Abstract.— Seasonal advection 

 of warm western-boundary current 

 water into the southern Benguela 

 eastern-boundary current region from 

 late spring to early autumn coincides 

 with increased coastal upwelling ac- 

 tivity. The combined effect of these 

 two processes is to stabilize the sys- 

 tem through intensification of tem- 

 perature fronts and thermoclines, pro- 

 viding conditions favoring anchovy 

 spawning and larval survival. These 

 conditions appear to be robust to short 

 periods of intense storm mixing and 

 downwelling during summer, and 

 weaken and disappear only with the 

 onset of winter when there is dimin- 

 ished influence of the warm western- 

 boundary current water, downweUing 

 conditions, and increased frequency 

 of storms. 



Ocean Stability and Anchovy 

 Spawning in the Southern 

 Benguela Current Region 



Peter A. Shelton 



Science Branch, Department of Fisheries and Oceans 



PO Box 5667, St John's. Newfoundland AlC 5X1, Canada 



Larry Mulchings 



Sea Fisheries Research Institute 



Private Bag X2, Rogge Bay 8012, Cape Town, South Africa 



Manuscript accepted 24 November 1989. 

 Fishery Bulletin. U.S. 88:323-338. 



The extension of the Hjort (1914) 

 hypothesis by Lasker and his cowork- 

 ers (Lasker 1975, 1978, 1981a, b) to 

 include the role of a stable ocean in 

 determining fish recruitment has had 

 a profound influence on research in- 

 to the early life history of marine 

 fish. Lasker's extended hypothesis 

 suggests that ocean stability influ- 

 ences food aggregations for larvae 

 and larval drift and, hence, recruit- 

 ment strength (Lasker 1981b). In sup- 

 port of Lasker's hypothesis, strong 

 wind events have been found to dis- 

 rupt anchovy larval food aggrega- 

 tions, and, by inference, to effect lar- 

 val mortality off California (Lasker 

 1975) and Peru (Walsh et al. 1980). 

 During such events, larvae can also 

 be entrained within increased off- 

 shore Ekman flow and placed outside 

 suitable feeding areas (Walsh et al. 

 1980, Lasker 1981b). A significant 

 linear relationship between anchovy 

 larval mortality rate and the frequen- 

 cy of calm, low wind-speed periods 

 during the spawning season off Cali- 

 fornia (Peterman and Bradford 1987) 

 provides further evidence of the im- 

 portance of event-scale disruption of 

 ocean stability. 



Parrish et al. (1983) compared the 

 seasonality and geography of sardine 

 and anchovy reproduction in the Cali- 

 fornia, Peru, Canary, and Benguela 

 Current systems with corresponding 

 features of the environment and sug- 

 gested that patterns of correspon- 



dence may indicate the most crucial 

 processes affecting reproductive suc- 

 cess. They found that spawning was 

 adapted to avoid times and places char- 

 acterized by intense ttirbulent mixing 

 and strong offshore transport. 



In this paper we use survey data to 

 provide a finer-scaled description of 

 ocean stability and anchovy spawn- 

 ing than that of Parrish et al. (1983), 

 and suggest that in the southern Ben- 

 guela Current system anchovy spawn- 

 ing is adapted to seasonal patterns of 

 ocean stability that are resistant to 

 event-scale processes. Local condi- 

 tions are briefly compared with those 

 in the Southern Californian Bight 

 where Lasker's studies took place, 

 and conclusions are drawn with re- 

 spect to the influence of ocean sta- 

 bility on survival of the planktonic 

 stages and recruitment in the south- 

 ern Benguela Current region. 



Methods 



Diu-ing the Cape Egg and Larval Pro- 

 gramme (CELP), plankton and the 

 environment were sampled at approx- 

 imately monthly intervals between 

 August 1977 and August 1978 from 

 a grid of stations in the southern 

 Benguela Current region. The pur- 

 pose of the program was to deter- 

 mine general spawning habitat and 

 time for the major fish species of com- 

 mercial importance (Shelton 1986). 

 The grid comprised 120 stations posi- 



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