Egg production, somatic growth and productivity of copepods in the Benguela Current system and Angola-Benguela Front

Copepods form an important link between phytoplankton and fish, so estimates of their growth rates are essential for understanding their role in the marine food web. To date, no empirical estimates of copepod growth rate in the northern Benguela system off Namibia or the Angola-Benguela Front system off Angola have been published. To redress this situation, we used bottle incubation techniques to measure the daily growth rate of 13 copepod species from this region during two BENEFIT cruises in April/May 1997 and July/August 1999, in terms of egg production by adult females and somatic growth of copepodite stages. Mean growth rate was fast (0.13-0.24 d(-1)) for small copepods (< 1800 mum total length: Calanoides carinatus stages C3 and C4, and female Centropages brachiatus) and slow (<0.1 d(-1)) for larger copepods (females of Nannocalanus minor, Pleuromamma spp., Scolecithrix spp., Metridia lucens, Candacia armata, Euchaeta spp., Undinula vulgaris, Calanoides carinatus, Eucalanus spp., Rhincalanus nasutus, Pontellidae spp. and Eucalanus elongatus). A strong negative relationship between mean growth rate (g) and mean body mass (BM) was found for the northern Benguela system (g = 0.866 x 10(-0.075BM), r(2) = 0.81, n = 15). For comparison, an equation was also derived for the southern Benguela system (g = 0.572 x 10(-0.042BM), r(2) = 0.89, n = 21) from previously published and unpublished data. These equations were used to calculate copepod production (growth rate x biomass). The estimate for the northern Benguela system of 39 g C m(-2) yr(-1) is less than previous indirect values of 52-69 g C m(-2) yr(-1). The estimate of annual copepod production for the southern Benguela system of 99 g C m(-2) yr(-1) is at the upper end of previous estimates, which range from 11 to 120 g C m(-2) yr(-1). The considerably lower production calculated for the northern Benguela may reflect the inadequate estimates of size-differentiated biomass available. The application of our growth rate-body size relationship to biomass estimates is a simple means of estimating copepod production in the study region.