Leptonychotes weddellii


Taxonomy [top]

Kingdom Phylum Class Order Family

Scientific Name: Leptonychotes weddellii
Species Authority: (Lesson, 1826)
Common Name/s:
English Weddell Seal
Taxonomic Notes: Rice (1998) notes that the specific name was frequently misspelled (weddelli) prior to Bonner (1988).

Assessment Information [top]

Red List Category & Criteria: Least Concern ver 3.1
Year Published: 2008
Date Assessed: 2008-06-30
Assessor/s: Gelatt, T. & Southwell, C. (IUCN SSC Pinniped Specialist Group)
Reviewer/s: Kovacs, K. & Lowry, L. (Pinniped Red List Authority)
Due to its widespread occurrence, paucity of known threats, and large population size, the Weddell Seal should be classified as Least Concern (LC).

IUCN Evaluation of the Weddell Seal, Leptonychotes weddellii
Prepared by the Pinniped Specialist Group

A. Population reduction
Declines measured over the longer of 10 years or 3 generations
A1 CR > 90%; EN > 70%; VU > 50%
Al. Population reduction observed, estimated, inferred, or suspected in the past where the causes of the reduction are clearly reversible AND understood AND have ceased, based on and specifying any of the following:
(a) direct observation
(b) an index of abundance appropriate to the taxon
(c) a decline in area of occupancy (AOO), extent of occurrence (EOO) and/or habitat quality
(d) actual or potential levels of exploitation
(e) effects of introduced taxa, hybridization, pathogens, pollutants, competitors or parasites.

Generation time in Weddell Seals based on the average age of reproducing adults is uncertain but has been estimated at approximately 12 yrs in an isolated population within McMurdo Sound. With sexual maturity attained at 3-6 years of age and a maximum longevity in excess of 30 years a generation time of 12-15 years seems reasonable. A population reduction of Weddell Seals has not been observed, estimated, inferred, or suspected in the past 30 years. However, global population abundance is difficult to estimate and has only been based on extrapolations from ship-based surveys. These extrapolations use seal density in a given area, extrapolated to an estimate based on ice available to be used by the seals.

A2, A3 & A4 CR > 80%; EN > 50%; VU > 30%
A2. Population reduction observed, estimated, inferred, or suspected in the past where the causes of reduction may not have ceased OR may not be understood OR may not be reversible, based on (a) to (e) under A1.

A population reduction of Weddell seals has not been observed, estimated, inferred, or suspected in the past 30 years.

A3. Population reduction projected or suspected to be met in the future (up to a maximum of 100 years) based on (b) to (e) under A1.

A population reduction of Weddell Seals is not suspected in the future as there are no compelling threats to Weddell Seals at this time. They are no longer hunted to feed dog teams and there is no other threats beyond potential habitat losses due to climate change via predicted reductions in sea ice habitats due to continued climate warming.

A4. An observed, estimated, inferred, projected or suspected population reduction (up to a maximum of 100 years) where the time period must include both the past and the future, and where the causes of reduction may not have ceased OR may not be understood OR may not be reversible, based on (a) to (e) under A1.

A population reduction of Weddell Seals has not been observed, estimated, inferred, or suspected in the past 30 years.

B. Geographic range in the form of either B1 (extent of occurrence) AND/OR B2 (area of occupancy)
Extent of occurrence (EOO): CR < 100 km²; EN < 5,000 km²; VU < 20,000 km²

The EOO of Weddell Seals is > 20,000 km².

B2. Area of occupancy (AOO): CR < 10 km²; EN < 500 km²; VU < 2,000 km²

The AOO of Weddell Seals is > 2,000 km².

AND at least 2 of the following:
Severely fragmented, OR number of locations: CR = 1; EN < 5; VU < 10
(b) Continuing decline in any of: (i) extent of occurrence; (ii) area of occupancy; (iii) area, extent and/or quality of habitat; (iv) number of locations or subpopulations; (v) number of mature individuals.
(c) Extreme fluctuations in any of: (i) extent of occurrence; (ii) area of occupancy; (iii) number of locations or subpopulations; (iv) number of mature individuals.

C. Small population size and decline
Number of mature individuals: CR < 250; EN < 2,500; VU < 10,000

The current abundance of Weddell Seals is poorly known, but the total number of individuals was last estimated at a minimum of 500,000.

AND either C1 or C2:
An estimated continuing decline of at least: CR = 25% in 3 years or 1 generation; EN > 20% in 5 years or 2 generations; VU = 10% in 10 years or 3 generations (up to a max. of 100 years in future)
C2. A continuing decline AND (a) and/or (b):
(a i) Number of mature individuals in each subpopulation: CR < 50; EN < 250; VU < 1,000
(a ii)
% individuals in one subpopulation: CR = 90–100%; EN = 95–100%; VU = 100%
(b) Extreme fluctuations in the number of mature individuals.

D. Very small or restricted population
Number of mature individuals: CR < 50; EN < 250; VU < 1,000 AND/OR restricted area of occupancy typically: AOO < 20 km² or number of locations < 5

The current abundance of Weddell Seals is poorly known, but the number of mature individuals is certainly > 1,000. AOO is > 20 km² and the number of locations is > 5.

E. Quantitative Analysis
Indicating the probability of extinction in the wild to be: Indicating the probability of extinction in the wild to be: CR > 50% in 10 years or 3 generations (100 years max.); EN > 20% in 20 years or 5 generations (100 years max.); VU > 10% in 100 years

There has been no quantitative analysis of the probability of extinction for Weddell Seals.

Listing recommendationPast, extrapolated, estimates of Weddell Seal abundance suggest a total population size of more than 500,000. Current abundance and population trend are unknown but there has been no evidence of significant changes in recent decades. Currently, the greatest threat to Weddell Seals may be the unknown effects of a changing climate. Recent work in McMurdo Sound suggested that sea ice in the area has expanded, but that there has been a local decrease in pup production. However, the overall effects of climate change on the world wide population of Weddell Seals are unknown. Weddell Seals do not appear to have changed in abundance in recent years and there are no known current threats. Thus, Weddell Seals qualify for listing as Least Concern.

Geographic Range [top]

Range Description:Circumpolar and widespread in the Southern Hemisphere, Weddell seals are the world’s southern-most breeding mammal and occur in large numbers on fast ice, right up to the shoreline of the Antarctic continent. They also occur offshore in the pack ice zone north to the seasonally shifting limits of the Antarctic Convergence. A small population lives all year on South Georgia. Weddell seals are present at many islands along the Antarctic Peninsula that are seasonally ice-free. Vagrants have been recorded in many areas north of the Antarctic in South America, New Zealand and southern Australia (Kooyman 1981, Rice 1998, Bengtson 2002).
Antarctica; South Georgia and the South Sandwich Islands
Argentina; Australia; Bouvet Island; Chile; Falkland Islands (Malvinas); French Southern Territories; Heard Island and McDonald Islands; New Zealand; South Africa; Uruguay
FAO Marine Fishing Areas:
Atlantic – Antarctic; Atlantic – southwest; Indian Ocean – Antarctic; Pacific – Antarctic
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population: The global population of Weddell seals has been variously estimated at 500,000 to one million or more (Erickson and Hanson 1990). This is a widespread species and, as with other Antarctic seals inhabiting the pack ice, population assessments are very difficult and expensive to conduct and therefore infrequently undertaken. Also, most past surveys do not account for seals in the water and unavailable for counting. To date a circumpolar survey has not been completed and reported. Published reports are limited to ship-based surveys of seals that produced a density estimate that is later extrapolated for the amount of land-fast sea ice present at the time (Erickson and Hanson 1990).
Population Trend: Unknown

Habitat and Ecology [top]

Habitat and Ecology: Adult males reach 2.9 m in length, while females reach 3.3 m. Adults in their prime weigh 400 to 450 kg, with females being somewhat heavier than males. Adult female weight fluctuates dramatically during the year with significant weight loss occurring after birth during lactation. Newborns are about 1.5 m long and average 29 kg. Females become mature at 3-6 years of age and males at 7-8 years. The annual pregnancy rate of mature females is 70-80%. Gestation lasts 11 months, including a delay of implantation of 2 months. Longevity is approximately 25 years (Reijnders et al. 1993).

Weddell seal pups are born from September through November and nursed for seven to eight weeks. Animals at lower latitudes pup earlier than animals living at higher latitudes. Males set up territories in the water around holes in the ice used by females to enter and leave the water. The only copulation that has been observed occurred underwater. The behavior of animals breeding in the pack ice or in the sub-Antarctic is not well known.

Weddell seals are not very social when out of the water, avoiding physical contact most of the time. However, they are loosely social; when in the shore-fast ice habitat they tend to congregate in groups along recurrent cracks, leads, and near access holes to the water. There is debate over whether or not this species is migratory or if there is just dispersal of weaned pups, juveniles and some older animals. Some individuals remain in residence year round in the fast ice at latitudes as high as 78°S in McMurdo Sound. Others, particularly newly weaned and subadult animals, move north from the continent and spend the winter in the pack ice. Seals living in fast ice areas or facing freezing over of access holes and leads, abrade and grind the ice to maintain access to and from the water. They bite at the ice and then rapidly swing the head from side to side to grind away the ice with their teeth. Seals living in areas where extensive grinding of ice is necessary for much of the year have accelerated wearing down of their teeth and decreased life expectancy. Predators include killer whales and leopard seals.

Weddell seals are prodigious divers, reaching over 600 m depth, and can undertake dives of at least 82 minutes. Deep dives are regularly used for foraging on Antarctic cod, and long dives occur when the animals are searching for new breathing holes, cracks and leads. In addition to Antarctic cod, the diet of Weddell seals primarily consists of Notothenid fishes. Squid and other invertebrates are also taken as small percentages of the diet.
Systems: Terrestrial; Marine

Threats [top]

Major Threat(s): Weddell seals served as an important source of food for men and dogs throughout the early periods of Antarctic exploration. They continued to be taken to feed sled dogs at one base in to the 1980s. Local populations of seals no doubt suffered declines from these harvests, but in the case of the population in McMurdo Sound the population has recovered in the 20 years since the harvest ended. At present, there are no immediate or significant threats to the Weddell seal.

The effects of global climate change on Antarctic seals are unknown. However, Learmonth et al., (2006) suggest that Weddell seal numbers may decline with increasing temperatures if Antarctic sea ice is significantly reduced. Loss of sufficient areas of fast ice habitat used for pupping, resting, avoidance of predators (possible), and access to preferred foraging areas because of changes from warming could lead to population declines of Weddell seals. This is in contrast to a paper by Proffit et al. (2007), which reports that the localized cooling and increased sea ice extent in the Ross sea is associated with decreased reproduction and lower weaning mass of Weddell seals in McMurdo Sound. Thus, the overall effects of global climate change on Weddell seals is unknown.

Two of the four species of Antarctic ice seal, the leopard and crabeater tested positive for antibodies to canine distemper virus (CDV). Weddell seals were tested and did not have any antibodies, and Ross seals were not tested. A mass mortality of crabeater seals occurred in 1955, and many animals displayed viral illness symptoms prior to death (Bengtson and Boveng 1991).

Seasonal tourism in the Antarctic and sub-Antarctic has increased steadily in the last 30 plus years, and is currently at all time high levels. The effects of increased vessel noise, disturbance from vessels passage, and close approach by people in small boats and on land or ice on Weddell seal behaviour, distribution, and foraging are unknown. There is also a risk of injury to a small number of animals from collision with boats or crushing from large vessel passage through and disruption of ice fields. There are no reports of significant fisheries interactions.

Conservation Actions [top]

Conservation Actions: The Weddell seal is not listed as endangered or threatened. Weddell seals are protected by the Antarctic Treaty and the Convention for the Conservation of Antarctic Seals, and any future commercial harvest would be regulated by these international agreements (Reijnders et al 1993).

Bibliography [top]

Bengtson, J. L. and Boveng, P. 1991. Antibodies to canine distemper virus in Antarctic seals. Marine Mammal Science 7: 85-87.

Bonner, W. N. 1982. Seals and man: A study of interactions. University of Washington Press.

Bonner, W. N. 1988. What shall we call the Weddell and Ross seals? Marine Mammal Science 4: 75-77.

Burns, J. M. and Castellini, M. A. 1998. Dive data from satellite tags and time-depth recorders: a comparison in Weddell seal pups. Marine Mammal Science 14(4): 750-764.

Burns, J. M. and Kooyman, G. L. 2001. Habitat use by Weddell seals and emperor penguins foraging in the Ross Sea, Antarctica. American Zoology 41: 90-98.

DeMaster, D. P. 1979. Weddell Seal. Mammals in the Seas,Vol. II: Pinniped species summaries and report on sirenians, pp. 130-134. FAO Fisheries Series No. 5.

Erickson, A. W. and Hanson, M. B. 1990. Continental estimates and population trends of antarctic ice seals. In: K. R. Kerry and G. Hempel (eds), Antarctic Ecosystems. Ecological change and conservation, pp. 253-264. Springer-Verlag, Heidelberg, Germany.

Green, K. and Burton, H. R. 1987. Seasonal and geographical variation in the food of Weddell seals, Leptonychotes weddelii, in Antarctica. Australian Wildlife Research 14: 475-89.

Kooyman, G. L. 1981. Weddell seal Leptonychotes weddelli Lesson, 1826. In: S. H. Ridgway and R. Harrison (eds), Handbook of marine mammals, pp. 275-296. Academic Press, London, UK.

Kooyman, G. L. 1989. Diverse Divers. Springer-Verlag, Berlin, Germany.

Learmonth, J. A., Macleod, C. D., Santos, M. B., Pierce, G. J., Crick, H. Q. P. and Robinson, R. A. 2006. Potential effects of climate change on marine mammals. Oceanography and Marine Biology: An Annual Review 44: 431-464.

Plotz, J., Ekau, W. and Reijnders, P. J. H. 1991. Diet of Weddell seals Leptonychotes weddellii at Vestkapp, eastern Weddell Sea (Antarctica), in relation to local food supply. Marine Mammal Science 7: 136-144.

Reijnders, P. J. H., Plotz, J., Zegers, J. and Grafe, M. 1990. Breeding biology of Weddell Seals (Leptonychotes weddellii) at Drescher Inlet, Riiser Larsen Ice Shelf, Antarctica. Polar Biology 10: 301-306.

Rice, D.W. 1998. Marine Mammals of the World: Systematics and Distribution. Society for Marine Mammalogy, Lawrence, Kansas.

Stirling, I. 1971. Population dynamics of the Weddell seal (Leptonychotes weddelli) in McMurdo Sound, Antarctica, 1966-1968. In: W. H. Burt (ed.), Antarctic pinnipedia, pp. 141-161. American Geogphysical Union.

Testa, J. W. and Siniff, D. B. 1987. Population dynamics of Weddell seals (Leptonychotes weddelli) in McMurdo Sound, Antarctica. Ecological Monographs 57: 149-165.

Testa, J. W., Siniff, D. B., Croxall, J. P. and Burton, H. R. 1990. A comparison of reproductive parameters among three populations of Weddell seals (Leptonychotes weddellii). Journal of Animal Ecology 59: 1165-1175.

Testa, J. W., Siniff, D. B., Ross, M. J. and Winter, J. D. 1985. Weddell seal-Antarctic cod interactions in McMurdo Sound, Antarctica. In: W. R. Siegfried, P. R. Condy and R. M. Laws (eds), Antarctic Nutrients Cycles and Food Webs, pp. 561-565. Springer-Verlag, Berlin, Germany.

Thomas, J. L. 2002. Leopard seal Leptonychotes weddellii. In: W. F. Perrin, B. Wursig and J. G.M. Thiewissen (eds), Encyclopedia of marine mammals, pp. 1300-1302. Academic Press, London, UK.

Citation: Gelatt, T. & Southwell, C. (IUCN SSC Pinniped Specialist Group) 2008. Leptonychotes weddellii. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 17 April 2014.
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