TOLERANCE LEVELS OF REDBREAST TILAPIA, TILAPIA-RENDALLI (BOULENGER, 1896) TO NATURAL SUSPENDED SILT

Tilapia rendalli was exposed to various silt concentrations for differ ent exposure periods to determine LC50 and sublethal concentration ran ges. The extent of damaging effects, if any, of suspended silt on the exposed fish, were investigated by means of scanning electron microsco py. The LC50 values ranged between 21 to 24 g 1(-1) and 42-48 g 1(-1) for juveniles and adults respectively. The silt concentrations tolerat ed by the test fishes were above those found in most natural waters. T he results therefore support the hypothesis that suspended silt is let hal to fish at concentrations higher than those naturally observed in surface water. The observed mortalities can be attributed to the combi ned physical effects of high silt concentrations and associated reduct ion in dissolved oxygen levels. To prevent fish mortalities and other impacts on biota, anthropogenic activities that affect the natural var iation in suspended silt concentrations in surface water should be car efully monitored and, where possible, be restricted.