International Journal of Aquaculture, 2013, Vol.3, No.4, 11
-
16
http://ija.sophiapublisher.com
13
the nitrate levels in site 1 and site 2 were significantly
different (ANOVA, p<0.05) from the control site
(
Site 3). There was no significant (ANOVA, p>0.05)
temporal variations in the mean nitrates of the study
area among the three sampled points.
1.7.2
Orthophosphate
Orthophosphate levels were generally low during the
study. Mean orthophosphate levels at site 1 however,
exceeded the local and international legal limits of
0.5
mg/L (Table 1). Statistical analysis indicated no
significant difference (
ANOVA,
p>0.05) amongst
the sampled sites and seasonal variations.
1.7.3
Ammonia
Ammoniawas generally high at site 1 and site 2 with
mean levels of
1.01
mg/L and 2.39 mg/L
recorded.
And these values were above the maximum
allowable limits (Table 1). Significant difference
(
ANOVA; p<0.05) were recorded between among
the three sites but no significant seasonal/temporal
variation (ANOVA; p>0.05) was observed between
site 1 and site 2 or with seasonal variations.
2
Discussions
An increasingly significant effect of intensive fish
culture is eutrophication of the water surrounding
rearing pens or the rivers/lakes receiving aquaculture
effluent (Tumbare, 2008). The socio-economic benefits
of cage aquaculture normally came at a cost to the
environment (FAO, 2007). Large scale aquaculture has
been practised in Lake Kariba since the early 1970
based on the wild kapenta (
Limnothrissamiodon
,
Boulanger, 1896) but has shifted towards the more
economical intensive farming of tilapia fishes and
crocodiles (Songore, 2002). The cost to the environment
has largely been ignored as the lake is deemed large
enough to self-purify (Magadza, 2011). However, the
results of this study, whose main objective was to
assess the water quality at the discharge points of two
large scale aquaculture enterprises relative to
international potable water quality guidelines, shows this
assertion to be inconsistent. Of the physical-chemical
parameters of water that were assessed, there was
depleted dissolved oxygen, higher turbidity and excess
ammonia relative to the local and international surface
water effluent discharge quality guidelines (EMA-SI,
1997;
WHO, 2006).
The fact that there was no significant temporal variation
in the parameters (turbidity, dissolved oxygen and
ammonia) may indicate the detrimental effect of
aquaculture effluent inflow (Westers, 2000). The
high turbidity observed at the aquaculture discharge
points, which was significantly different from that
recorded at the control point, indicates an increase in
the concentration of suspended matters in the water
as a result of effluent inflow. This finding is
corroborated by the level of total dissolved solids
that where significantly high at the discharge points
relative to that at the control This may have negative
implication on visual feeders especially predators
like the tiger fish (
Hydrocynusvittatus
,
Castelnau, 1816)
that dominates the lake (Mhlanga, 2000, Dalu et al., 2012).
Boyd (2003
)
reported that excessive concentration
of suspended and dissolved solids might be harmful
to aquatic life because they decrease water quality,
inhibit photosynthetic processes and eventually lead
to increase of bottom sediments and decrease of
water depth.
Oxygen plays a critical role in the physiology and
metabolism of aquatic organisms (Larinier, 2002;
Abdel-Tawwab et al., 2007
).
The depleted dissolved
oxygen levels recorded in this study, notably at the
discharge points, could be attributed to the presence of
high concentrations of degradable organic and
inorganic matters in the aquaculture effluent. This
degradable material is more oxygen demanding,
making oxygen less available to the desirable
organisms including fish (Nizzoli et al., 2005). Low
DO concentrations considerably affect the survival
and behaviour of aquatic organisms (Long et al., 2008).
Although the effect might not be prominent now the
continued pollution in the lake might mean some
points become anoxic and may lead to hypoxia
especially at turnover (Manganaro et al., 2009).
Nitrate levels were high at the two discharge points
with the highest being recorded at site 1 which was
slightly below the legal limit of 10 mg/L according to
the EMA-SI guideline values (1997). The nitrate
levels in site 1 and site 2 were significantly different
from the control site (Site 3). This shows that
aquaculture effluent may be playing a prominent role
in increasing nitrate levels in the lake. There were no
significant temporal variations in the mean nitrates of
the study area among the three sampled points and this
is attributable to the high inflows of water in Lake