Shelton and Hutchings: Ocean stability and anchovy spawning in southern Benguela Current region 



325 



Results and discussion 



Seasonality in temperature and spawning 



Both the mean and the coefficient of variation (CV) of 

 sea surface temperature measured over the CELP giid 

 peaked in summer (Fig. 2). This is caused by the sea- 

 sonal advection of warm water orginating from the 

 western-boundary Agulhas Current into the southern 

 Benguela Current region (Shelton et al. 1985) and the 

 simultaneous increase in coastal upwelling (Andrews 

 and Hutchings 1980). Strong cross-shelf temperature 

 gradients, or fronts, are set up between advected and 

 upwelled water, reflected by the increase in the sur- 

 face temperature CV. In winter the influence of west- 

 ern-boundary current water is diminished and upwell- 

 ing is less frequent and less intense (Shelton et al. 1985, 

 Andrews and Hutchings 1980). Consequently, cross- 

 shelf temperature gradients weaken and disappear and 

 mean SST values are lower and have a lower CV, re- 

 flecting cool, more isothermal conditions. 



Horizontal structure 



The changes in temperature structure described above, 

 as well as associated changes in the patterns of abun- 

 dance of microplankton and chlorophyll-a, can be seen 

 in contour diagrams of values for summer and winter 

 (Fig. 3). The strong temperature front that develops 

 along the west coast in summer is clearly visible. Al- 

 though this front is most persistent between Cape 

 Point and Cape Columbine, and was considered by 

 Bang (1973) to "pivot" at Cape Point, it may extend 

 as far south as Cape Agulhas when upwelling occurs 

 at capes east of Cape Point. At Cape Columbine 

 isotherms tend to diverge offshore, and further north 

 the front is generally weaker and more variable. The 

 density of potential food particles for early-stage larval 

 feeding, as indicated by microplankton and chloro- 

 phyll-a concentrations, are highest inshore of the tem- 

 perature front in summer, indicating that the front 

 constrains the offshore Ekman drift of productive 

 upwelled water. In winter when the front disappears, 

 microplankton and chlorophyll-a concentrations are 

 generally lower and more dispersed. 



Arrows indicating major directions of surface drift 

 over the survey area in summer (January 1978) and 

 winter (August 1977) based on drift-card recoveries are 

 plotted in Figure 4. Of the 24,000 cards released in each 

 month there was a 5% recovery in January 1978 and 

 a 7% recovery in August 1977. There was no clear 

 seasonality in the overall recovery rate. In summer the 

 southern Agulhas Bank area was characterized by on- 

 shore surface flow with a westerly component. Some 

 cards released over the western Agulhas Bank moved 



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 MONTH 



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Figure 2 



Monthly means and coefficents of variation (CV) of sea-surface 

 temperature (SST) values over the CELP survey grid, southern 

 Benguela Current region, for the period August 1977-August 1978. 



around Cape Point and up the west coast, and the gen- 

 eral pattern of flow inshore of the strong temperature 

 front on the west coast was northwards. North of Cape 

 Columbine there was some evidence of onshore flow. 

 There were very few southern African recoveries off- 

 shore of the front, although several cards were recov- 

 ered along the eastern coast of South America, indi- 

 cating general offshore flow into the South Atlantic 

 gyre at stations offshore of the front. 



In winter, flow in the south was again predominant- 

 ly onshore, although this time with a marked easterly 

 component east of Cape Agulhas and a westerly com- 

 ponent to the west. Several cards released on the 

 western Agulhas Bank were found substantial dis- 

 tances up the west coast, although one card released 

 in the extreme north of the grid travelled against this 

 flow and was recovered on the east coast. Again, few 

 cards were recovered on the coast of southern Africa 

 from releases on the west coast, although there were 

 10 recoveries from the coast of South America and one 

 recovery from southern California. In comparison with 

 summer, several of these recoveries came from cards 

 released at inshore stations, indicating widespread off- 

 shore flow along the west coast in the absence of the 

 front. 



Vertical structure 



Summer and winter vertical structure of temperature, 

 microplankton density, and chlorophyll-a concentration 

 are shown for three representative sections over the 

 survey area in Figures 5-7. A section over the Agulhas 

 Bank (Fig. 5) shows the existence of a strong thermo- 

 cline in summer between warm water of Agulhas 



