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The Agulhas Bank (, from Portuguese for Cape Agulhas, Cabo das Agulhas, "Cape of Needles") is a broad, shallow part of the southern continental shelf which extends up to south of Cape Agulhas before falling steeply to the abyssal plain.
It is the ocean region where the warm Indian Ocean and the cold Atlantic Ocean meet. This convergence leads to treacherous sailing 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 bank in 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 along the coast of the Agulhas Ecoregion spanning from below sea level. Many rocky sub-tidal reefs are of aeolianite or sandstone origin, but granite, quartzite 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.
As the current is diverged away from the coast, dynamic processes draws an onshore Ekman layer 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 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.
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 Province). 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 merge to contribute to the shedding of an Agulhas ring.
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 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 Sverdrup (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.
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.
The Cape Fold Belt (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 Thrust fault 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 (East Antarctica), and Ellsworth Mountains (West Antarctica). In the late Carboniferous and early Jurassic, 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.
The Agulhas-Falkland Fracture Zone (AFFZ) stretches across the South Atlantic. It is one of the largest and most spectacular 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 (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 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.)
One of the largest known slumps occurred on the south-eastern edge of the Agulhas Bank in the Pliocene or more recently. Stretching from a depth of , the so-called Agulhas slump is long, wide, and has a volume of . It is a composite slump with proximal and distal 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.
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 , fur seal, fish, , 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. 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. 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.
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 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 and anchovy and demersal hake 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 fishery to catch horse mackerel and chub mackerel; pelagic Longline fishing and pole fishing fishery to catch tunas and swordfish; while hook and line are used inshore to catch squid and teleost species, including Thyrsites atun and geelbek. All these species are relatively common and are considered having an important role in the ecosystem.
The bank is the spawning area of deep reef fish species, including the threatened endemic Petrus rupestris ( 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 have been reported off the western coast of South Africa, of which 21 are squaliformes.
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 and 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%.
A vulnerable population of fish-eating 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 has shown that there was a peak inter-oceanic migration events during the Eemian interglacial period, 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, colonised the North Atlantic and Mediterranean by following their prey — bluefin tuna and swordfish.
A vagrant Commerson's dolphin — 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 have been recovered by trawling from the seafloor off South Africa. Stranded pygmy sperm whales have been recorded on both the east and west coasts of South Africa.
Coastal MPAs:
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