Gastric evacuation, feeding periodicity and daily ration of sardine Sardinops sagax in the southern Benguela upwelling ecosystem

Abstract

Laboratory and field experiments were conducted to determine gastric evacuation rates of sardine Sardinops sagax, and stomach content analyses were undertaken to assess feeding periodicity and estimate the daily ration of this species in the southern Benguela. Gastric evacuation followed an exponential pattern and was influenced by food type; phytoplankton was evacuated at a much faster rate than zooplankton. Estimated gastric evacuation rates ranged between 0.05 and 0.29·h⁻¹, with mean values of 0.09 and 0.27·h⁻¹ for fish fed zooplankton and phytoplankton respectively. Despite this large range, no significant relationships were found between gastric evacuation rate and fish size, temperature, food particle size or meal size. Feeding periodicity was size-dependent; small fish had a peak in feeding activity at or around sunset, whereas larger fish appeared to feed continuously. This suggests that fish of different size exhibit different foraging behaviours, possibly reflecting differences in their diets. Estimates of daily ration ranged from 0.99 to 2.52% wet body mass·day⁻¹ for fish consuming zooplankton and from 2.97 to 7.58% wet body mass·day⁻¹ for fish consuming phytoplankton. Small fish consumed a bigger daily ration than large fish.