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Tuatara: Volume 15, Issue 3, December 1967

Population studies on the Weddell seal

page 133

Population studies on the Weddell seal


This Paper discusses current work on the population dynamics of the Weddell seal (Leptonychotes weddelli) in McMurdo Sound, Antarctica, and points out the suitability of this species for other studies.

The Weddell seal occurs in large numbers, is not normally dangerous, is unafraid of humans and thus easy to approach for observation at close range, and is easy to catch for detailed examination, measurements, or marking. No other seal offers all these advantageous characteristics.

The first work on Weddell seals by Canterbury University was originated to assess the effect of killing seals each year to feed the dogs at Scott Base. The official policy was to kill only males, but a number of femals were killed also. From 1957 to the present, at least 1,000 seals have been removed from the population.
Adult female Weddell seal with pup, after a blizzard.

Adult female Weddell seal with pup, after a blizzard.

page 134


The object of this study is to define the population parameters and movements of the Weddell seal in McMurdo Sound, i.e. the birth rate, longevity, life expectancy of age classes, pregnancy rate, dispersal, and daily and seasonal movements. This work is of value because of interest in the harvest of seals throughout the world. There has already been one Norwegian expedition to the Weddell Sea to harvest crabeater seals. It is possible that interest in harvesting the Weddell seal for pelts and blubber may grow in the future. Thus it is important to assess the population characteristics of the Weddell seal while it is still relatively undisturbed. Information gathered on this species may usefully be applied to other seals and to mammals generally.

Previous Work by Canterbury University

This study follows a four year study of the Weddell seal in McMurdo Sound by Dr. M. S. R. Smith. His primary research elucidated in detail the physiology of the male and female reproductive cycle. Besides this, Smith did population counts and recorded daily and seasonal movements during the period 1961-1965, studied the louse Antarctophthirius ogmorhini on the Weddell seal (Murray et al., 1965), assimilated data on injuries to the Weddell seal (Smith, 1966b), and recorded detailed body measurements and organ weights.

A general parasite study with special consideration of the bile duct cestode Glandiocephalus perfoliatus was done by D. W. Featherston (1965).

From 1963-65 branding of seal pups was carried out under the direction of Dr. Bernard Stonehouse. Autopsies were made on seals killed each year for dog food, and over 260 skulls with canine teeth were collected for the development of aging techniques. This work is in progress at present.

In addition, Dr. R. Balham of Victoria University, Wellington, and Dr G. L. Kooyman, formerly of the University of Arizona, have given free access to the data they collected on Weddell seals in McMurdo Sound during 1957 and 1963-1965 respectively.

The combination of these sources provides an excellent background of reference material for the present ovservations.

Life History

Weddell seals in McMurdo Sound pup on the sea ice from the middle of October to early November. They weigh about 65 Ib. at birth (Bertram, 1940), double their weight in 10 days, and are page 135 weaned in six to eight weeks. From this point until they return to breed as three year olds, little is known of their habits. It has been postulated they go into the pack ice or along the northern inaccessible coast (Smith, 1966a) although it is not indicated whether this means the Victoria Land Coast, the periphery of the continent, or both. However, this postulation is based on the negative evidence that young seals have never been seen in large numbers anywhere.

Females first become pregnant at three years of age and have one pup a year thereafter. One female was found pupping in her third year, thus having bred in its second year, but this is exceptional (Smith, 1966a). Twinning is virtually non-existent. Weddell seals may live to 15 or 16 years of age but the average age in McMurdo Sound is about eight years.

There is a resident winter population of Weddell seals in McMurdo Sound (Wilson, 1907) estimated to number about 250 (Smith, 1965). These seals live in the water during the winter and maintain air holes by gnawing the ice with their teeth. Most of the McMurdo Sound population returns in two waves; the first in early October consisting mainly of breeding females, and the second in December and January consisting of adult and sub-adult males and females. The maximum combined adult and sub-adult population is approximately 2,500 in mid-January and begins to decline again by the end of February (Smith, 1965). It has been suggested that the seals which leave McMurdo during the winter also go to the pack ice and inaccessible northern coast (Smith, 1966a).


Of primary importance in a population study of this sort is the marking of individual animals. On a short term basis this gives information on daily and seasonal movements while on a long term it provides information on dispersal, survival, and fidelity to breeding sites. Data are gathered by continuously checking for tagged seals throughout the season.

Initially the marking programme was concerned with the identification of year classes. Thus, pups were branded in three consecutive seasons with a brand to designate the year. In 1963, 85 pups were branded with the letters VL, in 1964, 280 pups with the letters IV, and in 1965, 90 pups with the letter ZX and 250 with the letter H. (Smith, 1966a). During 1963 through 1965, 604 seals (including pups, sub-adults, and adults) were tagged by Kooyman with numbered monel-metal tags.

During the initiation of my phase of this study in January through February, 1966, I tagged 252 Weddell seals in McMurdo Sound (including six at White Island) (Stirling, 1966a and b). These page 136 were all given individual combinations of coloured plastic sheep ear tags (Stirling, 1966c).

To understand what is happening in a population it is essential to determine how discrete it is. The study area here is defined as the west coast of Ross Island between Cape Royds and Scott Base, including the Delbridge Islands (Figure 1). To check on the movements of individuals in or out of this area, trips were taken to other locations to look for tagged seals from the study area, and to tag seals to see if they move into the study area. The following list summarizes the places visited and the number of seals tagged at each location in 1966-1967. Figures 1 and 2 show the location of these sites.
LocationNo. Seals Tagged
Study area884
Stranded Moraines43
Marble Point61
Cape Hallett119
Cape Crozier11
Franklin Island18
Cape Bird115
White Island3

The coast of Victoria Land and associated pack ice between McMurdo and Cape Hallett were examined for the presence of seals through the aid of U.S. Coast Guard Cutters “Staten Island” and “Glacier”. Two types of observations were made. The first was a census by helicopter over five sites along the coast; Cape Roberts, Nordenskjold Ice Tongue, Prior Island, Inexpressible Island, and Borchgrevink Glacier Tongue (Fig. 2). At one concentration of seals at Cape Roberts and again at Nordenskjold Ice Tongue, we were allowed onto the ice to look for tagged seals and obtain the approximate age and sex ratios of seals present. The second type of observation was a quantitative count from the bridge of the icebreaker of the numbers of each seal species seen in the pack ice.

Through the season aerial census of the study area was done by helicopter to follow the build up of total numbers of seals and seasonal population shifts.

To calculate the survival and productivity of a population it is necessary to obtain data on the age structure and reproductive status of the population. This is facilitated by the collection of canine teeth (from which individuals can be aged) and reproductive organs from seals killed for dog food at Scott Base. page 137


page 138


Sixty-eight percent of the seals tagged in January and February, 1965, were re-sighted in the next season. This is an excellent


page 139 return for a study of this nature and suggests high adult survival rate between years. Sixteen of the branded seals have been seen but only four brands were legible. Forty of Kooyman's tagged seals have been sighted, including twelve which were originally tagged as pups. These known age specimens are extremely valuable as they supply information on growth, movements from the birth site, and an irrevocable control for aging techniques.

Most breeding females originally tagged by Kooyman and resighted this season with pups, were breeding at the same place. This suggests fidelity to a pupping site.

The southward movement of the population as described by Smith (1965) was confirmed in this study by sighting of tagged individuals. Total numbers appear to be similar to past seasons. In the influx of seals in December through January many tagged seals were sighted that had not previously been seen in the pupping areas. This raises the possibility of seals breeding in other areas or the presence of a substantial proportion of non breeders and unsuccessful breeders.

Very little movement of tagged seals out of the study area during the breeding season was recorded. Five records were made outside the study area. Three were sub-adults, one was a pup, and one was an adult male. The adult male later returned to the study area. Seals were checked daily at Cape Bird and only one adult, one sub-adult and one pup from the study area were sighted. The remaining two sub-adults were sighted at Marble Point and Cape Crozier. No seals tagged in other localities were seen in the study area.

The concentrations of seals along the Victoria Land Coast appeared to be normal breeding populations similar to that in the study area. Large numbers of sub-adult seals were not seen in either the fast ice or the pack ice. Several Weddell seals were seen in the pack but only one was distinguished as a sub-adult.

The killing of seals for dog food is done in February. The population is mainly adult with some sub-adults and occasional pups. The age span and mean ages of seals killed in 1966 and 1967 are as follows:
YearSample SizeAge SpanMean Age
YearSample SizeAge SpanMean Age

Smith (1966a) determined pregnancy by the presence of extraembryonic membranes, but because the embryo does not implant for two to three months after mating (i.e. it page 140 implants mid-January to Mid-February), this method would miss some instances of pregnancy. In 1966, 95% of the mature females had a corpus luteum and in 1967, 97%. There may however be some pseudo-pregnancy, intra-uterine absorption, or natural abortion before the following spring, so these figures do not necessarily present the per cent of adult females that pup.


It is too early at this stage to draw conclusions further than the information given in the results. The purpose of the paper was only to describe the current work being done.

The Weddell seal is obviously suitable, because of its placid nature and availability, for detailed work in a variety of subjects such as population dynamics, metabolic rate, heat conservation, and underwater acoustics. It has, for example, been suggested that the Weddell seal has a sort of biological sonar similar to that of dolphins, which is used to find air holes and possibly fish during the continuous darkness of the Antarctic winter (Ray, 1965). Yet no experimental work has been done to date.


I am grateful to Dr. Bernard Stonehouse for advice and discussion of problems arising from this study. I am also extremely appreciative of the support and co-operation of the following: the Zoology Department, Canterbury University; the Antarctic Division of the D.S.I.R. (Wellington); the University Grants Committee (Wellington); the U.S. Navy Squadron VX-6, and the U.S. Coast Guard Cutters “Staten Island” and “Glacier”

Some individuals deserve special mention. R. East was my assistant for the 1966-1967 season. J. T. Darby and A. J. Peterson tagged seals at Cape Bird and made a daily check for tagged individuals. C. M. Clark, leader of Scott Base 1966-1967, gave support without which much of this work would have been impossible. The following Scott Base personnel assisted with field work: W. Orchiston, R. G. Rae, A. C. Rayment, and A. J. Simm.

Literature Cited

Bertram, G. C. L. 1940. The biology of the Weddell and crabeater seals, British Grahamland Expedition, 1934-37, Sci. Rept. 1(1): 1-139.

Featherston, D. W., 1965. Some aspects of the biology of Glandiocephalus perfoliatus (Raillient and Henry, 1912), A pseudophyllidean cestode found in the bile duct of the Weddell seal Leptonychotes weddelli, M.Sc. Thesis, University of Canterbury, 42 pp.

Murray, M. D., Smith, M. S. R., and Souck, Z., 1965. Studies on the ectoparasites of seals and penguins, II. The ecology of the louse page 141 Antarctophithirius ogmorphini Enderlein on the Weddell seal Leptonychotes weddelli Lesson, Austr. J. Zool. 13: 761-771.

Ray, C., 1965. Physiological ecology of marine mammals at McMurdo Sound, Antarctica, Bioscience 15(4): 274-277.

Smith, M. S. R., 1965. Seasonal movements of the Weddell seal in McMurdo Sound, Antarctica, J. Wildl. Mgmt. 29(3): 464-470.

——, 1966a. Studies on the Weddell seal (Leptonychotes weddelli) in McMurdo Sound, Antarctica, Ph. D. Thesis, University of Canterbury, 161 pp. plus xlii.

——, 1966b. Injuries as an indicator of social behaviour in the Weddell seal (Leptonychotes weddelli), Mammalia, 30(2): 241-246.

Stirling, I., 1966a. The seals at White Island: a hypothesis on their origin, Antarctic, 4(6): 310-313.

——, 1966b. A technique for handling live seals, J. Mammal. 47(3): 543-544.

——, 1966c. Seal marking in McMurdo Sound, Antarctic 4(7): 363-364. Wilson, E. A., 1907. Mammalia (Whales and Seals), In: National Antarctic Expedition 1901-1904, Natural History, Zoology, 2: 1-66.