Southern Benguela System Research Articles

Abundance and distribution of the mesopelagic fishMaurolicus muelleriin the southern Benguela system

Extensive sound-scattering layers of mesopelagic fish and macrozooplankton are found over the continental shelf and slope off the west coast of South Africa. They provide an abundant food resource for commercially valuable midshelf fish species such as Cape hake Merluccius spp. The sternoptychid Maurolicus muelleri (the lightfish) is a widespread and major component of the mesopelagic fish assemblage, occurring mostly between the 100 and 500 m isobaths. Estimates of lightfish abundance are derived from echo-integration data and from information on catch rate in research midwater trawls and egg abundance. Taking into account evidence for multiple-batch spawning behaviour, the most probable range of lightfish density within the regions and period surveyed was 4–10 tons·km−2. Assuming the population to be confined to the shelf and shelf-edge in most regions off the West Coast, it is proposed that the total biomass of lightfish in the Benguela system was of the order of a million tons. Diel patterns of vertical migration are shown, and factors influencing the distribution and abundance of the species are discussed.

Early life history and spawning ofGenypterus capensis(Smith, 1849) in the southern Benguela system

A description of the larval development of Genypterus capensis in the southern Benguela system from the earliest stages of development almost until metamorphosis is presented. Larval identification was based on morphological characteristics and the pigmentation pattern. The spatio-temporal distribution of kingklip larvae for a 12-month period provided information on spawning areas and seasons of the species. Different spawning strategies of the species over the western Agulhas Bank and on the West Coast are suggested. Over the Agulhas Bank, spawning starts when sea surface temperature begins to decrease and extends through the cooler periods (winter/spring). On the West Coast, spawning takes place only in those months in which upwelling is less intense (June–December).

Aspects of the variability in growth of juvenile anchovyEngraulis capensisin the southern Benguela system

Samples of juvenile anchovy Engraulis capensis were collected from commercial arid research catches made off the west coast of South Africa to investigate variations in the growth rate by month and by locality of capture. Age of fish was determined by counting daily increments in otoliths viewed by means of a scanning electron microscope. Estimated growth rates were highly variable, resulting in birthdate distributions spanning six or seven months for similarly sized fish caught in the same month. Significant differences in growth rate were recorded between months, the slowest growth rates occurring in anchovy which had hatched during the period of peak spawning in late spring and early summer. Growth rates of juvenile anchovy of the 1986 recruiting year-class were approximately 25 per cent lower than those of previous year-classes sampled for daily growth analysis.

Aspects of marine environmental variability near Cape Town, 1960–1985

Time-series of atmospheric pressure, winds, sea surface temperature, sea level and the Southern Oscillation Index are analysed in order to document aspects of marine environmental variability of and to provide suitable environmental indices for the nearshore southern Benguela system. The circannual cycle dominated, but a high degree of month-to-month and interannual variability was evident. Two contrasting environmental scenarios are postulated, representing extended summer or winter conditions respectively and probably related to decadal changes in the mean position of the South Atlantic Anticyclone. Principal components analysis pointed to possible teleconnections between the Pacific El Nino and the associated Southern Oscillation, and local environmental conditions.

Passive transportation of pelagic system components in the southern Benguela area

A current-closure system for pelagic species between the Agulhas Bank and St Helena Bay in the southern Benguela system is proposed, in particular for the anchovy Engraulis capensis. It is shown that current speeds for the most part exceed swimming speeds and that the species depends for its survival on passive transportation from spawning to nursery grounds and, in all likelihood, back to the spawning grounds. Such matters as active migration, navigation and orientation are briefly discussed as alternative or modifying influences on passive transport. A simple numerical procedure is used to study the initial conditions required for larvae to be able to enter the nursery grounds in St Helena Bay on a hypothetical current field by way of illustrating the possible effects of passive transport.

The role of regeneration from the sediments in the supply of nutrients to the euphotic zone in the southern Benguela system

Regeneration of nutrients from Benguela sediments taken between Lüderitz and Walker Bay, on the leeward side of upwelling centres, was studied by means of two methods. First, porewater samples were taken from sediment cores and analysed for nutrients. The observed concentration gradient was used to estimate the potential flux by diffusion of nutrients across the sediment/water interface. Second, intact sediment cores were incubated at the temperatures and concentrations of dissolved oxygen found at the interface, and the rate of increase of nutrients in the overlying water was monitored. The systems where samples were taken may be classified as semi-closed, as there is preferential settling of particulate organic carbon and regeneration of nutrients from the sediments. Here, the nutrient concentrations in bottom waters exceeded those in upwelling source water by up to 100 per cent, probably because of the regeneration process. More stable water-column conditions on the leeward side of upwelling centres may inhibit exchange of nutrients between bottom waters and the euphotic zone: implications of this possibility are discussed. A mechanism is proposed as to the manner in which nutrient-rich bottom water from St Helena Bay may be advected southwards during current reversals associated with coastal trapped waves. This mechanism would prime the upwelling centre at Cape Columbine and could result in advection of nutrients back into St Helena Bay euphotic zone waters.

Mesopelagic fish derivatives in the southern Benguela upwelling region

Discussion of mesopelagic fish within the southern Benguela system necessitates recognition of both oceanic and neritic realms, demarcated by the continental shelf-break. In the eastern South Atlantic (FAO Area 47), the stock size of two oceanic, mesopelagic families (Myctophidae, Sternoptychidae) have been estimated at 8–12 million and 750 000 metric tons respectively. Each possesses pseudoceanic species, populations of which constitute a possible alternative fishery resource to conventional pelagic stocks. Of these upper-slope/shelf derivatives, Lampanytodes hectoris (Myctophidae) and Maurolicus muelleri (Sternoptychidae) are most abundant. In the southern Benguela system (Orange River mouth to Cape Point), both species appear to be restricted mainly to the region east of the offshore divergence, corresponding roughly to the continental shelf-break. Maurolicus muelleri has not been taken by the fishery, but purse-seine landings of Lampanyctodes hectoris have accounted for up to some 10 per cent of the t...

Sedimentary flux and the formation of resting spores of selectedChaetocerosspecies at two sites in the southern Benguela System

Sediment traps were placed in St Helena Bay and Elands Bay in the southern Benguela system in December 1984 for 2–3 days. Comparisons of abundance of selected phytoplankton species in the water column, from the traps and in the sediment/water interface revealed differences in the sinking behaviour of the selected species. These differences are most likely exaggerated by the formation of resting spores. Although the water column was dominated by vegetative cells, the flux to the sediment occurred largely in the form of resting spores. The results suggest rapid formation and settling of resting spores from the water column. These transient events may be missed by typical field sampling procedures, thereby underestimating frequency of spore formation. High concentrations of resting spores dominated the phytoplankton assemblages of the sediment/water interface, suggesting that seeding by spores from the sediment is possibly an important factor resulting in the success of Chaetoceros species and the consistent...