The story of the two oceans - where two worlds meet
For anyone with an interest in the oceans and in marine life, southern Africa has one of the most fascinating coastlines in the world. The southern African continent is washed by ocean currents, which have a strong influence on plant and animal life. The marine environment is varied, complex and unpredictable. Let’s take a closer look at the Indian and Atlantic oceans.
Two seas, one country
At the southern tip of Africa, the cold current of the west coast meets that of the warmer east coast. There are few places in the world where a country is positioned between seas that are so different in temperature and in the flora and fauna that they support.
Due to the influence of the currents as well as the country’s latitude, South Africa experiences tremendous changes in weather and climate. This is illustrated by a comparison in temperature and rainfall received by Port Nolloth on the west coast and Durban on the east coast (these towns lie on the same latitude).
Port Nolloth experiences an average temperature of 14.1°C and an annual rainfall of 61mm, while Durban experiences average temperatures of 20.5°C and receives 1 000mm of rain every year.
Let’s meet in the middle?
There has always been some controversy about the true meeting point of the Indian and Atlantic oceans. Even scientists tend to disagree on this issue because the Agulhas Current shifts regularly. If there is such a thing, the boundary lies somewhere between Cape Agulhas (the southernmost tip of Africa) and Cape Point.
Based on the differences in plant and animal life occurring north-west and south-east of Cape Point, some people like to believe that Cape Point is the dividing line. Many consider this the most beautiful cape in the world.
The Agulhas Current, one of the most powerful currents in the world, flows southwards down the east coast of South Africa, bringing warm Indian Ocean water from tropical regions. This warm current on the east coast results in a humid climate with high rainfall. Vegetation is lush and dense forests occur.
The region stretching from southern Mozambique to Port St Johns is referred to as subtropical. It supports tropical species of corals, fishes and crabs, and also various temperate species from southern latitudes.
The coastal waters are warm, generally clear and low in nutrients. There is fierce competition for food and space on reefs and a greater proportion of predators than on the west coast. While there is a greater diversity of species on the east coast, they occur in lower numbers than on the west coast.
Off the west coast of South Africa, the cold Benguela Current flows sluggishly northwards. The temperate region of the west coast extends from Cape Point to Walvis Bay in Namibia and is characterised by cold waters, and various marine species either unique to this area or that occur in abundant numbers such as commercially important abalone (Haliotis midae) or west coast rock lobster (Jasus lalandii).
The cold current also has a significant influence on the climate of the region as the air flowing from the cold sea over the hot land yields low annual rainfall. The west coast is arid and desert-like and plant life relies heavily on frequent coastal fogs.
During summer, strong south-easterly winds blow across the surface water of the Atlantic coastline, resulting in upwellings of icy cold water. This water, originating from the ocean, carries a rich supply of nutrients, which fertilise the phytoplankton (microscopic plant life forms) and seaweeds. The phytoplankton flourish and form dense blooms so that the water is often murky and discoloured.
Prolific forests of giant kelp plants dominate the rocky shoreline. Kelp is one of the fastest growing seaweeds in the world and supplies plenty of food for the great numbers of invertebrates, such as abalone (perlemoen), crayfish, giant periwinkle, urchins, starfish and anemones, which find shelter within the forests.
Along the west coast, biological production is high and species diversity is low. The prolific growth of phytoplankton and seaweed stimulates productive food chains. Planktonic animals (zooplanktons) feed on the phytoplankton, which in turn are eaten by filter-feeding fishes like pilchards (Sardinops sagax) and anchovies (Engraulis japonicas).
The west coast is one of the richest fishing grounds in the world and not only supports huge commercial fisheries, but also large colonies of Cape fur seals (Arctocephalus pusillus pusillus) and seabirds such as endangered African penguins (Spheniscus demersus).
Endemic to the coastlines of South Africa and Namibia, African penguins have become highly threatened due to the fact that thousands were killed for food, and their eggs collected, by the early explorers. In more recent times, oil spills have had devastating effect on the African penguin population.
In June 2000, an oil spill off Robben Island seriously endangered the African penguin colonies on the west coast. Robben Island and Dassen Island (northeast of Robben Island) alone are home to some 41% of the world penguin population (there are 21 000 breeding pairs on Robben and Dassen Islands). In a dramatic rescue attempt, over 19 000 oil-free birds were evacuated to Port Elizabeth on the east coast and released there to swim some 900km back to Cape Town, giving the authorities time to clean up the spill.
Another 19 000 oiled penguins were taken to large warehouses in Cape Town to be washed and cared for by staff of South African National Foundation for the Conservation of Coastal Birds (SANCCOB), International Fund for Animal Welfare (IFAW) and thousands of volunteers. This was one of the most successful rescues of coastal birds in the world.
East meets west
One of the most noticeable differences between the east and west coasts of South Africa is the colour of fish. A variety of colourful fish, such as butterfly fish, wrasse, damsels, goldies and surgeons, are common in warm tropical waters.
The majority of fish found off the west coast tends to be silver to yellow-brown in colour. Even species such as klipvis (family Clinidae), which use colour to camouflage themselves for protection against predators, are dull in comparison to east coast tropical species.
Some deep-sea fish like red roman (Chrysoblephus laticeps) and jacopever (Helicolenus dactylopterus) are red, but this disappears as the light decreases in deeper waters (a universal phenomenon associated with depth).
Another contrast between east and west coasts is that there are more poisonous and venomous animals on the east coast, including stonefish (Synanceia verrucosa), devil firefish (Pterois miles) and various species of sea urchins.
Symbiotic relationships between animals also tend to be more common in warmer waters.
Mixing it up in the south
Most people think only in terms of an east and west coast, but there is another significant region. This is the warm temperate south coast region, which extends from Port St Johns to Cape Point.
The south coast represents a great mixing zone, resulting in great species diversity unique to this region. Various seaweeds exist here, but not the kelp forests that characterise the west coast. Many fish tend to migrate through this region during winter.
Schools of fish like pilchards migrate northwards followed by predators including elf (Pomatomus saltatrix), yellowtail (Seriola lalandi), garrick (Lichia amia) and mackerel (Scomber japonicas). Various shark species also join the hunt.
Along the east coast, vast numbers of pilchards are forced into the shallows and even onto the beaches. This annual phenomenon is known locally as the Natal sardine run and, during the run, humans and seabirds take great advantage of this bonanza. A carnival atmosphere prevails as they greedily gather huge quantities of the silvery fishes.
At this time, great numbers of dolphins migrate up the east coast coinciding with the annual migration of humpback whales (Megaptera novaeangliae) to Madagascar.
During these months, southern right whales (Balaena glacialis) also visit the Cape south coast. Birthing and mating of these whales occurs close inshore, resulting in one of the most spectacular land-based whale-watching opportunities in the world.
Sea for yourself
The 2012 International Aquarium Congress (IAC) committee has organised special tours and day trips to many of these fascinating spots along the South African coast.
Stay in touch
Beckley, L., Branch, G., Branch, M., & Griffiths, C. (1994). Two Oceans – A Guide to the Marine Life of southern Africa David Philip: Cape Town & Johannesburg
Branch, G. & Branch, M. (1995). The Living Shores of Southern Africa Struik Publishers: Cape Town
Payne, A.I.L. & Crawford, R.J.M (1989). Oceans of Life off Southern Africa Vlaeberg Publishers: Cape Town