The water quality in fish cages on Lake Kariba was studied. The Kruskal-Wallis test was used to determine whether the water quality parameters differed significantly among the cage-types (cage-type effect). Secchi disc readings ranged from 2 to 4 metres, while temperature ranged from 23 degree C to 29.5 degree C. The observed conductivity was between 98.7 mu S/cm and 102.3 mu S/cm. The pH was between 6.31 and 7.82. Concentrations of dissolved oxygen varied from 1.55 to 6.47 mg/l, while dissolved oxygen saturation ranged from 22.3% to 83.1%. Within the same month, temperature did not differ significantly among cages. Conductivity, pH, dissolved oxygen concentration and dissolved oxygen saturation differed significantly among the 3 cage-types. Temperature and pH levels in the cages were close to the optimum levels for the culture of cichlids. Sub-optimal levels of dissolved oxygen occurred occasionally in the octagonal cages

Mhlanga, W.

English

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A P íïrnry
'3nter Qulity in 3h
Kariba, Zimbabwe
(ariba Fisherie esearch
Institute
Project Report Number 78
1994
WILSON MHLANGA
LAKE KARIBA FISHERIES RESEARCH DJSTITUTE
P.O. Box 75
KARIBA
ZIMBABWE
ntof
on Lake
Abstract
The water quality in fish cages on Lake Kariba was studied. The Kruskal-Wallis test was used to
determine whether the water quality parameters differed significantly among the cage-types
(cage-type effect). Secchi disç readings ranged from 2 to 4 metres, while temperature ranged
from 23°C to 29.5°C. The observed conductivity was between 98.7pS cm and 102.3yS cm.
The pH was between 6.31 and 7.82. Concentrations of dissolved oxygen (DO) varied from 1 .55
to 6.47mg r1, while dissolved oxygen saturation ranQed from 22.3% to 83.1%.
Within the same month, temperature did not differ significantly among cages (pczO.05).
Conductivity, pH, dissolved oxygen concentration and dissolved oxygen saturation differed
significantly among the 3 cage-types.
Temperature and pH levels in the cages were close to the optimum levels for the culture of
cichlids. Sub-optimal levels of dissolved oxygen occurred occasionally in the octagonal cages.
Introduction
Culture of fish ¡n cages has increased rapidly in the last two decades. Several
studies have shown that the culture of fish ¡n cages results in increased levels
of nutrients and suspended solids, while the dissolved oxygen levels decrease
(Ackef ors and Enell, 1990).
The economic viability of a cage culture enterprise is heavily dependent on
choosing an optimum site. Several criteria are used in the selection of suitable
cage culture sites. Environmental factors that affect the growth and production
of the fish are an important component of the site selection criteria.
When selecting a site for cage culture, the ambient pH, temperature, dissolved
oxygen and salinity must be as close as possible to the optimum requirements
of the fish species to be cultured.
The objectives of this study were; i) to determine the water quality in the fish
cages to provide preliminary baseline data. This baseline data can then be used
in formulating more comprehensive monitoring programs, ii) to determine
whether the water quality in the cages was optimal for fish growth.
Study Area
Lake Kariba is a manmade reservoir created in 1958 by the damming of the
Zambezi River at the Kariba Gorge. The lake is situated on the border between
Zambia and Zimbabwe. There are five distinct basins (Figure 1). The lake ¡s 280
kilometres long, with a maximum width of 40 kilometres, and a surface area of
5 250 km2 at full supply level. Although the maximum depth is 120 metres,
30% of the lake surface area is shallower than 17 metres (Begg, 1970). The
lake is oligotrophic and monomictic. Mtada (1987) observed that thermal
stratification occurred from December to May, while complete mixing occurred
in June and July.
i
The study was carried out at a commercial cage farm established as a pilot project.
The cage farm is located in an area near Antelope Island, which is in Basin 5
(Figure 1). The farm consists of ten 36 m3 square cages and two 8 m3 octagonal
cages. The production system involves the use of mixed species of Tilapia (Family:
Cichlidae) of mixed sex. There are three species used; the Red-breasted bream
(Tilapia rendaii, Boulenger), Kariba Bream (Oreochromis mortimeri, Trewavas) and
Nile Tilapia (Oreochromis nh/oticus, Linnaeus).
Figure 1: Map of Lake Kariba. Inset: Location of Study Area.
2
Materials and Methods
Field Sampling
Sampling was carried out monthly from August to December 1993. The
following parameters were recorded for each cage; pH (WTW pH 96),
Conductivity (WTW LF 91m), Dissolved Oxygen and Temperature (WTW Oxi
96). Water Transparency was recorded 1 metre from the cages. Water
transparency was determined by means of a Secchi Disc (Wildco Instruments).
Sampling was carried out between 1000 hours and 1200 hours. Apart from
water transparency, all other parameters were determined in the surface layer
(less than 40 centimetres deep).
Statistical Analyses
Nonparametric statistical methods were used as the data did not fulfil the
assumptions of the Analysis of Variance. The Kruskal-Wallis test was used to
determine whether, the parameters under investigation, were significantly
different between the different cage types (cage-type effect).
The cages were grouped into three categories, based on their size. Cage Type i
represented all the square cages that had a volume of 36 m3; Cage Type 2
comprising all cages similar to Cage Type 2, which had been partitioned into
two sections of equal size, and they had a volume of approximately 16 m3.
Cage Type 3 consisted of octagonal cages with a volume of 8 m3.
Resu Its
The observed values of water transparency in the area adjacent to the cages are
shown in Figure 2. Transparency increased from August to September, and then
decreased in October and November. There was no difference in transparency
between November and December.
Figure 3 shows the mean temperatures with the standard deviations. The
lowest mean temperature during the study was recorded in August (23°C) while
3
the highest temperature was recorded in December (29.5°C). The Kruskal-Wallis
test showed that temperature did not differ significantly between the cage types
(p<O.05).
Conductivity results are shown graphically in Figure 4. The mean conductivity
values ranged from 98.7 to 102.3pS cm1. There were significant differences in
conductivity between the cage-types (p<O.05).
Figure 5 shows the pH levels in each cage-type. The pH values ranged from 6.31
to 7.82. Significant differences were observed in the pH levels of the 3 cage-types
(p<O.05).
Dissolved Oxygen (DO) concentrations are shown in Figure 6. The mean DO
concentrations for all the cages combined, ranged from 1.55 to 6.4 mg 11. The
lowest value occurred in the smallest cages, (i.e. Cage Type 3). Dissolved Oxygen
(DO) concentrations were significantly different among the 3 cage-types (p <0.05).
Figure 7 shows the percentage Saturation of Dissolved Oxygen in the different
cages. The mean Dissolved Oxygen Saturation for all the cages combined varied
from 22.3% to 83.1%. Percentage Saturation levels were significantly different
among the 3 cage-types (p<0.05).
4
Figure 2: Water trspaìcy in the areE î1::it to the cages
r-
D
Secchi Depth (m)
5
2
o
Aug Sep Oct Nov Dec
Month
Temperature
30
8
26
20
Aug Sep Oct Nov Dec
Month
Temperature is in Degrees Celsius
Figure 3: Mean temperatures recorded in the cages
o
104
103
102
10
Conductivity in Cage Type i
Conductivity (RS/cm)
104
103
10
Conductivity in Cage Type 2 Conductivity in Cage Type 3
Conductivity (uS/cm) Conductivity (uS/cm)
Oct Nov
Month
104
Oct Nov
Month
Figure 4: Mean conductivity values in each cage-type
7
pH readings in Cage Type i
Sep Oct Nov
Month
8.5
8
7.5
7
8.5
8
5.5
5
Dec
Au5 Sep Oct Nov Dec
Month
Figure : Mean pH levels in the 3 cage-types
8
pH readings in Cage Type 2 pH readings in Cage Types
Dissolved Oxygen (DO)
Concentrations in Cage Type i
DO (mg/I)
8
7
6
5
4
a
2
o
Aug Oct Nov Dec Aug Oct Nov
Month Month
Figure 6: Mean dissolved c;:r® (DO) concentrations in the
g
Oct Nov
Month
Dec
Dissolved Oxygen (DO)
Concentrations (n Cage Type 3
Dissolved Oxygen (DO)
Concentrations in Cage Type 2
Dissolved Oxygen (DO)
Saturation (%) in Cage Type 2
Saturalion (%)
loo
Aug
Seturatjoi, (%) SaturatFon (%)
loo loo
Oct Nov
Month
Dissolved Oxygen (DO)
Satumtion (%) in Cage Type i
Dec
Dissolved Oxygen (DO)
Saturation (%) in Cage Type 3
So
o
Dec - Aug Oct Nov Dec Dec
Month
Figure 7: Mean dissolved oxygen saturation (%) in the 3 c::ì-types
'o
Discussion
The major objective of this study was to make an inter-cage comparison of
water quality parameters. As there was no reference site in the study, no direct
comparison could be made between an area with cages and that without.
However, a general comparison was made using literature data. The aim of this
latter comparison, was to highlight the párameters that should receive priority
attention in future detailed studies.
Water transparency in the area outside the cages (Figure 2) was lower than that
observed by other workers. Begg (1970) reported that transparency in the
Sanyati Basin ranged from 3.25 in March to 4.55 metres in November, while
Ramberg (1987) reported a range of 3.0 to 6.5 metres.
The temperature values observed in this study are similar to those reported by
Ramberg (1987); who noted that surface temperatures in the Sanyati Basin
were 23°C and 29°C in August and December, respectively.
The temperatures observed in this study are within the range preferred by
cichlids. Beamish (1970), noted that O. nioticus that had acclimated to
temperatures between 15 and 30°C, showed preference for temperatures of
30°C. Those acclimated to 35°C preferred temperatures of 28°C. Denzer, 1968
(cited in Chervinski, 1982) reported that the upper lethal limit for O. niloticus
was 42°C.
Conductivity values (Figure 4) were within the range of 87.7 pS cm1 to
103.6 pS cm1 observed in the surface waters near Redcliff Island (Magadza et
aI. 1988).
The pH values in the cages were lower than those observed in this Basin by
earlier workers. Begg (1987) noted that the pH in the epilimnion usually ranged
between 7.8 and 8.5. Magadza et aI. (1987) also reported that the pH in the
11
area near Redcliff Island was alkaline with the observed levels being within a range
of about 7.1 to 8.7. The lower pH values that occurred in the cages are likely to
have been due to the presence of metabolic wastes that contain acidic compounds.
The pH ranges recorded in this study are ciose to the optimum levels for the culture
of cichlids. Swingle, 1961 (cited in Chervinski, 1982) noted that although Tilapia
could tolerate a pH range of 4 to 11, the recommended pH levels should be
between 7 and 8.
The concentrations of dissolved oxygen observed in this study were much lower
than those reported by other workers (Ibid.). The range reported in literature was
between 6.4 and 8.8 mg r1. This difference is probably related to the fact that the
area with the cages has a higher fish density than the rest of the surrounding
areas. The higher fish densities result in a higher oxygen demand, mainly due to
oxygen being taken up by the fish during respiration.
The concentrations of dissolved oxygen observed in this study were higher than
the lethal lower limit. Chervinski (1982) noted that for O. mossambicus and
O. niloticus, the lowest short-term dissolved oxygen limit was 0.1 ppm.
From Figure 7, it can be observed that, in Cage-types i and 2, the dissolved
oxygen saturation levels were greater than 50% in all the months. In Cage-type 3,
the mean dissolved oxygen saturation levels ranged from as low as 22.3% in
October, to as high as 73% in August. The low levels of oxygen saturation
observed in Cage Type 3 were partly because these cages had a comparatively
lower surface area that was in contact with the surrounding water. Consequently,
the deoxygenated water in the cages would be flushed out at a slower rate since
water exchange between the cages and the surrounding area (with more
oxygenated water) would be low.
The dissolved oxygen saturation levels in some cages may have had a negative
effect on the caged fish. Chervinski (1982) reported that Dissolved Oxygen
Saturation levels of less than 25%, resulted in reduced growth, and fish will die
12
when exposed to Saturation levels of less than 20% for a period of 2 to 3 days.
Conclusions and recommendations
Conductivity, pH and dissolved oxygen (concentration and percentage
saturation) differed significantly between the cages. Temperature did not differ
significantly between the cages (p<0.05).
When designing future sampling programmes, the different cage-types have to
be taken into consideration.
The water temperature and pH observed ¡n this study were close to the levels
that are optimum for the culture of cichlids. In some months, dissolved oxygen
levels in the cages, were at sub-optimal levels, especially in the octagonal
cages, and these may have had a negative effect on the fish.
The results of this study showed that apart from water transparency, dissolved
oxygen and pH, the other parameters that were monitored did not differ
significantly from the data reported in literature. The drop in water transparency,
pH and dissolved oxygen was most probably due to the introduction of the
cages. Detailed studies will be required to verify this observation. Future studies
should include a reference site in order to validate these observations.
Other parameters such as nutrient levels (especially Phosphorus and Nitrogen),
Ammonia, Chemical and Biological Oxygen Demand (COD and BOD), and
suspended solids were not monitored due to time constraints. However, future
work should include the measurement of these parameters.
As production is likely to increase, it is imperative that a long-term monitoring
programme be instituted in order to detect the changes in water quality that are
likely to occur.
13
Acknowledgements
I would like to thank the coxswains at Lake Kariba Fisheries Research Institute
(LKFRI) for their assistance during field sampling. I am also grateful to fellow
researchers at LKFRI for their comments on the draft copies of this report.
References
Ackefors, H.; Enell, M. 1990. Discharge of nutrients from Swedish fish
farming to adjacent areas. Amblo 19:28-35.
Baton, E.K.; Coche, A.G. 1974. (eds.). Lake Kariba: A Man-Made Tropical
Ecosystem in Central Africa. Dr. W. Junk, The Hague. 767pp.
Beamish, F.W.H. 1970. Influence of temperature and salinity acclimation on
temperature preferenda of the euryhaline fish T. niotica. J.
Fish. Res. Board Can. 27(7): 1209-1214.
Begg, G.W. 1970. Limnological observations on Lake Kariba during 1967 with
emphasis on some special features. Limno/. Oceanogr. 15(5):776-
788.
Chervinski, J. 1982. Environmental physiology of tilapias. In Pullin, R.S.V.;
Lowe-McConnell, R.H. (ads.) The biology and culture of tilapias.
ICLARM Conference Proceedings 7. International Centre for Living
Aquatic Resources Management, Manila, Philippines. pp 11 9-1 28.
Ramberg, L. 1987. Phytoplankton succession in the Sanyati basin, Lake Kariba.
Hydrobiologia. 153:193-202.
14
Mtada, O.S.M. 1987. The influence of thermal stratification on pelagic fish
yields in Lake Kariba, Zambia/Zimbabwe. J. Fish Biol. 30:127-133.
Magadza, C. H. D.; Dhlomo, E.J.; Gariromo, M.; Chisaka, J. 1988. Chemical Data
for the Sanyati Bay, Lake Kariba, and the Zambezi River below the
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15




A preliminary assessment of water quality in fish cages on Lake Kariba, Zimbabwe

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