Growth rates of copepods in the southern Benguela upwelling system: The interplay between body size and food

Copepod growth rates were estimated from shipboard measurements of egg prod uction of adult female Calanus agulhensis, Calanoides carinatus, Nannocalan us minor, and Centropages brachiatus and molting rates of juvenile stages ( N6-C5) of C. agulhensis. Data were obtained during austral spring and summe r of 1989-1995 in the southern Benguela upwelling system. While maximum gro wth rates showed less than a threefold decline over the body-size range exa mined (525-2,763-mu m total length), probably owing to allometric considera tions, mean growth rate decreased by one order of magnitude, suggesting lim itation of growth rate by an environmental factor Most of this decline in m ean growth rate was attributable to food limitation of large copepods. Freq uency distributions of growth rate under low food densities were severely s kewed toward slow growth rate for large copepods, whereas they were more sy mmetric for smaller copepods. In contrast, at high food concentrations, the frequency distributions had a high degree of symmetry for all copepods. Th ese frequency distributions were interpreted in terms of a probabilistic mo del describing the encounter rate of copepods with suitably sized food part icles. The effect of food limitation on growth rate was evaluated by regres sing the coefficient of variation of growth rate against body size. A stron g positive relationship was found (r(2) = 0.93, P < 0.001), indicating that small copepods were always sowing well, whereas the growth rate of large c opepods was more variable. It is suggested that this difference is a conseq uence of the ability of small copepods to consume small particles, which ar e present at a relatively constant background density.