Elephant seals support research of the Antarctic Ocean
Fig. 2. Pushing and shoving in the group generate stress. Scraps and deep threatening sounds are the result. The proboscis, a flap of skin that is extremely enlarged in males during the reproduction period, is clearly perceptible. (Photo: J. Plötz)
Icy air, ice-cold water, ice-cold food coupled with high pressure and darkness when diving to great depths under water – elephant seals live in an extraordinary habitat that is extensively unknown to us humans. To gain an insight into the underwater world of the seals, marine biologists from the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven have been using special satellite transmitters on the animals for several years. The data obtained in this way are of the utmost interest for oceanographers as well.
The southern elephant seal, Mirounga leonina, is the biggest species of seal (Figs. 1, 2) and its total number is estimated at 750,000 animals [1]. Every autumn, starting around mid-March, the powerful elephant seal bulls, weighing up to 4 tons and measuring 5 metres in length, gather on the rocky coasts of the South Shetland Islands at the tip of the Antarctic Peninsula. Traditional breeding and moulting sites are located in the Potter Bay on King George Island. In the water elephant seals tend to be loners, but on the beach they lie crowded next to each other in order to keep warm and for the moulting of their short hair within a few days.
Fig. 3. Elephant seal completely covered with hair and with a stuck-on satellite transmitter in a sea of algae: it measures the seal’s position and diving depth, the water temperature and salinity. The transmitter can hold out for a year and falls off during the next moulting at the latest. (Photo: J. Plötz)
This moulting takes place separately according to gender and the age of the males: first the females come on the beach beginning in mid-January only to return to the sea after about 14 days [2]. The younger males follow and only then – as of mid-March – the large, sexually mature bulls. Thus, the following rule applies: the later in the year, the bigger the elephant seals. Seal experts take advantage of this tight time window for moulting in order to attach transmitters the size of one’s palm, which operate with the satellite-based location system ARGOS, to the freshly moulted head section of the big bulls (Fig. 3). Once moulting is over, the bulls go back to their migratory life and do not return to their traditional beaches until six months later, when they mate with the considerably smaller females (3 m in length and weighing up to 900 kg) in the Antarctic spring. Prior to the arrival and during the presence of the females the males engage in violent fights with their rivals. The strongest bulls (the so-called beachmasters) dominate sections of the beach and guard around a dozen females. Pregnant animals give birth to their young shortly after their arrival and feed them with milk having a very high fat content for three weeks. After that the young are driven away because the subsequent mating leads to further conflicts with rivals, during which young animals and also mother seals may be crushed. The lifetime of elephant seals, approx. 15 years for males and about 23 years for females, is unusually short [3].
Fig. 1. Elephant seals (Mirounga leonina): Fully grown bull seals while sunbathing in Potter Bay on King George Island at the tip of the Antarctic Peninsula. The animals are moulting, during which the outer skin layers are also sloughed off. (Photo: J. Plötz)
During the annual migrations to their oceanic feeding grounds elephant seals cover thousands of kilometres. Some animals reach marine areas up to a latitude of 75° South [4]. They dive down to depths of over 2000 metres and remain under water for periods of over an hour. These figures are not unusual, but generally the diving depths range between 500 and 700 metres (shelf areas). Elephant seals prey on fish and squid. When they swim, these prey generate pressure waves that the seals perceive with their whiskers. But also hearing and sight presumably play a role in catching prey. However, food in the ocean is distributed unequally and it is not worthwhile for the seals to fish everywhere and at any time. This is one reason why researchers can draw conclusions about the spatial and temporal distribution of particularly productive zones in the Southern Ocean based on the migration behaviour of the animals. If stocks of fish and squid are decimated by virtue of excessive fishing or as a result of climate change, this changes the behaviour of elephant seals.
Remote satellite sensing can provide information on these changes since elephant seals have an unerring sense for waters with an ample supply of food. Individual animals even repeatedly return to certain marine regions, as impressively demonstrated by investigations conducted by the Bremerhaven seal biologists. Remote satellite sensing reveals that seals tend to go to shelf slopes where there are pronounced water eddies that stimulate plankton and fish production. During the extended migrations through the Southern Ocean the ARGOS transmitters not only send the geographic positions and diving depths of the respective seal, at the same time they supply data on the temperature and salinity of the waterbody crossed by the animal horizontally and vertically and thus provide important physical parameters from which oceanographers can draw conclusions on the currents in the ocean. Seals lead a nomadic life in the ice desert of the Antarctic Ocean and are always looking for lucrative feeding grounds. Based on seasonal changes in the migration behaviour of elephant seals, biologists thus obtain indications of when, where and at what depth exceptionally high numbers of fish and squid occur and with what oceanographic conditions a good supply of food correlates. However, seals furnish information on their migrations only for a relatively short period of time since the transmitters fall off during the next moulting (i.e. after 12 months) at the latest. Currently 14 bulls are equipped with a transmitter, but together with the previously marked animals scientists have already been able to gain crucial new insights into otherwise inaccessible marine regions.
[1] J. Plötz, H. Bornemann, L. Kindermann: Robben und Robbenschlag in der Antarktis. In: J. L. Lozán et al. (ed.): Warnsignale aus den Polarregionen - Wissenschaftliche Fakten. Büro Wissenschaftliche Auswertungen. Hamburg 2006.
[2] H. Bornemann et al., Antarctic Science 12, 3 (2000), doi: 10.1017/S0954102008001557.
[3] R. R. Reeves et al.: Guide to Marine Mammals of the World. National Audubon Society Guide Series. Alfred A. Knopf. New York 2002.
[4] C. A. Tosh et al., Antarctic Science 21, 113 (2009), doi:10.1017/S0954102008001557.