The living environment: Vertebrate populations

2016

Vertebrate populations

Antarctic vertebrates encompass a variety of flying seabirds and penguins, several seals and whales, and numerous fish. The species diversity, especially on the Antarctic continent, is much lower than in temperate and tropical regions, and even the Arctic. However, the abundance of many species is very high. All highly depend on the Southern Ocean for food, and their breeding areas include terrestrial, fast ice and marine regions. Many of the large air-breathing vertebrates are also highly migratory and explore areas far outside the Antarctic and the Southern Ocean.

Status and trend data are available for only a few species—notably, 2 penguin species on the continent, some albatross populations, the giant petrel populations, and fur and elephant seals at Macquarie Island. Long-term population data do not exist for the ice-breeding seals and whales, most of the flying birds, and some of the penguins at Macquarie Island. Hence, trends and status are difficult to establish. Heard Island and McDonald Islands are visited infrequently, and data are largely lacking.

Fish

The fish fauna of Antarctica is unique. Their species composition and, to a large extent, their distribution in the Southern Ocean have been well documented. Some 322 species are recognised in the Antarctic waters, but only about half (161 species) live in the high Antarctic—that is, south of the Antarctic Polar Frontal Zone (Eastman 2005). Of these, the vast majority (77 per cent) are notothenioids (a group of about 120 species that are found almost exclusively in the Southern Ocean). This is the most diverse group, with 129 species belonging to 5 families (Near & Cheng 2008). Their biomass makes up 91 per cent of the Antarctic fish fauna (Eastman 2005). Notothenioids have lived in the Antarctic environment for millions of years and are well adapted to life in a polar ocean. These fish evolved to produce glycoproteins that act as antifreeze agents in their blood, which is key to their survival in the freezing temperatures (Devries 1971).

There is little information on the status and trends of Antarctic fish populations. This is partly because of the vast area covered by the Southern Ocean, which renders population surveys near impossible, especially those frequent enough to estimate abundance and trends. Hence, formal stock assessments are only available for some of the exploited fish populations.

Historically, vessels from the Soviet Union and other Eastern Bloc countries conducted large-scale fishing operations in the Southern Ocean off the AAT in the mid-1960s. Marbled rock cod (Notothenia rossii) was caught in such quantities that the stock had declined by the 1970s and was depleted by the end of the 1980s. Off South Georgia, stocks had all but disappeared after only 2 years of fishing (Kock 1992). Currently, only 2 species of finfish are harvested in Australia’s exclusive economic zone at Heard Island and McDonald Islands, and Macquarie Island: Patagonian toothfish and mackerel icefish. The latter is being targeted only at Heard Island and McDonald Islands.

CCAMLR regulates all legal commercial catches, but illegal, unreported and unregulated fishing still occurs in the high seas of the CCAMLR area, albeit at probably lower levels than in the 1980s, 1990s and early 2000s. It is worth noting that, since 2004, no illegal fishing was reported in the exclusive economic zone around Heard Island and McDonald Islands.

Whales

Of the 14 recognised species of baleen whales, 8 frequent the Southern Ocean. Some 28 species of toothed whales also use the southern polar regions, and at least 22 appear to remain in the Southern Ocean year round (Van Waerebeek et al. 2010). Toothed whales are less well known in the Antarctic region than baleen whales (Leaper et al. 2008a). Some species, such as the baleen humpback whale (Megaptera novaeangliae), spend summer in Antarctic waters, but in autumn migrate north to warmer waters where they give birth to their young.

By the mid-1900s, several great whale species—for example, blue (Balaenoptera musculus), humpback and sei (B. borealis) whales—living in the Southern Ocean had become nearly extinct after decades of intensive hunting (Hutchinson 2006). Currently, despite efforts to protect them by banning commercial whaling and declaring the Southern Ocean an international whale sanctuary, rates of recovery vary among species and by region, and some populations still show no sign of recovery (Leaper et al. 2008b). The reasons for this are largely unknown. Blue, fin (B. physalus) and sei whales are listed by the IUCN as endangered, and sperm whales (Physeter macrocephalus) are classified as vulnerable to extinction. Officially, blue whales have not been hunted for 65 years, but there are only very limited indications of a possible population recovery (Ballance et al. 2006). The reason for this is probably illegal and unreported hunting. For example, in the 1960s, the Soviet Union reported catching 156 blue whales; however, the actual number appeared to be 1433 (Yablokov 1994). A more recent study of blue whales provided evidence of a slow increase in their global population, but their numbers still remain well below pre-exploitation levels (Branch 2007). In comparison, humpback and southern right whales (Eubalaena australis) are comparatively abundant again and are listed as being of least concern. However, the vast abundance of whales from pre-industrial times will likely remain a thing of the past (DEWHA 2009).

Knowledge about the size and structure of whale populations is very limited. Estimating the abundance of whale populations is a complex and complicated process. Issues can arise with species identifications, the timing of surveys, the areas covered and even the response of species to the presence of vessels (Leaper et al. 2008b). Many species have a circumpolar distribution, but their species-specific behaviours differ significantly. For example, although sperm whales use the Southern Ocean during the austral summer, it is generally only the large males that visit the regions south of the Antarctic Polar Frontal Zone (Leaper et al. 2008a). Sperm whales rank among the least well known whale species. Their population was severely reduced through commercial activities, particularly from 1945 to 1975. The most recent assessment concluded that, globally, sperm whales are currently at 32 per cent of their pre-whaling population levels (Whitehead 2002). A survey off Western Australia also documented that this species shows no sign of recovery (Carroll et al. 2014).

From 1978–79 to 2003–04, 3 circumpolar ship-based surveys of Antarctic minke (B. acutorostrata), sperm, fin, blue, humpback and killer (Orcinus orca) whales were done in the ice-free areas of the Southern Ocean (south of 60°S) to estimate their abundance (Branch 2006). Despite some methodological differences, the estimates for the populations of sperm, fin and killer whales increased from the second to the third survey. Minke whale numbers, however, were lowest during the third survey compared with previous estimates. Possible explanations include that more minke whales had travelled into the pack ice and that a larger number of whales were missed on the survey track (Branch 2006).

The most comprehensively studied whale is the humpback whale; its distribution and stock abundance are probably the best known of any whale species. The International Whaling Commission distinguishes 7 separate breeding stocks of humpback whales in the Southern Hemisphere, and an eighth that occupies the northern Indian Ocean but does not migrate to Antarctic waters (Branch 2011). The 3 circumpolar surveys comprehensively assessed the whales’ summer abundance. The various breeding stocks of humpback whales vary in size (Branch 2006). The largest stock is probably breeding stock D, which migrates annually from summer feeding grounds in Antarctica to north-western Australia for winter (Kent 2012). This stock has a long history of exploitation (Chittleborough 1965). However, recent surveys indicate that the stock is increasing to a point that its delisting as a threatened species under Australian legislation has recently been proposed (Bejder et al. 2016).

Seals

Four species of seals—crabeater (Lobodon carcinophagus), leopard (Hydrurga leptonyx), Ross (Ommatophoca rossii) and Weddell (Leptonychotes weddellii)—inhabit the sea ice zone that surrounds Antarctica. They rely on the sea ice at critical stages of their lives, particularly in their reproductive and moulting periods. Their populations are difficult to study because these seals are highly mobile, are dispersed across large and inaccessible regions, spend long periods of time foraging in the ocean where they are difficult to survey, and do not appear to occupy set territories. Sightings are usually of individuals or very small groups. Surveys to estimate their population sizes are infrequent, because the studies are expensive and labour-intensive. Consequently, population trends are largely unavailable.

In the past, estimates of the global population of crabeater seals ranged from 2 to 5 million individuals in the mid-1950s (Scheffer 1958), to about 75 million in the early 1970s (Erickson et al. 1971) and 11–12 million in 1990 (Erickson & Hanson 1990). Based on surveys conducted in 1972–73, Laws (1984) estimated that there were 772,000 crabeater seals in the Wilkes Land region of East Antarctica, and postulated that these seals should increase in number because of all the ‘excess’ krill available after many krill-eating whales had been removed from the Southern Ocean. A detailed aerial survey of 1.5 million km2 from 64°E to 150°E, roughly coinciding with the area where Laws operated, was conducted in 1999–2000. If Laws’s krill surplus hypothesis had been correct, several million crabeater seals could have been expected. However, the survey estimate for crabeater seals yielded fewer than 1 million individuals in the survey area, with a range of 0.7–1.4 million. Thus, it appears that crabeater seals are abundant, but that earlier estimates were too high. Leopard and Ross seals are probably also abundant, but less so than crabeater seals, with numbers in the tens of thousands (Southwell et al. 2008a,b).

Crabeater, leopard and Ross seals inhabit the northern region of the sea ice that consists of ice floes of varying sizes and density, known as pack ice. Weddell seals, on the other hand, are found on the fast ice—the sea ice that is attached to the continent. How the pack ice seals respond to environmental stressors may vary among species (Ainley et al. 2015). However, changes in the structure and size of ice floes could lead to the loss of pupping platforms. A reduction in sea ice persistence may decrease the availability of Antarctic krill, an important food source for all pack ice seals. However, if coastal ice-free areas increase in size, crystal krill (Euphasia crystallorophias) may become more abundant and may partially offset the loss of Antarctic krill (Pakhomov & Perissinotto 1996). Leopard seals have the most diverse diet among the ice seals and are the least likely to be immediately affected by changes in food availability. However, depending on the rate, kind and magnitude of change, they are likely to be affected eventually.

Fur seals (Arctocephalus spp.) and southern elephant seals (Mirounga leonina) inhabit the subantarctic islands, but can be encountered as far south as the Antarctic continent. Southern elephant seals, particularly young males, have numerous haul-out sites on the Antarctic continent, where they spend most of the austral summer moulting. Fur seals are infrequent visitors. Although several fur seal populations still appear to be increasing, albeit at varying rates depending on the location, the numbers of southern elephant seals at Macquarie Island appear to be generally declining (Figure ANT12). The reasons for this are unknown and difficult to investigate, because this species migrates across long distances for 8–10 months per year (Learmonth et al. 2006). However, elephant seals are probably subjected to different pressures throughout the year, as well as at various stages of their lifecycle. For example, sea ice extent in the summer foraging areas of females may be linked to survival of their offspring. Extensive sea ice may exclude the seals from high-quality foraging areas, which in turn negatively affects the survival of their pups (van den Hoff et al. 2014).

Flying seabirds

Globally, 28 per cent of seabirds are threatened, and 5 per cent are critically endangered, making them the most threatened group of birds. Among them, petrels and albatrosses (Procellariiformes), and penguins (Sphenisciformes) make up 43 per cent of threatened taxa (Croxall et al. 2012). Populations comprising less than 100 breeding pairs are inherently vulnerable. About one-third of global albatross populations are in this category, including most breeding populations at Australia’s subantarctic islands.

Seabirds typically live for several decades; they mature late and lay only 1 or 2 eggs per year, which usually do not get replaced when lost. Also, some albatross species breed only every second or sometimes third year. Although adult survival is usually very high (around 95 per cent of adults return the following year to their colonies), their low annual reproductive output does not enable seabirds to withstand even small increases in their natural mortality rates.

One of the most serious threats to seabirds, particularly those breeding at lower latitudes on the subantarctic islands, is commercial fishing operations. Within the Australian jurisdiction, incidental seabird mortality is strictly controlled and regulated. However, seabirds fly enormous distances and often forage in the high seas in international waters, where they interact with the pelagic longline fisheries. Seabirds get hooked when they scavenge for food behind the vessels; as the line sinks, they drown. Progress is being made, particularly through the efforts of the Agreement on the Conservation of Albatrosses and Petrels, in developing and improving best practices and procedures for minimising seabird deaths in fisheries. Efforts include improvements in line weighting in longline fisheries, development of underwater bait-setting devices (which deliver baited hooks out of reach of most diving seabirds) and implementation of new technologies to avoid bycatch in trawl fisheries.

On land, flying seabirds may experience disturbance by humans, loss of breeding habitat, increased competition for nest sites, and increased exposure to parasites and pathogens. On subantarctic islands, their breeding success can be reduced directly by non-native predators, such as cats, rats and mice. This is what happened, for example, on Macquarie Island (see Box ANT1). Introduced predators are a key threat, because they could increase the natural mortality among adult birds and reduce breeding success when chicks are preyed upon. Heard Island and McDonald Islands have so far remained free from introduced vertebrates. Introduced species, such as rabbits, can also have an indirect effect when overgrazing leads to destabilisation of the substratum, which in turn can lead to an increase in landslides across breeding areas (Scott & Kirkpatrick 2007).

Most seabird populations in the Antarctic are only infrequently surveyed, because it is difficult to access their colonies or because of their cryptic behaviour (the ability of an animal to avoid observation or detection) during breeding, or both. Hence, for many species, it is difficult to assess their population status reliably. In June 2015, the members of the Antarctic Treaty officially recognised the recently published report on Important Bird Areas in Antarctica, which identified some 204 areas as important for the conservation of flying seabirds and penguins (Harris et al. 2015).

Penguins

Penguins make up about 90 per cent of the biomass of seabirds in Antarctica (Bargagli 2005). Like all seabirds, they are long-lived and produce only 1 or 2 eggs per year. They often live in large colonies in the coastal areas of subantarctic and Antarctic islands. During the breeding season, the foraging areas of the breeding population are limited, because they need to return regularly to their colonies to feed their offspring. Of the 18 species in the penguin family, 7 live and breed in the AAT and Macquarie Island, but only emperor (Aptenodytes forsteri) and Adélie (Pygoscelis adeliae) penguins inhabit colonies in the eastern high Antarctic. Adélie penguins spend the winter months at sea and return to their breeding colonies during the southern summer, whereas emperor penguins breed during the winter months and fledge their young in summer. Consequently, these 2 species are subject to marine and terrestrial processes at different times of the year.

From 2005 to 2011, a large-scale aerial survey was conducted along 3800 kilometres of coastline in East Antarctica, extending from 59.4°E to 136.0°E. Some 13 previously unreported breeding sites were discovered, and no breeding sites appeared to have been abandoned, indicating that the breeding distribution may have expanded (Southwell & Emmerson 2013a). The populations at these new breeding locations ranged from 425 to 6130 individuals (Southwell & Emmerson 2013b).

In 2009, satellite imagery obtained a first count of the global population of emperor penguins and estimated that some 238,000 penguins occupied 46 colonies around the Antarctic coastline (Fretwell et al. 2012). In 2015, the IUCN revised the listing of emperor penguins to near threatened because of the predicted decrease of their population as a result of changes in the sea ice environment (BirdLife International 2015). In East Antarctica, the size of 3 colonies has decreased significantly since the mid-1970s; the reasons for this are largely unknown. A link between breeding success and sea ice extent was suggested for 2 colonies, but could not be confirmed for the third (Barbraud et al. 2011, Robertson et al. 2014).

The greatest threats for penguins in East Antarctica are likely to be loss of breeding habitat (in the case of emperor penguins), a reduction in food availability because of climate change and ocean acidification. Changes in sea ice conditions have varied consequences. For example, a reduction in the sea ice extent potentially shortens foraging distances, but also means reduced krill production (Bretagnolle & Gillis 2010). It is difficult to predict to what extent penguins may be able to adapt to environmental change, particularly as the rate of change is likely to increase once the ozone loss is reversed, making adaptation difficult for these long-lived species.

Klekociuk A, Wienecke B (2016). Antarctic environment: The living environment: Vertebrate populations. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/antarctic-environment/topic/2016/living-environment-vertebrate-populations, DOI 10.4226/94/58b65b2b307c0