Investigation on limnological parameters in relation to composition of
fish catches in Lake Tanganyika in the Kigoma area
Student: Nzinza Makala
Mentor: Dr. Pierre-Denis Plisnier
Introduction
Lake Tanganyika is the largest, deepest (1470m) and oldest of the western part of the East African rift lakes
(Cohen et al, 1993). This lake is located between 3o 20’- 8o 45’S and 29o 05’-31o 15’E. Its surface area is
32,600km2 and its maximum length is 650km and mean width is 50km. During the dry season from May to
September, the southeast winds tilt the thermocline northward over the entire lake. In the south basin, the
thermocline is close to the surface resulting in an upwelling of nutrients into the epilimnion (Coulter, et al
1991).
The productivity of Lake Tanganyika, like the sea and probably like other large deep tropical lakes,
depends to a large extent on the regeneration of nutrients from the water column. Nutrient rich waters are
observed below the thermocline. Their accesses to the upper photic layers are highly dependent of
hydrodynamic events driven by climatic conditions (Plisnier et al,1999).
Also Lake Tanganyika has a vitally important pelagic fishery, composed of Lates stapersii (migebuka) and
two sardines, Stolothrissa tanganicae and Limnothrissa miodon (dagaa). Usually, fishing is done during
the night. There is a very high variability in fish catches.
The objective of this study was to determine the limnological parameters in relation to the composition of
fish catch. Especially, by studying per haul during the night fishing at different positions or sites on Lake
Tanganyika in the off shore from Kigoma bay, we can try to understand possible relationships.
Materials and Methods
The following parameters were measured in the field: water temperature, dissolved oxygen, pH,
conductivity and turbidity. In the laboratory, water samples were analyzed for soluble reactive phosphorus,
alkalinity, nitrate, silica and chlorophyll a. The sampling depth for temperature and dissolved oxygen were
done at every 10 meters and the rest parameters taken at a 20 meters interval up to 100 meters. Water
temperature was measured using thermometer coupled to a dissolved oxygen meter (YSI model 58)
connected with a cable reaching 90 meters depth. At a hundred meter water sampler with a capacity of 7.4
liters was used to collect water, thus temperature and dissolved oxygen at that depth was taken by dipping,
carefully, an oxygen probe into the water sampler. pH was measured using an Orion pH meter (model
210A). Conductivity (µS/cm) was measured using a (Cole/Parmer) conductivity meter (model 19820-00).
Turbidity measurements (NTU) were taken using a HACH turbidity meter (model 210A).
Water samples were collected in plastic bottles for soluble reactive phosphorus, nitrate, silica, alkalinity
and chlorophyll a analysis in the lab. Alkalinity was determined by titrimetric method using a digital
titrator with continuous stirring using a magnetic stirrer. Chlorophyll a was measured by reading
absorbances at 665nm and 750nm using a spectronics 21D spectrometer after methanol extractions. The
nutrients such as soluble reactive phosphorus, nitrate and silica were measured by using a HACH DR/2010
spectrophotometer.
Results
Generally, water temperature decreased with depth. The maximum and minimum water temperature was
26.0oC at the surface and 23.7oC at 100m on July 19 and on July 26 respectively. The thermocline was at
70m on 16th and 19th,July but after a storm it dropped to 85m, 75m and 75m on 24th, 26th and 30th July
respectively (Figure 1).
Dissolved oxygen
vs Depth
D.O. (m g/l)
T( oc)
23
24.5
26
0
60
Depth (m )
60
60
60
80
80
100
100
100
120
120
120
120
120
D.O. (m g/l)
0
0
20
40
80
pH
0 1.5 3 4.5 6 7.5
26
8.4
8.6
Turbidity (NTU)
8.8
9
9.2
Conductivity(uS/cm )
580
0.2 0.3 0.4 0.5 0.6
0
20
20
20
20
40
40
40
40
60
60
80
80
100
100
100
100
100
120
120
120
120
120
80
24.5
D.O. (m g/l)
60
80
pH
0 1.5 3 4.5 6 7.5
26
8.4
Depth (m )
0
Depth (m )
0
60
Turbidity (NTU)
8.6
8.8
9
9.2
580
0
20
20
20
20
40
40
40
40
40
60
60
80
80
100
100
100
100
100
120
120
120
120
120
Depth (m )
D.O. (m g/l)
T( oc)
24.5
pH
0 1.5 3 4.5 6 7.5
26
8.4
8.6
8.8
Depth (m )
0
Depth (m )
0
80
Turbidity (NTU)
9
9.2
80
580
0
20
20
20
20
20
40
40
40
40
40
60
60
80
80
100
100
100
100
100
120
120
120
120
120
T( oc)
23
24.5
80
pH
D.O. (m g/l)
26
8.4
0 1.5 3 4.5 6 7.5
8.6
8.8
Depth (m )
0
Depth (m )
0
Depth (m )
0
80
Turbidity (NTU)
9
9.2
580
20
20
20
20
20
40
40
40
40
40
60
60
80
80
Depth (m )
0
Depth (m )
0
Depth (m )
0
80
60
80
100
100
100
100
100
120
120
120
120
120
Fig. 1-5: Physical parameters v/s Depth
700
80
0
80
660
Conductivity(uS/cm )
0.2 0.3 0.4 0.5 0.6
60
620
60
0
60
700
Conductivity(uS/cm )
0.2 0.3 0.4 0.5 0.6
60
620 660
60
0
60
700
Conductivity(uS/cm )
20
80
660
80
0.2 0.3 0.4 0.5 0.6
60
620
60
0
60
700
40
80
24.5
660
20
100
80
620
0
100
Depth (m )
Depth (m )
Depth (m )
0.2 0.3 0.4 0.5 0.6
0
40
23
Depth (m )
9.2
40
0
Depth (m )
9
40
T( oc)
30-July-01
Figure 1e—5e
8.8
40
23
26-July-01
Figure 1d—5d
8.6
20
T( oc)
24-July-01
Figure 1c—5c
8.4
0
20
23
19-July-01
Figure 1b—5b
C onduc t i vi t y ( uS/ c m)
580
20
60
Conductivity vs
Depth
Turbidity (NTU)
20
Depth (m )
Depth (m )
16-July-01
Figure 1a—5a
pH
0 1.5 3 4.5 6 7.5
0
Turbidity vs Depth
pH vs Depth
Depth (m )
Temperature
vs Depth
Date
620
660
700
The study showed that pH was decreasing with depth yet showed deviation from the observed trend
(Figure). The pH ranged between 8.96 at the water surface and 8.55 at 100m. The average pH was 8.76
(Figure 3).
The highest dissolved oxygen was 7.5mg/l at depth of 10m on July 19th. Dissolved oxygen profile
remained generally constant from 0m to 60m deep. The oxycline was at 80m, 70m, 80m, 75m and 70m on
July 16th, 19th, 24th, 26th, and 30th respectively (Figure 2 and Table 1).
Turbidity varied from one day to another. The average turbidity was 0.40 NTU, 0.37 NTU, 0.36 NTU,
0.43 NTU, and 0.41 NTU on 16th, 19th, 24th, 26th and 30th, July respectively. This said, the water was more
turbid on July 26th and less turbid on 19th July (Table 1).
There was a significant variation in conductivity to everyday samples. On July 24th there was an increase
in conductivity although at 20m and 60m it decreased. Also conductivity remained stable on other days
except July 16th where it decreased sharply from 90m to 100m (Figure 5).
Nitrate remained constant from the surface to 60m where the chemocline was observed. The average
nitrate concentration at the water surface was 0.02mg/l and 0.08mg/l at 100m (Figure 7).
Soluble reactive phosphorus concentration showed a decrease at 40m and then increased to 100m. Except
on July 19th, when the concentration decreased rapidly from 90m to 100m. The average concentration at
the water surface was 0.07mg/l and 0.16mg/l at 100m (Figure 8).
Silica trend showed an increase with depth. The average concentration of silica at the water surface was
1.25mg/l and 3.14mg/l at 100m (Figure 9).
Alkalinity profiles showed a high variability but generally increased with depth. The highest alkalinity
(287 mg/l CaCO3) was recorded on 24th July (Figure 6c).
Chlorophyll a concentration (0.75mg/l average) at the surface was lower compared with that at 20m and
40m was 0.81mg/l.
Fish catch showed some correlation with limnological parameters, especially turbidity. The Lates stappersi
(adult) catches were very high on July 24th when turbidity was 0.36NTU. And Stolothrissa tanganicae
catches were very high on July 26th where turbidity was 0.43NTU. Other species such as Limnothrissa
miodon and Lates stappersi (juvenile) catch composition were generally low (Table 1 and Figure 4).
Discussion
The results show that nitrate and soluble reactive phosphorus average was 0.02mg/l and 0.07mg/l at the
surface respectively. They showed a small increase at 100 meters. The lake water generally showed
oligotrophic characteristics near the surface but have high concentration of nutrients in deep water, which
depends on, to a large extent, the regeneration of nutrients from the hypolimnetic water (Plisnier et al,
1999). The decrease in nitrate and soluble reactive phosphorus on July 16th and 19th at 40m is probably due
to considerable high primary production at the respective depth as it can be observed by Chlorophyll a
concentration on the same dates.
The average pH for all sampling nights was 8.76. It was high in the photic zone where primary production
is taking place (Figure.3). Deep water characterized by lower pH, compared to the surface, affected the pH
in the surface waters at the end of the dry season. The nutrient rich deep waters probably strongly
influenced primary production when it was brought near the surface. Consumption of HCO3- during
photosynthesis results in higher pH values (Plisnier, et al, 1999).
Silica was low on the last three sampling nights (24th, 26th& 30th July) compared with the first two nights
(16th & 19th July) (Figure.9). Since diatom algae use silica for their cell walls, this probably indicates
plankton abundance on those nights. For that reason, silica can be a limiting element for phytoplankton
Alkalinity vs
Depth
Date
Alkalinity (m g/l)
16-July-01
Figure 6a—10a
Depth (m)
200
250
300
0
0.05
Depth (m )
0
1
2
3
4
0.00
5
40
60
60
80
80
80
80
100
100
100
100
60
120
120
120
120
70
250
40
0
0
20
10
60
0
0.05
0.1
40
50
SIO2 (m g/l)
PO43--P (m g/l)
0.15
0
0
0.1
0.2
0.3
0
0
0
20
20
20
20
40
40
40
40
60
60
60
60
80
80
80
80
100
100
100
100
120
120
120
120
250
300
0.05
0.1
0
0
0.1
0.2
0.3
0
80
80
80
100
100
100
100
120
120
120
120
60
0
0
0.05
0.1
2
3
4
Chl. a (m g/l)
5
0
0
0.15
0.1
0.2
0
1
2
3
4
5
0.00
0
20
20
40
40
40
60
60
60
80
80
80
100
100
100
100
60
120
120
120
120
70
40
60
80
250
300
0
0.05
0.1
0
0
0
0
20
20
40
40
60
0.1
0.2
20
30
40
50
SIO2 (mg/l)
0.3
0
1
2
3
4
Chl. a (m g/l)
0
5
0
0
20
20
10
40
40
20
60
60
60
80
80
80
80
100
100
100
100
60
120
120
120
120
70
Fig. 6 - 10: Chemical Parameters v.s. Depth
1.00
10
20
PO43--P (m g/l)
0.15
0.50
0
0
NO3--N (m g/l)
Alkalinity (m g/l)
200
1
Chl. a (m g/l)
SIO2 (m g/l)
0.3
0
20
0.5
0
10
20
30
40
50
60
70
PO43--P (m g/l)
NO3- -N (mg/l)
300
1
60
80
250
5
40
40
60
A l kal init y ( mg / l)
4
0
60
200
3
20
20
40
2
SI O 2 ( mg / l )
0
0.15
20
40
1
PO43--P (m g/l)
NO3--N(m g/l)
0
1.00
30
60
NO3--N(m g/l)
0.50
20
40
300
20
Depth (m )
Chl. a (m g/l)
SIO2 (m g/l)
0.3
20
200
Depth (m )
0.2
0
40
0
Depth (m )
- P ( m g/ l )
20
Alkalinity (m g/l)
30-July-01
Figure 6e—10e
0.15
3-
0.1
20
0
26-July-01
Figure 6d—10d
0.1
0
0
Alkalinity (m g/l)
24-July-01
Figure 6c—10c
PO4
NO3--N (m g/l)
Chlorophyll a vs
Depth
Silica vs Depth
0
200
19-July-01
Figure 6b—10b
Phospate vs
Depth
Nitrate vs Depth
30
40
50
0.5
1
Date
Thermocli
ne
Turb
idity
Oxy
cline
Sardine+
Juvenile.
Lates
stappersi
Wind
speed
(m/s)
Lake
conditions
Surface
Temp.
(oC)
16/07/
01
70
0.4
80
80.7
19.3
2.4
Little
calm
25.8
19/07/
01
70
0.37
70
45.5
54.5
0.0
calm
26.0
24/07/
01
85
0.36
80
0
100.0
2.4
calm
25.6
26/07/
01
75
0.43
75
100
0.0
4.0
Little
calm
25.5
30/07/
01
75
0.41
70
100
0.0
5.2
calm
25.5
Location
(GPS)
04o58'00''
S
029o33'00'
'E
04o47.37'
S
029o30.61
'E
04o53'20.
9''S
029o31'30
.7''E
04o51'27.
2''S
029o30'28
.0''E
04o49'28''
S
029o31'20'
'E
0-15
m
1530
m
3045
m
4560
m
6090
m
Ave
rage
S.D.
23.3
14.9
12.4
23.4
9.2
16.6
6.5
28.8
17.2
27.8
6.1
1.6
16.3
12.3
42.7
32.6
18.1
18.1
2.2
22.7
15.5
32.2
24.1
27.8
43.4
4.3
26.4
14.3
90.6
53.3
26.6
31.7
13.2
43.1
30.2
Table 1:Summary on limnological parameters, fish catch composition in percentage and zooplankton
abundance at the different positions near Kigoma.
r = - 0.93
Fig. 11a: Surface
temperature in
relation with wind
speed
Date
%
0.4
0.35
NTU
0.45
100
80
60
40
20
0
0.3
16
/0
7/
19 01
/0
7/
24 01
/0
7/
26 01
/0
7/
30 01
/0
7/
01
6.0
4.0
2.0
0.0
16
/0
7/
01
24
/0
7/
01
30
/0
7/
01
26.5
26.0
25.5
25.0
24.5
Wind Sp.
(m/s)
Surf.
Temp.(Oc)
r = -0.92
Fig. 11b: Lates stappersii in
relation with Turbidity
Date
Surface Temp. (oC)
Lates stappersi
Fig. 11c:
Thermocline v.s.
Depth
Therm ocline
r = 0.45
Depth (m )
20
40
60
Fig. 11d: Zoopl. Abund.
(average) in relation with
Turbidity
0.45
100
80
60
40
20
0
0.4
0.35
0.3
16
/0
7
19 /01
/0
7
24 /01
/0
7
26 /01
/0
7
30 /01
/0
7/
01
Zoopl.
Abund.
(n/m 3x103)
Date
0
Turbidity
Date
80
100
Zoopl. Abund. (average)
120
Turbidity
Fig. 11: Limnological parameters in relation with fish catch composition
NTU
Wind speed (m/s)
0.44
0.42
0.4
0.38
0.36
0.34
0.32
100
%
growth when diatoms are the predominant algae
(Plisnier, 2001). This is also shown by Chlorophyll a
concentration on respective days (Figure10).
r = 0.93
80
60
40
20
16
/0
7/
19 01
/0
7/
24 01
/0
7/
26 01
/0
7/
30 01
/0
7/
01
0
NTU
Fig. 11e: Sardine+Lates
juvenile in relation with
Turbidity
Chlorophyll a concentration was generally higher at
20m and 40m (Figure 10). This may be attributed to
high primary production. There is a general decline
in Chlorophyll a concentration at 60m. This may
partly be due to little light reaching the depth. Also
Chlorophyll a concentration was high on the 4th and
5th nights. We recorded higher wind speed, which
might have caused more mixing which in turn could
have resulted into higher primary production (table
1).
Date
On an average basis, turbidity was high on 26th and
30th of July (Table 1 and Figure 11). Turbidity in
Sardine+Juvenile.
Turbidity
water is caused by suspended inorganic and organic
matter such as clay, silt, carbonate particles, fine
organic particulate matter, and plankton (phytoplanton and zooplankton) and other small organism (Wetzel,
and Likens, 1990). The observed high turbidity may have been caused by high concentrations of
microorganisms (plankton and colonies of bacteria). Also turbidity showed to be very important parameter
because the clupeid and Lates stappersi (juvenile) catches depends much on high turbidity when Lates
stappersi (adult) is vice versa (Figure 11)
Water temperature remained almost constant to about 60m depth with the thermocline fluctuating between
70m and 90m during the sampling period. This fluctuation of thermocline could be explained by internal
waves and wind mixing which might have caused dampening of the thermocline.
Dissolved oxygen, like temperature, remained almost constant to 60m with very sharp decline to almost 1.0
mg/l. We observed an oxycline between 70m and 80m (Figure.2). This is almost embedded in the
thermocline.
Conductivity depends on dissolved solutes/free ions (nutrients) in the water column. It will increase, at a
given temperature, whenever there is an abundance of dissolved nutrients (cations and anions) in the water.
In Lake Tanganyika conductivity increases with depth (Kimirei& Nahimana,2000). The general trend was
depicted although there are signs that deeper waters were coming to the surface, probably due to internal
waves and turbulence (Figure 5).
A large percentage of adult Lates stappersi caught on July 24th corresponded with a low turbidity
concentration and half moon phase. Those conditions could possibly be favorable. On July 26th turbidity
was high where catches for Stolothrisa tanganicae was high also compared to other species (Table 1).
Conclusion
It seems from the results that fish catch of a certain species could be tied to limnological parameters,
especially turbidity. The moon phases are probably also important. High turbidity correlated positively to
Stolothrissa tanganicae catch and inversely to Lates stappersi (Figure 11). However, this study of three
weeks of limnological parameters cannot be considered conclusively because few data were collected and
there were no strong hydrodynamic changes during the period of investigation.
Acknowledgements
I would like to thank Willy Mbemba and Ismael Kimirei for their help in the field and during laboratory
work. Special thanks go to Dr. Plisnier for his guidance and support for this project. I would like to thank
the Nyanza Project organizers for giving me the opportunity to explore my career interests. I will not
forget to thank World Wildlife Foundation (WWF) for financing the Nyanza project. Also I thank all
members of the nightfishing team that helped in sampling.
References
Coulter, G.W. (Ed), 1991. Lake Tanganyika and its Life. Oxford University Press New York, 354.
Plisnier, P.D., 2001. Limnology notes and field mannual, Nyanza Project, 25.
Plisnier, P.D., Chitamwebwa, L. Mwape, K. Tshibangu, V. Langenberg & E. Coenen, 1999.
Limnological annual cycles inferred from physical-chemical fluctuations at three stations of Lake
Tanganyika. Hydrobiologia 407: 45-58, 1999.
Wetzel R. G., Likens, G. E., 1990. Limnological Analyses. Second edition. Springer – verlag, New
York, 391.