Fish community ecology in relation to land use and lotic parameters in the Mnembo River catchment (southern Africa)

Endorheic Lake Chilwa is one of the most productive lakes in Africa, contributing up to 24% of total fish production in protein-starved Malawi. High population density and agricultural practices in the Chilwa catchment have been linked to declines in the number and size of the commercially important Barbus species. The Mnembo River is a major inflow into Lake Chilwa which has received little scientific study to date. In 2003/2004, water quality parameters and fish abundance and distribution were monitored monthly at 3 sites in the Mnembo River to provide data for a lake management plan. Studies on smaller inflows into Lake Chilwa have implicated sediment yield, discharge, conductivity and total suspended solids (TSS) as influences on Barbus migration. Barbus catch was negatively correlated with discharge and pH in the Mnembo River. Female Barbus spawning condition (Gonadosomatic Index) was positively correlated with rainfall and water temperature and negatively correlated with TSS concentrations. Within Lake Chilwa's watershed, sediment yield in the Mnembo River (56t km -2 yr-1) was significantly lower than in the Likangala (374t km-2yr-1) and Domasi Rivers (315t km -2yr-1) likely due to lower agricultural activity in the Mnembo catchment. Elevated rates of soil loss (0.30t km-2yr -1) in the Mnembo catchment were mainly attributed to steep slopes rather than poor land use practices as is the case in the Likangala and Domasi catchments. Compared to Lake Chilwa's other catchments, the Mnembo River catchment is in better condition, however current land use practices will cause degradation to rapidly increase until a sustainable management strategy for the Lake Chilwa watershed is implemented.