Student: Mrs. Catherine Mwakosya
Mentor: Dr. Pierre-Denis Plisnier
Introduction
The main pelagic fishery of Lake Tanganyika is composed of centropomid species Lates stappersi
(Boulenger) and two sardines Stolothrissa tanganicae (Regan) and Limnothrissa miodon (Boulenger)
(Coulter, 1991). These pelagic fish rely heavily on zooplankton as a source of their food during part of their life cycle. Thus the understanding of zooplankton dynamics and their role in the energy transfer through the food chain is necessary for estimates of the potential production of the fishery (Bosma et. al.
1997). Fishing is the main economic activity in Lake Tanganyika. Its effectiveness depends probably on limnological characteristics of the Lake.
This study intended to determine the trend of fish catches and possibly correlate them with zooplankton abundance and stomach contents. I wanted to investigate possible relationship with the changes in limnological parameters.
Materials and Methods
The experiment involved one research vessel R.V Echo and one fishermen unit (Catamaran). The five night trips took place on July 16 th
60.9” E), 24
30 th th
July (04
July (04 o o
49’ 28” S, 029 o
(04 o 58’ 00” S, 029
53’ 20.9” S, 029 o o 33’ 41” E), 19
31’ 30.7” E), 26 th th
July (04 o
July (04 o 47’ 36.6” S, 029 o
51’ 27.2” S, 029 o
30’
30’ 28” E) and
31’ 20” E). Those nights were used to collect fish, zooplankton and water samples in the pelagic zone. Fishing was conducted by local fishermen’s using two-canoe catamaran.
Upon reaching the site of their choice the fishermen lit their lanterns to attract the fish for more than
30min. After attracting the fish they then joined the two canoes and the lift net was lowered to about 70
–120 m, then waited for one and half hours or more before they hauled their net. After hauling, fish total catch was recorded and the fishes were sorted by species. A subsample was taken for length-frequency and stomach content analysis. The gutting procedure was done immediately after the hauling and the digestive tract (stomach) removed and preserved in 4% formalin. In the laboratory the relative fullness was determined by giving ranks from 0 indicating emptiness to 5 for fullness. Stomach contents were examined using numerical analysis method whereby the samples examined at several levels of magnification for food items present were identified and counted. Composition of the gut contents was also recorded and where possible counting of the individuals and weighing was done. Data obtained were used to calculate the index of occurrence, abundance and weight according to Plisnier et al (1988) where by:
Index of occurrence I
O
= NA/NT x 100 %
NA = number of stomach in with food category A (A = Shrimps or fish or Zooplankton)
NT = Total number of stomach ‘not empty’ analyzed.
Index of abundance I ab
= NX/NXT x 100 %
NX = Number of individual of food category A ( X= Number of shrimps or fish ......etc)
NXT = The total number individuals of different food category obtained in the sample
Index of weight I w
= (wX * NX) /WT x 100 % wX = average weight of food category A
NX= number of individuals of food category A
WT= The total weight of different food category obtained in the sample
Zooplankton and water samples were also taken in the same night fishing trips. The detailed information on the zooplankton abundance and other limnological parameters may be found in the reports by other
Nyanza 2001 participants (Allgeier, 2001; Nzinza 2001, this publication).

Results
1. Catches
During the five sampling trips, the species caught were Lates stappersi , Stolothrissa tanganicae and
Limnothrissa miodon . In this study L. stappersi were grouped into two categories: “juveniles” (0-15cm total length) and “adults” (16-48cm). Total catch weights were between 2.1 kg and 180 kg for the first haul sampled (Table 1). “Adults” L. stappersi were major contributors in the total catches with peaks on
16 th July (99.4%) and 24 th July (100%). S. tanganicae was the 2 peaks on 19 th July (40.9%) and 26 th nd contributor to the total catches with
July (91.7%). L. miodon did not make a great contribution on the total catches and was only caught on July 26 th (7.6%) and 30 th July (4.7%) (Figure 1). In comparing with data from Tanzania Fisheries Research Institute (TAFIRI) for Lake Tanganyika monitoring programme on the two stations Kibirizi and Katonga, S. tanganicae composed large value of the total catches per unit boat (1180 kg ) on 18 kg) and on July 31 st
(Figure 3a).
th July, while L. stappersi reached the maximum on 25 th July (1600
only S . tanganicae was caught with the value of total catch per unit boat of 315 kg
The average total length (TL) of L. stappersi “adults” increased on July 24 while “juveniles” L. stappersi increased on July 16 th (Fig 2a) . S. tanganicae on the other side showed an increase on the average total length on July 16 and decreased on July 30 th (Fig 2b). In the TAFIRI statistics L. stappersi also shows an increase on the average total length on July 24 th at Katonga station (Fig 4a.). S. tanganicae at Kibirizi on July 18 and Katonga July 31 st
4b).
shows double peaks on the average total length (Figure 3b and Figure
The limnological parameters which related to the trends of catches during the study period are summarized in Table 1 below:
Table 1. Summary of the sampling results
Date
Lake condition
Moon condition
Surface Temperature
16July 2001
Little Calm
19 July 2001
Calm
24 July 2001 26 July 2001 slight winds Little calm
30 July, 2001
Slight winds
New moon
25.8
o C 26.0
o
Half moon
C 25.6
o C 25.5
o
Full moon
C 25.5
o C
Turbidity (NTU) average.
Maximum
Chlorophyll a ug/l average.
Maximum
Zooplankton abundance
(n/m 3 x 10 3 )
0-15m
15-30m
30-45m
45-60m
60-90m
Fish catches (kg)
Lates stappersi (A)
Lates stappersi (J)
Stolothrissa tanganicae
Limnothrissa miodon
0.40
0.45 (0m)
0.753
0.82(20-40m)
23.3
14.9
12.4
23.4
9.2
0.37
0.54 (20m)
No data
28.8
17.2
27.8
6.07
1.6
0.36
0.6 (0m)
0.433
0.51 (40m)
42.7
32.6
18.1
18.1
2.2
0.43
0.57 (60m)
1.033
1.07(20-40m)
32.2
24.1
27.8
43.4
4.3
Total catches (kg) 30.2 22.0 180.0 6.5 2.1 contents
A total number of 123 stomachs were examined of which 35 were of L. stappersi adults, 26 L. stappersi juveniles, 48 S. tanganicae and 14 L. miodon. The levels of stomach fullness of the analyzed samples were between empty and half full (50.4% empty and 17.1% half full). Very few were full (4.1%).
There were three main food items observed: shrimps, fish and zooplankton.
The “adults” L. stappersi were found to feed mainly on shrimps and fish ( S. tanganicae ). In only a few cases zooplankton were observed. “Juveniles” L. stappersi were feeding mainly on shrimps and copepods. The index of occurrence for shrimp as prey was between 60% and 84% during the study period. Fish prey was between 23% and 50%, and zooplanktons were only 20% and 37%. The
0.41
0.45 (0- 40m)
0.828
0.92(20m)
90.6
53.3
26.6
31.7
13.2

% composition by weight of the fish catches
(16/7/01)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
L. st a ppe rsi (A) L. st a ppe rsi (J ) L. miodon S . t a nga nic a e
% composition by weight of the fish catches
(19/7/01)
6 0 . 0 0
5 0 . 0 0
4 0 . 0 0
3 0 . 0 0
2 0 . 0 0
1 0 . 0 0
0 . 0 0
L. s t apper si ( A) L. st apper si ( J ) L. miodon S . t an gan icae
% Composition by weight of fish catches
(24/7/01)
1 00%
90%
80%
70%
60%
50%
40%
30%
20%
1 0%
0%
L. s t a pp e r s i (A) L. s t a p pe r s i (J ) L. mi o don S. t a n ga n i c a e
% composition of fish catches on 30/7/2001
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
L. stapper si (A) L. stapper si (J) L. mi odon S. tanganicae
% composition by weight of the fish catches
(26/7/01)
6 0%
4 0%
1 00%
8 0%
2 0%
0%
L. sta p p e rsi (A) L. sta p p e rsi (J) L. mio d o n S. ta n g a n ic a e
Total catch by w eights of the fishes
200.00
160.00
120.00
80.00
40.00
0.00
16/ 07/ 2001. 19/ 07/ 2001.
24/ 07/ 2001. 26/ 07/ 2001. 30/ 07/ 2001.
D a t e s

(site 1)
80
60
40
20
0
N=281
32 mm
46 mm
60 m m
74 mm
88 m m
Length
(site 2)
80
60
40
20
N=449
0
32 m m
44 mm
56 m m
68 mm
80 mm
92 mm
Length
80
60
40
20
0
(site 3)
Length
N=275
Fig 3b: Length - frequency distribution of
L. stappersi at Kibirizi station on
18/7/2001(site 4)
15
10
5
0
30
0mm
32
0m m
34
0mm
36
0m m
38
0mm
40
0m m
42
0mm
44
0m m
Length
Fig 4: Length- frequency distribution of
L. stappersi at Katonga station on
25/7/2001
20
15
10
5
0
17
0m m
20
0m m
23
0m m
26
0m m
29
0m m
320 mm
35
0m m
380 mm
Length
40
20
0
site 1
N=215
Length
40
20
0
site 2
N=136
Length
40
20
0
site 3
N=214
Length site 4
N=297
40
20
0
40 m m
52 m m
64 m m
76 m m
88 m m
10
0m m
Length
40
20
0 site 5
Length
Length frequency distribution of S. tanganicae at Katonga on 31/7/01 on 5 different units.

L e n g t h - F r e q u e n c y d i s t r i b u t i o n o f
S t o l o t h r i s s a t a n g a n y i c a e o n 1 6 / 7 / 0 1
20
1 5
1 0
5
0
3cm 4cm 5cm 6cm 7cm
L e n g t h c m )
8cm 9cm 1 0cm
L e n g t h f r e q u e n c y d i s t r i b u t i o n o f j u v e n i l e s L a t e s s t a p p e r s i o n 1 6 / 7 / 2 0 0 1
1.2
1
0.8
0.6
0.4
0.2
0
1
Lengt h
I n d e x o f o c c u r r e n c e f o r v a r i o u s f o o d i t e m s i n t h e L a t e s s t r a p p e r s i o n
1 6 / 7 / 2 0 0 1
80. 00
60. 00
40. 00
20. 00
0. 00
Shr i mps Fi sh Zoop.
C a t e g o r y o f f o o d i t e m
I n d e x o f a b u n d a n c e s o f v a r io u s f o o d it e ms o n t h e
L a t e s s t a p p e r s i o n 1 6 / 7 / 2 0 0 1
100.00
80.00
60.00
40.00
20.00
0.00
Shr imps Fish Z oop.
C a t e r o r y o f f o o d i t e m
1 50
1 00
50
0
L e n g t h f r e q u e n c y d i s t r i b u t i o n o f
S t o l o t h r i s s a t a n g a n i c a e o n t h e d a y
1 9 / 7 / 2 0 0 1
5cm 6cm 7cm
L e n g t h ( c m )
8cm
L e n g t h - F r e q u e n c y d is t r ib u t io n o f L a t e s s t a p p e r s i ( A ) o n t h e d a y 1 9 / 7 / 2 0 0 1
3
2
5
4
1
0
7
6
1
L e n g h t ( c m )
1.5
1
0.5
0
Length frequency distribution of adults Lates stappersi on
16/7/2001
1
Length
I n d e x o f w e i g h t o f v a r i o u s f o o d i t e ms o n t h e
L a t e s s t a p p e r s i o n t h e d a y 1 6 / 7 / 2 0 0 1
80. 00
60. 00
40. 00
20. 00
0. 00
Shr imps Fish
C a t e g o r y o f f o o d it e m
Z oop.
1 . 5
1
0 . 5
0
L e n g t h - f r e q u e n c y d i s t r i b u t i o n o f L a t e s s t a p p e r s i ( J ) o n t h e d a y 1 9 / 7 / 2 0 0 1
1
L e n g t h
I n d e x o f o c c u r e n c e ( I
O
) o f v a r i o u s f o o d i t e m s i n L a t e s s t a p p e r s i o n t h e d a y
100
80
60
40
20
0
Shr i mps Fi sh
C a t e gor y of f ood
Zoop.
1 5
1 0
5
0
Lengt h - f r equenc y di s t r i but i on of
La t e s st a ppe r s i ( A ) on t he day
2 4 / 7 / 20 0 1
1
L e n g t h
I n d e x o f a b u n d a n c e I
A B
o f v a r i o u s f o o d i t e m s o f n t h e L a t e s s t a p p e r s i o n t h e d a y
1 9 / 2 0 0 1
100
50
0
Shrimps Fish Zoop.
C a t e gor y of f ood i t e ms
I nde x of we i ght of v a r i ous f ood i t e m on t he La t es s t apper s i on t he da y 1 9 / 7 / 20 0 1
1 00
50
0
Shr i mps Fi sh c a t e g o r y o f f o o d i t e m
Zoop.
I n d e x o f o c c u r e n c e f o r v a r i o u s f o o d i t e m s o n t h e l a t e s s t a p p e r s i ( A ) o n t h e d a y 2 4 / 7 / 2 0 0 1
45. 0
40. 0
35. 0
30. 0
25. 0
20. 0
1 5. 0
1 0. 0
5. 0
0. 0
Shr i mps Fi sh Zoop.
C a t e g o r y o f f o o d i t e m
Leng t h - f r eq uency o f Lat es st ap p er si ( J) o n 2 6 / 7 / 2 0 0 1
1.2
1
0.8
0.6
0.4
0.2
0
1
Lengt h
I nde x of a bunda nc e of v a r i ous f ood i t e ms on t he La t e s st a ppe r si on t he da y 2 4 / 7 / 2 0 0 1
60.0
40.0
20.0
0.0
Shrimps Fish Zoop.
C a t e gor y of f ood i t e m
Leng ht - f r eq uency d ist rib ut io n o f St o lo t hrissa t ang anicae o n t he d at e
2 6 / 7 / 2 0 0 1
1 40
1 20
1 00
80
60
40
20
0
4cm 5cm 6cm 7cm
Le ngt h
8cm 9cm
I nde x of we i ght of v a r i ous f ood i t e m s on t he La t e s st a ppe r si on t he da y
2 4 / 7 / 2 0 0 1
1 00. 0
80. 0
60. 0
40. 0
20. 0
0. 0
Shrimps Fish Zoop.
C at eg o ry o f f o o d it ems
Le ngt h - f r e que nc y di s t r i but i on of Li mnot hr i s sa mi odon on t he dat e 26 / 7/ 2 00 1
20
10
0
Frequency

abundance were: shrimps: 68% to 94%, fish: 5%-32% and zooplankton: 14% -20%. The weight indices for shrimps were between 5% and 28% while for fish they were between 17% and 95 % (Figure 6).
The Clupeid Stolothrissa were found to feed mostly on shrimps and zooplankton (copepods). The occurrence of shrimps as prey was 90 – 100%, with abundance of 20%. The other clupeid samples for
Limnothrissa were found to be empty. It was thus not possible to establish their diet composition although several papers indicate that shrimps and zooplankton are found to be important diet of the two clupeids S . tanganicae and L. miodon (Kurki et al., 1999)
Discussion
The trend of fish catches in the present study was dominated by L. stappersi in percent composition for the first three sampling nights followed by domination of S. tanganicae in the last two nights of the study period (Figure 1). The same trend was also depicted by statistics taken from TAFIRI on the dates close to our study period. Such types of information suggest several predator-prey interactions or movements of fishes for limnological reasons.
There was a large percentage of the “adult” L. stappersi caught on July 24. This value corresponds with the low turbidity level of 0.36 NTU in average on that day. The two first sampling nights led to low turbidity levels in average compared to two last sampling days (Table 1). The data obtained throughout the study period suggest that there is inversely relationship between turbidity and the catches of “adult”
L. stappersi, while there is positive relation between turbidity and catches of “juvenile” L. stappersi and
Stolothrissa. When there is decrease in turbidity level, an increase of L. stappersi is observed .
Therefore, turbidity seems inversely related to L. stappersi catches (Plisnier, 1997).
The trend of zooplankton abundance was increasing in the water column at the depth of 0-45m and decreasing at the depth between 45-90m almost throughout the study period (Table 1). The trend does not correlate with the trends of fish catches, as they were different in various days. Coulter (1991) observed that there is spatial distribution when fish were abundant, zooplankton abundance was lower and nannoplankton abundances were higher. He also noted that there is an inverse relationship between fish abundance and the abundance of crustacean's zooplankton.
The diet of the "adult" L. stappersi from the stomach contents analysis was based mainly on the shrimps and in few cases clupeid Stolothrissa (Figure 5) although the abundance of them in the water column was lower than those of copepods (Allgeier 2001 this report). The "juvenile" L. stappersi were found to feed on shrimps and copepods. This may suggest that the feeding ecology of the L. stappersi depends on the availability of the food and the size of the fish itself or probably related to limnological conditions.
Chlorophyll a was much higher on July 19 th th and July 26
respectively (Table 1). th . This indicates a high abundance of phytoplankton which might lead into higher abundance on the zooplankton on the following sampling nights on July 24 and July 30
There were significant changes in the water movements occurring during the sampling period. The thermocline depth was constant for the first two nights followed by changes on the third night (Table 1).
The third night followed a strong west wind on July 23. The movement of water probably led to mixing of nutrients from the deep water toward upper layer and this may also have contributed into the high fish catches on July 24.
Conclusion
The data collected during the study period does not depict the direct effect of zooplankton abundance on the fish catches. The number of zooplankton found in the water column does not seem to be the only factor contributing to the fish catches. There might be other limnological factors together with the abundance of zooplankton that determine the trends of the fish catches. Sometimes size of the prey rather than their abundance appears to be more determining factor of prey selection. Thus adult L. stappersi were found to prey almost entirely upon fishes, all of which found to be S. tanganicae , while juvenile L. stappersi were found to eat shrimps and copepods. The catch effort including the number of lamps, mesh size, time taken for attracting fishes or position of the moon can also be questioned if they contribute to trend of fish catches. A better understanding of the effect of zooplankton abundance on fish

catch may be obtained by conducting day fishing because night fishing seem to be biased by light attraction of the plankton rather than it is in the natural situation.
Acknowledgements
I would like to thank my mentor Dr. Plisnier, Willy Mbemba and Ismael Kimirei for their supervision. I would also like to thank the National Science Foundation and World Wildlife Foundation for their financial support. In addition, I thank Tanzania Fisheries Research Institute and The Nyanza Project for providing this opportunity, facilities, and equipment for this research.
References
Coulter , G.W., 1991. Lake Tanganyika and its Life: Oxford University Press, Oxford U.K. 90-150.
Plisnier , P.D., J.c. Micha and V. Frank 1988. Biologie et exploitation des poissons du lac Ihema
(Bassin Akagera, Rwanda) Presses Universitaires de Namur, 170.
Plisnier, P.D., 1997. Climate, limnology and fisheries changes of Lake Tanganyika FAO/FINNIDA
Research for the Management of the Fisheries on the Lake Tanganyika GCP/RAF/271/FIN-TD/73(En):
50.
Kurki , H., P. Mannini, I. Vuorinen, E. Aro, H. Molsa and .V. Lindqvist, 1999. Macroplankton communities in Lake Tanganyika indicate food chain differences between the Northern part and the main basins. Hydrobiologia, 407: 123-129.
Windell , J.T. and S.H. Bowen, 1978. Methods for study of fish diets based on analysis of stomach contents. [In methods for assessment of fish production in fresh water by T. Bagenal] IBP Handbook
No3 Blackwell Scientific Publications, Oxford London Edinburg Melbourne 219-226p