MEPS 254:239-251 (2003)  -  doi:10.3354/meps254239

Seasonal and event-scale variation in growth of Calanus agulhensis (Copepoda) in the Benguela upwelling system and implications for spawning of sardine Sardinops sagax

Anthony J. Richardson1,3,*, Hans M. Verheye2, Betty A. Mitchell-Innes2, Justine L. Fowler2, John G. Field1

1Marine Biology Research Institute, Zoology Dept., University of Cape Town, Rondebosch 7701, South Africa
2Marine and Coastal Management, Private Bag X2, Rogge Bay 8012, South Africa
3Present address: The Sir Alister Hardy Foundation for Ocean Science (SAHFOS), The Laboratory, Citadel Hill, Plymouth PL1 2PB, United Kingdom

ABSTRACT: We investigated the growth rate and egg production of Calanus agulhensis, the dominant copepod on the western Agulhas Bank region of the southern Benguela upwelling system (South Africa), and assessed the implications for the spawning of sardine Sardinops sagax. Daily weight specific growth rate (SGR) of copepod developmental stages (N6 to female) was measured monthly between September and March 1993/94 and 1994/95. Seasonally, SGRs of small stages (N6 to C2) remained relatively constant. By contrast, SGRs of large stages (C3 to female) mirrored changes in chl a related to seasonal warming and wind patterns, with a moderate peak in September/October and a larger peak in January to March. Superimposed on this seasonal cycle were fluctuations in response to winds on the event scale. At the onset of upwelling, mean female SGR across the shelf was slow. During sustained upwelling, female SGR increased, with a peak associated with enhanced chl a in the upwelling front and slower rates inshore in newly upwelled water and offshore in oligotrophic water. Female SGR was fastest during prolonged quiescence when high chl a levels extended over most of the shelf. During downwelling, female SGRs decreased again, with fastest rates inshore. Overall, SGRs were not related to temperature, but were related to chl a, with stronger relationships for larger stages. This suggested food limitation of larger stages, which may be a consequence of their preference for larger phytoplankton cells that only occur at high concentrations in restricted locations in time and space in the southern Benguela upwelling system. The spawning of sardine varied seasonally and coincided with the maximum chl a concentration and production of C. agulhensis eggs (daily egg production x female density), both of which are a good source of food for fish larvae. Thus the timing of sardine spawning maximizes the food available to their larvae.

KEY WORDS: Copepods · Specific growth rate · Calanus · Egg production · Somatic growth · Sardine · Spawning · Larval food

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