The influence of open and closed mouth phases on the marine fish fauna of the Swartvlei estuary

Abstract

The Swartvlei estuary fish community was sampled during open and closed phases using seine and gill nets. Results showed a marked decline in the abundance of juvenile fish <50 mm TL but not of larger size groups following mouth closure. Both the catch per unit effort and size composition of gill-net catches revealed no major changes between the open and closed phase, indicating that no substantial emigration of large juveniles and adults occurred prior to mouth closure. Atherina breviceps, which breeds both in the estuarine and marine environment, showed minimal fluctuations in abundance and size distribution when comparing the open and closed phase. Salinities in the Swartvlei estuary remain above 10 °0/00 during the closed phase thus reducing the possibility of fish kills which have been recorded in the Bot River estuarine system.