Abstract
M.Sc.
Developments in the catchment area of the Olifants River, such as poor agricultural practices
and overgrazing, caused increased amounts of erosion material to enter the river. This leads
to the silting up of the Phalaborwa Barrage, situated in the river, on the border of the Kruger
National Park. Because of the occasional flushing of the Barrage, in order to retain storage
capacity, the Olifants River inside the Kruger National Park is subjected to very high sediment
loads. This has led to poor water quality and subsequent fish mortalities. The objective of this
study was therefore to investigate the possible effects of silt loads released from the Barrage
on the survival of selected life stages of Oreochromis mossambicus.
Silt was collected from the Phalaborwa Barrage, and the silt/fish interaction was invesigated
under controlled laboratory conditions. Oreochromis mossambicus was exposed to various
silt concentrations for different exposure periods to determine the LC50 and sub-lethal
concentration ranges. After determination of sub-lethal values, 0. mossambicus was exposed
to these concentrations to determine whether they have any effects on the oxygen
consumption and selected haematological parameters of exposed fish.
Lethal levels for suspended sediment are interrelated with age differences of the fish, particle
size of the suspended sediment and concentration. Increased suspended sediment disrupted
activity and respiratory patterns, changed orientation responses and ultimately resulted in the
death of the fish. The gills and the opercular cavities of the dead fish were usually clogged
with a thick layer of mucus and silt. The laboratory experiments revealed that LC5 0 values for
0. mossambicus ranged from 26,78 to 54,20g silt/1 for larvae, 35,10 to 39,20g silt/1 for
juveniles and 32,93 to 35,43g silt/1 for adult fish. The larval stage was generally less tolerant
of suspended silt than juveniles and adults.
Sub-lethal levels for adult fish ranged from 0 to 27g silt/l, where no mortalities occurred.
Exposure of adult fish to sub-lethal concentrations of silt did alter routine oxygen consumption
by causing increased rates (9 to 330% from initial) of oxygen consumption. This indicates that
sub-lethal levels of suspended silt concentrations result in an increase in oxygen consumption
by the fish due to an increase in metabolic rate associated with increased activity and stress.
Silt exposure
Exposure of adult fish to sub-lethal silt concentrations (<20g silt/l) resulted in increases in
haemotocrit, red blood cell counts, white blood cell counts and blood glucose concentrations,
but decreases in haemoglobin and mean corpuscular volume when compared to that for the
control fish. These differences became more apparent as the silt exposure concentration
increased with statistically significant differences (P<0.05) between most of the selected