Gilchristella aestuaria (Pisces: Clupeidae) biomass and consumption of zooplankton in the sundays estuary

Abstract

Gilchristella aestuaria is an important resident zooplanktivorous fish species in most southern African estuarine systems. The dry mass of this species in the Sundays Estuary during 1986/87 ranged from 11 g·m-3 in the lower reaches to 178 g·m-3 in the middle reaches. The average total biomass of G. aestuaria in the estuary was 219 kg, with a maximum value of 723 kg dry mass recorded in November 1986. Daily food consumption by the estuarine round herring ranged from 0,6 mg·m-3·day-1 in the lower reaches to 20,4 mg·m-3·day-1 in the middle reaches. On average, G. aestuaria consumed almost 19 kg dry mass of food per day in the Sundays Estuary during 1986/87, or 6,9 tons dry mass per year. Most food consumption occurred during summer (37 kg · day-1), compared to only 1 kg·day-1 in winter. The higher summer consumption coincides with increased zooplankton abundance in the Sundays system during the warmer months. Calculations indicate that production by the calanoid copepod Pseudodiaptomus hessei in the Sundays Estuary could provide at least 50% of the food requirements of the resident G. aestuaria population.