Data collected off the south-west coast of South Africa suggest that easily measured hydrographic or biological parameters may be used as indicators of the suitability of the food environment for those mesozooplankton which preferentially utilize phytoplankton >10 μm in size. It is proposed that, in the southern Benguela during the upwelling season, there is a temperature window from 12 to 15°C when conditions are generally good for diatom population development and chlorophyll concentrations exceed 3 mg·m−3. At lower (<12°C) or higher (>15°C) temperatures, chlorophyll concentrations are <mg·m−3. There is generally a shift in size structure of the phytoplankton population at about 15°C, high biomass, diatom-dominated populations being replaced by low biomass, flagellate-dominated populations. At temperatures between 12 and 15°C when chlorophyll concentrations exceed 3 mg·m−3, the nutritional requirements for egg production by a common herbivorous mesozooplankter will be satisfied. Conversely, at lower (<12°C) or higher (>15°C) temperatures and lower concentrations of chlorophyll, the fecundity of this same mesozooplankter will be reduced severely. These parameters, in conjunction with satellite imagery, may be used to assess minimum potential secondary production of mesozooplankton in South African west coast shelf waters.