The relative importance of food and temperature to copepod egg production and somatic growth in the southern Benguela upwelling system

The fecundity and somatic growth rates of Calanus agulhensis and Calanoides carinatus, the dominant large calanoid copepods in the southern Benguela u pwelling system, as well as the fecundity of several other common copepods, were measured between September and March of 1993/94 and 1994/95. Mean egg production of most copepods was low at <30 eggs female(-1) day(-1) (Calano ides carinatus 23.7, Calanus agulhensis 19.0, Neocalanus tonsus 16.1 and Rh incalanus nasutus 26.1), whereas the mean fecundity of Centropages brachiat us was significantly greater (83.6 eggs female(-1) day(-1)). This study als o presents the first comprehensive field estimates of the fecundity of Nann ocalanus minor (mean: 26.1 eggs female(-1) day(-1), range: 0.0-96.2 eggs fe male(-1) day(-1)) and of somatic growth of N6 and all copepodite stages of Calanoides carinatus (decreasing from 0.58 day(-1) for N6 to 0.04 day(-1) f or C5). Somatic growth rates of Calanus agulhensis also declined with age: from 0.57 day(-1) for N6 to 0.09 day(-1) for C5. Data on growth rates were used to assess the relative importance of food [as measured by total chloro phyll (Chl) a concentration], phytoplankton cell size (proportion of cells >10 mu m) and temperature to the growth of copepods.' Multiple regression r esults suggested that fecundity and somatic growth rates were positively re lated to both Chi a concentration and phytoplankton cell size, but not to t emperature. Although it was not possible to separate the effects of Chi a c oncentration and phytoplankton cell size, data from previous laboratory exp eriments suggest that copepod growth is not limited by small cells per se, but by the low Chi a concentrations that are associated with these particle s in the field. Despite growth not being directly related to temperature, a dome-shaped relationship was evident in some species, with slower growth r ates at cool (<13 degrees C) and warm (>18 degrees C) temperatures. The sha pe of this relationship mirrors that of Chi a versus temperature, where poo r Chi a concentrations are associated with cool and warm temperatures. It i s concluded that the effect of food limitation on growth of copepods outwei ghs that of temperature in the southern Benguela region. Sources of variabi lity in relationships between growth and Chi a concentration are discussed.