Cape fur seals: A unique marine phenomenon with a controversial history

By Maggi Barnard

The presence of Cape fur seals in Namibian waters is not only a unique phenomenon, as these animals are usually associated with sub-polar climates, but also the cause of controversy in both fishing and conservation circles.

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Namibia’s unusual coastline is known for its cold sea temperatures, dense sea fogs, harsh desert conditions, high marine productivity, rich fish resources and the presence of seals and penguins at a sub-tropical latitude. Cape fur seals (Arctocephalus pusillus pusillus) occur from Cape Frio on the northern section of Namibia’s coast southwards as far as the Cape Peninsula and Algoa Bay on South Africa’s east coast.

There are about 21 colonies along the Namibian coast. The two largest breeding colonies are at Cape Cross north of Henties Bay, and Wolf Bay and Atlas Bay near Lüderitz. These colonies are responsible for 75% of Namibia’s seal pup production.

A dense fur and blubber layer gives Cape fur seals the necessary protection against cold-water temperatures. On land though, seals have thermoregulatory problems in high temperatures, especially on sunny, windless days or when the hot east wind blows. When the cooling effect of the wind and spray are absent on windless days, they have to reach the water to cool down.

By Ron Swilling.

This means disaster for pups shortly after birth when they are less mobile and far from the water. They can rapidly become thermally stressed and if unable to reach shade or sea and their body temperature increases to more than 43 degrees Celsius, they fall into a coma and die within hours.

Researchers have found that when young pups are far from the water and there is a drop in wind speed, over 30 per cent can die in one day. This means a few windless days between mid-November and the end of December can reduce a cohort substantially, according to Jean-Paul Roux in an article published in Namibia Brief.

The breeding season for Cape fur seals begins in mid-October when bulls come ashore and establish territories, which they defend for about six weeks. Pregnant cows start coming ashore in early November. Male territories contain from seven to 66 cows, with an average of 28. Most pups are born in November and December after a gestation period of about eight months. They weigh between 4.5 to 6.4 kilograms and are 60 to 70 centimetres in length. By July, many pups start eating solid food.

Cape fur seals are carnivorous and classified as a top predator in the Benguela system. The size of the population depends on the abundance of prey. According to the latest study published in the African Journal of Marine Science last year, seals consume a wide range of prey, of which the four most important are Cape horse-mackerel, Cape hake, lantern fish and pelagic goby.

Sustainable utilisation

At Independence in 1990 the national policy objectives for the fisheries sector were set, the main objective is to utilise fisheries resources on a sustainable basis. Seals were regarded as an exploitable resource. Since then, Government has noted that the flourishing population of Cape fur seals is a real threat to other marine life.

In 2006 about 60 000 pups and 7 000 bulls were allocated to two concession holders, making it the second-largest seal harvest in the world. This quota caught the attention of international conservation groups, resulting in a huge outcry against the Namibian Government to stop culling seals, as was done by South Africa in 1990.

The International Fund for Animal Welfare challenged Government’s justification for the annual harvest, saying scientific studies show that over-fishing stocks and not predation by seals is the main culprit causing the demise of fish. Yet, according to an article in A Decade of Namibian Fisheries Science published in 2001, it has been estimated that Cape fur seals in the Benguela system consume about a million tons of fish annually, approximately the same amount as the total annual fish catch of Namibia and South Africa combined.

Read more here: Utilisation of Seals – Culling still a controversial issue

Controlled use

The Benguela is a complex system influenced by a large number of factors. Of these, environmental conditions are the most important in controlling patterns of prey abundance and distribution. Understanding the relationship between the environment and the composition of the seal diet may shed light on the factors determining prey abundance and distribution, which would be vital to fisheries management, the study concluded.

The exploitation of seals along the Southern African coast represents one of the oldest commercial ‘fisheries’ in the region, said Dr Ben van Zyl in a paper on culling delivered to the World Conservation Trust. In the early years sealing activities were entirely uncontrolled. In 1610 Dutch sealers had already killed about 45 000 seals near the Cape of Good Hope. By the end of the 19th century, the regional population had been severely depleted and more than 20 breeding colonies eradicated.

South Africa introduced legislation to control the exploitation of seals in 1893, but nothing was done to control it in Namibia until 1922. The population showed a remarkable recovery at the beginning of the 20th century, although more than 2.5 million pups and bulls were still harvested in the two countries between 1900 and 1983.

In Namibia, harvests between 1920 and 1950 took around 10 000 pups per year. This increased sharply to over 50 000 per year from the 1970s. In the early 1980s, the sealing industry faced a virtual collapse of the pelt market following international campaigns against culling. The annual pup harvest declined steadily between 1982 and 1988, while the harvesting of bulls increased. The demand for bulls remained high and after 1988 the pups were harvested again, increasing to exceed 30 000 each year.

In 1994 and 1995 the Namibian Cape fur seals population was hit by an episode of mass mortality affecting all age and sex classes. An estimated 300 000 deaths, thought to have been triggered by anomalous oceanographic conditions, occurred over the two seasons.

Today, about one million Cape fur seals inhabit the coast, with the majority found at Cape Cross, known as the largest seal colony in the world.

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This article was originally published in the 2007 Travel News Namibia magazine edition.