An estimation of the standing stock and population structure of Upogebia africana (Crustacea : Thalassinidae) in the Knysna Estuary

Abstract

Within the Knysna Estuary Upogebia africana has an extensive distribution from north of Leisure Isle to the Red Bridge, occupying 62% of the available intertidal zone. Six sites were surveyed within the above distributional range. U. africana was found from the high-water level (Spartina zone) to the shallow subtidal zone (to about 1.5 m depth). The density, biomass and population structure of the mud prawns were found to be highly variable within the estuary. Only small animals (carapace length –× = 10.9 mm) were found in the upper estuary at the Red Bridge. Although large animals were present, the Invertebrate Reserve had a low density and biomass of U. africana (– × = 11.7 m–2; 3.9 g.m–2 dry weight). By contrast a relatively inaccessible centre mud bank (Oyster Bank) in the middle reaches of the estuary had much larger populations (–× = 176.5 m–2; 65.3 g.m–2 dry weight). This site is, therefore, a natural mud prawn reserve. Within the intertidal zone, mud prawn density (74–76 m–2) and biomass (26–27 g.m–2 dry weight) was usually greatest in the Spartina and lower Zostera zones. Sex ratios throughout the estuary did not deviate significantly from 1:1.