Where pinnipeds are concerned, historically it would appear that there have been only two or three species immigrating into the Antarctic. The main immigration was of an ancestral phocid. possibly related to the nearly extinct tropical phocids of todav known as monk seals, which gave rise to the four most closely related Antarctic phocids: the crabeater. Wed-dell, Ross, and leopard seals. At some later date it is likely that elephant seals arrived. Although they extend their distribution into south temperate latitudes, as much as 90% of the world population relies on the Southern Ocean as a critical habitat.

Figure 1 Waters of warmer northern seas meet the perpetually colder waters around Antarctica to create the Antarctic Convergence. This confluence of relatively warm and frigidly cold waters occurs in some of the roughest seas known.

These were likely to have been followed, or perhaps even preceded, by Antarctic fur seals. The taxonomie status of Southern Hemisphere fur seals, a group within which eight species are currently recognized, is uncertain and it seems probable that many of these are not true species but are instead subspecies. Therefore, the Antarctic fur seal may simply be an Antarctic race or subspecies of the southern fur seal.

Among cetaceans, there are only three Antarctic species within the highly diverse family Delphinidae. which includes all of the dolphins and porpoises. These three are the hourglass dolphin, long-finned pilot whale, and killer whale. The beaked whales are represented by only three species, but because these species are very difficult to identify in the field, it is possible that among the very large number of these individuals that are found in the Southern Ocean, several other species could be present.

II. Ecology of Antarctic Marine Mammals

The presence of a large biomass of marine mammals in the Antarctic is probably a result of the unusual food chain structure of the Southern Ocean. The marine mammals of the Antarctic with large numbers, such as crabeater seals and Antarctic fur seals, rely on krill as their main food source (see Section IV). This is in contrast to marine mammal communities elsewhere that rely mainly on a fish-based diet. Energy enters die food chain through photosynthesis and carbon sequestration by phytoplankton. The relative efficiency with which this energy is passed up the food c-hain to predators with a krill- or fish-based diet is illustrated in Fig. 2. The efficiency of energy transfer at each step in the food chain can be as low as only a few percent. The fewer steps there are between phytoplankton and marine mammals, the more efficiently will energy be transferred to marine mammals. In the Antarctic, there is 011 average one less step than there is in other oceanic ecosystems, which has led to the very large biomasses of marine mammals found in the Southern Ocean.

III. Distribution and Abundance

Antarctic marine mammals can be divided ecologically among those associated with fast ice, pack ice, or found in the open ocean. Weddell seals are most associated with fast ice. Ross seals with open water or pack ice. Leopard seals are animals mainly of the pack ice zone, but they may also be found feeding at penguin and seal colonies north of the pack ice zone. Crabeater seals travel extensively within the pack ice zone and individuals may have a potential range that extends to the total area of the Antarctic pack. The same may be true for Ross seals, although relatively little is known about the biology of these animals. They have been recorded to migrate north of the pack ice zone. Weddell seals appear to be relatively sedentary, forming more or less isolated populations around the coast of Antarctica.

TABLE I

Species and Common Names. Abundances, Trends in Abundance, and Conservation Status of Antarctic Marine Mammals

“Cetacean abundances are from most recent estimates, but are not known as precisely as shown here. The number of asterisks denotes the range in the size of the populations: “, 0 to 1000; 1000 to 10,000; 10,000 to 100.000; “**, 100,000 to 1,000,000; 1,000,000 to 10.000.000; and 7,000.000 to 14,000,000.

”Minke whale abundance is for the two species combined.  

Elephant seals are known to feed within the pack ice zone on occasion, but they are mainly animals of the open oceans north of the pack ice zone. Antarctic fur seals are sometimes found along the boundary between the pack ice and the open ocean but, again, they are mainly animals of the open ocean. Perhaps up to half of both the Antarctic fur seal and southern elephant seal populations migrate north of the polar front during the winter.

Toothed whales have a stratified distribution within the Southern Ocean relative to the polar front and the edge of the pack ice (Fig. 3). Some species, such as long-finned pilot whales and hourglass dolphins, are more closely associated with the polar front, whereas others, such as killer whales, are more often present close to the pack ice. Bottlenose and killer whales are the only cetaceans regularly associated with a distribution within the pack ice zone, but they are also present within the open ocean. Bottlenose whales appear to be able to survive comfortably among almost continuous sea ice cover. Killer whales have been recorded in pack ice in winter, but the only baleen whale found regularly in the pack ice zone is the minke whale. Other larger species, including the sperm whale, are restricted to the open ocean, but during the summer they may feed along the boundary between the pack ice and the open ocean. In general, these species are absent from the Southern Ocean during the winter. In the case of the sperm whale, only males are found within the Southern Ocean as females remain north of the polar front throughout the year.

Figure 2 Simplified diagram of energy flow to marine mammals as top food chain predators in marine food chains. This shows the contrast in efficiency of transfer when energy mainly takes a direct route, as in the case of marine mammals in the Southern Ocean, compared with the more indirect route involving more steps in the food chain. The percentage of the energy taken in by phytoplankton that subsequently reaches the top predators is shown at the top of the diagram.

Figure 3 Distribution of odontocete whales in the Southern Ocean relative to the southern polar front and the edge of the pack ice. Reprinted from Kasamatsu and Joyce (1995) with permission of Cambridge University Press.

Marine mammal distributions are also affected by bathy-metric and oceanographic conditions. Southern right whales, which are possibly from the same population that winters at Peninsula Valdes, Argentina, and along the coast of South Africa, spend the summer foraging over the continental shelf of South Georgia within the Southern Ocean. Baleen whale and Antarctic fur seal abundance around south Georgia is also influenced by the local oceanography so that there are regions of predictably high abundance of these marine mammals at specific points along the edge of the continental shelf. Southern elephant seals also appear to migrate from breeding and molting grounds on sub-Antarctic islands to shallow regions along the coast of Antarctica. Most of these types of preferences for different locations are assumed to reflect the distribution of food so that marine mammals migrate to the areas of greatest food abundance.

Figure 4 Populations of blue whales (Balaenoptera musculus) have declined around the world and especially in Antarctica.

The crabeater seal is probably the most abundant seal in the world, with a population of somewhere between 7 and 14 million. There are considerably fewer Weddell seals and leopard seals (Table I). Ross seals are rarely seen and the total number is very uncertain, but it is probably the least abundant Antarctic pinniped. The Antarctic fur seal population is >3 million and is increasing at about 10% each year. In contrast, the southern elephant seal population within the Antarctic appears to have been relatively stable since the early-1960s, even though the number of elephant seals breeding at sites outside the Antarctic has declined steadily over the same period. The elephant seal population at South Georgia is estimated at 470,000, which probably represents 58% of the world population of the species.

In general, whale populations are in a highly depleted state (Table I). Blue whales are numbered in the hundreds for the whole of the Antarctic, and the sighting of a blue whale is a rare event. The number of fin whales appears to be increasing, as are humpback whales and southern right whales.

Within the Antarctic, there are no significant threats to pinniped species. However, some cetacean populations have been depleted to such a high degree that several are endangered. In particular, blue whales are so rare in the Antarctic that they are possibly close to extinction from the area (Fig. 4). Similarly, severely depleted southern right whale and humpback whale populations have very specific migratory routes between summering grounds in the Antarctic and winter grounds in temperate and tropical regions, which make them more vulnerable to threats such as disturbance, habitat loss, and reduced genetic diversity.

IV. Diet

Among seals, there is a progression of dietary specialization from those that mainly eat krill to those that mainly eat fish (Fig. 5). The leopard seal has seabirds and other seals as a major component of its diet, and it is probable that some individuals specialize in feeding on other seals or penguins instead of krill, fish, or squid. Among whales, dietary specializations are divided along taxonomie lines between odontocetes that mainly eat squid and mysticetes that forage primarily on zooplankton.

The crabeater seal is one of the most ecologically specialized of all seals because it feeds almost entirely on Antarctic krill that it gathers from the underside of ice floes where the krill themselves feed on the single-celled algae that grow within the brine channels in the ice. Antarctic fur seals also feed on krill to the north of the Antarctic pack ice edge, and many of the other Antarctic seals rely, to varying degrees, on krill as a source of food. Antarctic krill probably sustains more than half of the world’s biomass of seals and also sustains a substantial proportion of the biomass of the world’s seabirds and whales.

Although the dentition of crabeater seals is modified to help strain krill from the water, the feeding apparatus of the baleen whales is the most highly modified for a diet of plankton. Krill is the major component in the diet of most of the Antarctic baleen whales, although copepods may also be strained from the water, especially by right whales. The Antarctic krill, Eu-phausia superba, often occurs in dense swarms in the open ocean, and the baleen whales have probably evolved to exploit these dense patches of food. Baleen whales eat 30-50 million tons ol krill in the Antarctic each year and seals probably eat a similar or slightly lower total amount as whales. Consumption of squid by beaked whales and sperm whales is estimated to be about 14 million tons each year. Killer whales prey on fish and squid but also hunt seals and penguins. Pods of killer whales have been observed tipping over ice floes to push crabeater seals into the water in an effort to catch them.

Figure 5 Pie charts showing the composition of diets of Antarctic seals. The charts are atranged with those species that depend most on krill at the top and those that depend most on fish at the bottom: Betiueen these are those species that have squid as a major component of the diet.

V. Exploitation

Throughout the 19th and early 20th centuries, the Antarctic was viewed as an almost limitless source of marine mammals to be hunted for skins, oil, and other products that found expanding markets in Europe and North America. However, industrialization of whale and seal hunting brought both greater efficiency and the inevitability that the resources would be exhausted, much to the detriment of the ecology of the Antarctic and its populations of marine mammals.

There were three phases of exploitation: exploratory sealing (late 18th and earl}’ 19th centuries), preindustrial sealing and whaling (19th century), and industrial whaling (20th century). There are very few records of the exploratory sealing and the preindustrial era. During the exploratory era, exploitation was mainly targeted at fur seals to supply skins for the Chinese market, where they were turned into felt to supply the European market. By about 1830, fur seals in the Antarctic and elsewhere in the Southern Hemisphere had been all but extinguished. In 1825, James Weddell, himself the captain of a sealing vessel, noted that “the number of skins brought from off Georgia cannot be estimated at fewer than 1,200,000.” He was referring to South Georgia, where >95% of the current world population of Antarctic fur seals resides. This species was considered to be extinct until the early 1920s when whalers saw several individuals at South Georgia. Since then, the numbers have increased rapidly and the population is conservatively estimated to now be on the order of 3 million. The preindustrial era was mainly targeted at whaling and the larger seals, particularly elephant seals, for their oil. This activity was mainly undertaken from sailing vessels. The introduction of steam power to the Antarctic was largely responsible for the transition to industrial whaling.

Industrial whaling began in the early years of the 20th century. This industry operated for more than 60 years and in that time it removed about 71 million tons of whale biomass involving 1.4 million individual whales from the Antarctic; about 10% of these were taken at South Georgia. Antarctic fur seals feed on krill (Fig. 5), and may have benefitted by the reduction in numbers of krill-feeding baleen whales and therefore had less competition for their food.

The industry was selective about which species of whales it targeted. The largest and most profitable were selected first, followed by progressively smaller species (Fig. 6). Eventually, the industry became unprofitable because only minke whales were left to exploit and these were too small to be profitable.

VI. Conservation Measures

Concerns about the effects of industrial whaling on the populations of whales began early in the industrial era. By the early 1920s, the “Discovery Investigations” had been established to determine whale populations mainly around South Georgia. These were funded by a levy on the industry, but they were free from control of the industry. They are one of the first examples of the fledgling field of ecology being used to solve a wildlife management problem. Even though the “Discovery Investigations” made ground-breaking scientific progress and were influential in the introduction of some conservation measures, they came too late to influence the power of the industry and the fate of the populations of whales in the Southern Ocean.

Figure 6 Changes in the number of each species of whale caught in the industrial whale harvest in the Southern Ocean.

The story of overexploitation of a marine resource in the Southern Ocean repeated itself in the 1960s and 1970s when industrial fisheries targeted fin fish populations and reduced them to uneconomic levels. This stimulated a renewed effort to ensure that there was proactive conservation of marine living resources in the Southern Ocean. The result was the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR) and the Convention for the Conservation of Antarctic Seals (CCAS), which came into effect in 1982 and 1978, respectively. One of the unique features of the CCAMLR convention is that it accepts that exploitation has effects on components of the ecosystem far beyond those that are being targeted for exploitation. This means that any proposals for the exploitation of living resources in the Antarctic must consider the effects that such exploitation is likely to have on marine mammals, whether or not they are the target species. Therefore, even though marine mammals enjoy legal protection in the Antarctic from unregulated exploitation under the environmental protocol within the Antarctic Treaty, they are also protected from other activities within the Southern Ocean ecosystem. Only time will tell if this is sufficient to ensure their long-term survival.