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



333 



Figure 12 



Anchovy egg and larval abundance patterns over the CELP survey grid, southern Benguela Current region, in January 1978, a month of 

 peak spawning. 



1986). With the next pulse of upwelhng, the isotherms 

 flip to a more vertical orientation and the front moves 

 offshore again, to an average position over the inner 

 shelf break (200-m contour). 



Anchovy spawning, transport, and 

 recruitment in relation to ocean stability 



Anchovy eggs are generally absent or in very low abun- 

 dance in the plankton in autumn and winter when the 

 region is cool and isothermal (March- August, Fig. 11). 

 With the onset of spring (September-November) both 

 egg and larval abundance increases rapidly, reaching 

 a peak in late spring or early summer when both the 

 mean temperature and the CV are high, and then 

 decreasing to low numbers over the autumn. 



Anchovy spawning is typically most intense south 

 and east of Cape Point over the broad Agulhas Bank 

 (Fig. 12). The vertical distribution of anchovy eggs in 

 the spawning area on the Agulhas Bank (Fig. 13a) is 

 restricted almost entirely to the warm upper mixed 

 layer, above the strong thermocline that forms in sum- 

 mer at about 50 m. On the west coast a transect bisect- 

 ing the front in the vicinity of the Cape Peninsula shows 

 that anchovy eggs extending up the west coast are con- 

 centrated within the frontal zone (Fig. 13b). Anchovy 

 larvae (Fig. 12) are generally more widespread over 

 the Agulhas Bank than the eggs as a result of disper- 



sal, and in particular extend further up the west coast, 

 often as far as Cape Columbine, in the vicinity of the 

 front. Recent samples from a more extensive grid of 

 stations confirm these patterns (Fig. 14). 



Based on these patterns of anchovy eggs and larval 

 abundance and the catch pattern of recruits in the com- 

 mercial fishery (Crawford et al. 1980 and Crawford 

 1980), the habitat of the anchovy in the southern 

 Benguela region can be divided into spawning, trans- 

 port, and recruitment areas (Fig. 15). The role of ocean 

 stability in each of these three areas can be evaluated. 



Anchovy spawning is highly seasonal, peaking in 

 summer when the southern Benguela region is most 

 structured by temperature fronts and thermoclines 

 under the simultaneous influence of advected warm 

 western-boundary current water and coastal upwell- 

 ing. The spawning area is characterized by a strongly 

 stratified, stable water column throughout summer. 

 Stratification is resistant to event-scale mixing pro- 

 cesses and only weakens with the reduced influence of 

 eastern-boundary current water on the Bank and the 

 retraction of the cold bottom layer with the onset of 

 winter. Winter storms are then able to mix the entire 

 water column creating isothermal conditions which per- 

 sist until the onset of summer. The warm, uniform sur- 

 face layer over the Agulhas Bank in summer provides 

 conditions conducive to rapid anchovy egg develop- 

 ment; below 14 °C anchovy egg development in the 



