Análisis de precios de transferencia

Antarctic fur seals (Arctocephalus gazella) are rapidly becoming one of the most abundant seal species in the Antarctic. From a state of near-extinction in the early ANTARCTIC: DEFINITIONS AND BOUNDARIES

twentieth century, the Antarctic fur seal has recovered to a population that could now exceed 5 million. The story of this rapid change reflects both the change in human attitudes to the exploitation of marine mam- mals and changes in the ecosystem of the Southern Ocean, probably also mediated by the impacts of humans on that environment.

The Antarctic fur seal is the only member of the family Otariidae (fur seals and sea lions) that is en- demic to the Southern Ocean. (Otariidae belong to the larger taxon Pinnipedia, which includes all seals, fur seals, sea lions, and the walrus.) Antarctic fur seals are one member of a group known as the southern fur seals, eight species that are distributed from the tro- pics to the sub-Antarctic. The taxonomic status of the southern fur seals is uncertain, and there is almost certainly a degree of introgression between the popu- lations of each species. Antarctic fur seals are known to hybridize with sub-Antarctic fur seals (A. tropica- lis) at Macquarie Island, and some species are suffi- ciently similar morphologically that it might be difficult to recognise them from the local species even if they were present on the breeding colonies. However, Antarctic fur seals are distinct in that they have a breeding chronology that differs from other southern fur seal species. Lactation lasts only 4 months compared with around 9 months in the other species of southern fur seals.

The Antarctic fur seal has a circumpolar distribu- tion. Breeding colonies are located mainly on sub- Antarctic islands, but more southerly colonies are found at the South Shetland and South Orkney Islands. Overall, there is little evidence of strong ge- netic isolation between any of these populations but there may be a weak division between a western re- gion containing the populations of South Georgia and Bouvetøya, which were the probable sources for populations at Marion Island, the South Shetland Islands, and Heard Island, and an eastern region containing the panmictic populations of Iˆles Kergue- len and Macquarie Island. The latter region may be a result of a pronounced founder effect, or represent a remnant population that survived sealing at Iˆles Kerguelen.

During the early twentieth century Antarctic fur seals were thought to be extinct, as a result of harvest- ing of their fur, but in 1922, three juvenile males were recorded at South Georgia by the captain of a sealing vessel. Surveys in the late 1950s showed a small but rapidly increasing population at the western end of South Georgia and the increasing trend appears to have continued ever since. By the early 1990s, the population at South Georgia had reached 1.5 million and colonisation had spread to other islands where rapid rates of increase were also being observed. The

population at South Georgia probably represents more than 98% of the world population of the species and is now so large that surveying its size has not been possible in the recent past. However, given the esti- mated rates of increase through the 1990s, this popu- lation could now exceed 5 million seals.

This rapid increase in the number of fur seals is only partly reflective of the present lack of a directed harvest on the species. Changes in the structure of the community of predators that exploit Antarctic krill (Euphausia superba), which is an important compo- nent of the diet of Antarctic fur seals, is likely to have been another driving force behind the increase in the number of fur seals. Fur seals at South Georgia are likely to consume about 4 million tonnes of krill each year and, before commercial whaling reduced the number of whales, much of this consumption may have been taken up by whales.

Studies of diving behaviour have shown that fe- male fur seals mainly feed within the upper 50 metres of the water column where the water is mixed by the effects of wind and tide. Males dive deeper—to more than 200 metres—and probably have a greater pro- portion of fish in their diet than females do, especially in winter. The Antarctic fur seals in the eastern re- gion, where krill are either absent or less abundant, feed mainly on lantern fish (myctophids).

Mothers have restricted foraging ranges when they are supporting their pup. The pups remain at the breeding grounds while mothers travel up to 300 km to feed. In winter, when not constrained by having to return regularly to feed their pups, female fur seals travel much further afield. From South Georgia, they will travel south to the ice edge in the northern Wed- dell Sea and also as far north as the River Plata and the Falkland Islands. Males appear generally to mi- grate south in the late summer and then follow the ice edge northwards as the winter sets in.

Fur seals are almost certainly predated by killer whales, and some leopard seals appear to specialise on feeding on fur seals close to colonies. Ultimately, the population of Antarctic fur seals, at least at South Georgia, may be regulated by the availability of krill in the vicinity of the island. Large-scale variability in the trophic structure of the Southern Ocean caused by factors such as the El Nin˜o Southern Oscillation is evident in the growth patterns of fur seals and there is accumulating evidence of their sensitivity to inter- annual variation in krill availability. This sensitivity is now being used as a way of monitoring changes occurring in the marine environment of the Southern Ocean as part of a system for assessing the effects of fisheries on the environment within the Convention on the Conservation of Antarctic Marine Living Resources.

Estimated Sizes and Trends of Antarctic Fur Seal (Arctocephalus gazella) Populations

Site Pup Numbers Total Population Year of Census Mean Annual Rate of Change Macquarie Island 152a 164a 1999/00 2001/02 increasing (1988/89–1999/2000)a increasing Heard Island 248 1,012 1987/88 2000/01 + 23% (1962/63–1987/88) + 20.1% (1962/63–2000/01) McDonald Island 100 300 1979/80 increasing Iˆles Nuageuses (Iˆles Kerguelen) 2,500e 5,000 ? 1984/85 2000 increasing increasing Courbet Peninsula (Iˆles Kerguelen) 2 >200 1,500–1,700 1,332 ? ? 1984 1998 2000 increasing increasing increasing Iˆle de la Possession (Iˆles Crozet) 67 234 ? ? 1992/93 1999/00 + 21.4% (1983–1992) + 16.9% (1992–1999) Marion Island 251c 796c 1,205d 3,821 1994/95 2003/04 + 17% (1988/89–1994/95) +13.8% (1994/95–2003/04) Prince Edward Island 400 200

2000i 1981/82 2001/02 increasing + 16.2% Nyrøysa (Bouvetøya) 2,000 15,523c >9,501 66,128 1989/90 2001/02 +7.0% (1978/79–1989/90 +0.1% (1996/97–2001/02) South Georgia <600,000e 2,700,000f,g 4,500,000 –6,200,000f,g 1990/91 1999/00 + 9.8% (1976/77–1990/91) +6% – 14% (1990/91–1999/2000)

South Sandwich Islands <500 346

<2,000 1962/63 1997/98

? stable South Orkney Islands <1,000 1970/71 ? South Shetland Islands 9,300

10,057h 1991/92–1995/96 2000/01 + 11% (1994/95–1995/96) + 0.9% (1995/96–2001/02) Cape Shirreff (SSSI No32, S.

Shetland Is.) 5,313 8,455 8,577 21,190 1991/92 1999/00 2001/02 + 14%i (1986/87–1991/92)+ 6%i (1991/92–1999/00) +4.6%i (1992/93–2001/02) a

For populations of both A. tropicalis and A. gazella.

b

Corrected for observer undercount.

c

Corrected for precount mortality.

d

Recalculated from population values in publication.

e

Number of breeding females.

f

Estimated from the number of breeding females.

g_{Standard deviation = 300,000.} h_{Standard error = 140.} i_{Calculated from pup counts.}

Data supplied by the Scientific Committee for Antarctic Research.

Antarctic fur seals are colonial breeders. Anybody who visits South Georgia during December will be stunned by the density of seals occupying the avail- able coastline towards the western end of the island. Fur seals are also aggressive animals and are unfor- giving towards visitors.

Male fur seals set up territories during early No- vember and females begin to arrive at the breeding grounds during late November. The peak of births occurs at about 7 to 10 December and almost all pups have been born by the end of December. The small, aggressive, inquisitive black-coated pups become the dominant wildlife feature on the beaches.

After giving birth, mothers spend about 6 days with their pups. They are then mated by one of the dominant males holding a territory on the breeding beaches before going to sea to feed for a period of 3 to 6 days. Each mother then returns to feed her single pup for about 2 days and this alternation between feeding at sea and returning to provision the pup continues for a period of about 4 months. Weaning appears to be initiated when the pup abandons the breeding grounds to go to sea during late March or April.

Males hold territory on the breeding grounds for periods of just a day or two to over 30 days. The longer they spend in territory then the more offspring they will sire so there is much competition for terri- tories. This results in overt fighting between males, which has led to advantages for males with large body sizes. Consequently, males are up to 5 times the weight of females (males weigh up to 200 kg whereas females weigh about 40 kg). Males have highly devel- oped neck musculature and, at the beginning of the breeding season, they have an impressive mane of tough hair, which helps to provide some protection against the slashing canine teeth of an opponent.

Owing to their commitment to holding territory, males fast through the breeding season. This means that by late December most males have lost up to one- third of their original body weight. As a result of these stresses, males do not live as long as females (up to 14 years for males compared with over 20 years for females). Many males die as a result of these stresses and large numbers die on the breeding grounds. About half of all these adult males die each year.

Males are also rarely able to breed before they are 7 years of age, whereas females normally begin to breed at 3 years of age. Thereafter, around 80% of females will breed each year. Studies have shown that males often return to breed close to where they were born. This may be because there are advantages to competing with males that are known to one anoth- er. Females are probably less likely to return to the

location in which they were born; the advantage to females is that pups have a better chance of survival if their parents are not closely related. Once estab- lished as a breeder, females return to give birth in the same location in successive years.

Antarctic fur seals have proved to be a highly valuable species for scientific research. Not only is the history of the population recovery a fascinating case study, fur seals are themselves remarkably robust to being the subject of research. Studies of their div- ing, foraging, diet, behaviour, and population struc- ture have provided insights into the biology of seals. Those who first encounter fur seals are usually over- whelmed by their aggressiveness, but pausing to watch them for a short time will reveal a fascinating world of a species with a complex social structure much of which we still do not understand.

IANL. BOYD

See also Bouvetøya; Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR); Div- ing—Marine Mammals; Killer Whale; Leopard Seal; Macquarie Island; South Georgia; Sub-Antarctic Fur Seal; Zooplankton and Krill

References and Further Reading

Boyd, I. L. ‘‘Pup Production and Distribution of Breeding Antarctic Fur Seals (Arctocephalus gazella) at South Georgia.’’ Antarctic Science 5 (1993): 17–24.

———. ‘‘Estimating Food Consumption of Marine Preda- tors: Antarctic Fur Seals and Macaroni Penguins.’’ Jour- nal of Applied Ecology 39 (2002): 103–119.

Boyd, I. L., D. J. McCafferty, K. Reid, R. Taylor, and T. R. Walker. ‘‘Dispersal of Male and Female Antarctic Fur Seals.’’ Canadian Journal of Fisheries and Aquatic Sci- ence 55 (1998): 845–852.

Boyd, I. L., and J. P. Roberts. ‘‘Tooth Growth in Male Antarctic Fur Seals from South Georgia: An Indicator of Long-Term Growth History.’’ Journal of Zoology (London) 229 (1993): 177–190.

Boyd, I. L., I. J. Staniland, and A. R. Martin. ‘‘Spatial Distribution of Foraging by Female Antarctic Fur Seals.’’ Marine Ecology Progress Series 242 (2002): 285–294.

Lento, G. M., M. Haddon, G. K. Chambers, and C. S. Baker. ‘‘Genetic Variation, Population Structure, and Species Identity of Southern Hemisphere Fur Seals.’’ Molecular Biology and Evolution 12 (1997): 28–52. Reid, K., and J. P. Croxall. ‘‘Environmental Response of

Upper Trophic-Level Predators Reveals a System Change in an Antarctic Marine Ecosystem.’’ Proceedings of the Royal Society B 268 (2001): 377–384.

Wynen, L. P., S. D. Goldsworthy, C. Guinet, M. N. Bester, I. L. Boyd, I. Gjertz, G. J. G. Hofmeyr, J. Sterling, R. W. G. White, and R. W. Slade. ‘‘Post-Sealing Genetic Vari- ation and Population Structure of Two Species of Fur Seal (Arctocephalus gazella and A. tropicalis).’’ Molecu- lar Ecology 9 (2000): 299–314.