Biological Journal o f the Linnean Society, 10: 109-137.With 5 figures
March 1978
The ecology of the fishes in Nyumba ya Mungu
reservoir, Tanzania
ROLAND G. BAILEY
Department of Zoology, Chelsea College, The University of London,
London SW10 OQX
S. CHURCHFIELD
Westjield College, The University of London, London NW3 7ST
T. PETR*
Department of Zoology, Makerere University, Uganda
AND
R. PIMM
Westjield College, The University of London, London NW3 7ST
Accepted f o r publication June I977
Twenty species of fish were found in Nyumba ya Mungu reservoir on the River Pangani which,
with two exceptions, were indigenous t o the river basin. Their distribution is described and
some information on their growth, reproduction and feeding habits is presented. The highest
densities of fish were found in the littoral and the broad, shallow, upstream region of the lake.
The lacustrine environment has favoured the endemic tilapias, Sarorherodon jipe and
S. pangani. which graze periphyton. and the introduced species, S. esculentus which feeds on
phytoplankton. Males of the endemic forms may grow to a large size, but t h e introduced
species, including Tilapia rendalli, were heavier for a given length. A reduction in tilapia stocks
since 1970 is reflected in experimental catches during 1972-74 and was attributed t o the
combined effects of heavy commercial exploitation and a loss of spawning grounds. In the same
period a striking increase in the populations of small carnivores, Rhabdalestes leleupi and
Haplochromis gr. blqveti, was recorded. b'ully plan tivorous and piscivorous fish are absent but
omnivores are represented by Barbus species and Synodontis punctulatus. Tilapias exhibited a
high incidence of nematode infection and the available evidence indicates that fish-eating birds
are important predators of cichlids in Nyumba y a Mungu.
KEY WORDS: -- tropical - man-made lake
reproduction - parasites - predators.
-
fish - fishery
-
ecology
--
Present addrcss: Department of %oology,Monash University, Clayton, Australia.
109
growth
-
diet -
110
K. G . BAlLEY ET AL.
CONTENTS
. . . . . . . . . . . . .
Introduction
Methods
. . . . . . . . . . . . . .
Results and discussion
. . . . . . . . .
. . . . . . . . . . .
Species list
Ecology of individual species
. . . . .
Mormyrus kannume
. . . . . .
Rhabdalestes leleupi
. . . . . .
Labeo forskalii
. . . . . . . .
Garra dembeensis
Barbus species
. . . . . . . .
. . . . . .
Clarias mossambicus
Synodontis punctulatus
. . . . .
. . . . .
Haplochromis gr. bloyeti
. . . . . . . . . .
Tilapias
. . . .
Ecology of the fish community
. .
Distribution-some quantitative data
. . . . .
Feeding interrelationships
. . . . . . . . . .
Parasites
Predators
. . . . . . . . . .
. . . . . . .
Breeding ecology
. . . . . . .
Commercial fishing
Summary . . . . . . . . . . . . . .
. . . . . . . . . .
Acknowledgements
. . . . . . . . . . . . .
References
. . . . . . . . .
110
111
113
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. 116
117
. 1 18
. 119
. 120
. 121
. 121
*
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. 128
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. 132
. 133
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. 134
. 135
. 136
. 136
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INTRODUCTION
Nyumba ya Mungu reservoir (the House of God) is a man-made lake in
northern Tanzania, situated in the Pangani river valley 50 km south of Moshi.
When full, it has a maximum surface area of 180 km2 and whilst it is
considerably smaller than some other impoundments created on major rivers in
Africa in recent decades, it remains for the present the largest river lake in East
Africa. Completed in 1965, it was constructed primarily for the purposes of
flood control, hydro-electric power and irrigation, with in addition, the
possible development of a fishery supplying fresh fish to the important centres
of Moshi and Arusha. In a pre-impoundment survey the fish species which
occur in the Pangani basin were reviewed by Bailey (1965)who suggested that
a useful fishery would develop based mainly on the endemic tilapias. Records
of the Fisheries Department show that a flourishing fishery had become
established by 1969. Large numbers of fishermen were drawn to the lake and
spectacular yields were obtained in 1970-71.Since that time the industry has
declined (Annual Reports, 1970-1974).
During a period of ten weeks in July, August and September 1974, as
members of a multidisciplinary team engaged in a hydrobiological survey of the
reservoir, three of us (Bailey, Churchfield & Pimm), undertook an investigation
of the fish community. Prior to this, the fourth contributor, (Petr) had made
three, shorter visits t o the lake in June 1972,January and April 1973,devoting
some time to fish sampling. A general account of this earlier work is given
elsewhere (Petr, 1975a), but in the present paper unpublished data on fish
diets, reproduction and experimental catches are combined or compared with,
the results of the later extended visit, to enable comment on both seasonal
changes in fish biology and developments in the community, over a three-year
period. Some additional information for 1972-73 on tilapias in NYM is
available from Okorie (1972,1974).
FISH ECOLOGY OF NYUMBA YA MUNGU
111
The shape and dimensions of the reservoir are shown in Fig. 1. Denny (1978)
provides a full description of its physical features and water budget. It has a
maximum depth of 41 m at the dam and an overall mean depth of 6 m. The
lake lies in a semi-arid area experiencing two limited wet-periods, the ‘long’
rains from March to May and the ‘short’ rains in November. Most of the annual
input however, is derived from two major inflows, the rivers Ruvu and
Kikuletwa draining Lake Jipe, the southern slopes of Mt. Kilimanjaro and Mt.
Meru. This catchment area is affected by wet seasons of approximately similar
duration but with increased precipitation in the mountains. The subsequent rise
in lake level takes place chiefly in April and May but continues throughout
June to reach its maximum in July. A second smaller increase in lake level may
follow the short rains. Annual reductions in water level are brought about by
the combined effects of evaporation and a continual discharge through the
power station into the River Pangani.
The inflows provide a good supply of minerals and plant nutrients t o the
lake, which is for the most part well mixed by the direct effect of the trade
winds, and supports rich communities of planktonic and periphytic algae
(Denny, Harrnan, Abrahamsson & Bryceson, 1978; Bowker & Denny, 1978).
The inflow deltas are characterised by swamps which from the Kikuletwa have
encroached into the shallow north end of the lake, (Welsh & Denny, 1978). In
the remainder of this area and in the littoral of south lake, the emergent
branches of drowned trees were apparent.
METHODS
Fish sampling
Fish samples were obtained in three ways. Gill net fishing was carried out
using a fleet of monofilament nylon nets with stretched mesh sizes ranging
from 20 t o 140 mm. Individual nets were 27 or 33 m in length with a mounted
depth of 1.5 m. In 1972 and 1973, use of the experimental fleet was confined
to Damsite and the Ruvu inflow at Korogwe. In 1974 it was deployed at
Damsite, the open water of south lake and at Magadini, Mikocheni, Samanga
and Korogwe in the north. Nets were set close to the water surface to avoid
snagging on submerged trees, and fished from a few hours to a full 24 hour
period, depending upon circumstances. On two occasions only, in 1972 and
1973, gill nets were also used to collect fish in the River RUVU,10 km north of
the lake.
Two further methods were used to collect fish in 1974. A suitably rigged
length of mosquito-netting (8.0 x 1.5 m) and a 20 mm mesh gill net were used
as improvised inshore seines at three locations and samples from commercial
catches were examined at Damsite, Spillway, Ngorika A and C,Kiti cha Mungu,
Langata, Handeni, Mikocheni, Samanga and Korogwe (see Fig. 1). Samples
were either examined fresh or preserved in 5% formalin for later study. Total,
fork or standard lengths of fish were measured usually to the nearest 0.5 cm
and weight to the nearest 5 g. Some small fish were subsequently re-weighed to
the nearest 1 g. The treatment of length and weight data varied and will be
described when necessary in the results.
R. C. BAILEY ET
112
AL.
r,
Legend
.
Swamp
-- - .
,. ....-.
.. ,
, ........
. ., ., ., , , I VWbb drolMd
,
, . . . . . . . I
I
I
LA_..._
I
VIII.g.r
I
Figure 1. Map of Nyumba ya Mungu reservoir, to show its general features and the location of
major fishing villages.
FISH ECOLOGY OF NYUMBA YA MUNGU
113
Reproduction
Sex was determined from external characters where possible and from gonad
inspection in preserved material. Gonad state (GS) was assessed using a scheme
adopted from Nikolsky (1963) in which:I immature, gonads were very small; virgin fish not yet reproducing.
I1 gonads small, eggs indistinguishable; maturing virgin or resting spent fish.
111 gonads occupying about half of abdominal cavity, eggs visible to the naked
eye; maturing fish.
IV gonads at maximum size, more or less filling abdominal cavity; mature or
ripe fish.
V sexual products extruded with slight pressure; spawning fish.
VI products discharged, a few eggs may remain in deflated ovary, some
inflammation of genital pores; spent fish.
Measurements and counts of eggs were made for some species. When
numbers were high, estimates were made from subsamples taken on a
volumetric basis.
Diet
The food of fish was investigated by gut analysis. Contents were identified
and then, the number of guts in which each food item occurred was recorded
and expressed as a percentage of the number of guts containing food. An
indication of the relative abundance of different foods in individual guts was
obtained by counting or visual estimation.
Parasites and predators
Samples of cichlid fishes were examined for parasites and few large
fish-eating birds were shot and their gut contents analysed.
A bbre viations
The following abbreviations are used: NYM, Nyumba ya Mungu reservoir;
TL, total length; FL, fork length; SL, standard length; GS, gonad state; n,
number; w, weight.
RESULTS A N D DISCUSSION
Species List
The following twenty species of fish were recorded in the reservoir or its
inflows:
Family Mormyridae
Family Characidae
Mormyrus kannume Forskal
Rhabdalestes leleupi Poll
R. G. BAILEY ET AL.
114
Family Cyprinidae
Family Clariidae
Family Mochokidae
Family Anguillidae
Family Cichlidae
Garra dernbeensis Riippell
Labeo forskalii Ruppell
Barbus oxyrhynchus Pfeffer
B. paludinosus Peters
B. kerstenii kerstenii Peters
B. jac kson ii Gunther
B. Eineomaculatus Boulenger
B. usambarae Lonnberg
Barbus sp. nov.
Clarias mossambicus Peters
Synodontis punctulatus Gunther
Chiloglanis deckenii Peters
Anguilla nebulosa labia ta Peters
Haplochromis gr. bloyeti
Sarotherodon jipe (Lowe)
S. pangani (Lowe)
S. esculentus (Graham)
Tilapia rendalli Boulenger
Apart from two of the tilapias, all of these species are indigenous t o the
Pangani basin. The list includes an undescribed species of small Barbus and i t is
possible that more than one taxon of Haplochromis will be identified when a
study of the material is undertaken. Large samples of the latter provided fish
which, whilst they were apparently similar in general morphology, exhibited
variable markings and colour patterns in detail.
So far as is known NYM was not stocked with tilapias. Lowe (1955)
described four species which are endemic to the Pangani basin. Of these
Sarotherodon korogwe appears to be confined to the lower river, S. pangani
occurs throughout its course and S. jipe and S. girigan are found in Lake Jipe
which in wet seasons, connects with the river via a series of swamps. Bailey’s
(1965) suggestion that S. pangani and the Lake Jipe forms were likely to
become established in NYM has proved to be the case, except for the apparent
absence of S. girigan. The latter is similar in appearance, body proportions, gill
raker, scale and fin ray counts to S. pangani, and can only reliably be separated
from it using differences in the character of the lower pharyngeal bone. In
S. girigan, this has a larger toothed area with coarse teeth, well scattered on the
bone, compared with S. panguni, in which the toothed area is small, with fine
teeth set close together (Lowe, 1955). In terms of precise measurements Dr
E. Trewavas (pers. comm.), has found the proportions of the pharyngeal bones
to differ in two ways:S. girigan
length of blade
length of toothed area
width of bone
head length x 100
=
0.4-0.7
(28.2)31.3-32.0(34.0)
S. pangani
0.9-1.1
28.0-30.9
Pharyngeal bones from NYM tilapias agree with Lowe’s description for
S. pangani and calculations of the ratios above for 10 specimens, SL 14-47cm,
FISH ECOLOGY OF NYUMBA Y A MUNGU
115
gave ranges of 0.8-1.2and 27.8-30.5respectively, which approximate closely to
Dr Trewavas’ data for this species. On this basis all tilapias of this sort in NYM
have been referred to S. pungani.
The two other tilapias in NYM, S. esculentus and Tilapia rendalli, are the
legacy of previous pond and dam stocking in the drainage of the River Pangani.
Stocks of both species were held at the Fisheries Department’s ponds in
Korogwe town on the lower river during the 1950s from where they were
widely distributed (Bailey, 1968). They are recent arrivals in NYM;
S. esculentus first appeared in the experimental gill nets at Damsite in 1973 and
T. rendalli in 1974.
The occurrence of Mormyrus kannume and Barbus jucksonii in the reservoir
is interesting and supports the evidence of a faunal relationship between some
non-cichlid fishes in the eastern rivers of Kenya and northern Tanzania with
those of the Lake Victoria basin, (Whitehead, 1962;Bailey, 1969). Two species
of fish, Alestes affinis and Bagrus orientalis, have been recorded in the River
Pangani but not so far in NYM.
Ecology of individual species
Mormyrus kannume
Distribution. Confined to the inflows, small samples of this fish were
obtained from fishermen at Samanga and Korogwe.
Size range. n = 7, FL 15.0-21.5 cm, w 55-180g.
Reproduction. All fish were immature except the largest, a female, in GS 111.
Its single ovary contained 2352 ova, ranging from 1.0 to 2.5 mm in diameter.
Food. All stomachs were full of aquatic insects. Small mayfly nymphs
(Caenis sp.) and dipteran larvae were numerically predominant, but larger
odonatan, coleopteran and lepidopteran larvae contributed the greater volume.
In Lake Victoria, M. kannume is essentially an insectivore (Corbet, 1961).
Rhabdalestes leleupi
Distribution. Abundant throughout the lake, especially in the open surface
waters of bays, lagoons and the limnetic zone. Rare in running water of the
inflows and River Ruvu.
Size range. Rhabdalestes exhibits sexual dimorphism. Males, n = 60, FL
4.5-6.5(mode 5.5)cm, w 0.85-3.00g, having a slender, shorter body than
females and with elongated and strengthened anterior rays in the anal fin,
imparting a notched shape to its outer edge. Females, n = 725, FL 6.0-9.5
(mode 7.0)cm, w 2.35-9.80g,with a deeper, longer body and a straight or
slightly concave outer edge to the anal fin.
Reproduction. Males were rarely caught in gill nets and a comparison of two
catches in a small bay at Damsite produced a striking inequality in the sex
ratio. On 21 July 1974,the mosquito-net seine fished inshore caught 74 fish of
which 68% were males, whereas the 20 mm gill net set in the open water of the
bay three days later caught 124 fish of which 99% were females. How far the
inequality in the second catch (and in gill nets generally) resulted from net
selectivity acting on the size difference between the sexes, cannot be
116
R G. BAILEY ET AL.
ascertained. However, since large males overlap females in size it may
reasonably be assumed that had males been present in some quantity rather
more would have been retained. It appears that males tend to inhabit inshore
areas whilst females predominate in more open water. Bowmaker (1969)found
a similar situation in his study of Alestes macrophthalmus in L. Bangweulu.
In both sexes paired gonads are present usually with one larger than the
other. On all visits and in all localities sampled, ripe females, GS IV, were
recorded, typically in large numbers, and in July 1974, the only month for
which data are available, males were in GS 111 or IV.Breeding evidently extends
over most of the year but the spawning sites in the lake remain unknown.
The number of eggs ranging in maximum diameter from 0.20 to 2.00mm,
carried by ripe females, for 20 fish, FL 7.0-7.5 cm, was between 836 and 2530,
with a mean figure of 1542.
Food. Rhabdalestes is carnivorous, feeding in mid t o surface waters on
zooplankton, aquatic and terrestrial insects (Fig. 2). Chironomid pupae
provided the major source of food but these small fish could also ingest
relatively large organisms including dragon and damselfly larvae. A wider
variety of insects was consumed in north lake and the occurrence of crustacean
zooplankton in the diet was restricted to fish from the south. The quality of
insect food eaten over 24 hours was shown to vary in April 1973.Chironomid
pupae were only consumed by night and terrestrial insects, by day.
Labeo forskalii
Distribution. Juveniles and adults were caught inshore in south lake and at
all of the northern stations. The largest catches in July and August 1974 were
obtained from the inflows. Additional collections were made in the River Rum
in December 1972 and April 1973.
Size range. n = 344,FL 6.5-25.0cm, w 4-194g. Ninety-three per cent of the
juveniles were between FL 7.0-9.0cm, w 5-11 g, and 75% of the adults fell into
the ranges, FL 13.0-18.0cm, w 30-89g.
Reproduction. A small number of ripe and spawning males and females (GS
IV and V), were caught at the beginning of the rains in April 1973,both in the
lake and in the River Ruvu. None were found in these states at other times.
Thus, in common with other Labeo species, for example, L. victorianus
(Cadwalladr, 1965)L. forskalii is a seasonal spawner. The presence of traps and
statements of fisheries staff indicate that there is some movement of these fish
through inflow areas during the rains, but flooded lake margins may also
provide suitable spawning sites, since the ripe lake fish obtained in 1973,were
caught at Damsite. The latter, FL 17.5-25.0
cm, were considerably larger than
ripe fish caught in the river, FL 8.0-11.0cm. Whether the riverine fish were
slow-growing adults or precocious spawners was not ascertained.
Food (Fig. 2). The occurrence of organic detritus and sand grains in guts
suggests that this species feeds chiefly off bottom deposits and the associated
phytobenthos. The latter consisted mainly of diatoms, (including Achnanthes.
Cyclotella, Cymbella, Nitzschia and Synedra) and green algae (including
Cosmarium and Scenedesmus). Larger filamentous algae, for example
Rhizoclonium, were also recorded which together with the chironomid larvae
found in riverine specimens indicates that Labeo also browses on aufwuchs.
I
,dds iawnsuon
I
n 3 N n W VA VaWnAN d 0 A 3 0 7 0 3 3 HSld
L11
R. G. BAILEY ET AL.
118
Barbus oxyrhynchus
Distribution. A ‘large’ Barbus species, which was occasionally collected at
most stations, including the inflows.
Size range. n = 40, FL 6.5-32.0 cm, w 5-345 g.
Reproduction. None examined were beyond GS 11.
Food. Barbus oxyrhyncltus is an omnivore. In 1 1 fish, FL 11.0-18.0 cm, the
foods recorded together with the number of occurrences were: Cladocera 2,
Caenis nymphs 3, chironomid larvae 4, other insects 3, fish remains 3 and
macrophyte fragments 7.
Barbus paludinosus
Distribution. This was the commonest species of Barbus found at most
perimeter stations and in the River Ruvu.
Size range. n = 443, F L (6.5) 7.08.5 (10.5) cm, w 5-16 g.
Reproduction. No clear breeding pattern was established. On all occasions
most fish examined were in GS I to 111, but a few ripe fish (GS IV) were found
at the end of the rains in June 1972, at their commencement in April 1973 and
in the dry season of July-August 1974. Some ripe and spent fish (GS IV-VI)
were found in the River Ruvu in April 1973. In Lake Victoria B. paludinosus
breeds during the rainy season entering affluent streams for this purpose
(Greenwood 1966). The size range and counts of eggs obtained are given in
Table 1.
Table 1. The size and numbers of eggs in four species of small Barbus
Species
n
FL
(crn)
Egg count
mean
range
Max. diam. of
eggs, range
(mm)
B. paludinosus
B. lineomaculatus
B. kerstenii
B. jacksonii
8
14
7
1
7.0-10.5
6.0- 8.0
5.0-7.0
10.5
1805
1559
811
2250
1240-2280
1164-2338
488-1200
-
0.26-0.57
0.27-0.64
0.37-0.66
0.35-0.55
Food (Fig. 2). In the reservoir this species is omnivorous. No seasonal change
in the diet was detected but guts containing zooplankton, notably cladocerans,
were confined to samples from south lake. The frequent occurrence of seeds,
up to 50 in some individual guts, is interesting. Samples from the River Ruvu
had fed almost exclusively on chironomid larvae and pupae. In the Lake
Victoria basin this species has a varied diet which includes insects and snails
(Greenwood, 1966).
Barbus lineomaculatus
Distribution. A fairly common species found in south lake and the inflow
areas.
Size range. n = 1 5 5, FL 3.5-8.0 cm, w 3-7 g.
Reproduction. A large number of ripe females, GS IV, was caught in April
FISH ECOLOGY OF NYUMBA Y A MUNGU
119
1973, notably at Damsite. Evidently as a result of an increase in girth
consequent upon the maturation of the ovaries, ripe females become more
vulnerable t o capture in the 20 mm mesh gill nets. Some ripe females were also
recorded in June 1972, but none in the dry season of 1974. Thus it is probable
that breeding commences in the lake and its inflows at the onset of the rains
and continues after their peak while lake levels are still rising. Fecundity data
for this species are given in Table 1.
Food (Fig. 2). Cladocerans, aquatic and terrestrial insects, algae and
vegetable debris were found in the guts of dissected fish; two specimens
contained fish scales.
Barb us juc ksonii
Distribution. Recorded at Damsite and Korogwe only. This species was rare
in the lake but more common in the River Ruvu collections.
Size range. n = 66, FL (6.5) 7.0-8.0(10.5) cm, w (4) 5-7 (16) g.
Reproduction. A single ripe female was caught at Korogwe in April 1973,
none in the River Ruvu. The egg count from this specimen is included in
Table 1.
Food (Fig. 2). A mixed diet including insects, snails, fish fry, seeds and other
vegetable matter was founp for the lake fish whereas those from the river had
fed exclusively on chironomids. Insect larvae and bottom debris are reported as
the food for this species in Lake Victoria (Greenwood, 1966).
Barbus kers tenii kers tenii
Distribution. This species was restricted to running water in the inflow deltas
and the River Ruvu.
Size range. n = 26, FL 3.5-7.0 cm, w 2-5 g.
Reproduction. The river sample at the onset of the rains in April 1973
contained ripe females. They had fewer but rather larger eggs than the
preceding species (see Table 1).
Food. Riverine fish contained chironomids and vegetable debris which agrees
with the diet described for this species in the Lake Victoria basin (Greenwood,
1966).
Burbus usumbarue (n= 10, FL 3.0-5.0 cm) and Burbus sp. nov. (n = 20, FL
2.5-4.0 cm) were too small to be retained by gill nets but both were caught in
the mosquito-net seine in the lake margins.
Clarius mossambicus
Distribution. Only 1 3 fish were caught in the experimental gill nets and all of
these were taken at Damsite. A study of fishermen’s catches in 1974 however,
showed that Clarius was distributed throughout the lake but that it was most
common in the northern area where, in the extensive reed swamp and delta
channels, large catches were made.
Size range. n = 79, TL 12.0-80.0 cm, w 20-4,500 g. Eighty per cent of the
sample examined fell into the ranges TL 25.0-49.0 cm, w 140-910 g.
R. G . BAILEY ET AL.
120
Reproduction. Gonad states of 12 fish from Damsite were: GS I, 6 fish, TL
12.5-19.5 cm; GS 111, 3 males, TL 22.0-57.0 and 3 females, TL 23.0-49.0 cm.
All but two of this small sample were obtained in the dry season of 1974. In
Lake Victoria this species enters streams to spawn in the rainy season
(Greenwood, 1955) and according to Matthes (1967) spawning in the Great
Ruaha River, Tanzania, takes place during the floods.
The egg count for a single female, TL 31.5 cm, was 2670 and the maximum
diameter of the eggs ranged from 0.55-0.80 mm.
Food (Fig. 2). Small fish, among which Barbus and Haplochromis were
identified, and insects, are the items of most importance in the diet.
Synodontis punctulatus
Distribution. This catfish was widely distributed in the main body of the
reservoir, including the deep water of south lake, but it was absent in
collections from the inflow deltas and the River Ruvu.
Size range. n = 201, FL 5.0-19.0 cm, w 5-195 g. Ninety per cent of the
sample fell within the ranges, FL 11.0-17.0 cm, w 25-175 g.
Reproduction. The gonad states of adults, FL 11.5-19.0 cm, caught at
Damsite may be summarised as follows:
males,
n = 9, GS 111-100%
females,
n = 13, GS IV-100%
males,
n = 25, GS III-96%,GS IV-4%
April 1973
females,
n = 20, GS IV-lo%, GS V-90%
July-August 1974 males n = 26, GS III-92%,GS IV-8%
females n = 15, GS III-33%, GS IV-67%.
June 1972
Data for the peak of the dry season are lacking, but it is likely that breeding
commences at the onset of the rains and continues as the lake level rises
through June into July. Spawning probably occurs within the lake basin.
The number of eggs, ranging in maximum diameter from 0 . 3 5 to 1.60 mm,
carried by 23 ripe fish, FL 13.0-18.0 cm, was between 1240 and 4956, with a
mean value of 3220. In general the highest numbers were contained in the
ovaries of the largest fish.
Food (Fig. 2). Data from juveniles and adults were combined since they
appeared to share a similar diet. Synodontis punctulatus is an omnivore, in the
diet of which aquatic insects, notably chironomid larvae, and filamentous green
algae, were the principal components. Occasionally stomachs filled almost
exclusively with one or other of these items were found, giving individual
counts in excess of 300 for chironomid larvae. More frequently however, a
mixed content was determined in which mayfly nymphs, caddis larvae,
terrestrial insects or organic debris made some contribution. Rare occurrences
were ostracods, chydorids and fish scales or fins.
Chiloglanis deckenii (n = 9, FL 2.0-4.5 cm) was recorded in delta streams at
Samanga.
Anguilla nebulosa labiata A single large eel was examined; it had a total
length of 154 cm and weighed 9.3 kg. Two other specimens of similar size were
seen and all of them had been captured by east bank fishermen. These mature
FISH ECOLOGY OF NYUMBA YA MUNGU
121
eels have become land-locked by the creation of the dam, escaping only when
the lake spills. At such times it is possible that elvers may enter the lake via the
spillway, a phenomenon which has been observed in Lake Kariba (Balon, 1974).
Haplochromis gr. bloyeti
Distribution. Common throughout the reservoir, including the inflows; a
small sample of Haplochromis was also obtained from River Ruvu.
Size range. n = 1174, TL 4.5-12.5 cm, w 2-25 g. The majority of fish fell
within the range TL 5.0-7.0 cm; those larger than TL 9.0 cm were usually
males.
Reproduction. In dissected samples, a few ripe and spawning fish (GS IV and
V), of both sexes, were found in June 1972, April 1973 and July-August 1974.
The number of eggs ranging in maximum diameter from 0.20 to 2.00mm,
carried by 24 ripe females, TL 5.5-7.0cm, varied between 22 and 106 with a
mean figure of 50.
Food (Fig. 2). A mixed diet of animal and plant material is indicated.
However these fish are primarily carnivorous since compared with food of
animal origin only very small quantities of plant materials were found in spite
of the fact that they occurred in a significant proportion of the guts examined.
No seasonal differences in diet were apparent but an abundance of
zooplankton and occurrence of fish remains provided a striking difference
between the gut contents of ‘small’ and ‘large’ Haplochromis respectively. In
Fig. 2, samples from the southern and northern ends of the lake were treated
together but it should be noted that zooplankton was absent in fish from the
swamp zone and inflows which, on the other hand, contained a wider variety o f
insects.
Three Haplochromis from the River Ruvu had fed on chironomid larvae and
in one, fragments of terrestrial insects were found.
Tilapias
Relative abundance and distribution
Prior to 1974, Sarotherodon jipe and S. pangani, the indigenous tilapias,
were not satisfactorily separated. Together they constituted the entire
experimental catch of 1 5 1 fish in June 1972, and 84% of 204 tilapias in April
1973. On the latter occasion the remaining 16% comprised S. esculentus. In
July and August 1974, this situation was remedied. A total sample of 1428
tilapias was examined and the species composition was as follows: S. jipe45.03%, S. pangani-27.38%, S. esculentus-20.73% and T. rendalli-6.86%.
Since it was made up of all experimental catches and random subsamples from
the commercial fishery, which between them covered virtually all parts of the
lake, the calculated percentages may be taken as giving a good indication of the
relative abundance of the four species. The predominance of S. jipe over
S. pangani is not surprising in view of its lacustrine origin but the data also
show the significant contribution made by S. esculentus to the tilapia stocks in
1974 and the lowly status of T. rendalli.
The distribution of tilapias in NYM in the same period is shown in Table 2.
The indigenous species were caught chiefly in littoral areas of south lake where
R G. BAILEY ET AL.
122
S. jipe was clearly predominant. In north lake they occurred in almost equal
proportions but S. jipe became more common again, in the inflows.
Sarotherodon esculentus accounted for almost one-quarter of the total
tilapias sampled in both parts of the lake. In the south it tended to occupy
deeper offshore areas whilst at the shallow northern end it was generally rare
except on cleared seining beaches near Handeni, where it comprised 78%of the
tilapia catch.
Small numbers of T.rendalli were found inshore close to the fringing reed
swamp. It is surprising that so few were recorded in north lake which has a
marked development of macrophytes.
Size range, length frequencies and length-weight relationships
Sarotherodon jipe and S. pangani grow to a large size in NYM attaining
almost 50 cm in length and 1.75 kg in weight (Table 3). This is considerably
larger than the maximum sizes recorded for the introduced species.
Length frequencies of tilapias from all samples examined in which sexing was
carried out in 1974 are shown in Fig. 3. In each species the modal range for
both sexes is approximately the same but smaller peaks of large male fish are
apparent in the plots for endemic species.
The length-weight relationship was investigated in two ways using data from
all fish above TL 9.5 cm, obtained from commercial and experimental fishing
samples in 1974.
(i) The mean weights of individuals of each species at length intervals of
Table 2. The distribution of tilapias in Nyumba ya Mungu reservoir in July and
August 1974. (The total numbers of fish examined, less 14 of unknown origin,
have been broken down on a regional basis for this purpose)
Species
South lake
no.
%
North lake
no.
%
Ruvu inflow.
no.
%
49.44
16.53
23.05
10.98
208
216
150
28
34.55
35.88
24.93
4.65
63
T. rendalli
311
104
145
69
-
-
Totals
629
100.00
602
100.00
98
100.00
S. jipe
S.pangani
S.esculentus
35
-
64.29
35.71
-
Kikuletwa inflow.
no.
%
50
35
-
58.82
41.18
-
-
-
85
100.00
* At Korogwe and Samanga, respectively.
Table 3. The size range of tilapias in Nyumba ya Mungu reservoir in July and
August 1974
Species
S. jipe
TL
(n = 643)
W
S.pangani
TL
(n = 391)
W
S.esculentus
TL
(n = 296)
W
T.rendalli
TL
(n = 98)
W
Males
Females
All fish
9.5-48.0
15-1740
10.5-47.0
20-1800
12.0-22.0
30-250
10.0-24.5
20-3 15
9.5-27.5
15-360
12.0-31.5
25-565
12.5-28.5
40-400
10.0-22.0
20- 190
5.0-48.0
10-1740
5.0-47.0
10-1800
10-28.5
20-400
10.0-24.5
20-3 15
FISH ECOLOGY OF NYUMBA YA MUNGU
123
60
S esculentus
T rendolfi
40
30
-20
I
t i/
I
S pongoni
0
!I
2o
-'$?
o=o,'.',
0
0
10
I
I
1
14
18
22
'\
/
pH.\
l .I- . ' -oO
-
26
\ @
30
34
38
42
46
50
Total body length (2cm intervals)
Figure 3. Length frequencies of NYM tilapias (see text). 0 , Males; 0,females.
1 cm were plotted in Fig. 4. I t was found by inspection that the points
for the introduced species, T. rendalli and S. esculentus were closely
aligned and slightly separate from those of the endemic species S. iipe
and S. pangani, over the equivalent part of their length range. Thus, in
the figure, separate curves have been fitted by eye to these two sets of
points. The curves appear to have approximately similar gradients but
their separation suggests that the introduced tilapias are heavier for their
length than the endemic forms.
(ii) The validity of these statements was tested by an examination of the
regression statistics of each species. Length-weight regressions were
computed by the method of least squares using the full range of
available weights for length intervals of 0.5 cm. The following values
were obtained:
R G . BAILEY E T A L .
124
Species
a (intercept)
b slope
S. jipe
S. pangani
S. esculentus
T. rendalli
-1.8277
-1.8233
-1.7132
-1.8663
3.0435
3.0391
3.0093
3.1475
An analysis of covariance showed that there were no significant
differences (D0.05)between the slopes but that a highly significant
difference (KO.01) occurred between the adjusted means of length and
weight. A pooled slope was then calculated for the four species
combined. The values of a were adjusted for the pooled length-weight
relationship and the significance of the difference between them was
tested (Le Cren, 1951). This was done by inspecting the calculated 95%
confidence limits of the adjusted values of a . The following results were
obtained:
Species
Confidence limits
lower
upper
adjusted a
~~
S. jipe
5'. pangani
S. esculentus
T. rendalli
-1.8300
-1.8315
-1.7591
-1.7501
-1.8605
-1.8625
-1.7899
-1.7789
-1.7995
-1.8005
-1.7282
-1.7212
These data show that there is no significant difference ( D O . 0 5 ) between the
intercepts of S. jipe and S. pangani or those of S. esculentus and T. rendalli, but
that the difference between any pair of intercepts taken from these two groups,
is significant.
The validity of the deductions from Fig. 4 is therefore confirmed. In NYM
during the period of the investigation, for any given length, the introduced
tilapias had a heavier body than the endemic species. This is probably a
taxonomic difference reflecting the rather deep body of T. rendalli and the
plump body of S. esculentus.
Reproduction
In 1974, the sexes were separated externally by the form of the genital
papilla in most fish above 10 cm in length. No marked inequalities were
apparent in the sex ratio of the full tilapia samples; males were slightly more
numerous in three species and females in the fourth. Among larger tilapias
however, 8047% of S. jipe and S.pangani greater than TL 25 cm and virtually
all fish above TL 30 cm, were males (Fig. 3).
Lowe (1955), in samples from Lake Jipe (mostly S. girigan with some S. jipe,
TL 16-32 cm) and the nearby Taveta Fish Ponds in Kenya (S. girigan and
S. jipe, TL 1 4 2 8 cm), found an unequal sex ratio in favour of males, (61 and
88% respectively). Males grew faster than females, but the latter began to
spawn at a smaller size. She questioned how far the better growth of males was
FISH ECOLOGY OF NYUMBA YA MUNCU
125
Total length (cm)
Figure 4. The length- weight relationships of NYM tilapias. Curves fitted by eye (see text). a, S.
jipe and S.pangani; 0,S. esculenrus and T. rendalli.
an inherent character of these species and how much it may be influenced by
the inequality of the sex ratio, giving the more numerous males more energy
for growth than the females who may be constantly engaged in reproduction.
Differences in the sex ratio of S. jipe and S. pangani in N Y M favoured males
but to a much lesser extent than the populations examined by Lowe, except
among larger fish. In the experience of fishermen at NYM, large fish are
invariably males, and moreover, males in a sexually non-active state. In every
case where gonads of large males were investigated in this study, they were
found to be in condition 11-111. Thus, it appears that a proportion of the male
population of S. jipe and S. panguni have outgrown the females by putting their
energy into growth rather than reproduction.
The gonad states of fishes from experimental fishing are given in Table 4,
together with data derived from Okorie (1974). In the months covered by this
investigation very few of the indigenous tilapias were ripe. The highest number
occurred in June 1972 with considerably fewer in April 1973 and none at all in
July and August 1974. On commercial landing beaches in the latter period
R. G. BAILEY ET AL.
126
Table 4. The gonad states of tilapias caught in experimental gill nets (40100 mm mesh sizes) in various months during 1972-74
Species
date
n
S. jipe and S. pangani
6/72
126
7/72.
154
1/73.
68
4/73,
115
S. esculentus
4/73
171
7-8/74
273
4/73
32
7-8/74
66
~
~
_
_
_
_
GS
1-111
IV
V
VI
98
289
-
143
5d, 29
Id, 29
l?
34
71
13d, 119 3013.119
2d, 59
3d
3d
168
39
-
273
-
16
12d,39
65
-
19
-
-
Id
*, Data derived from Okorie 1972,1974.
however, some ripe females were seen. A small percentage of Okorie’s July
sample in 1972 was in state IV or above, and substantially more, 50% and 38%,
in January and April 1973 respectively.
Under natural conditions most species of Sarotherodon are seasonal
breeders, but in some breeding may occur all the year round (Fryer & Iles,
1969). Where seasonal breeding has been established it is usually positively
related to the wet period of the year when river and lake levels rise. No clear
correlation is apparent between the incidence of mature S. jipe and S. pangani
and the rise in lake level following the long rains at NYM.Okorie’s results for
April 1973 conflict with those for the same month from this investigation
although his results for January 1973 are indicative of increased breeding
activity following the short rains in November 1972. Examined together,
however, the available data for NYM suggest that these species spawn
throughout most of the year, but information for the late dry season,
September-October, is lacking.
From the small samples of ripe females obtained in the reservoir the
minimum and modal sizes at maturity for both S. jipe and S. pangani were TL
20.0 cm and TL 22.5-25.0 cm respectively, compared with the equivalent
sizes derived from Okorie (1974) of TL 17.0 cm and TL 21.0-23.0 cm. A single
mature female of S. jipe from the River Ruvu had TL 18.0cm. In her
observations on Lake Jipe and pond populations of S. jipe and S. girigan Lowe
(1955), found the minimum breeding size of females to range from T L
15.0-17.0 cm, while males were consistently larger at TL 21.0 cm.
Ripe males and females of S. esculentus were found at the onset of the rains
in April 1973, but none in a larger sample examined in July and August 1974
(Table 4). These limited observations are consistent with the breeding
periodicity of Lake Victoria populations in which spawning coincides with the
wet season (Lowe, 1956). The minimum and modal ranges in size at maturity
for both sexes were TL 17.0cm and TL 17.0-18.5 cm, compared with a
minimum size of about 20.0 cm before breeding commences in Lake Victoria.
Only eight adults of T. rendulli were caught at NYM and none were mature.
The numbers of ova found in both ovaries from small samples of
Sarotherodon species in the reservoir, with GS 111 or IV, are given in Table 5 .
Compared with these data, rather fewer ripe eggs (200-250) were found in the
ovaries of small S. jipe (TL 15.0-21.0 cm) examined by Lowe (1955).
The form of the nest depressions of S. pangani and possibly S. jipe were
FISH ECOLOGY OF NYUMBA Y A MUNGU
127
Table 5. Egg counts for three species of Sarotherodon in Nyumba ya Mungu
reservoir
n
Species
-
111
IV
S. pangani
S. escirlen tiis
mean
____
~~
S. jipe
T L (cm)
IV
Ill
IV
7
3
2
1
2
18.0-27.5
23.5-24.5
20.0-26.0
17.5
17.0-18.5
1372
484
718
1180
480
Egg counts
range
1100-1984
43 7- 570
446-990
-
424-536
<2.0
>2.0
>1.4
<1.0
>1.0
described by Lowe (1955). Unfortunately no nests were observed in the course
of this investigation and there are no documented records of the location of the
spawning grounds in the reservoir. It is however probable that the shallow
northern part of the lake has been and continues to be of major importance in
this respect. Fishermen stated that the areas of water which remain open off
Mikocheni (see Fig. 1 ) are used for nesting sites and this is given strong support
by the occurrence of large numbers of juvenile S. jipe and S. pangani which
were seen, and on one occasion caught by seining close inshore, in this area of
the lake.
Food. The occurrence of different foods and extraneous material in the gut
contents of 85 adult tilapias is summarised on a percentage basis in Fig. 2.
Larger samples of the endemic species were examined since there are few
published data on their dietary preferences.
As a group adult tilapias in the reservoir are primarily herbivorous with a diet
in which microscopic algae form a major part. Predominant among these were
the diatoms, Cyclotella, Melosira, Nitzschia and Synedra and the blue-green
alga, Microcystis, all of which occurred both in the planktonic and periphytic
associations investigated in the lake.
So far as the endemic species, S. jipe and S. pangani were concerned, a
detailed study of the algal content of gut samples (reported upon separately by
Denny, Bowker & Bailey, 1978), the occasional occurrence of fragments of
macrophyte tissue (notably in S. pangani), together with their essentially
littorine distribution, indicates that they are chiefly browsers of periphyton in
the lake; whereas the presence of detritus and sand grains in samples from the
inflow stations, suggests that in these areas they feed additionally on bottom
deposits and associated phytobenthos. Largely on the basis of their possession
of a dense felt of pharyngeal teeth, Lowe (1955) deduced that S. jipe fed on
fine particulate matter, probably algae, and Bailey (1965) reported that
S. pangani which has slightly coarser phawngeal teeth, fed upon diatoms,
filamentous algae and macrophyte tissue in the River Rum.
Gut contents from samples of S. esculentus caught in the vicinity of the
dam, were made up almost entirely of algae, dominated with the exception of
Synedra, by the genera noted above. From the absence of macrophyte, detrital
and inorganic material it may be concluded that these were taken from the
plankton, (see also Denny, Bowker & Bailey, 1978). Thus, as Graham (1929)
and several later workers showed for Lake Victoria populations, adults of
S. esculentus in the reservoir may be regarded as feeding more or less
128
R. G. BAILEY ET AL.
exclusively on phytoplankton. Guts examined from a small sample of Tilapia
rendalli were filled with the macerated remains of aquatic vascular plants
together with associated periphytic algae. Chironomid larvae and fish remains,
including vertebrae and both ctenoid and cycloid scales, were also noted. The
ability of this species t o feed on macrophytes is well known, and for this reason
its use in pond and dam stocking has been widely advocated.
Ecology of the fish community
Distribution-some quantitative data
The results of experimental fishing with the fleet of gill nets provides some
information on the standing crop in different parts of the lake.
In Table 6 data from 24 hour fishing periods at Damsite in 1972, 1973 and
1974 are compared. The same species were caught on all three occasions with
two exceptions in 1973-74,-the omission of Barbus oxyrhynchus and the
appearance of S. esculentus. The latter reflects a recent expansion of this
species in the lake, but in spite of its introduction into the catch, there is a
marked decline in the numbers and biomass of all tilapias since 1972.
Numercially this is more than offset in the total catch by a striking increase of
the small fishes, Rhabdalestes and Haplochromis. In terms of biomass, however,
the total for 1974 is less than one third of the figure for 1972. Moreover the
average weight of all tilapias in the experimental catches has dropped
considerably; 234 g-1972, 130 g-1973 and 75 g-1974.
There are obvious limits to comparisons of these data, which were derived
from single 24 hour fishing periods at different times of the year and using
fleets of nets differing slightly in composition and considerably in their
positioning. At the same time it is reasonable to comment that the decrease in
tilapias parallels the decline in yield which has occurred in the commercial
fishery. Results of experimental fishing at 4 other locations are given in
Table 7. The data from Mikocheni and the inflow stations demonstrate the
importance of tilapias, especially S. jipe and S. pangani at the shallow northern
end of NYM. Higher biomasses of tilapia were obtained in a few hours of
day-time fishing than over 24 hours at Damsite. At these places, tilapias
contributed from 66 to 79% of the total biomass. However, it must be noted
that catfishes, which appear in the commercial fishery, were not caught in the
experimental nets, probably as a result of their nocturnal habits.
The poor catch from the limnetic zone contrasts with all other areas. In
addition to the experimental fleet arid during the hours of darkness, a strong
torch was fixed above a 20 mm gill net, rigged to hang vertically in the water to
a depth of about 15 m. A single Haplochromis was caught.
Feeding interrelationships
The diets of fish in NYM are now for the most part, adequately known, and
a simplified food web has been constructed in Fig. 5 .
Among primary producers, three associations of algae may be recognised,
phytoplankton, periphyton and phytobenthos. A high density of phytoplankton was found at NYM. Together with bacteria and particulate organic
matter, phytoplanktonic algae are grazed by zooplankton, and their importance
in the diet of S. esculentus and to a lesser extent the endemic tilapias, has been
Table 6. Total numbers and biomass from 24 hours experimental fishing at Damsite in 1972, 1973 and 1974.
?:
Date
location
no.
Sarotherodon jipe
S. pangani
S. esculen ms
Haplochromis sp.
Rhabdalestes sp.
Barbus oxyrliynchus
Barbus Spp.
Labeo sp.
Synodontis sp.
Clarias sp.
Totals
Nets applied
i’
120
2-3 June 1972
4-5 April 1973
26-27 July 1974
Fisheries Office Bay
From intake tower-westwards
Off dam wall
%
no.
%
30.38
28100.0
87
32
3
47
41
64
1
22.03
8.10
0.76
11.90
10.38
16.20
0.25
309.9
126.0
279.2
214.7
3670.1
3376.4
244.5
0.83
0.35
0.77
0.59
10.11
9.30
0.67
395
100.00
36312.8
100.00
77.38
-
-
20,40, 60,80, 100 mm
.f
%
%
0.97
4.38
7.79
20.92
22.63
194.0
4346.0
2511.4
295.5
426.9
122
9
45
2
29.68
2.19
10.95
0.49
669.6
1210.4
2590.1
1692.8
411
100.00
13936.7
4
18
32
86
93
-
-
-
20.40, 50, 70, 100mm
no.
%
Wf
(g)
v)
2
%
13
20
11
444
547
1.15
1.76
0.96
39.15
48.24
1053.0
1313.0
938.0
2200.0
2010.0
9.99
12.45
8.90
20.87
19.07
4.80
8.68
18.58
12.15
7
63
19
0.62
5.56
1.68
0.88
47.0
370.0
1115.0
1495.0
0.45
3.51
10.58
14.18
100.00
1134
100.00
10541.0
100.00
1.39
31.18
18.02
2.12
3.06
-
-
10
-
-
20,40,60,80,
100 mm
-
c.
W
0
Table 7. Total numbers and biomass from experimental fishing (20, 40, 60, 80 and 100 mesh) at four localities
Date
Location
Duration
20-21 August 1974
Open water, southern end
24 hours
no.
Sarotherodon jipe
S.pangani
S.esculentus
T. rendalli
Haplochromis sp.
Rhabdalestes sp.
Barbus oxyrhynchus
Barbus spp.
Labeo sp.
Garra sp.
Synodontis sp.
Clarias sp.
Totals
%
wt (g)
%
-
11
69
-
1
-
10
-
93
2.15
-
11.83
74.19
-
1.08
-
135
-
30
300
-
5
-
-
28 August 1974
Mikocheni
12.00-18.00 hrs
Korogwe
no.
%
wt (g)
46
18
3038
517
2.65
-
11.47
4.49
3.97
39.74
3
173
141
0.75
43.14
35.16
130
1025
645
2
6 August 1974
-
0.66
-
-
6
14
-
-
10.0~12.00hrs
%
54.49
9.27
-
2.33
18.39
11.57
-
-
-
1.50
3.49
30
190
0.54
3.41
no.
%
53
11
20.85
4.42
-
6
96
2
18
64
10.75
400
52.98
-
-
-
-
-
-
100.00
870
100.00
401
100.00
5575
100.00
249
-
-
26 August 1974
Samanga
16.00-18.00 hrs
-
2.41
38.55
0.80
7.23
25.70
-
100.00
(g)
96
no.
o/;
2370
1115
50.80
23.90
19
12
14.29
9.02
Wt
-
45
430
15
95
595
-
-
0.96
9.22
0.32
2.04
12.75
-
32
-
-
24.06
-
2
3
64
1
1.50
2.26
48.12
0.75
133
100.00
Wt
(g)
3295
540
%
67.94
11.13
-
-
-
-
1
r
130
2.68
<
-
7
10
865
3
-
0.14
0.21
17.84
0.06
-
4665 100.00
P
.0
4850 100.00
rn
h
g
FISH ECOLOGY OF NYUMBA YA MUNGU
131
ddestes
1
Periphyton
I/
I
Barbus spp
Mocrophytes
\Fphemeroptero
Trichopterq
'.
1
Hemiptero
* Synodantis
Phytobenthos
Labeo
Organic detritus
Figure 5. A simplified food-web,for NYM.
demonstrated. Algae living on soft bottom deposits, which in some cases may
have sedimented out of the plankton, make up the phytobenthos, which is
consumed by bottom dwelling invertebrates and fish.
The richest source of food for primary consumers in NYM, however, appears
to be provided by periphyton. This comprises assemblages of larger filamentous
and microscopic algae associated with the solid surfaces of stones, macrophytes
and submerged trees. Compared with population densities of open water
plankton, ranging from 12,000 to 19,000 individuals ~ m - a~ dense
,
growth of
periphyton, 1 cm thick and attached to 1 cm2 of substratum, could constitute
a population density of up to 2,000,000 individuals ~ m - over
~ , 100 times richer
than the plankton (Bowker, pers. comm.). At NYM the most abundant and
ubiquitous littoral surfaces for the attachment and production of periphyton
were the submerged parts of emergent aquatic plants and branches of
inundated trees. When the lake level falls during the dry season, exposed algal
communities are destroyed but their redevelopment when it rises again after
the rains, was found to be rapid, in the order of 0.34-0.82 g organic weight mw2
lake bottom day-'. In this connection it may be noted that underwater forests
in deeper water provide a more permanent supply of surfaces for periphytic
production than marginal reed swamps. Submerged trees are a feature common
to many tropical man-made lakes, and their importance in providing a large
surface area for the establishment and growth of periphytes, and hence food
for herbivorous invertebrates and fish, has recently been reviewed by Petr
(1975b). The results of this investigation suggest that while periphytes are
included in the diet of several species of fish, they are of special importance to
the principal, commercially exploited tilapias, S. jipe and S. pangani, (Denny,
Bowker & Bailey, 1978).
Macrophytes form a minor part of the diet of several fish, including
catfishes, but they are probably only significant in the case of Tilapia rendalli
R.
132
G.
BAILEY ET A t .
and Barbus species. A recurrence of seeds in the guts of Barbus was noted and
where the outer casing was broken, digestion appeared to have begun.
Macroinvertebrates, especially aquatic insects were shown to be heavily
exploited by the omnivores, Synodontis and Barbus species, and the large
numbers of small carnivores Haplochromis and Rhabdalestes. The greatest
diversity of invertebrates was found at the northern end of the lake, (Bailey,
Churchfield & Pimm, 1978) and this was reflected in the results of gut analyses.
Except in northern lagoons and inflow channels, zooplankton was relatively
abundant (Bailey et al., 1978), and whilst a fully planktivorous predator was
absent, planktonic crustaceans were included in the diet of Haplochromis,
Rhabdalestes and Barbus paludinosus from south lake. Cladocerans were
recorded with greater frequency than cyclopoid copepods in spite of the fact that
they were less abundant in the environment. The tendency of planktivorous
fish to select cladocerans (and calanoid copepods when present) in preference
to cyclopoids, has been demonstrated in many temperate and tropical habitats
(for examples, see Brooks & Dodson, 1965 and Green, 1967).
Small fish are preyed upon to some extent by large Haplochromis, Barbus
oxyrhynchus and B. jacksonii, but only Clarias mossambicus could be said to
nearly approach the status of a top carnivore. It is the absence of a fully
piscivorous fish which provides the most striking omission from the food web
at NYM. However, none are recorded in the Pangani basin which unlike the
eastward flowing rivers t o the south, does not contain the tiger-fish,
Hydrocynus vittatus (Bailey, 1969).
In Table 8 the data from 24 hour catches at Damsite have been used to show
the proportions of the major feeding categories represented. Significant changes
in the trophic structure become apparent during the years covered by this
investigation some of which, for example in the ratio of herbivores to
carnivores, undoubtedly reflect real alterations in the balance of the fish
community.
Parasites
The pericardial cavities of cichlids in NYM commonly contained larvae of
the nematode, Contracaecum. The incidence of infection in a sample of 184
tilapias, including all three species of Sarotherodon was 70.6%. The mean
number of worms per fish was 4 and the highest individual count was 14.
Okorie (1974) also observed the high incidence of parasitism of tilapias by
nematodes, in NYM. Haplochromis was less frequently parasitised, nematodes
Table 8. Changes in the trophic structure of the fish community at Damsite,
1972-74. (Obtained from the conversion of the data in Table 6, where
1 = tilapias, 2 = Barbus spp, Synodontis and bottom feeder Labeo, and
3 = Rhabdalestes, Haptochromis and Clarias)
% biomass
% numbers
1
2
3
Year
1972
1973
1974
1972
1973
1974
Herbivores
Omnivores
Carnivores
30.4
39.2
30.4
13.0
42.8
44.0
3.0
7.8
88.3
77.4
20.8
1.8
50.6
32.1
17.3
31.4
14.5
54.1
F I S H ECOLOGY OF NYUMBA Y A MUNGU
133
occurring in only 13.8% of 116 pericardial cavities examined. Tilapias are
presumably infected as juveniles when planktonic crustaceans, which act as the
first host, may be included in the diet. The parasite’s life cycle is completed in
fish-eating birds in which two further moults ensue before the adult condition
is attained (Khalil, per. comm.). Larvae and immature adults of Cvntracaecirm
species including C. microcephalus, were found in the guts of the white pelican
(Pelicanus onocrotaltis) and the white-necked and long-tailed cormorants
(Phalacrocorax carbo and P. africanus).
Metacercariae of the trematode, Clinostomum, were occasionally found in
the branchial region of Sarotherodon species and immature adults were
identified in the guts of the white-necked cormorant and fish eagle, (Cztncurna
vocifer). Gastropods provide the first host environment for flukes.
Predators
Crocodiles were common and a wide diversity of fish-eating birds, including
kingfishers, cormorants, herons, the white pelican and fish eagle, were observed
perched upon the emergent branches of drowned trees, often in large numbers.
The gut contents of four, freshly-killed birds were examined and the remains
of S. jipe or S. pangani (TL 12.0-16.0 cm) were found in three of them, the
white-necked cormorant, fish eagle and pelican. In addition 1 5 Haplochromis
(TL 7.0-10.0cm) were found in the pelican and two (TL 8.0cm) in the
long-tailed cormorant. From these limited data it appears that cichlids,
including tilapias, are important in the diet of the larger fish-eating birds at
NYM. Cott (1952) found that whereas Haplochromis species were commonly
eaten by some birds on Lake Victoria, including the cormorants, tilapias were
rarely encountered in their diet.
Breediug ecology
Tilapias require relatively soft bottom deposits beneath quiet, shallow water,
in which to excavate their nests and they utilise marginal weed beds as nursery
areas for their young. We have no direct observations on the location of the
main spawning areas for tilapias, but the available evidence suggests that these
were not in short supply during the early development of NYM and that the
endemic species and Haplochromis, may breed throughout most of the year.
Since the lake was filled however there has been a massive encroachment of
dense beds of emergent macrophytes into and spreading from the inflow deltas
(Welsh & Denny, 1978) and from 1972 to 1974, not withstanding small
biannual fluctuations, there has been an overall drop in the level of the lake
(Denny, 1978). Both of these events have had their greatest effect across the
shallow northern region of the lake where large areas of what were probably
favourable spawning and nursery grounds have been lost by becoming
overgrown, or by exposure.
Amongst other fishes, it appears that Synodontis punctulatus and
Rhabdalestes leleupi have also been favoured by lacustrine conditions. An
extended breeding season for the latter species was demonstrated. In contrast,
the breeding of Barbus species, Labeo forskalii and Clarias mossambicus
probably remains associated with river and stream regimes. These are essentially
anadromous fishes migrating into the inflows and peripheral streams when they
become swollen during the rains. These investigations were carried out chiefly
5
R. G. BAILEY ET AL.
134
in dry periods but support for the above supposition is afforded by the
distribution of these species in the lake, the location of fish traps in inflow
channels, and statements by Fisheries Department staff.
Commercial fishing
Man has had a considerable impact on the ecology of the fish community.
Within four years of the closure of the dam a flourishing tilapia fishery was
established. Annual yields from the Tanzanian Fisheries Department show a
peak of 28,508tonnes in 1970, falling to 2282 tonnes in 1974. Whilst there are
grounds for questioning the accuracy of these computations, it is clear that a
drastic decline in the fishery occurred and that this was initiated and
accelerated by heavy overfishing. During 1970.71, the lake was subjected to an
extraordinarily high fishing pressure in relation to its size. Large numbers of
itinerant fishermen were drawn to it and 26 fishing settlements sprang up
around its perimeter. In 1973-74 however, as a result of falling catches an
exodus had commenced.
An impairment of recruitment is a major facet of overfishing, thus in 1974,
the size of tilapias in commercial catches was analysed from random samples
taken from gill-net, seine-net and rod and line fishermen. The results are
summarised in Table 9. A substantial proportion of the crop of endemic species
comprises fish of less than TL 20 cm, the majority of which are unlikely to
have begun breeding, and Sarotherodon esculentus is most heavily exploited in
the size range within which breeding occurs. Two thirds of the small sample of
Tilapia rendalli were less than TL 15.0 cm and it is probable that these were
immature virgin fish.
To the removal of under-sized fish must be added the detrimental effects of
beach-seining and the practice of driving fish into gill nets, both of which were
observed in probable spawning areas. Seine nets may physically destroy nest
sites and both methods are liable to disturb breeding behaviour and result in
the capture of mouth brooding females.
Apart from the tilapias, Labeo and Clarias are also exploited mainly in
inflow deltas and during 1974 some lake fishermen had begun to fish for the
less profitable catfish, Synodontis. However, the fall in the total biomass
caught by experimental fishing and the change in the trophic structure of the
Table 9. Size selection of tilapias by commercial fishing. (Based upon the
percentages of the total samples of commercially caught fish falling into each
5 cm division of the length range)
Species
n
S. jipe
415
S. pangani
S. esculentus
T.rendalli
All species
176
230
92
913
15.4
42.9
23.9
7.7
5.5
2.2
1.4
1.o
4.5
48.3
34.1
6.3
3.4
2.8
0.6
2.6
67.8
29.1
0.4
67.4
28.3
4.3
15.3
48.7
25.2
4.8
3.2
1.5
0.8
0.4
TL divisions
1o+
15+
20+
25+
30+
3 5+
40+
4 5+
FISH ECOLOGY OF NYUMBA Y A MUNGU
135
fish community observed at Damsite during these investigations may be
attributed primarily to the heavy selective exploitation of tilapia stocks.
Jackson (1966) elaborated upon the positive and negative aspects of the
fishery potential of new river lakes in the tropics. The extent to which these
have been realised in NYM is discussed elsewhere (Bailey & Denny, 1978).
SUMMARY
An investigation of the fish community in Nyumba ya Mungu reservoir, the
largest man-made lake in East Africa, was carried out in the period 1972-1974.
Twenty species of fish were recorded, all but two of which, Sarotherodon
esculentus and Tilapia rendalli, are indigenous to the Pangani basin.
Among the fish present, Mormyrus kannume, Chiloglanis deckenii, Garra
dembeensis and Barbus kerstenii kerstenii were confined to the inflows and
Clarias mossambicus and Labeo forskalii were more abundant in the shallow
northern region of the lake and the inflow deltas. High numbers of
Rhabdalestes leleupi and Haplochromis gr. bloyeti were frequently obtained
around the lake’s perimeter and together with Synodontis punctulatus and
Sarotherodon esculentus, they were the main species to occupy the limnetic
zone. Sarotherodon jipe and S. pangani were widely distributed, but in the
deeper southern part of the lake, they chiefly inhabited littoral areas.
Sarotherodon esculentus and T. rendalli are recent arrivals and their first
recorded appearances came in the course of this study. In 1974, S. jipe was the
most abundant of the tilapias, comprising 45% of the total sample, followed by
S. pangani-27%, S. esculentus-21% and T. rendalli-7%. Catches from experimental gill-net fishing near the dam in 1972, 1973 and 1974 were compared.
They show a progressive decline in the total number and biomass of tilapias and
a striking increase of Rhabdalestes and Haplochromis. The shallow northern
and inflow areas yielded much higher biomasses of tilapia than were found at
Damsite. Very poor catches were had in the limnetic zone of south lake and
fishing with gill nets in the deepest water was prevented by the presence of
submerged trees.
Most species of fish in NYM are small, only Clarias. Anguilla nebtilosa labiata
and the tilapias commonly exceeded a total length of 20 cm. Of the latter,
S. jipe and S. pangani grow to a larger size than S. esculentus and T. rendalli,
and the indigenous species over 30 cm in length, were invariably males in a
sexually non-active state. The introduced species however, were heavier for
their length than the endemic forms.
From an investigation of the diets of fishes the feeding interrelationships
in the reservoir were elucidated. Tilapias are herbivores and those in NYM were
shown to have complementary feeding habits so far as the available food plants
were concerned. Sarotherodon esculentus feeds on phytoplankton, the endemic
species chiefly graze periphytic algae and T. rendalli consumes both macrophytes and the periphyton associated with them. Benthic algae and organic
detritus provide Lubeo with food. Zooplankton, especially Cladocera, is eaten
by Haplochromis, Rhabdalestes and Barbus paludinosus, but both
Haplochromis and Rhabdalestes exhibited a preference for larger invertbrates,
notably chironomid larvae and pupae and mayfly nymphs, whilst
B. puludinosus had a mixed diet. Macroinvertebrates also form an important
136
R. G. BAILEY ET AL.
part of the diet of Clarias and the omnivores, Synodontis, Barhtts oxyrhynchus,
B. lineomaculatus, and B. jacksonii.
Fish themselves are preyed upon to some extent by Barbus species, large
Haplochromis, and Clarias. Of these, Clarias is the top carnivore, but a fully
piscivorous fish is lacking. An analysis of the gut contents of a few larger birds
however, indicated that the cormorants, pelican and fish eagle, may be
important predators of fish, including the tilapias, at NYM.
Tilapias were shown to be heavily parasitised by nematodes (Contracaecum
spp.) which inhabit the pericardial cavity.
The gonad states of fishes were analysed and egg counts were made for some
species. The lacustrine environment has favoured the spawning of Rhabdalestes,
Synodontis, Haplochromis and Sarotherodon spp. but the breeding of Clarias,
Labeo, and probably Barbus spp. remains associated with access to affluent
rivers and streams during the rainy season.
I t is suggested that since the lake filled, tilapia spawning grounds have been
reduced by an encroachment of swamp and low lake levels in 1972-74. This,
combined with a heavy commercial exploitation (involving in 1974 at least, the
removal of under-sized fish), has impaired recruitment to tilapia stocks and has
contributed to the change in the balance of this fish community observed at
Damsite.
ACKNOWLEDGEMENTS
We are extremely grateful for the generous financial help and co-operation of
many organizations without which this investigation would have been
impossible. A full acknowledgement of these bodies is given elsewhere (Denny
& Bailey, 1978). However, we would especially like to thank Mr M. Kayuza,
Regional Fisheries Officer, Moshi, his staff and students at NYM, for their
enthusiastic assistance in the field. We are also indebted to Dr E. Trewavas
(British Museum, Natural History), Dr L. F. Khalil (Commonwealth Institute of
Helminthology), Mr J. Martin and Mr A. Dearsley (Chelsea College) for their
advice or comments on sections of the manuscript, and to Miss H. Haworth,
our typist.
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