In addition to these general scientific problems (which are not unique to the Benguela), there are particular scientific and operational problems and threats to management in each country, which differ according to the nature of the fisheries and the economic realities and research and management capacity in each country.
In Angola, the resources and their environment have been significantly less studied than elsewhere in the Benguela, and the history of fisheries research is too short to have provided a long time-series of observations and a strong scientific foundation for the proper analysis of trends in population size (Neto 1998). There are limited national data for long-term retrospective analyses of major fluctuations in the marine ecosystem, large deficiencies in the understanding of fundamental life history characteristics (e.g. stock delineation, location of spawning grounds, distribution of ichthyoplankton, nursery grounds, migration patterns) of commercially important stocks, and no population models which can be used to evaluate management options. Partly because of the large number of remote landing points, and a small corps of compliance officials, catch and effort statistics for many of Angola’s fisheries tend to be unreliable, making it difficult to implement even basic management measures. Research capacity is limited because of the small number of people involved, the lack of appropriate tertiary education in fisheries science in the country, and the severe macroeconomic problems in the country resulting from the protracted civil war and resultant breakdown of services. There is clearly a desperate need in Angola for education and training in fisheries science and resource management at all levels, but this can only proceed if there is stability, and basic services and infrastucture are in place.
In Namibia, there are fairly reliable catch statistics for all of the exploited species, and control measures are effectively implemented. The greatest general scientific problem is the development of rigorous methods of assessing sardine, hake and horse mackerel biomass from survey and other information, and the building of these assessments into formal, testable management procedures which take assessment errors into account. In view of the dramatic effects which the major environmental perturbations of the mid 1990s had on the abundance and distribution of Namibian resources (particularly sardine and their predators), a strong need has been perceived to improve understanding of the effects of the environment on the country’s marine resources, particularly with a view to predicting recruitment and anticipating changes in distribution. The major operational constraint is a severe shortage of scientific and technical staff within the Ministry of Fisheries and Marine Resources for the large number of resources that have to be studied, and the amount of effort necessary merely to maintain the current level of resource monitoring. The often prolonged absence of staff attending training courses and studying outside Namibia for higher degrees places an additional load on remaining research staff, further reducing the amount of time available for detailed analysis of results, innovative research and the publishing of results in the primary literature. Although the opportunities for local tertiary education in marine science may improve in due course, lessening the need for distance-education, this problem is likely to persist for some time.
In South Africa, there are long and generally reliable time-series of catch statistics for the major exploited species, and effective exploitation control measures for most of them. The major exception is the linefishery, where attempts to limit effort have so far been ineffective. The problem is being exacerbated by technological improvements and increased capitalisation in the sector, and increasing demands on already over-exploited coastal linefish resources by a rapidly expanding subsistence sector.
Although the scientific basis for fisheries management in South Africa is very much stronger than elsewhere in the region, there have been a number of developments which in recent years have weakened national capacity in marine science, and which threaten to weaken it further. State funding for marine research, both within statutory organisations and at universities is shrinking, with a consequent shortage of funds for equipment, running expenses travel and education. There is a shortage of funds to maintain, man and operate ageing research vessels, and at present little provision for replacing them. Because of these factors, increased difficulty is being experienced in maintaining even routine resource-monitoring cruises essential for recommending TACs, and there is almost no ships’ time available for developmental work and supporting (e.g. environmental) research. This jeopardises the substantial progress which has been made in the past two decades in understanding the effect of the environment on fish resources in the southern Benguela, at a time when the use of such understanding in fisheries management is being pioneered in the region.
Also, in recent years there has been a weakening of scientific and research management capacity within the Department of Environment Affairs and Tourism due to loss of senior staff brought about largely by moves to reduce the size of the Public Service. The loss of expertise, and in many cases, posts, has added to the routine commitments of remaining staff, further limiting their opportunities for undertaking innovative work and otherwise pursuing their scientific careers. The weakening of state-funded capacity in marine reseach, unless compensated for by an increase in the marine research done by non-statutory bodies and through regional and international co-operative programmes, could well lead to a decline in the quantity and quality of marine science in South Africa.
With the broadening of participation in the fisheries sector under the new Act, and the increased number of landing sites, monitoring is becoming increasingly difficult, which is likely to result in a decline in the reliability of data. The pressure on already hard-pressed staff within the M&CM Chief Directorate brought about by growing demands on the MCS Unit and new responsibilities to advise on quota allocation (previously the function of the Quota Board) is likely to add to staffing problems in the years ahead.
We acknowledge generous assistance given by staff at IIP Luanda, NatMIRC, Swakopmund and M&CM, Cape Town in providing data needed for this Report. We also thank the Reprographics Section, M&CM, Cape Town for the use of figures and other artwork from their files. Drs A.I.L.Payne and P. de Barros are thanked for extensive and helpful comments on an earlier draft of the manuscript, which led to a number of improvements, and generally tightening of the text.
11. REFERENCES AND OTHER LITERATURE
During the past decade several thousand scientific publications on the Benguela ecosystem and its resources have been published in the primary scientific literature, the majority of them dealing with the southern Benguela. A listing of these is beyond the scope of this document. Instead, readers are referred to Crawford et al. (1987) and references therein for a comprehensive summary of the published work on the major fish and inverterbrate resources of the Benguela up to that date, and to the book Oceans of Life off Southern Africa (Eds. A.I.L. Payne and R.J.M. Crawford, 1989, revised in 1995), which gives a popular but authoritative account of the same body of work. Much of the more recent work has been published in the South African Journal of Marine Science, particularly in the three “symposium volumes” viz. Vol. 5: The Benguela and Comparable Ecosystems (Payne et al. 1987), Vol.12: Benguela Trophic Functioning (Payne et al. 1992) and Vol. 19: Benguela Dynamics: Impacts of Variability on Shelf-Sea Environments and their Living Resources (Pillar et al. 1998). A wealth of information on the Namibian and South African fishing industries, fisheries, catches, etc. may be found in the Fishing Industry Handbook series, edited by M. Stuttaford and published annually by Marine Information CC. The references listed below give details on these publications, a short selection of other useful works, and the articles cited in the text, which have largely been restricted to work which has appeared since publication of the BENEFIT Science Plan (Shannon and Hampton 1996, 1997).
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ANON 1998c – Cruise Report of Dr Fridtjof Nansen horse mackerel survey, June 1998. National Marine Information and Research Centre, Swakopmund.
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BAKUN, A. 1995 – Patterns in the Ocean: Ocean Processes and Marine Population Dynamics. Published jointly by Centro de Investigaciones Biologices di Nord Ovest, La Paz, Mexico and University of California Sea Grant, San Diego, USA: 320pp.
BARANGE, M. and I. HAMPTON 1994 – Influence of trawling on in situ estimates of Cape horse mackerel Trachurus trachurus capensis target strength. ICES J.mar.Sci.51: 121-126. BARANGE, M., PILLAR, S.C. and I. HAMPTON 1998 – Distribution patterns, stock size and life-history strategies of Cape horse mackerel Trachurus trachurus capensis, based on bottom trawl and acoustic surveys. In Benguela Dynamics:Impacts of Variability on Shelf-Sea Environments and their Living Resources. Pillar S.C., Moloney, C.L., Payne, A.I.L. and F.A.Shillington (Eds). S.Afr.J.mar.Sci. 19: 433-449. BARANGE, M., HAMPTON, I. and B.A. ROEL (in press) – Trends in the abundance and distribution of anchovy and sardine on the South African continental shelf in the 1990s, deduced from acoustic surveys. S.Afr.J.mar.Sci.21: 367-391. BIANCHI, G. 1992 – Demersal assemblages of the continental shelf and upper slope of Angola. Mar. Ecol. Prog. Ser.81: 101-120. BOYER, D. 1996 – Stock dynamics and ecology of pilchard in the northern Benguela. In Proceedings of the Seminar and Workshop The Benguela Current and Comparable Eastern Boundary Upwelling Systems, Swakopmund, Namibia, May 1995: 79-82.
BOYER, D., BOTES, F., BURMEISTER, L., CLOETE, R., FOSSEN, I., HOLTZHAUSEN, H., KIRCHNER, C., KLINGELHOEFFER, E., LETH, N., MAARTENS, L., STAALESEN, B., STABY, A., and E.VOGES 1998 – Collected papers; First Regional Workshop, Benguela
Current Large Marine Ecosystem (BCLME) Programme, UNDP, Cape Town, South Africa, 22-24 July 1998: 14pp.
BRANCH, T.A. 1996 – Baseline biomass estimates for orange roughy off Namibia, using a swept-area technique. Unpublished ms. Namibian Deep Water Fisheries Working Group: 13pp.
BRANCH, T.A. and R.D. ROBERTS 1998. - Swept-area estimates for Namibian orange roughy. Namibian Deep Water Fisheries Working Group Doc. WG/01/98/DWFWG/ORH:5: 21pp.
BROUWER, S.L., MANN, B. Q., LAMBERTH, S.J., SAUER, W.H.H. and C. ERASMUS. 1997. A survey of the South African shore angling fishery. S.Afr.J.mar.Sci.18: 165 - 177. BUTTERWORTH, D.S. and H.F.GEROMONT 1997 – Evaluation of a range of possible simple interim management procedures for the Namibian hake fishery. Report to the Ministry of Fisheries and Marine Resources, Namibia: 28pp.
COLE, J.F.T. and J. McGLADE 1998 – Temporal and spatial patterning of sea surface temperature in the northern Benguela upwelling system: possible environmental indicators of clupeoid production. In Benguela Dynamics: Impacts of Variability on Shelf-Sea Environments and their Living Resources. Pillar S.C., Moloney, C.L., Payne, A.I.L. and F.A.Shillington (Eds). S.Afr.J.mar.Sci. 19: 143-157. CONSTANÇA, L. J. 1995 – An evaluation of the trawl surveys used for assessment of stock abundance of demersal fish in Angolan waters. M. Phil. thesis, University of Bergen, Norway: 60 pp.
CRAWFORD, R.J.M. 1998 – Responses of African penguins to regime changes of sardine and anchovy in the Benguela system. In Benguela Dynamics:Impacts of Variability on Shelf-Sea Environments and their Living Resources. Pillar S.C., Moloney, C.L., Payne, A.I.L. and F.A.Shillington (Eds). S.Afr.J.mar.Sci. 19: 355-364.
CRAWFORD, R.J.M. (in press) – Seasonal responses to long-term changes of prey
resources off South Africa. In. Proceedings of 22nd International Ornithological Congress, University of Natal, Durban, South Africa Adams,L. and R. Slotow (Eds).
CRAWFORD, R.J.M. and B.M. DYER 1995 - Responses by four seabird species to a fluctuating availability of Cape anchovy Engraulis capensis off South Africa. Ibis, 137(3): 329-339.
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Benguela ecosystem. 4. The major fish and inverterbrate resources. In Oceanography and Marine Biology. An Annual Review. 25. Barnes, M. (Ed). Aberdeen University Press: 353-505.
CRAWFORD, R.J.M., UNDERHILL, L.G. and J.D.VENTER 1990 – Handline catches and stock identity of snoek Thyrsites atun off South Africa and Namibia. S.Afr.J.mar.Sci .9: 95-99.
DELGADO, F. and P.A. KINGOMBO 1998 – (Effect of) Coastal development and marine environment on artisanal fisheries (in Angola). Collected papers, First Regional Workshop, Benguela Current Large Marine Ecosystem (BCLME) Programme, UNDP, Cape Town, South Africa, 22-24 July 1998. 11pp.
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FOWLER, J.L. and A.J.BOYD 1998 – Transport of anchovy and sardine eggs and larvae from the western Agulhas Bank to the West Coast during the 1993/94 and 1994/95 spawning season. In Benguela Dynamics:Impacts of Variability
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GAMMELSRØD, T., BARTHOLOMAE, C.H., BOYER, D.C., FILIPE, V.I.L. and M.J. O’TOOLE 1998 – Intrusion of warm surface water along the Angolan-Namibian coast in February-March 1995: the 1995 Benguela Niño. In Benguela Dynamics:Impacts of Variability on Shelf-Sea Environments and their Living Resources. Pillar S.C., Moloney, C.L., Payne, A.I.L. and F.A.Shillington (Eds). S.Afr.J.mar.Sci. 19: 41-56.
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HAMUKUAYA, H., O’TOOLE, M.J. and P.M.J. WOODHEAD 1998 – Observations of severe hypoxia and offshore displacement of Cape hake over the Namibia shelf in 1994. In Benguela Dynamics:Impacts of Variability on Shelf-Sea Environments and their Living Resources. Pillar S.C., Moloney, C.L., Payne, A.I.L. and F.A.Shillington (Eds). S.Afr.J.mar.Sci. 19: 57-59.
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– The diurnal vertical dynamics of Cape hake and their potential prey. In Benguela Dynamics:Impacts of Variability on Shelf-Sea Environments and their Living Resources. Pillar S.C., Moloney, C.L., Payne, A.I.L. and F.A.Shillington (Eds). S.Afr. J.mar.Sci. 19: 365-376.
HUTCHINGS, L., BARANGE, M., BLOOMER, S.F., BOYD, A.J., CRAWFORD, R.J.M., HUGGETT, J.A., KERSTAN, M., KORRÛBEL, J.L., DE OLIVEIRA, J.A.A., PAINTING, S.J., RICHARDSON, A.J., SHANNON, L.J., SCHÜLEIN, F.H., VAN DER LINGEN, C.D. and H.M.VERHEYE 1998 – Multiple factors affecting South African anchovy recruitment in spawning,transport and nursery areas. In Benguela Dynamics: Impacts of Variability on Shelf-Sea Environments and their Living Resources. Pillar S.C., Moloney, C.L., Payne, A.I.L. and F.A.Shillington (Eds). S.Afr.J.mar.Sci. 19: 211- 225.
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KIRCHNER, C., SAKKO, A.L. and J.I. BARNES (in press) – The economic value of the Namibian recreational shore-angling fishery. S.Afr.J.mar.Sci. 22.
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ATLANTNIRO Atlantic Research Institute for Fisheries and Oceanography (Kaliningrad, former USSR)
AVHRR Advanced Very High Resolution Radiometer
BCLME Benguela Current Large Marine Ecosystem (Programme)
BENEFIT BENguela Environment Fisheries Interaction & Training (Programme)
BEP Benguela Ecology Programme
CPUE Catch Per Unit Effort
CAF Consultative Advisory Forum (South Africa)
CSIR Council for Scientific and Industrial Research (South Africa)
DANIDA Danish International Development Agency
DIFD Department for International Development (United Kingdom)
EEZ Exclusive Economic Zone
ENVIFISH Environmental Conditions and Fluctuations in Distribution of Small Pelagic Fish Stocks (Programme)
EU European Union
FAO (United Nations) Food and Agriculture Organisation
GTZ Deutsche Gesellschaft für Technische Zusammenarbeit
ICCAT International Commission for Conservation of Atlantic Tunas
ICEIDA Icelandic International Development Agency
ICSEAF International Commission for the South-East Atlantic Fisheries
IIP Instituto de Investigação Pesqueira (Angola)
IPA Instituto de Desenvolvimento da Pesca Artesanal (Angola)
IMR Institute of Marine Research (Bergen, Norway)
IOW Institüt für Ostseeforschung (Warnemunde, Germany)
IRD (French) Research Institute for Development
M&CM (Chief Directorate) Marine and Coastal Management (Department of Environmental Affairs and Tourism, South Africa)
MFMR Ministry of Fisheries and Marine Resources (Namibia)
MSC Monitoring, Surveillance and Control
NatMIRC National Marine Information and Research Centre (Namibia)
NORAD Norwegian Agency for Development Co-operation
ODA Overseas Development Agency (United Kingdom)
ORSTOM French Research Institute for Development through Co-operation
SADC Southern African Development Community
SADCO South African Data Centre for Oceanography
SAC Satellite Applications Centre (CSIR, South Africa)
SAMLMA South African Marine Linefish Management Association
SEAFO South East Atlantic Fisheries Organisation
SFAC Sea Fisheries Advisory Committee (South Africa)
SFRI Sea Fisheries Research Institute (South Africa)
SIDA Swedish International Development Agency
SST Sea surface temperature
TAC Total Allowable Catch
UCT University of Cape Town
UIA United Nations Implementing Agreement
UNAM University of Namibia
UNCLOS United Nations Convention on the Law of the Sea
VPA Virtual-population Analysis
VIBES Variability of exploited pelagic resources in the Benguela ecosystem in relation to Environmental and Spatial aspects (Programme)
SYNTHESIS AND ASSESSMENT OF INFORMATION ON THE BENGUELA
CURRENT LARGE MARINE ECOSYSTEM (BCLME)
THEMATIC REPORT NO.2
INTEGRATED OVERVIEW OF THE OCEANOGRAPHY AND
ENVIRONMENTAL VARIABILITY OF THE BENGUELA
By L.V. SHANNON and M.J. O’TOOLE
Windhoek, Namibia November 1999 1.INTRODUCTION
The Benguela is one of four major current systems which exist at the eastern boundaries of the world oceans, and the oceanography of the region is in many respects similar to that of the Humboldt Current off Peru and Chile, the California Current off the west coast of the U.S.A. and the Canary Current off north-west Africa. These eastern boundary currents are characterized by upwelling along the coast of cold nutrient-rich water, and are important centres of plankton production which support a global reservoir of biodiversity and biomass of fish such as sardine (pilchard), anchovy and horse-mackerel and also sea birds and marine mammals.
The coastal upwelling area of the Benguela Current ecosystem extends from southern Angola along the west coast of Namibia and South Africa around the southernmost part of the continent. While the area shares many of the generic characteristics of other eastern boundary currents, it is unique in that it is bordered at both northern and southern ends by warm water systems viz. the Angola Current and Agulhas Current respectively. These equatorward and poleward boundaries are not fixed in space and in time, but are highly dynamic, and their pulsing impacts on the ecosystem as a whole and on its harvested resources. With a western boundary approximating to the 0 meridian, the Benguela thus encompasses the coastal upwelling regime, the eastern part of the South Atlantic gyre and a complex system of fronts and transition zones. In terms of the Benguela Current Large Marine Ecosystem (BCLME) Programme, the Benguela is viewed in a broader context than is customarily defined, and includes the full extent of Angola’s Exclusive Economic Zone (EEZ), with a northern boundary at 5S at the Angola Front. (The latter is the boundary between the BCLME and the equatorial current system.)
The earliest physical observations in the South Atlantic and Indian Oceans were those necessary for the safe and efficient passage of sailing ships along the trade routes between Europe and the East. The early Portuguese, Dutch and other navigators accordingly compiled comprehensive records of winds and currents – records which display a remarkable amount of information about the underlying physical oceanography of the region. The first published work of scientific merit was, as may be expected, of currents around the Cape of Good Hope and was compiled by James Rennel in the 18th century (Rennel 1778). It was, however, the cruise of the H.M.S. Challenger in the 1870s which initiated the global science of oceanography, and pioneering studies were conducted on that expedition in the Benguela region during 1873. The next half-century witnessed the age of the great oceanographic expeditions inter alia those of the Valdivia, the Gauss, the Planet and the Möwe. It was, however, work undertaken during the expedition of the German Meteor in the South Atlantic between 1925 and 1927 that resulted in a quantum jump in human understanding of the oceanography of the Benguela and adjacent regions.
The scene for development of regional oceanography was set by the arrival in Cape Town of Professor J. D. F. Gilchrist in 1896. Gilchrist is regarded as the father of southern African oceanography, and he undertook a host of marine studies in Angola, Namibia, South Africa and Moçambique. During the second half of the twentieth century it was oceanographers such as Drs T. J. Hart, R. I.Currie, A. J. Clowes, A. H. B. De Decker, N. D. Bang, J. R. E. Lutjeharms and L. Hutchings who contributed so much to the understanding of the complex physics, chemistry and biology of the Benguela.
This overview is a brief summary the oceanography and environmental variability of the Benguela Current Large Marine Ecosystem. As such it draws principally on the published scientific literature which now comprises several thousand authoritative articles. In preparing this overview, we have synthesised available information and ideas and have attempted, where possible and where appropriate, to simplify and to explain concepts in such a way that they will be intelligible to non-oceanographers, yet still useful to marine scientists with an interest in the Benguela system. It is not, however, a Benguela science review per se: There are a number of authoritative reviews in the international scientific literature, and these are referred to in the text, together with other key publications.
The overview commences with a discussion of the main physical features and processes in the Benguela - the bathymetry, windfield, temperature, upwelling, currents, fronts and boundaries. Key aspects of the chemistry and chemical processes follow, including major and minor elements and all-important dissolved oxygen. The next section deals with plankton, primary and secondary production and the foodweb and carbon budget. We have also devoted several paragraphs to a discussion of environmental variability and the ecosystem consequences thereof. Finally we have provided a perspective on the various issues, problems and threats facing the Benguela, and have identified what we believe are the major gaps in knowledge and understanding. It is hoped that the various sections collectively will provide a useful introduction to, or at least background information for, those overviews dealing with more specific aspects of Benguela resources.
PHYSICAL FEATURES AND PROCESSES 2.1 Bathymetry The continental shelf along the west coast of southern Africa is variable in width and depth. It is narrow off southern Angola (20km), south of Lüderitz (75km) and off the Cape Peninsula (40km) and widest off the Orange River (180km) and in the extreme south where the Agulhas Bank extends over 200km polewards from Cape Agulhas, the southernmost tip of Africa. The edge of the continental shelf, or shelf break as it is generally termed, lies at depths between about 200m and 500m. The position of the 200m depth contour (isobath) is shown in Fig. 1. By global standards the Benguela continental shelf is relatively deep, and the slope and configuration of the shelf is by no means uniform. Indeed, double shelf breaks are common off the west coast of Southern Africa: for example near Walvis Bay (23S latitude) there are inner and outer breaks beginning at depths of about 140m and 400m respectively. This is illustrated graphically in Fig. 6, which also shows a similar feature at 32S off South Africa. Between about 31S and 35S several convoluted submarine canyons intersect the shelf, the best known of which is the Cape Canyon which lies approximately 80 north-west of Cape Town. The variable topography of the Benguela shelf is of particular significance for near shore circulation and for fisheries.
The continental shelf is covered by layers of sediments primarily of biological origin (biogenic) and large areas of shelf sediments contain more than 75% calcium carbonate. Significant features of the Benguela shelf are two extensive mud belts, each about 500km long. The southern belt extends from the Orange River southwards, up to 40km wide with an average thickness of 15m, and is mainly of river origin. The northern belt which lies over the middle shelf off Namibia comprises organically rich diatomaceous oozes (originating from planktonic plants). In places the organic carbon content of these diatomaceous muds exceeds 15%!
West of the shelf break is a steep continental slope area which descends to a depth of about 5000m where it meets the abyssal plain of the South-east Atlantic Ocean. This plain comprises two large ocean basins, the Cape Basin and the Angola Basin, separated by a submarine mountain chain, the Walvis Ridge which runs from its abutment with the continental shelf at latitude 20S (northern Namibia) in a south-westerly direction for more than 2500km towards the Mid-Atlantic Ridge. The steep continental slope and a cross-section of the Walvis Ridge is illustrated in Fig.6. As may be expected, the latter feature forms a barrier to deep circulation in the South-east Atlantic. Other prominent bathymetric features are the Agulhas Ridge which forms the southern boundary of the Cape Basin, and the Agulhas Plateau – both shown in Fig. 1, and numerous seamounts of volcanic origin, of which Vema is perhaps the best known.
Winds significantly influence the oceanography of the Benguela region on various time and space scales, ranging from basin-wide seasonal and longer period processes to local inshore events of only a few hours duration. The prevailing winds along the west coast of southern Africa are controlled by anticlockwise (anticyclonic) motion around the South Atlantic High pressure system, the seasonal low pressure field over the land and eastward-moving cyclones which cross the southern part of the subcontinent. The South Atlantic High (Anticyclone) is part of a discontinuous belt of high pressure which encircles the southern hemisphere and is maintained throughout the year. Small seasonal differences occur. On average the anticyclone is centred at about 28S: 8E, and undergoes seasonal shifts, being at a more northerly and easterly position in winter than in summer. The pressure over the subcontinent alternates between a well-developed low during summer and a weak high in summer, and consequently the atmospheric pressure gradient – and hence wind – is seasonably variable. The coastal plain, much of which is arid, acts as a thermal barrier to cross-flow, and hence winds tend to be predominantly southerly (longshore) over most of the Benguela region, being “topographically steered” along the coast. These longshore winds produce coastal upwelling which gives much of the Benguela its cool surface water characteristics (discussed in the next subsection).
The essential seasonal differences in the intensity of the upwelling-producing longshore winds is best illustrated in Fig.2. From this diagram it is evident that the principal perennial area of strong southerly winds lies near Lüderitz (27S) with a secondary area near Cape Frio (18S). In winter the northward shift in the atmospheric pressure systems has a strongest influence south of 31S, where there is a relaxation of the southerly winds and a greater frequency of westerlies. Off central Namibia wind speeds are generally lower on average, and display less seasonality. Off northern Namibia, the longshore wind is strongest during autumn and spring. North of about 15S, the latitude of Namibe in southern Angola, the winds are much weaker than off Namibia and South Africa, although they remain longshore on average and reach maximum intensity during winter.
A common feature of the wind field during autumn and winter are “Berg” winds. These catabatic winds occur when there is a pronounced high pressure over the subcontinent, and they blow down from the central plateau across the escarpment and over the coastal plain and then seawards. They are hot, dry winds, often laden with fine particles of dust which is visible in some satellite pictures. They exert little direct physical effect on the sea, however, as being warm, they tend to blow above the cool marine atmosphere layer.
Apart from seasonal changes in the windfield, coastal winds are modulated in the southern Benguela during summer by the passage of the easterly-moving cyclones (low pressure cells) which move past the tip of the subcontinent. These result in periodic changes in winds from northerly – initially with an easterly component, before blowing from the northwest – to southerly (southwesterly to southeasterly). The latter can be quite intense and are often characterised by a “tablecloth” of cloud over Table Mountain at Cape Town. These wind relaxation-reversal-strengthening events typically occur on periods of 3 to 10 days.
Diurnal changes in coastal wind intensity and direction are common throughout most of the Benguela region north of St Helena Bay (near 33S). These are associated with the differential heating and cooling of the sea and the adjacent land mass, typical of the classical land-sea breeze effect. Off much of Namibia coastal fog is often associated with the night time and early morning slacker winds, and tends to dissipate around noon when the southerly wind intensifies.
Readers wishing to know more about the climatology of southern Africa are referred to a definitive book on the subject by Tyson (1986), while comprehensive accounts of the winds over the ocean are contained in Nelson and Hutchings (1983), Shannon (1985), and Shannon and Nelson (1996). For comparisons of the winds and oceanography between the Benguela and the other three eastern boundary current systems, Parrish et al (1983) and Bakun and Nelson (1991) are recommended texts.
2.3 Upwelling and surface temperature
Coastal upwelling is the process whereby cold water is brought to the surface near the coast under the influence of longshore equatorwards winds. The essential process is illustrated in Fig. 3. In simple terms, the longshore wind can be viewed as displacing warm surface water northwards and, as a consequence of the earth’s rotation, offshore. This results in a drop in sea level against the coast, which serves as a non-permeable boundary. To balance the displaced water, the deeper water wells up inshore, and compensatory circulations and longshore currents over and adjacent to the continental shelf are set up. In a simple one cell system, the thermocline (layer where there is a strong vertical temperature gradient) is displaced vertically upwards, and may result in a front between the warm oceanic water and the cool upwelled water, with water moving at depth over the shelf and upwards, and sinking at the front. This is in reality an over-simplification – two or three fronts may develop with rather complex circulations in between, while the actual extent of upwelling and the intensity and direction of shelf currents will be influenced by “coastal trapped waves” – a type of internal wave within the ocean. The existence of these coastal trapped waves can result in enhanced or reduced upwelling and larger sea level changes than might simply be inferred from the wind. Nevertheless, as a general rule, the areas along the west coast of southern Africa where the southerly winds are consistently strongest are also the areas where upwelling is most pronounced. It follows, therefore, that coastal upwelling in the Benguela is neither uniform in time or in space.
The wind field, topographic features (bathemetry and land features) and orientation of the coast result in the formation of a number of areas where upwelling is more intense. The principal upwelling centre or cell is in the central Benguela in the vicinity of Lüderitz (27S). Strong upwelling occurs there throughout the year (Stander, 1964), with some slackening during autumn, and the extensive zone characterised by cold surface water, weak stratification and high turbulence which results there appears to be an important determinant of the biology of the system – effectively dividing the Benguela into two quasi-independent subsystems (Fig. 7). There are several secondary upwelling cells viz Cunene, northern Namibian and central Namibian cells (at approximately 18, 20 and 24S) and the Namaqua, Columbine and Cape Peninsula cells (at about 31, 33 and 34S). The last two are seasonal, with maximum upwelling occurring between September and March, whereas off northern and central Namibia upwelling is more perennial, but with a late winter maximum. Several smaller ephemeral upwelling cells develop to the west of headlands along the south coast. Although upwelling does occur along the coast of Angola at times, it is not pronounced, and the water column remains stratified throughout the year.
In the northern Benguela peak upwelling and insolation (solar heating) are out of phase, and sea surface temperatures over the shelf follow a distinct seasonal cycle. In the south off the Cape Peninsula, maximum insolation and the upwelling season coincide, and average sea temperatures inshore vary seasonally by little more than 1C. Viewing the Benguela in terms of a heat budget, the central zone is a major heat sink, with negative climatological sea surface temperature anomalies of 5 - 6C in the Lüderitz vicinity. South of Africa in the area influenced by the Agulhas Current positive climatological sea surface temperature anomalies of 2-4C exist and the area is a major heat source for the atmosphere.
Off Angola, north of about 14S, there is a positive offshore climatological sea surface temperature anomaly during summer. The dramatic temperature differences between surface waters of the Angolan and Agulhas Currents and those associated with upwelling off Namibia and South Africa are illustrated in Figs 4 and 8.
2.4 Water masses and general circulation Like temperature, salinity is an important physical property of sea water, and also affects density, and density and pressure (which is approximately proportional to depth) are key parameters in terms of ocean dynamics – just as they are in the atmosphere, the only difference being that sea water is less compressible than air. Salinity is measured in “practical salinity units” (psu) and one psu corresponds to one part per thousand or one tenth of one percent. The salinity of sea water is typically about 3.5% or 35 psu, but like temperature can vary. Salinity is influenced inter alia by fresh water input from rivers, by evaporation, precipitation, freezing of sea water and melting of sea ice.
Water masses are defined by specific temperature-salinity properties. There are a number of different water masses present off the west and south coasts of southern Africa, and their distribution and essential characteristics have been described by various authors and reviewed by Shannon (1985), Chapman and Shannon (1985) and Shannon and Nelson (1996).
The principal water masses in the Southeast Atlantic are Tropical and Subtropical Surface Waters, Thermocline Waters (comprising South Atlantic and Indian Ocean Central Water), Antarctic Intermediate Water (AAIW), North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW). The “core” characteristics of these are annotated in Fig. 5 which shows the typical temperature-salinity curves (relationships) for the water masses present in the South-east Atlantic. The linear part of the curve (approximately 6C, 34.5 psu - 16C, 35.5 psu) spans the Thermocline Water layer, and this is the water which upwells along the coast, and which constitutes, often in highly modified form, the waters present over the continental shelf in the Benguela system. From this figure it can be seen that there are quite marked differences between the Thermocline Water present in the northern and southern parts of the Benguela and true South Atlantic Thermocline Water. At the core of the layer, however, i.e. at 10 - 12C, it is impossible to distinguish between the Thermocline Waters of different origins on the basis of temperature and salinity only. The flow of the Thermocline Water tends to be similar to that of the overlying surface water, which is discussed a little later. In a recent paper Poole and Tomczak (1999) using optimum multiparameter analysis show a clear separation between Thermocline Water in the Benguela Current system south of about 25 S and that further north, the latter being 80% Western South Atlantic Central Water.
Thermocline Water overlies Antarctic Intermediate Water, which is formed in the Southern Ocean and which is characterised by the salinity minimum in the temperature-salinity curve. The core of the AAIW in the Benguela has a salinity in the range of 34.2 – 34.5 psu and a temperature of 4 - 5C, and is present in the region at an average depth of 700 – 800m. In terms of volume, AAIW accounts for about 50% of the water present in the upper 1500m. The AAIW in the southern Benguela is generally much fresher (younger and less mixed) than that present off Angola and Namibia, and is also fresher than that from the Indian Ocean. The differences in “freshness” of AAIW at three areas in the Benguela is illustrated in Fig. 6. While there is some northward movement evident in the south of mixed South Atlantic and South Indian AAIW, the greater part of this water mass in the Benguela region (at least in the area adjacent to the continental shelf) evidently moves southwards from the tropical South Atlantic, having reached the area by a somewhat circuitous route. Further offshore, the AAIW flows in a north-westerly direction. AAIW does not upwell to the surface anywhere in the Benguela.
North Atlantic Deep Water corresponds to the deep salinity maximum (see Fig. 5) and has a salinity typically 34.8 psu, and lies below the AAIW stratum. As its name suggests it is formed in the North Atlantic. It then sinks and spreads southwards. At the equator it comprises a thick layer between 1000m and 3500m of relatively warm (for its depth) and saline water. West of the Benguela continental shelf it flows generally polewards, becoming diluted en route south. Volumetrically, NADW is the main water mass present in the South-east Atlantic.
Underlying NADW in the Cape Basin is Antarctic Bottom Water. AABW forms near the edge of the Antarctic continent in the Weddel Sea area, and spreads throughout the Southern Ocean. Unless prevented by topography, it also tends to spread northwards in the South Atlantic, Indian and Pacific Oceans. In the Cape Basin it flows slowly clockwise, moving southwards at depths greater than 4000m west of the Benguela continental shelf. The Walvis Ridge forms a virtually non-penetrable barrier to the northward flow of the AABW, which as a consequence is not significantly present in the Angola Basin. The effect of the Walvis Ridge on the distribution of AABW is dramatically illustrated in Fig. 6.
Whereas the general movement of deeper water masses viz. AAIW, NADW and AABW in the Southeast Atlantic is polewards, the flow of the Subtropical Surface Water (STSW) tends to be more closely aligned to the direction of the prevailing wind – at least in the area south of 15S latitude where the flow is generally in a north-westerly direction with speeds typically in the range 10 – 15 cm/s. with an average of around 17 cm/s. As a consequence of seasonal warming and cooling of surface water, the temperature – salinity characteristics of STSW can be quite variable, and can range from 15 to 23C and 35.4 psu to 36.0 psu. (see Fig. 5). The surface water present off Angola is mainly of tropical/equatorial origin. Temperatures in excess of 25C are common, reaching 28 - 29C in summer and there is usually a very strong and shallow thermocline present. (This and the contrast with the area further south is highlighted in Fig. 6.) Surface salinity in the area is highly variable, ranging from very low values close to the mouths of major rivers such as the Congo, to levels in excess of 36psu further offshore. The influence of the Congo River at the surface can be substantial, and water from this source can be traced as far south as Namibia. (Dr M. E. L. Buys, pers. comm.) It is generally observed as a thin surface layer, lens-like in places overlying a strongly stratified surface sea water layer.
The principal features of the flow of surface water, and away from the coast also of Thermocline Water, is illustrated schematically in Fig. 7. The broad arrows between 15 and 35S represent the Benguela Current, which can best be defined as the integrated equatorward flow of the upper layers in the South Atlantic east of the 0 meridian. The circulation has been described in some detail by Stramma and Peterson (1989) and summarised by Shannon and Nelson (1996), and readers are referred to these papers for further particulars. In terms of absolute volume, the total equatorial flow of the Benguela Current, including that of surface, Thermocline and AAIW is thought to be about 15 – 25 Sv (one Sv is 106 m3/s). North of 15S, which encompasses the Angola system, the main features of the currents within the surface, Thermocline and AAIW layers are (a) a large cyclonic gyre centred around 12S, 4E viz. the Angola Dome (b) the Angola Current which flows southwards along the edge of the continental shelf and (c) the Equatorial Undercurrent and South Equatorial Current which feed the Angola Current from the north-west. The oceanography of the Angola system (including the Angola Dome) and the offshore northern Benguela area was well documented by Moroshkin et al (1970). The existence of the Angola Dome and cyclonic flow around it was confirmed by Gordon and Bosley (1991). The volume transport around the Angola Dome is of the order of 3 Sv. The major dynamics affecting the eastern tropical Atlantic Ocean were summarised Voitureiz and Herbland (1982) and by Picaut (1985).
Of the 15 – 25 Sv equatorward flow in the main Benguela Current (as defined earlier), some 7 Sv is of Indian Ocean origin (Van Ballegooyen et al 1994). The latter comes from the Agulhas Current which flows southwards and westwards along the east coast of South Africa. The Agulhas Current is the major western boundary current in the Indian Ocean (75 Sv), and is rather analogous to the Gulf Stream and the Brazil Current. The Agulhas tends to follow the edge of the continental shelf and on reaching the Agulhas Bank turns southwards and then eastwards, flowing back into the Indian Ocean. This turning back is termed “retroflection”. Small periodic meanders termed the Natal Pulse, may develop in the main body of the Agulhas Current off Natal and these grow downstream, resulting in the Current becoming unstable and the shedding of large eddies or rings. About six or eight Agulhas rings are shed each year, and these fast spinning rings move slowly (typically 5 –8 cm/s) in a west-north-westerly direction into the South Atlantic, transporting about 7 SV of Indian Ocean water on average. At the surface shallow filaments of Agulhas Current water also may round the Cape of Good Hope, just outside the upwelling area, but the mass of water associated with these filaments is usually fairly small although occasionally the southern Benguela is flooded by warm water of Agulhas origin. The retroflection of the Agulhas Current and the leakage of warm Indian Ocean water into the South Atlantic is well illustrated in Fig. 4, and shown diagrammatically in Fig. 7.
In summary the main features of the surface currents are:
Offshore: flow of about 15 – 20 cm/s (i.e. about 1/3 knot in a northwesterly direction over most of the region between 15S and 35S (viz. the Benguela Current)
Cyclonic circulation around the Angola Dome and periodic intrusion of tropical water from the north and north west into the northern Benguela
Leakage of Agulhas Current water into the South Atlantic, mainly via rings and to a lesser extent via shallow filaments
Shelf circulation Circulation over the continental shelf and in the oceanic area adjacent to the shelf has been the subject of intensive investigation during the past twenty years, although most of the effort has been focussed on the southern Benguela. The various observations were summarised by Shannon and Nelson (1996) and again recently by Shillington (1998).
Surface currents over much of the Benguela shelf are largely influenced by the prevailing winds. In the south there is a general convergent flow of surface water from the Agulhas Bank westwards and northwards around the Cape of Good Hope which funnels into a frontal jet west of the Cape Peninsula (discussed later). Typical speeds are in the range of 25 –75 cm/s. Near Cape Columbine (33S) the surface current divides into an offshore flow and a northward alongshore flow, partially into St Helena Bay. A southward moving current often occurs near the surface close inshore over the entire region, particularly during winter and also periodically during the rest of the year when reversals take place on a time scale of several days. Over the Namibian shelf, the surface currents are generally in a northerly direction, closely aligned to the wind. However, periodic and episodic reversals in the surface currents can occur, the most pronounced and extended reversals occurring during Benguela Niños (discussed in Section 5). The most prominent circulation feature off the coast of Angola in the southward flowing Angola Current. Whether this current is a permanent or seasonal feature is not clear as most of the more comprehensive investigations appear to have been undertaken during the autumn and winter months. Certainly the existence of the Angola Current was well documented by Moroshkin et al (1970). It is a coastal current, generally present as a poleward flow over the upper part of the continental slope (i.e. west of the shelf break), detectable between the surface and a depth of about 200m. Literature on the current is sparse, but there is some evidence of seasonality, with the most intense flow occurring during late summer (March), when surface speeds as high as 70cm/s have been reported, and subsurface speeds of up to 88 cm/s (Dias 1983). The dynamics of the Angola Dome and Angola Current are linked with those of the system of equatorial currents and the Benguela Current and upwelling processes. However, to what extent the Angola Current contributes to the Benguela system off Namibia is uncertain – at the surface and subsurface at least. In this respect Dias (1983) showed that most of the southward flow of the current at a depth of 100m turned between 16and 17S latitude to flow westwards just north of the Angola-Benguela front. At greater depths e.g. 400m, the poleward flow from Angola into the northern Benguela does however seem to be more continuous, and this has been the focus of recent cooperative research between German, Norwegian, Angolan, Namibian and South African scientists.
In the southern Benguela, in particular off the Cape Peninsula, but also in the vicinity of Cape Columbine there exists a strong equatorward flowing jet. First predicted and discovered by Bang and Andrews (1974) the existence of this jet has subsequently been confirmed as a semi-permanent feature of the upwelling system. Current velocities are highest at subsurface depths, being typically in the range 25 – 75 cm/s. The jet is important biologically in that it is known to transport eggs and larvae of various fish species from the spawning grounds on the Agulhas Bank to the nursery areas inshore north of Cape Columbine. To what extent the jet is continuous throughout the Benguela, or indeed even if it exists in the central and northern Benguela, is not known, although there is a suggestion in the work of Gordon et al. (1995) that a jet like feature is present in the upper 100m over the mid-shelf near Lüderitz.
Apart from the shelf-edge jet, the most significant discovery during the past two and a half decades has been of a poleward undercurrent. The idea of a poleward flow in upwelling regions seems to have originated from ideas put forward by Hart and Currie (1960) in their classic text on the Benguela Current viz. as some form of compensation for the water displaced from the inner shelf by upwelling. Nelson (1989) showed that, as in the case of in other upwelling systems, a poleward undercurrent does exist in the Benguela, but which is much more extensive than that required as compensation for coastal upwelling, stretching from the coast, across the shelf and out into the Cape Basin. Current meters have revealed that subtidal currents on the shelf are dominated by the presence of coastal trapped waves (previously commented on), which have periods of 3 to 8 days. These result in a net polewards flow of about 5km/d or 5.8 cm/s (Nelson 1989). On occasions this southwards-moving current may reach the surface inshore, resulting in episodes of poleward flow at the surface. In St Helena Bay, such events result in episodic flushing of the Bay. Aspects of the poleward current will be discussed further in Section 3.1 which addresses dissolved oxygen.
The shelf-circulation can thus be briefly summarised as follows:
Wind driven surface currents over the shelf
Poleward subsurface flow over shelf and in deeper water adjacent to shelf throughout the region (4-5 km./d)
Poleward flowing Angola Current in extreme northern part of the Benguela region i.e. along coast of Angola
Jets associated with the upwelling system and located near the edge of the continental shelf in the southern Benguela
Coastal trapped waves are characteristic of the shelf area throughout the Benguela (periods typically 3-8 days).
System boundaries, fronts and filaments Whereas a thermocline refers to a layer where there is a rapid change in temperature with depth, a halocline and a pycnaocline are vertical discontinuities of salinity and density respectively. Fronts refer to areas where there is a sharp change in temperature, salinity (or some other parameter such as colour) horizontally. Reference back to Fig 3 which illustrated a simplified concept of upwelling, it can be seen that where the thermocline intersects with the sea surface, the vertical discontinuity is translated into a horizontal discontinuity viz. a front. Physical boundaries of, and within, the Benguela are usually associated with fronts of one form or another, and these fronts tend to form barriers to horizontal movement of water and small particles such as plankton. They may be less important as barriers for highly motile animals such as fish and marine mammals.