Agulhas Current large marine ecosystem
The Agulhas Current large marine ecosystem (LME) is characterized by its mixed climate. Situated in the Indian Ocean, it lies off the southeastern margin of the African continent. It includes the continental shelf areas of Mozambique, South Africa, the Comoros Islands, the Seychelles, Madagascar and Mauritius. Its dominant large-scale oceanographic feature is the swift and warm Agulhas current, a western boundary current that forms part of the anticyclonic Indian Ocean gyre. The Global Environment Facility (GEF) is supporting an LME project, presently in the preparation (Block B) stage, for the Agulhas Current and the neighboring Somali Coastal Current LME. Book chapters and articles pertaining to this LME include Andrew et al, 1998, Beckley, 1998, Nguta, 1998, Ragoonaden, 1998, and Tomks Forbes and Cyrus, 1998.
Contents
Productivity
Detailed information on the Agulhas current, which flows southwestward and follows the edge of the continental shelf, and a description of oceanographic conditions, can be found in Beckley, 1998. For zones, ocean currents and winds in the East African region,(Nguta, 1998, p. 62). The region’s mangroves, seagrass beds, and coral reefs reflect high degrees of biodiversity and endemism (for instance the Zanzibar butterfly fish, the African butterfly fish, the ear-spot angelfish, and the tail-barred parrotfish). South Africa’s De Hoop Nature Reserve is important to several species of dolphins and the southern right whale. South Africa has several species of sea turtles, porpoises, jackass penguins, great white sharks, and abalone. The Agulhas Current LME is considered a Class II, moderately productive (150-300 grams of Carbon per square meter per year (gC/m2-yr)), ecosystem based on SeaWiFS global primary productivity estimates. Nutrient enrichment and mixing is current-associated in this LME. For a study of sea level variation and ocean circulation in the Southwest Indian Ocean, see Ragoonaden, 1998 and the Southwest Indian Ocean Fisheries (Fisheries and aquaculture) Program (www.SWIOFP.org). Several studies have suggested that the Agulhas Current LME is responsible for the dispersal of the early life history stages of various fish species. For distribution and abundance of clupeoid larvae (pilchard, round herring and anchovy) and scombrid larvae (tuna and chub mackerel), see Beckley, 1998. The role of estuaries in providing sheltered areas for juvenile organisms in this high energy marine environment is discussed in Cooper et al, 1998.
Fish and Fisheries
This LME contains fish such as Cape hake, blackhand sole, and yellowfin and albacore tuna. The LME is distinguished by a very high percentage of crustacean catches (over 40% of the total catch). The Food and Agriculture Organization (FAO) 10-year trend shows very diverse catch trends and fluctuations. The catch decreased from 32,000 metric tons in 1990 to 20,000 metric tons in 1999, with peak years between 1992-1994 with an average of 40,000 metric tons. Non-oceanic tunas represented over 40% of the catch in that time period (see FAO 2003, appendix figure 18). Tuna is heavily exploited by foreign vessels. Recent indications show over-exploitation. Artisanal fishermen have noticed a marked decrease in catches of large pelagic migratory species. Fishing with trawlers is becoming increasingly common in the region, although the impact of trawlers on benthic communities and on the recruitment of juveniles is presently not known.
Fishing methods have improved, resulting in greater numbers and more variety of fish being caught. As a consequence fish stocks are shrinking and several species face potential extinction. There are gaps in the scientific understanding of coastal and marine systems. Mozambique’s Bazaruto Islands, a narrow chain of tidal mud flats, inland salty lakes and coral reefs, contain the only viable dugong population on the East African coast. Mauritius, Reunion, the Seychelles and Madagascar have reef fish. The deep waters surrounding the Comoro Islands are the home of the unique coelacanth, a living fossil. The University of British Columbia Fisheries Center has detailed long term fish catch statistics for this LME. The ten year 1990-1999 FAO catch data is available by clicking the graph below.
Pollution and Ecosystem Health
Health concerns focus on habitat destruction (degradation of seagrass, loss of mangroves and siltation of coral reefs), and the pressures of growing populations and tourism. The rapid increase in population is accompanied by urbanization and industrialization. Mining of titanium and zirconium, and mining-related activities in general, have adverse impacts and disturb sand dune systems, wetlands and estuaries. Mangroves are being cleared for mariculture ponds or for salt production. Coral reefs are under increasing pressure from urbanization, tourism, dredging and extraction of coral material, and destructive fishing methods. Coral bleaching associated with sea temperature rise is also a problem. For more information on the effects of sewage, industrial wastes, and the use of ecologically harmful chemicals, see Nguta, 1998. Marine-based sources of pollution stems from the exploitation of the seabed for oil, minerals, sand and corals.
Estuaries have a reduced freshwater inflow; their degradation prevents the natural regulation of water quality and the natural cycling of nutrients. The Matola River in Maputo and the St. Louis River in Mauritius also experience some form of pollution from industry. Shipping around Cape Agulhas is exposed to extremes of weather and sea conditions, which greatly increases the risk of major marine pollution incidents, particularly from oil tankers. Some ships carry lethal cargoes such as nuclear waste. The Global International Waters Assessment (GIWA) has issued a matrix that ranks LMEs according to the destruction and degradation of [[ecosystem]s], habitat and community modification, pollution and global change. GIWA characterizes the Agulhas Current LME as severely impacted in the area of unsustainable exploitation of fisheries (Fisheries and aquaculture), i.e. overexploitation, excessive bycatch and discards, and destructive fishing practices such as the use of fine-mesh nets. The LME is also seriously affected by economic impacts of global change. These impacts and other pollution indicators are increasing. There is a need to assess and monitor pollution in the region. The poor sewage infrastructure, industrial effluents, solid wastes, agricultural run-off, polluted ballast water and oil spills are some of the problems requiring further study and monitoring. For a list of academic institutions and agencies that have the potential to assess and monitor aquatic pollution in the region. (Nguta, 1998)
Socioeconomics
Fishing and aquaculture are major economic activities. Crustaceans, Cape hake, blackhand sole, and yellowfin and albacore tuna are some of the species harvested. Fishing (subsistence, commercial, and recreational), and related industries including boat construction employ tens of thousands of people. The large triangular Agulhas Bank is of major economic and biological importance. Extensive exploration for oil and gas has been undertaken there. South Africa’s industrial heartland, including its gold mines, depends on port facilities for the export of processed products. Coastal cities, commercial ports and industrial centers are rapidly developing. The region borders one of the world's busiest shipping routes for the transport of crude oil from the Middle East to Europe and the Americas. South Africa and the Seychelles Islands have the most successful tourist industries.
Governance
The fragmented nature of coastal and marine resource management is a legacy of the colonial past and of subsequent political turmoil in South Africa, Mozambique, Madagascar, the Comoro Islands, the Seychelles and the island of Mauritius. The languages and cultures of the foreign occupiers were different, as were the management systems and laws they bequeathed to the now independent and democratic countries of the region. There are regionally incompatible laws, a paucity of environmental regulations and a relative absence of inter-ministerial frameworks for the management of the marine environment as a whole. The Global Environment Facility (GEF) is supporting an LME project, presently in the preparation stage, for the Agulhas Current and the neighboring Somali Coastal Current LME. The project will assist in the conservation of coral reefs and to enhance their socioeconomic value. Regional organizations involved in the management of coastal and marine resources in the West Indian Ocean a: the Western Indian Ocean Fishery Sub-Commission (WIOFC), the Inter-governmental Oceanographic Commission’s Regional Committee for the Cooperative Investigation of the North and Central Western Indian Ocean (IOCINCWIO), the Indian Ocean Commission (COI), and the UNEP Regional Seas Programme.
References
Articles and LME Volumes
- Andrew, J., Cooper, G., Harrison, TD and Ramm, AEL, 1998. The role of estuaries in Large Marine Ecosystems: examples from the Natal Coast, South Africa. In K. Sherman, E.N. Okemwa, and M.J. Ntiba, eds. Large Marine Ecosystems of the Indian Ocean: Assessment, Sustainability, and Management. Blackwell Science, Inc. Malden MA. 237-246. ISBN: 0632043180 (Agulhas Current large marine ecosystem) .
- Beckley, Lynnath E. 1998. The Agulhas Current ecosystem with particular reference to dispersal of fish larvae. In K. Sherman, E.N. Okemwa, and M.J. Ntiba, eds. Large Marine Ecosystems of the Indian Ocean: Assessment, Sustainability, and Management. Blackwell Science, Inc. Malden MA. 255-276. ISBN: 0632043180.
- FAO, 2003. Trends in oceanic captures and clustering of large marine ecosystems—2 studies based on the FAO capture database]. FAO fisheries technical paper 435. 71 pages.
- Nguta, C. Mweu. 1998. An overview of the status of marine pollution in the East African region. In K. Sherman, E.N. Okemwa, and M.J. Ntiba, eds. Large Marine Ecosystems of the Indian Ocean: Assessment, Sustainability, and Management. Blackwell Science, Inc. Malden MA. 61-71. ISBN: 0632043180.
- Ragoonaden, S., 1998. Mean monthly sea level variation and its relation to large-scale ocean circulation in the Southwest Indian Ocean. In K. Sherman, E.N. Okemwa, and M.J. Ntiba, eds. Large Marine Ecosystems of the Indian Ocean: Assessment, Sustainability, and Management. Blackwell Science, Inc. Malden MA. 193-214. ISBN: 0632043180.
- Tomks Forbes, A., and DP Cyrus, 1998. Status and future of the St. Lucia lake system, a large estuary of the Southwestern Indian Ocean. In K. Sherman, E.N. Okemwa, and M.J. Ntiba, eds. Large Marine Ecosystems of the Indian Ocean: Assessment, Sustainability, and Management. Blackwell Science, Inc. Malden MA. 283-296. ISBN: 0632043180.
Other References
- NOAA. 1991. Report of the ad hoc Committee on Large Marine Ecosystems. NOAA Technical Memorandum NMFS-F/NEC-92, 19p.
- Quartly G.D. & M.A. Srokosz, 1993. Seasonal Variations in the Region of the Agulhas Retroflection: Studies with Geosat and FRAM J. Phys. Ocean. 23, 2107-2124.
- Quartly. G.D. & M.A. Srokosz. Seasonal Variability in the Agulhas region. Results from Altimetry, Models and AVHRR. With maps and graphs.
- Uenzelmann-Neben, G., 2002. Contourites on the Agulhas Plateau, SW Indian Ocean: Indications for the evolutions of currents since Paleogene times. In: Stow, D., Pudsey, C., Howe, J., Faugeres, J.C. (eds), Atlas of deepwater contourite systems.
| Disclaimer: This article contains some information that was originally published by the National Oceanic and Atmospheric Administration (NOAA). Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the National Oceanic and Atmospheric Administration (NOAA) should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content. |
