Influence of natural silt on the survival of Oreochromis mossambicus yolk sac larvae

Abstract

This study investigates the tolerance of Oreochromis mossambicus yolk sac larvae to natural silt from the Phalaborwa Barrage. Larvae survived silt concentrations lower than 13.2 g silt/I, but were highly active, indicating sublethal stress effects. When silt concentrations exceeded 29 g silt/I, stress reactions such as floating at the surface, gulping air, reduced fin and opercular movements and partial and complete loss of equilibrium were observed. Fish that succumbed exhibited darker pigmentation, and silt and mucus covered bodies. LC,() values were derived as 53.4 g silt/I, 31.7 g silt/I and 19.1 g silt/I for 1 hour, 24 hour and 48 hour exposure periods, respectively. Since these concentrations are much lower than the silt released from the Phalaborwa Barrage during flushing, it can be concluded that the release of high silt concentrations can severely impact the larval fish populations in the Olifants River below the Barrage.