Variations in size compositions of populations of Upogebia africana (Ortmann) (Decapoda, Crustacea) within the Swartkops estuary and possible influencing factors

Abstract

The size frequencies of various populations of the mud prawn Upogebia africana sampled within the Swartkops estuary were determined and the size compositions of these populations varied along the length of the estuary, as well as vertically within the intertidal zone. Ovigerous prawns were recorded from sites representing the extremes in population structures and their mean sizes differed significantly (P < 0,05) from each other. The variations in the size compositions were therefore attributed to differences in the growth of the prawns, rather than a spatial separation of adults and juveniles. The mean sizes of the adult prawns from the various stations were determined and plotted against factors such as water temperatures and salinities, nitrogen and organic contents of the substrata, population densities and distances of the populations from the estuarine mouth. The latter factor, which was used to give a relative measure of the exposure (in terms of duration and current velocity) of the populations to the flow of water entering the estuary during flood tides, gave the most significant (P < 0,01) correlation with the mean size of the adult prawns and could satisfactorily account for patterns observed.