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Fishery Bulletin 88(2|. 1990 



Figure 15 



A generalized representation of anchovy spawning, transport, and 

 recruitment areas in the southern Benguela Current region. 



laboratory is very slow and hatching larvae are de- 

 formed (King et al. 1978). Below the warm surface 

 layer a strong thermocline at about 50 m is associated 

 with subsurface maxima of microplankton and chloro- 

 phyll-fl, facilitating the survival of early-stage larvae 

 that require high concentrations of food particles within 

 a suitable size range (Lasker et. al. 1970). 



Spawning conditions in the southern Benguela ap- 

 pear to be in marked contrast with conditions in the 

 spawning ground of the Northern anchovy EngrauUs 

 mordax in the Southern California Bight where 

 Lasker's studies were carried out. Surface tempera- 

 tures in the Southern California Bight given in Lasker 

 (1981b) are some 3-5°C lower than over the Agulhas 

 Bank during the respective spawning seasons. Vertical 

 temperature structure during the spawning season off 

 southern California, as shown in Lasker (1978), is quite 

 variable, with generally weak stratification susceptible 

 to mixing. The chlorophyll-a layer disrupted by storm 

 mixing in April 1984 was at about 15 m (Lasker 1975), 

 in contrast to the deeper maxima on the Agulhas Bank. 

 Although maximum chlorophyll-a values before storm 

 mixing in the layer off California in April 1974 are 

 similar to those recorded in the Agulhas Bank layer, 

 microplankton particle concentrations over the Agulhas 

 Bank do not reach the high values recorded by Lasker 

 (1981b) because dinoflagellates are not as common as 

 in the Southern California Bight, possibly due to per- 

 sistent strong winds (Parrish et al. 1983), which 

 thoroughly mix the relatively deep, upper mixed layer. 



From the the spawning area in the south, the drift 

 of eggs and early-stage larvae is primarily northwest- 



wards, into the west coast transport area. Bang (1973) 

 and Bang and Andrews (1974) have documented the 

 presence of a strong northward-flowing jet current 

 associated with the front which is a prominent feature 

 of this area in summer. The role of this jet in transport- 

 ing anchovy eggs and early-stage larvae from the 

 Agulhas Bank spawning ground to the west coast 

 nursery area has been examined and described in 

 Shelton and Hutchings (1982). Drift-card recoveries for 

 summer support their interpretation. In the transport 

 area, enhanced levels of chlorophyll-a and microplank- 

 ton associated with the inshore and frontal zone may 

 be important in early-larval survival. Despite predomi- 

 nantly southeast winds in summer, facilitating offshore 

 Ekman flow and upwelling at the coast, the offshore 

 loss of eggs and larvae along the west coast may be 

 small because of the constraining influence of the front. 

 However, there is frequently an offshore divergence 

 of the front off Cape Columbine, following the orien- 

 tation of bottom contours, and this may be associated 

 with the "leaking" of water containing larvae from the 

 productive coastal region (Shelton et al. 1985, Shelton 

 1986). 



Onshore flow north of Cape Columbine facilitates 

 penetration of the productive recruitment area by 

 larvae. Positioned downstream of the major sites of 

 upwelling at Cape Columbine and Cape Point, the fron- 

 tal structure dominating the transport area in summer 

 gives way to more gently sloping isotherms north of 

 Cape Columbine where the shelf widens. Because of 

 lower average wind speeds (Hutchings et al. 1988), 

 upwelling is milder than in the transport area and a 

 vertically stable water column is a persistent feature. 

 Enhanced levels of microplankton and chlorophyll-a 

 characterize the inner shelf in this area throughout the 

 year (Hutchings 1981), providing suitable conditions for 

 the survival and growth of anchovy recruits. 



The patterns of ocean stability described above, 

 based largely on survey data, appear to have played 

 a major role in shaping the spawning behavior observed 

 by anchovy in the southern Benguela region. Upwell- 

 ing generates the high levels of plankton production 

 that support the spawning biomass of up to 2 million 

 tons of anchovy estimated for the area (Armstrong 

 et al. 1988), but the associated processes of turbulence 

 and offshore Ekman flow present special problems to 

 the plankton stages that have to be solved through 

 evolutionary adaptation. In the southern Benguela 

 region this appears to have been brought about by the 

 selection of times and areas of spawning which make 

 best use of seasonal and spatial patterns of ocean 

 stability to provide suitable food concentrations and 

 favorable transport. 



Survey data for the southern Benguela therefore 

 tends to confirm the conclusions from the larger-scale 



