| dc.description.abstract | 1. This study deals with some aspects of the general
biology and ecology of Synodontis (Pisces:
Mochokidae) found in Lake Victoria. The material
used were sampled at the mouths of two important
inflows of the lake: the Rivers Sondu and Kuja.
A combination of field and laboratory studies was
carried out under the following major lines of
investigation: .
(a) the environment where the fish were caught;
(b) biological studies of Synodontis, which included
size distribution, re·production, feeding habits
(and incidentally, stomach parasites), and age
d~termination; and (c) catch statistics of all
fish species caught at the two sampling
stations.
2. The mean physico-chemical measurements taken at R.
Sondu were as follows: temperature 25.20C, pH 7.5,
D.O 5.8ppm., conductivity O.9/,Scm-l, turbidity 7.0ppm.,
sediment P.O.lmg-l, sediment Ca. 4.9m - equiv. %,
sediment pH 5.8. The mean readings for R. Kuja were:
temperature 26.30C, pH 7.4, D.O. 6.3ppm., conductivity
1.OjJScm-l, turbidity 36.6ppm., sediment P.O. lmg-l,
sediment Ca. 7.6m equiv.% and sediment pH 6.8. The
highest readings were recorded at R. Kuja: temperature
28°C, turbidity 96npm.t sediment Ca. 8.7m - equiv.%,
and sediment nH 7.2. Discharge data suggests a moreerratic
flow regime for R. Kuja than R. Sondu. The
significance of these findings in relation to fish
abundance are discussed.
3. The sizes of S. afrofischeri from both stations
ranged between length classes 7.0 - 7.9 cm. to
15.0 - 15.9 cm; while those of S~ victoriae were
between length classess 8.0 - 8.9 cm. to
19.0 - 19.9 cm.
fish (and modal
R. Sondu.
In either species the longest
lengths of adult fish) came from
The calculated mean size of S. afrofischeri from
both stations were 9.3 cm FL (weight 13.3 g) for
males and 11.5 cm FL (weight 27.9 g) for females.
The corresponding figures for S. victoriae were
13.8 cm FL (weight 44.6g) for males, and 12.9 cm
FL (weight 35.5 g) for females. Hence in
S. afrofischeri females were the bigger sex, whereas
in S. victoriae the opposite was true. In both
species length was positively correlated to weight.
Throughout the study period, the distribution
(in time) of S. afrofischeri was more or less even
at the two stations. ~ victoriae, however, was
rare at R. Kuja during the dry months. Possible
reasons to account for the observed sizes and
distribution of Synodontis are discussed.
5. The breeding of Synodontis takes place predominantly
during the rainy seasons, the long rains of March -
~~y being the most prominent. The average lengths
of mature (stages V-VI) fish were: ~ afrofischeri
12.0 cm FL (male) and 11.6 cm FL (female);
~victoriae 14.2 cm FL (male) and 13.5 cm FL (female) •
•
6. The sex ratio tended to favour males at immature
and developing stages, but this changed dramatically
in favour of females at the mature and breeding
stages •.
7. High condition factors were recorded at a period
just prior to the rainy seasons of November -
December (short rains) and March - April (long
rains). The condition factors were least
in-between these periods.
8. The mean fork length of mature and ripe ~
afrofischeri was 13.3 cm and the mean fecundity
was 6,592 eggs; while the respective values for
~ victoriae were 14.8 cm and 7,140 eggs.
9. ~ afrofische~~ at R. Sondu fed mainly on, in
in order of importance, insect adults and immatures,
detritus, mixed food items* and molluscs; while
at R. Kuja their main food items were detritus, food
of mixed items, insect adults and immatures, and
some vegetable matter. A high percentage (49.1%)
of well-fed fish were caught at R. Sondu. Possible
reasons to explain the differential feeding habits
and status of nutrition of fish from the two
" sampling stations are given.
10. In the course of stomach content analysis several
cases of (unencapsulated) nematode worms were
recorded from both species of fish. The mean
intensity of infestation was about 2.4 paraSites
per fish, infestation frequency being highest in
female fish.
•
11. Age determination for Synodontis using the otolith
method gave negative results.
It was concluded that the otolith is probably
not the best material to use in ageing these
species of fish.
* i.e. insects, sand, vegetable matter etc.
12. At the mouth of R. Kuja the most abundant fish
species were Labeo ~ictorianusl Barbus spp.,
Haplochromis spp. and Synodontis. At R~ Sondu
the most predominant species were Synodontis,
Lates niloticusa Barbu~ spp. and Schilbe mystus.
Fish numbers and species diversity were higher
at R. Kuja than at R. Sondu.
13. The sign.ificance of the whole study in relation
to management of river fishery is discussed | en |
