Chondrichthyes (cartilaginous fish,
including sharks, rays and chaemeras) >
Habitats of cartilaginous fish off the coast of southern Africa
The seas of southern Africa support a varied and diverse
fauna of cartilaginous fishes, which can be roughly classified by habitat
relative to the continental land mass. The majority of species occur on and
around the edges of the continental shelves, from the seaside to approximately
200 m, but the area also has a varied upper continental slope fauna of deepwater
sharks, rays (mostly skates), and chimaeras ranging from 200 to over 1000 m
depth, and a less diverse oceanic fauna of widespread sharks and rays, which
occur in the great ocean basins mostly in the top few hundred meters, but in
some species reach the ocean floor. Southern Africa features a large number of
cartilaginous fishes found nowhere else, the endemics. Others, particularly
tropical shelf, oceanic, and deep-slope species, are wide-ranging, and occur in
the Atlantic, western Indian Ocean, Indo-Pacific, or are circumtropical; some
cool-water sharks are amphitemperate, and are absent from the tropics although
widespread in temperate waters of the northern and southern hemispheres. Some
rays have a more limited amphitemperate range, and occur in the north Atlantic
and off the west coast of southern Africa.
Shelf cartilaginous fishes have a complex composition off
southern Africa: the temperate-water species divide into wide-ranging
amphitemperates, Atlantic amphitemperates, and restricted endemic species; the
tropical species have major components from the Western Indian Ocean, Indo-West
Pacific, and circumtropical species, and additional components from the Eastern
Atlantic and Eastern Hemisphere. A few sharks and rays have a wide range from
European seas through tropical West Africa to southern Africa. Several species
show a bicoastal distribution, and are present off Namibia and the east coast
but are missing from off the western Cape.
Shelf and oceanic cartilaginous fishes are much influenced
by the two major current systems off southern Africa. The west coast, washed by
the cold Benguela current, supports a temperate, depauperate fauna. The southern
and southeast Cape coast has a richer temperate fauna with most of the west
coast species and additional temperate endemic cartilaginous fishes. Mozambique
and Natal are washed by the warm Agulhas current and have many tropical shelf
sharks and rays, but southern Natal and Transkei are a transitional zone to the
temperate eastern Cape fauna. In the summer, particularly in years with an "el
Nin~o" flux of warm water, Natal sharks and rays follow the current to the
eastern and southwestern Cape. A tropical shelf species, the Zambezi shark, has
a broad salinity tolerance and regularly ascends tropical rivers and occurs in
tropical lakes; it has been taken in rivers in Kruger Park and in Mozambique.
Only one chimaera, the St Joseph, occurs on the shelf in temperate waters in
southern Africa, but also extends onto the uppermost slope.
The oceanic cartilaginous fish fauna is dominated by
tropical sharks, with lesser numbers of South Atlantic temperate-tropical sharks
and a minor amphitemperate component. Oceanic sharks are wide-ranging and have
no local endemics; some visit the shelf but normally occur far out at sea. Some
of these give birth to their young offshore, which then migrate outwards to the
open ocean. They include both the smallest and largest of sharks, as well as a
variety of near-surface and deep-water species. Tropical oceanic sharks show a
similar tendency to shelf sharks and rays in ranging southwestwards in the
summer, and some of these turn up as far south as Cape Point. Oceanic rays are
few, and include the unique pelagic stingray and possibly the manta. No
chimaeras are oceanic.
The deep slope cartilaginous fishes include wide-ranging bottom species that
cross the temperate-tropical boundary because of the generally cold waters of
the slope in both temperate and tropical areas. There are, however, numerous
slope endemics, and important differences between the deep-slope cartilaginous
fish fauna of Natal and southern Mozambique and that off the western Cape. Some
slope species are virtually circumglobal in all temperate and tropical seas,
others are broadly shared with other regions, while some are endemic. Off the
southwestern Cape there is a broad faunal transition zone at 200 to 600 m depth
on the hake fishing grounds, with an intermediate cartilaginous fish fauna
between the outer shelf and deep slope faunas. The slope ray fauna is dominated
by skates, with a few species of stingrays (east coast) and electric rays. Most
southern African chimaeras occur on the continental slope.
Text by Leonard J.V. Compagno, David A. Ebert
and Malcolm J. Smale |