Predation impact of juvenile Gilchristella aestuaria (Clupeidae) and Atherina breviceps (Atherinidae) on the zooplankton in the temperate Kariega estuary, South Africa
Seasonal changes in the predation impact of juvenile (SL 18-25mm) Atherina breviceps and Gilchristella aestuaria on zooplankton was investigated at three stations in the middle reaches of the permanently open Kariega estuary during 1999 and 2000. Daily rations were estimated using the gut fullness index approach. Variations in zooplankton biomass demonstrated a seasonal pattern with maximum values (generally >40mg dwt m-3) recorded in summer and minimum values during winter (generally <25mg dwt m-3). Total biomass of G. aestuaria during the study ranged between 0.04g dwt m-2 and 0.2g dwt m-2 and between 0.01g dwt m-2 and 0.04g dwt m-2 for A. breviceps. There were no seasonal patterns in biomass for either species. Results of the gut content analysis indicated that both species consume mainly copepods, amphipods and mysids. There were no seasonal patterns in the dominant prey in the guts of the juveniles. Peaks in feeding activity for G. aestuaria were recorded during daylight hours while maximum feeding activity of A. breviceps occurred at night. Gut evacuation rates (k, h-1) of G. aestuaria and A. breviceps during summer were estimated at 0.41h-1 and 0.68h-1 respectively. During winter, evacuation rates were lower and were estimated at 0.37h-1 for A. breviceps and 0.28h-1 for G. aestuaria. Daily rations of A. breviceps during summer and winter were equivalent to 8.3% and 4% body dwt d-1, while for G. aestuaria daily ration was estimated at 7.9% body dwt during summer and 4.3% body dwt d-1 during winter. Results indicate that the predation impact of G. aestuaria ranged between 2.5% and 33.6% of the zooplankton secondary production while A. breviceps impact was equivalent to between 3.3% and 11.1% of the zooplankton secondary production.