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The Agulhas Bank (, from Portuguese for
Cape Agulhas Cape Agulhas (; pt, Cabo das Agulhas , "Cape of the Needles") is a rocky headland in Western Cape, South Africa. It is the geographic southern tip of the African continent and the beginning of the dividing line between the Atlantic and Indian ...
, ''Cabo das Agulhas'', "Cape of Needles") is a broad, shallow part of the southern
Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
n continental shelf which extends up to south of
Cape Agulhas Cape Agulhas (; pt, Cabo das Agulhas , "Cape of the Needles") is a rocky headland in Western Cape, South Africa. It is the geographic southern tip of the African continent and the beginning of the dividing line between the Atlantic and Indian ...
before falling steeply to the
abyssal plain An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between and . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth's surface. ...
. It is the ocean region where the warm
Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by t ...
and the cold
Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...
meet. This convergence leads to treacherous
sailing Sailing employs the wind—acting on sails, wingsails or kites—to propel a craft on the surface of the ''water'' (sailing ship, sailboat, raft, windsurfer, or kitesurfer), on ''ice'' (iceboat) or on ''land'' (land yacht) over a chosen cou ...
conditions, accounting for numerous wrecked ships in the area over the years. However, the meeting of the oceans here also fuels the nutrient cycle for marine life, making it one of the best
fishing grounds Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life; or more commonly, the site where such enterprise takes place ( a.k.a. fishing ground). Commercial fisheries include wild fisheries and fish farms, both ...
in
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...
.


Extent and characteristics

South African marine ecoregions from the 2011 classification The Agulhas Bank stretches approximately along the African coast, from off
Cape Peninsula The Cape Peninsula ( af, Kaapse Skiereiland) is a generally mountainous peninsula that juts out into the Atlantic Ocean at the south-western extremity of the African continent. At the southern end of the peninsula are Cape Point and the Cape ...
(18°E) to Port Alfred (26°E), and up to from it. The bank slopes down relatively steeply from the coast to about deep and reaches before dropping steeply to on its southern edge. The shelf spans an area of with a mean depth slightly over . It is entirely within the exclusive economic zone of South Africa. The National Spatial Biodiversity Assessment 2004 recognised 34 biozones nested within 9 bioregions (of which four were offshore). The National Biodiversity Assessment 2011 replaced these ecozones and biozones with the terms ecoregions and ecozones. In 2011, the Agulhas Ecoregion was divided into four distinct ecozones: Agulhas inshore, Agulhas inner shelf, Agulhas outer shelf, and Agulhas shelf edge. 33 different benthic habitats types were identified on the Agulhas Bank. There are dozens of warm temperate
reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock o ...
s along the coast of the Agulhas Ecoregion spanning from below sea level. Many rocky sub-tidal reefs are of
aeolianite Eolianite or aeolianite is any rock formed by the lithification of sediment deposited by aeolian processes; that is, the wind. In common use, however, the term refers specifically to the most common form of eolianite: coastal limestone consisting ...
or
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...
origin, but
granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...
,
quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tec ...
and siltstone reefs are also present. The Agulhas reefs are very heterogeneous and include several possible different sub-types. Some of the reefs are within protected areas, but only a few of those protected areas include protection from fishing.


Oceanography

The Agulhas Bank is a natural boundary between
ocean current An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours ...
s from the
Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...
,
Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by t ...
, and
Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...
, resulting in one of the most turbulent waters of the world oceans.


Agulhas Current

The Agulhas Current flows south along the African east-coast and along the south-eastern edge of the bank. It then retroflects back into the Indian Ocean south-west of the bank. This retroflection results in intense
eddy Eddie or Eddy may refer to: Science and technology *Eddy (fluid dynamics), the swirling of a fluid and the reverse current created when the fluid flows past an obstacle * Eddie (text editor), a text editor originally for BeOS and now ported to Lin ...
activities such as meanders, eddies, and filaments. In upper layer water, the Agulhas rings and eddies move warm and salty water into the large South Atlantic
gyre In oceanography, a gyre () is any large system of circulating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction dete ...
, which exports it to the tropics. In the lower ocean layers water is transported in the opposite direction.


Upwellings

Cyclonic eddies is another source of edge upwelling west of Port Elisabeth. Plumes of warm surface water migrate onto the bank along its eastern edge, providing subtropical surface water from the Indian Ocean. In summer, easterly winds can intermittently drive coastal
upwelling Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nut ...
along the South African south coast. The Agulhas Bank is dominated by westerly winds and most of the upwelling on the bank is related to the interaction of the Agulhas Current on the eastern edge, but easterly winds do occur, especially in summer and fall, and can generate local upwelling cells. As the current is diverged away from the coast, dynamic processes draws an onshore
Ekman layer The Ekman layer is the layer in a fluid where there is a force balance between pressure gradient force, Coriolis force and turbulent drag. It was first described by Vagn Walfrid Ekman. Ekman layers occur both in the atmosphere and in the ocean ...
of cold water from below the warm shelf-edge flow. In spring and summer, at a depth of , a semi-permanent ridge of cold water is present on the eastern and central shelf. In summer, there is mixture of subtropical water separated by
thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...
s from cool waters, but there is a considerable seasonal variation. On the shelf, bottom waters exhibit characteristics of the central Indian Ocean in the east and central Atlantic Ocean waters in the west.


Agulhas meanders and Natal pulses

As the Agulhas Current flows south along the African east coast, it tends to bulge inshore frequently, a deviation from the current's normal path known as Agulhas Current meanders (ACM). These bulges are occasionally (1-7 times per year) followed by a much larger offshore bulge, known as Natal pulses (NP). Natal pulses move along the coast at per day. An ACM can bulge up to and a NP up to from the current's mean position. The AC passes offshore and an ACM can reach offshore. When the AC meanders, its width broadens from to and its velocity weakens from to . An ACM induces a strong inshore counter-current. Large-scale cyclonic meanders known as Natal pulses are formed as the Agulhas Current reaches the continental shelf on the South African east-coast (i.e. the eastern Agulhas Bank off
Natal NATAL or Natal may refer to: Places * Natal, Rio Grande do Norte, a city in Brazil * Natal, South Africa (disambiguation), a region in South Africa ** Natalia Republic, a former country (1839–1843) ** Colony of Natal, a former British colony ( ...
). As these pulses move along the coast on the Agulhas Bank, they tend to pinch off Agulhas rings from the Agulhas Current. Such a ring shedding can be triggered by a Natal pulse alone, but sometimes meanders on the
Agulhas Return Current The Agulhas Return Current (ARC) is an ocean current in the South Indian Ocean. The ARC contributes to the water exchange between oceans by forming a link between the South Atlantic Current and the South Indian Ocean Current. It can reach veloc ...
merge to contribute to the shedding of an Agulhas ring.


Agulhas leakage and rings

Agulhas rings are large anticyclonic eddies or
warm core ring A warm core ring is a type of mesoscale eddy which forms and breaks off from an ocean current, such as the Gulf Stream or the Kuroshio Current. The ring is an independent circulatory system of warm water that can persist for several months before ...
s of ocean water that are pinched off the Agulhas Current along the eastern edge of the Agulhas Bank from where they move into the
South Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe an ...
. As the Agulhas Current reaches the east coast of South Africa, large solitary meanders known as Natal pulses form at irregular intervals. 165 days after the appearance of a Natal pulse, an Agulhas ring is formed off Durban. The Agulhas rings are among the largest eddies in the world and play an important role in the Agulhas Leakage, the transport of warm water from the Indian Ocean to the Atlantic Ocean, which affects the global climate. The average diameter of the Agulhas rings is , but they can reach 500 km. They extend down to the ocean floor; circulate at ; and move into the South Atlantic at /day. Only half of the Agulhas eddies that leave the Cape Basin manage to cross the
Walvis Ridge The Walvis Ridge (''walvis'' means whale in Dutch and Afrikaans) is an aseismic ocean ridge in the southern Atlantic Ocean. More than in length, it extends from the Mid-Atlantic Ridge, near Tristan da Cunha and the Gough Islands, to the Africa ...
and those that do tend to lose half their energy before reaching the ridge within six months. The Agulhas rings transport an estimated 1-5 Sv (millions m²/s) of water from the Indian Ocean to the South Atlantic. The Agulhas rings are thought to be of global climatic importance. Their delivery of warm water from the Indian to the Atlantic Ocean can control the rate of thermohaline overturning of the entire Atlantic. Other factors contribute, to various degrees, to the inter-ocean exchanges in the region, including filaments from the Agulhas Current and intrusions of water from Antarctica. Cold, cyclonic eddies have been observed in the southwestern Atlantic. Based on model simulations, researchers have found that the interaction of the Agulhas Current and the eastern edge of the bank can result in the Agulhas rings. The provenance of ocean sediments can be determined by analysing terrigenous strontium isotope ratios in deep ocean cores. Sediments underlying the Agulhas Current and Return Current have significantly higher ratios than surrounding sediments. Analyses of cores in the South Atlantic deposited during the Last Glacial Maximum (LGM, 20 000 years ago), show that the Agulhas leakage (shedding of Agulhas rings) was significantly reduced. It has been hypothesised that the reason for this was that the Agulhas Current was stronger which resulted in a more eastward retroflection and therefore less leakage. However, analyses of such cores south of Africa show that the trajectory of the current was the same during the LGM and that the reduced leakage must be explained by a weaker current. Consequently, it can be predicted that a stronger Agulhas Current will result in its retroflection occurring more eastward and an increased Agulhas leakage.


Benguela Current

Compared to the Agulhas Current, the Benguela Current on the west and south-west coast of Africa is more intense and steadier. Its dynamic southern upwelling system is driven by the prevailing northward winds that produce an intense off-shore Ekman transport. Most of this upwelling is concentrated to a few upwelling cells in the southern region: Namaqua (30°S), Cape Columbine (32.5°S), and
Cape Peninsula The Cape Peninsula ( af, Kaapse Skiereiland) is a generally mountainous peninsula that juts out into the Atlantic Ocean at the south-western extremity of the African continent. At the southern end of the peninsula are Cape Point and the Cape ...
(34°S). The wind is most intense from October to February, and the contrast in sea surface temperature between the open sea and the shelf is most prominent during summer. Coastal upwelling is also common on the western bank, but the more stable atmospheric condition results in larger cold water plumes that sometimes merge to form a continuous upwelling regime along the South African south-west coast. This upwelling zone is the southernmost extension of the Benguela Current Large Maritime Ecosystem. The Agulhas Current regularly flows around the southern tip of the bank and brings warm water to the western bank along the bank's western edge. Regularly, the mesoscale eddies from the east interact with the Benguela upwelling system on the African west coast.


Deep water eddies

Flowing south along the South American continental slope, the Deep Western Boundary Current (DWBC) carries
North Atlantic Deep Water North Atlantic Deep Water (NADW) is a deep water mass formed in the North Atlantic Ocean. Thermohaline circulation (properly described as meridional overturning circulation) of the world's oceans involves the flow of warm surface waters from the ...
(NADW) into the South Atlantic. At about 8°S and at a depth of , the DWBC breaks into anticyclonic eddies during periods of strong
meridional overturning circulation Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to tem ...
. One such NADW eddy was observed in 2003 and the researchers speculated that a deeply penetrating Agulhas ring pinched it off the NADW slope current. Spinning at , these deep-water eddies move around the southern tip of the Agulhas Bank and into the Indian Ocean. Most of the NADW flow (more than 7 Sv) meanders east around the Agulhas Plateau together with the surface Agulhas Return Current, but a smaller portion (3 Sv) continues north along the African east-coast as the Agulhas Undercurrent. Of 89.5 Sv released from the North Atlantic, 3.6 Sv leaves the South Atlantic south of the Agulhas Bank. However, 0.9 Sv recirculate in the basin north of the Walvis Ridge for centuries, of which 50-90% end up flowing south of the Agulhas Bank within 300 years, increasing the net inter-oceanic exchange with 4.1-4.5 Sv.


Alphard Banks

The Alphard Banks are a small group of long extinct volcanic seamounts on the Agulhas Bank south of Cape Agulhas. They rise from the bottom at about 80 m to about 14 m on the top.


Geology

The oldest rock found along the coastline of the Agulhas Bank are eugeosynclinal sediments of the up to thick Kaaimans Group deposited during continental rifting some 900 million years ago (Mya). The proto-South Atlantic closed during the Saldanian orogeny to form part of the supercontinent Gondwana (700-600 Mya). The Cape granites were emplaced and the Kaaimans Group rocks were folded and thermally metamorphosed during this period. The formation of the main basin in the Cape Province commenced 570 Mya and lasted for 200 My. The
Table Mountain Table Mountain ( naq, Huriǂoaxa, lit= sea-emerging; af, Tafelberg) is a flat-topped mountain forming a prominent landmark overlooking the city of Cape Town in South Africa. It is a significant tourist attraction, with many visitors using the ...
Group is thick and an erosional unconformity marking its base is composed of both terrestrial and marine sediments. Synclines along the coast of the southern Cape contains sediments from the Bokkveld Group. The
Cape Fold Belt The Cape Fold Belt is a fold and thrust belt of late Paleozoic age, which affected the sequence of sedimentary rock layers of the Cape Supergroup in the southwestern corner of South Africa. It was originally continuous with the Ventana Moun ...
(CFB) rocks and the Karoo Basin were deposited 450 Mya; the Cape Supergroup 450-300 Mya during a series of transgression- regression cycles.
Pan-African Pan-Africanism is a worldwide movement that aims to encourage and strengthen bonds of solidarity between all Indigenous and diaspora peoples of African ancestry. Based on a common goal dating back to the Atlantic slave trade, the movement ext ...
thrusts were reactivated 270-215 Mya to form the CFB which was then part of a continuous fold belt that developed during the Gondwanide orogeny together with Sierra de la Ventana (Argentina),
Pensacola Mountains The Pensacola Mountains are a large group of mountain ranges of the Transantarctic Mountains System, located in the Queen Elizabeth Land region of Antarctica. Geography They extend 450 km (280 mi) in a NE-SW direction. Subranges of th ...
(East Antarctica), and
Ellsworth Mountains The Ellsworth Mountains are the highest mountain ranges in Antarctica, forming a long and wide chain of mountains in a north to south configuration on the western margin of the Ronne Ice Shelf in Marie Byrd Land. They are bisected by Minnesota ...
(West Antarctica). In the late Carboniferous and early
Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
, the Karoo Supergroup was deposited in the Karoo Basin north of where the CFB is located today, and covering nearly two-thirds of present-day
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...
.


Gondwana breakup

Basaltic lavas were extruded 183 Mya to form the Karoo large igneous province; a volcanism caused by the Bouvet hotspot which is linked to the Gondwana break-up. The Bouvet hotspot was located in or near present-day South Africa from the late Triassic 220 mya and until the Africa-Antarctica breakup 120 mya. The Bouvet hotspot track stretches south-east from the African continent, near the South Africa-Mozambique border, and east of the AFFZ down to
Bouvet Island Bouvet Island ( ; or ''Bouvetøyen'') is an island claimed by Norway, and declared an uninhabited protected nature reserve. It is a subantarctic volcanic island, situated in the South Atlantic Ocean at the southern end of the Mid-Atlantic R ...
/ Bouvet Triple Junction in the South Atlantic. 100 Mya, the region where the triple junction was located passed over the hotspot, resulting in a continuous eruption that lasted to about 94 Mya and the seafloor spreading that still separates Antarctica, Africa, and South America. The Agulhas-Falkland Fracture Zone (AFFZ) stretches across the South Atlantic. It is one of the largest and most spectacular fracture zones on Earth. It developed during the Early Cretaceous as West Gondwana (South America) broke up from Africa. The AFFZ is characterized by a pronounced topographic anomaly, the
Agulhas Ridge Agulhas (''needles'' in the Portuguese language — or ) may refer to: * Cape Agulhas, the southernmost point of Africa * L'Agulhas, a town near the Cape * Cape Agulhas Lighthouse, located at the Cape * Cape Agulhas Local Municipality, the municipa ...
(41°S,16°E-43°S,9°E) which rises more than 2 km above the surrounding sea floor. The only equivalent in size are the neighbouring Diaz Ridge and the Falkland Escarpment. The Agulhas Ridge is unique because it was not formed during the continental breakup during the Cretaceous and because it separates oceanic crusts of different age, and not oceanic crust (~14 km thick) from continental crust (25 km thick). North of the AFFZ is the Outeniqua Basin which is a complex system of sub-basins separated from each other by faults and basement arches; there are several smaller fault-bounded sub-basins in the north (Bredasdorp, Infanta, Pletmos, Gamtoos, and Algoa) and a distinctively deeper sub-basin in the south, the South Outeniqua Basin. The sedimentary fill of these basins developed as the northern edge of the Falkland Plateau separated from the South African southern margin during the early
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
. The Diaz Marginal Ridge (DMR) separates these basins from the AFFZ. The DMR is buried under of sediments and sedimentary rocks and of this sedimentary material is undisturbed Cretaceous sediments younger than the oldest Cretaceous sedimentary rocks in the Southern Outeniqua Basin. The DMR must therefore have formed after the initial West Gondwana breakup 130-90 Mya. The DMR probably formed when new, hot oceanic crust slid past old, cold continental crust and the contrast in temperatures induced a thermal uplift. As West Gondwana drifted away from Africa roughly 125 Myr, the South Atlantic seafloor formed between them and magnetic anomalies north of the AFFZ reflects phase of the seafloor spreading. South of the AFFZ traces can be found of how the Falkland Plateau and the Agulhas Bank moved relative to each other. On a modern map, the Falkland Plateau can still be rotated and fitted into the Natal Valley in the Indian Ocean east of South Africa. The Agulhas Plateau is located southeast of the shelf, separated from it by the Agulhas Passage (through which the Agulhas Current flows.)


Pliocene

The Alphard Tertiary Igneous Province includes Palaeocene
tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...
s,
trachybasalt Trachybasalt is a volcanic rock with a composition between trachyte and basalt. It resembles basalt but has a high content of alkali metal oxides. Minerals in trachybasalt include alkali feldspar, calcic plagioclase, olivine, clinopyroxene and li ...
s, aegirine–augite
trachyte Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and al ...
s and aegirine–augite phonolitic trachytes, which have been radiometrically dated at about 58 million years old. The intrusions appear to be tectonic effects. One of the largest known slumps occurred on the south-eastern edge of the Agulhas Bank in the
Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58allochthonous sediment masses separated by a large glide plane scar. In the western part, the sediments are dammed by basement ridges, but, in the eastern part, they have spread into the Transkei Basin. A series of slump scarps along the western edge of the shelf are 18–2 Mya, but covered by younger sediments brought there by the Benguela upwelling.


Human evolution

Anatomically modern humans Early modern human (EMH) or anatomically modern human (AMH) are terms used to distinguish '' Homo sapiens'' (the only extant Hominina species) that are anatomically consistent with the range of phenotypes seen in contemporary humans from exti ...
evolved around 200  kya. The genetic diversity in the human lineage is relatively low, which indicate one or several population bottlenecks late in our lineage. It has been estimated that the population was limited to perhaps 600 individuals during the MIS 6 glacial stage (195-125 kya), one of the longest cold periods in the Quaternary of Africa. A technological and behavioural revolution that occurred globally about 50 kya led to a cultural complexity which happened in South Africa around 120-70 kya. The Cape Floral Region is a thin coastal strip and a botanic hotspot which developed at the confluence of the Benguela Upwelling and Agulhas Current. According to what professor Curtis Marean calls the "Cape Floral Region – South Coast Model" for the origins of modern humans, the early hunter-gatherers survived on shellfish, as well as geophytes,
fur seal Fur seals are any of nine species of pinnipeds belonging to the subfamily Arctocephalinae in the family '' Otariidae''. They are much more closely related to sea lions than true seals, and share with them external ears (pinnae), relatively l ...
,
fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...
,
seabird Seabirds (also known as marine birds) are birds that are adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same enviro ...
s, and wash-ups found on the exposed Agulhas Bank. The bank slopes into the sea and a reconstruction of how the coastline has changed over 440 kya shows that the coast during the Pleistocene was located as far as from the present coast. The present South African southern coastal plain (SCP) is still separated from the rest of Africa by the
Cape Fold Belt The Cape Fold Belt is a fold and thrust belt of late Paleozoic age, which affected the sequence of sedimentary rock layers of the Cape Supergroup in the southwestern corner of South Africa. It was originally continuous with the Ventana Moun ...
. During glacial maxima the sea-level dropped . This not only left large parts of the Agulhas Bank exposed, which greatly expanded the area of the SCP, but it also reconnected the SCP to the rest of Africa by the shallow water shelves, which broke the isolation of the SCP. Modern humans evolved on the SCP and the fluctuation in sea-levels would have resulted in a significant variation in
selective pressure Any cause that reduces or increases reproductive success in a portion of a population potentially exerts evolutionary pressure, selective pressure or selection pressure, driving natural selection. It is a quantitative description of the amount of ...
. No fossil records are known from the now submerged shelf, but a series of key fossil sites along the coastal margin of the present SCP provide earliest traces of anatomically modern humans and the use of marine resources.


Commercial importance

South Africa began oil exploration on the Agulhas Bank in the 1980s. Of more than 200 offshore wells in South Africa, most are found on Bredasdorp Basin on the Agulhas Bank.


Fishery

The Agulhas Bank is also significant for fisheries who use demersal trawling, demersal
longline fishing Longline fishing, or longlining, is a commercial fishing angling technique that uses a long ''main line'' with fish bait, baited fish hook, hooks attached at intervals via short branch lines called ''snoods'' or ''gangions''.< ...
, and midwater trawl fishing on the bank. Squid and small
pelagic fish Pelagic fish live in the pelagic zone of ocean or lake waters—being neither close to the bottom nor near the shore—in contrast with demersal fish that do live on or near the bottom, and reef fish that are associated with coral re ...
es are also caught. Before the introduction of the EEZ, foreign fisheries used roch-hopper gear trawling on the bank. Most of the catches are short-lived shelf-zone pelagic species and more long-lived deep-water species. The large populations of sardine and anchovy also present on the shelf follow an annual cycle. Anchovy spawn on the western Agulhas Bank in early summer while the sardines span over a broader season and area — eggs are transported by currents to the nursery area in the
St Helena Bay Saint Helena Bay ( af, St. Helenabaai) is a settlement in West Coast District Municipality in the Western Cape province of South Africa. Nicknamed by locals as ''Agterbaai'',''St Helena Bay.'' CapeWestCoast.org. http://www.capewestcoast.org/reg ...
on the South African west coast from where juvenile then migrate back to the Agulhas Bank to spawn. South Africa has a relatively large fishing industry mostly catching pelagic
pilchard "Sardine" and "pilchard" are common names for various species of small, oily forage fish in the herring family Clupeidae. The term "sardine" was first used in English during the early 15th century, a folk etymology says it comes from the ...
and
anchovy An anchovy is a small, common forage fish of the family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water. More than 140 species are placed in 1 ...
and demersal
hake The term hake refers to fish in the: * Family Merlucciidae of northern and southern oceans * Family Phycidae (sometimes considered the subfamily Phycinae in the family Gadidae) of the northern oceans Hake Hake is in the same taxonomic order ( ...
on the south and western coasts. Though the east coast has fewer commercial fisheries, the large human population along there has resulted in overexploitation of coastal fish and invertebrate stocks by recreational and subsistence fishers. A small aquaculture industry produces mussels and oysters offshore. Several pelagic species are heavily harvested by the commercial fleet: purse-seine fishery is used to catch sardines, anchovies, and round herring; mid-water
trawl Trawling is a method of fishing that involves pulling a fishing net through the water behind one or more boats. The net used for trawling is called a trawl. This principle requires netting bags which are towed through water to catch different speci ...
fishery to catch horse mackerel and chub mackerel; pelagic longline and
pole Pole may refer to: Astronomy *Celestial pole, the projection of the planet Earth's axis of rotation onto the celestial sphere; also applies to the axis of rotation of other planets *Pole star, a visible star that is approximately aligned with the ...
fishery to catch tunas and swordfish; while hook and line are used inshore to catch squid and teleost species, including snoek and geelbek. All these species are relatively common and are considered having an important role in the ecosystem.


Biodiversity

There are at least 12,914 marine species in South Africa, but small bodied species are poorly documented and the abyssal zone is almost completely unexplored. Almost a quarter of South Africa's coast line is protected, excluding deeper water. A third of the marine species are endemic to South Africa (though poor levels of taxonomic research in adjoining countries probably affects the apparent endemism.) The degree of endemism varies considerably among taxa:
Bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...
64%, Mollusca 56%,
Echinodermata An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
3.6%, Porifera 8.8%, Amphipoda 33%, Isopoda 85%, or
Cumacea Cumacea is an order of small marine crustaceans of the superorder Peracarida, occasionally called hooded shrimp or comma shrimp. Their unique appearance and uniform body plan makes them easy to distinguish from other crustaceans. They live in sof ...
71%. Fisheries are one of the major threats to the biodiversity of the Agulhas Bank.


Crustaceans

Copepods comprise 90% of the zooplankton carbon on the Agulhas Bank, and are thus an important source of food for pelagic fish and juvenile squids. The population of '' Calanus agulhensis'', a large species that dominates the copepod community in terms of biomass, has a center of distribution on the central Agulhas Bank. Since 1997 the copepod biomass on the central Agulhas Bank has declined significantly while the biomass of pelagic fish has increased significantly. While it is likely that predation has played an important role in the copepod decline,
global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
(sea surface temperature and chlorophyll A abundance) is believed to have contributed to a smaller population.


Fishes

The shelf edge along the bank's southern tip is subject to sporadic upwelling. This slope and its surrounding seamounts are the spawning ground for
sardine "Sardine" and "pilchard" are common names for various species of small, oily forage fish in the herring family Clupeidae. The term "sardine" was first used in English during the early 15th century, a folk etymology says it comes from the It ...
,
anchovy An anchovy is a small, common forage fish of the family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water. More than 140 species are placed in 1 ...
, and
horse mackerel Horse mackerel is a vague vernacular term for a range of species of fish throughout the English-speaking world. It is commonly applied to pelagic fishes, especially of the Carangidae (jack mackerels and scads) family, most commonly those of the gen ...
. Eddies help transport water inshore and link the spawning habitat with important nursery areas. Eggs and larvae laid by the anchovy are transported via the Good Hope Jet to Africa's southwestern coast where they mature. Young anchovies then return to the Agulhas Bank to spawn. Young sardine and anchovy congregate along the west coast between March and September before they migrate to their spawning grounds on the Agulhas Bank. Sardines of intermediate age are present on the western Agulhas Bank between January and April before migrating to KwaZulu-Natal for winter. The spawning on the Agulhas Bank takes place offshore from September to February. The bank is the spawning area of deep reef fish species, including the threatened endemic red steenbras (''Petrus rupestris''). Other species have been overexploited, including daggerhead seabream or dageraad ('' Chrysoblephus cristiceps''), black musselcracker (''Cymatoceps nasutus''), and silver kob (''Argyrosomus inodorus''). 57 species of
shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s have been reported off the western coast of South Africa, of which 21 are squaloid sharks.


Birds

The main food source for African penguins (''Spheniscus demersus'') is anchovy and sardine which they forage between Cape Columbine and the central Agulhas Bank. The birds have colonies on
Dassen Island Dassen Island is an uninhabited South African island in the Atlantic Ocean. It is situated about west of Yzerfontein and north of Cape Town. The flat and low-lying island measures about long northwest-southeast and wide, with an area of . It ...
, on the South African west coast, and Bird Island, on the south coast. African penguins breed opportunistically, following the anchovy and sardine: from February to September on the Western Cape but from January to July on St Croix Island off Eastern Cape. After breeding, the birds forage further offshore: off the western coast and up to from their colonies off Eastern Cape. In 2005, when Korean and Philippine vessels started longline fishing along the edges of the Agulhas Bank, seabird bycatch became a huge problem. Large numbers of albatrosses and
petrel Petrels are tube-nosed seabirds in the bird order Procellariiformes. Description The common name does not indicate relationship beyond that point, as "petrels" occur in three of the four families within that group (all except the albatross f ...
s were killed — in average 0.6 birds per 1000 hooks, but up to 18 birds per 1000 hooks were reported. Since 2007, however, more restrictive permit conditions for foreign-flagged fleets and the use of birds scaring lines have decreased the number of killed birds by 85%.


Fur seals

Cape fur seals are present along the South African coast. Fur seals are protected in South Africa since 1893 although a small number are occasionally culled to protect sea birds. Many seals are caught in fishery nets and boat propellers, but the seals are also regularly accused of stealing fish from the fisheries. Sharks are known to prey on them, but in 2012 a cape fur seal was observed preying on and consuming a mid-sized blue shark.


Cetaceans

51 species, or more than 50%, of the recognized species of cetaceans are present in the southern African subregion (between the equator and the Antarctic ice edge), of which 36 have been sighted in South African and Namibian waters. A vulnerable population of fish-eating
killer whale The orca or killer whale (''Orcinus orca'') is a toothed whale belonging to the oceanic dolphin family, of which it is the largest member. It is the only extant species in the genus ''Orcinus'' and is recognizable by its black-and-white ...
s are present offshore on the Agulhas Bank. Observations peak in January while few are sighted in April and May. The killer whales move in pods of 1-4 individuals and are mostly sited over the shelf edge off the south-east coast. An analysis of killer whale
mtDNA Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial DNA ...
has shown that there was a peak inter-oceanic migration events during the Eemian
interglacial period An interglacial period (or alternatively interglacial, interglaciation) is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene in ...
, 131-114 kya. This peak coincides with a period of maximal Agulhas leakage which promoted a rapid and episodic interchange of killer whale lineages. During this period killer whales and other marine top predators, such as the
great white shark The great white shark (''Carcharodon carcharias''), also known as the white shark, white pointer, or simply great white, is a species of large Lamniformes, mackerel shark which can be found in the coastal surface waters of all the major ocean ...
, colonised the North Atlantic and Mediterranean by following their prey —
bluefin tuna Bluefin tuna is a common name used to refer to several species of tuna A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, th ...
and swordfish. A
vagrant Vagrancy is the condition of homelessness without regular employment or income. Vagrants (also known as bums, vagabonds, rogues, tramps or drifters) usually live in poverty and support themselves by begging, scavenging, petty theft, temporar ...
Commerson's dolphin Commerson's dolphin (''Cephalorhynchus commersonii''), also referred to by the common names jacobita, skunk dolphin, piebald dolphin, panda dolphin, or tonina overa (in South America), is a small oceanic dolphin of the genus ''Cephalorhynchus'' ...
 — a species with two isolated populations, one along the southern coast of Argentina and the other around the Kerguelen Islands — was sighted on the Agulhas Bank in 2004. It is not known from which population the sighted individual stems. The Kerguelen Islands are located and South America from the Agulhas Bank, but the west-ward direction of the Antarctic Circumpolar Current would force the dolphin to swim against the current from the Kerguelen Islands. Fossil beaked whales have been recovered by trawling from the seafloor off South Africa. Stranded
pygmy sperm whale The pygmy sperm whale (''Kogia breviceps'') is one of two extant species in the family Kogiidae in the sperm whale superfamily. They are not often sighted at sea, and most of what is known about them comes from the examination of stranded speci ...
s have been recorded on both the east and west coasts of South Africa.


Conservation

There are several
marine protected area Marine protected areas (MPA) are protected areas of seas, oceans, estuaries or in the US, the Great Lakes. These marine areas can come in many forms ranging from wildlife refuges to research facilities. MPAs restrict human activity for a conse ...
s on the Agulhas Bank. These include: Coastal MPAs: * (Nelson Mandela Bay, Port Elizabeth, Eastern Cape, 2019) * (Western Cape, 2000) * (Eastern Cape, 2004) * (South of Cape Town, Western Cape, 2019)( * (Western Cape, 2000) * (Western Cape, 2000) * (Western Cape, 2000) * (Eastern Cape, 2000) * (Western Cape, 2000) * (Eastern Cape, 2000) * (Western Cape, 2004, partly in the area which may be considered part of the Agulhas Bank) * (Eastern Cape, 2000) * (Western Cape, 2001, seasonal) Offshore MPAs: * (South of Cape Agulhas, Western Cape, 2019) * (South of Port Elizabeth, Eastern Cape, 2019) * (South of Cape Agulhas, Western Cape. 2019) * (Offshore of Port Elizabeth, Eastern Cape, 2019) * * *


References


Notes


Sources

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * {{Marine biodiversity of South Africa Fishing areas of the Atlantic Ocean Fishing areas of the Indian Ocean Fishing in South Africa Geography of South Africa Temperate Southern Africa Undersea banks of the Atlantic Ocean Undersea banks of the Indian Ocean