Initial findings on the distribution and abundance of two dipteran larvae in an ecologically and environmentally changed northern Lake Victoria are presented Results suggest that density of Chironomid larvae seem to have gone up about four-fold since the pre-perch era and t hat of chaoborid larvae maintained at more or less the same level. Possible reasons for this increase are proposed. Inshore stations held higher densities of larvae when compared with the offshore station of Bugaia

Mbahinzireki, G.

English

Aquatic Commons

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DISTRIBUTION AND ABUNDANCE OF TWO DIPTERAN C 
LARVAE IN NORTHERN LAKE VICTORIA 
,.. 
ABSTRACT 
Initial findings on the distribution and abundance of two dipteran 
larvae in an ecologically and environmentally northern 
Lake Victoria are presented Results suggest that density of 
Chironomid larvae seem to have gone up about four- fold since the 
pre-perch era and t hat of chaoborid larvae maintained at more or 
less the same level. Possible reasons for this increase are 
proposed. Inshore stations held higher densities of larvae when 
compared with the offshore station of Bugaia. Type of the sediment 
and physico-chemical factors seem to influence production and 
distribution of the insect larvae in the lake. Body length did not 
suggest anything conclusive yet as the identity of the individual 
larval groupsis still unclear. More work remains to be done. 
INTRODUCTION 
Recent studies on the benthic fauna of the northern Lake Victoria 
ave revealed the existance of Chironomid and Chaoborid larvae in 
substantial quantities. The two groups of insect larvae were singled 
of many other zoobenthos that are found in the lake because they 
most common, abundant, (Mbahinzireki, 1991) and important in the 
Iisheries. Examination of the gut contents of many fish specimens 
the lake shows that larvae of insect groups constitute 
a 3igni ficant proportion of the total food items 
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Therefore, like many other aquatic invertebrates, the larvae are 
important in the foodweb the lakes ecosystem. They are also 
important converters of organic materials into animal protein for 
use by organisms at higher trophic levels including fish. 
Despite the demonstrated importance of these invertebrates, 
there are hardly any previous studies on them to compare with. 
The information available on work by Macdonald (1956) and Tjonneland 
(1962) mainly refers to the biology and flight activities, respectively, 
of the African midge. Yet, the current information refers to studies 
conducted on the lake long after it has undergone some environmental 
and ecological changes 
The introduction of the exotic fish species, namely the Nile perch 
and Nile tilapia in L.Victoria more than two decades ago has given 
rise to other ecological changes in the lake. These include reduction 
in fish stocks, fish species diversity and trophic groups. As a result 
most of the native fish species have declined to near extinction 
(Ogutu-Ohwayo 1990). 
Furthermore, the lake basin itself is undergoing considerable 
changes as a result of increasing human population, hence increased 
human activity in form of land and water use for agriculture, 
recreation, urbanisation and industrialisation. There is some 
evidence to suggest that the lake is becoming eutrophied as indicated 
by frequent algal blooms, localised fish kills, anoxia in bottom 
waters and high nutrient input through effluents and precipitation 
(Reeky & Bugenyi, 1989). All these activities are having great 
on the functioning of the ecosystem and investigations 
carried out on the lake have to take into account of such dynamic 
changes 
3
 
The present paper attempts to provide some information on the 
present status regarding distribution and abundance of Chironomid and 
Chaoborid larvae in the cha nging ecosystem of L.Victoria. 
MATERIALS AND METHODS 
(a)	 Sampling 
Most of the inshore stations (10-28m) of the lake, namely Napoleon 
Gulf, Buvuma Channel and Pilkington Bay and, the offshore deep 
station of Bugaia (60-65m), Fig.1, were sampled at least once 
a month. Sediment samples were collected using the Ponar Grab, 
whose grabbing surface area is 262.5 square cm. 
At each station, the following activities were carried out: 
(i)	 Measurement of the lake depth using the Hydrolab or simply by 
sounding; 
ii) Measurement of temperature and dissolved with the Hydrolab; 
iii) Collecting two samples of t he bottom sediment, noting the nature 
and type of the sediment; 
iv) Washing the sediment in a sievenet of 400 Urn mesh size; 
v) Preservation of the recovered animal specimens in formalin . 
(b)	 Treatment of the sample in the laboratory. 
The animals were sorted by taxon and identified as far as possible 
using a stereo microscope of X30 magnific a tion. All the specimens 
of each taxon were counted to compute their densities and total 
length of Chironomid larvae from each sample was measured on a 
millimetre graph paper. Ea ch taxon was finally stored separately 
in formalin. 
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RESULTS' 
Tables 1 and 2 present densities of Chironomid and Chaoborid larvae 
per square metre, respectively. The results so far show that the 
larval organisms are well represented in almost every sediment 
sample taken from the various stations. Densities of Chironomid 
2fluctuated and 4600 individuals per m in the 
inshore lake sediment and, between around 20 and 250 individuals 
per square metre, exceeding 500 larvae per square metre in 
the offshore station. A high density of 2057 was recorded in May, 
91 in Bugaia. Chaoborid larvae densities varied between 38 and 
around 1600 individuals per metre squared in inshore stations; 
highe s t record of 3296 being registered in July, 90, in Buvuma 
Channel. Like in the case of Chironomid larvae, offshore waters of 
Bugaia registered the lowest amount of Chaoborid larvae, between 19 
and 100, rarely exceeding 150 larvae per square metre. 
The tables also show that there are several months which were 
not sampled. 
Figure 2 shows fivej raPhs of the body lengths of Chironomid 
larvae collected from Pilkington Bay during certain months of 1991. 
In anyone sample taken and at anyone time, almost all the size 
groups were well represented . Most of their total length sizes 
ranged between 3 and 16mm. 
. .• /5 ••
 
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5
 
DISCUSSION 
The initial research findings show that substantial quantities of 
Chironomid and Chaoborid larvae abound mostly in the inshore sediments 
of the northern lake. The two groups of dipteran larvae are so common 
that they are found in almost every sample of sediment taken at anyone 
time. Their densities fluctuate within certain limits whose levels and 
influencing factors are yet to be determined 
Inshore lake bottom is richer in larvae Compared with the deep 
offshore station of Bugaia. The tables also seem to suggest that the 
period from the month of to August, a period of complete mi xing 
yields high of these larvae and get poorer towards the end 
of the year. More investig3tions are needed in order to confirm the 
assertion. 
There are a few evidences to suggest that the two insect larvae, 
particularly those of Chironomid have increased in de nsities over time. 
Macdonald (1956) reports that he estimated an average density of 1000 
and 2000 - 2500 of Chr ronomid and Chaoborus larvae, respectively per 
square metre in Ekunu Bay (Lingira bay) . Chironomid larvae are 
presently more abundant than the Chaoborid larvae which become plank­
tonic during the night . Analysis of stomach contents of the catfish, 
Synodontis afrofischeri, caught in the Napoleon Gulf last year revealed 
that over of the food content was contributed by larvae and pupae 
(10%) of Chironomidae. The Nile tilapia, a well - known phytodetritus 
feeder is presently reported to in- corpora te a good amount of Chironnomid 
larvae in its diet (Balirwa, 1989) 
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Further evidence is given by the frequent masses of lakeflies which 
are over t he lake, a phenomenon which was rare in the 
past. 
The increase in insect larvae may partly be explained by a few 
ecological changes that have t aken place in the lake in the recent 
past. The decline in the cichlid haplochromines, some of which were 
insectivorous and , other carnivorous catfishes has also led to the 
reduction of predation pressure. Haplochromines used to constitute a 
large proportion of lakes ichthyomass (Kudhongania and Cordone, 
Although the juvenile Nile perch often feed on these insect 
larvae, the bulk of their diet is mainly Caridina nilotica as shown 
by examined stomach contents. Yet, to date over 70% of the lake's 
ichthyomass is Lates nilo t i cus . It c a n also be argued that increased 
phytobiomass and that of Zooplankton has favoured increased production 
of the larvae as most of them are carnivores and detritivores . However, 
increased anoxic conditions of the deeper layers of lake may have 
affected the biomass of Chaoborid larvae which are to low 
levels of dissolved Oxygen but with low effects on Chironomid larvae. 
All possible factors the distribution and abundance of 
the benthic fauna are not yet fully investigated and understood. 
However, the type of the bottom sediment seems to affect the distribution 
and abundance of the larvae. For instance, the density of Chironomid 
larvae was generally low in hard rocky bottom , gravel and compact sandy 
bottom, higher in loose sandy bottom and, far higher in soft The 
bottom substrate of most stations sampled, especially those in bays is 
mostly soft mud. This could account for the observed high quantities 
Chironomid larvae in those areas. Certain localities of the bottom in 
Napoleon Gulf and Buvuma Channel have hard gravel substrata or 
&f~relatively
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sometimes loose sand. The lake bed of Bugaia waters consists of 
hardened sandy rock and poor in benthic fauna. 
On a few occasions sediment samples of this station contained of 
these larvae. This finding may have some implication in the distribution 
of the bottom feeding fish in these waters. Fish may be scarce. 
It should be pointed out that compared to other lakes, like Albert 
and Kyoga, whose benthic fauna are being L. Victoria is by 
far richer in the larvae of the two insects (Mbahinzireki, unpublished 
data). 
Not much can be concluded at this on the various length sizes 
represented in the samples of the Chironomid group. The various size­
classes may be representing particular instars of the v a rious specie s 
or that the group is simply a continuous breeder. This problem can only 
be overcome after the larvae have been identified into their genera/species. 
However, at least 4 to 5 genera of Chironomidae are common, namely, 
Chironomus, Tanypus, Forcipomyia, procladius and Ablabesmyia. There 
could be more genera in the lake as reported by Freeman and Cranston 
(1980). Macdonald (op . cit.) could only recognise two species of 
Chaoborus, C. anomalus and C. species B in Lake Victoria. 
-
More work remains to be done in order to fully understand the 
biology and of the benthic insect fauna and how their distribution 
and abundance could influence the produdtion of fish in the lake. There 
are still gaps to fill especially in respect to production, seasonality 
and taxonomy. Such knowledge would help in formulation of management 
and development policies for the new and altered fisheries of L.Victoria 
mnting
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ACKNOWLEDGEMENT 
My special thanks go to the members of Limnology division of UFFRO, 
especially Messrs J. Were and H. Ochieng for assisting in sampling 
and analysis. I am grateful to Ms Rose Ikanza for word processing 
and, of the manuscript. The project is being carried out 
with the aid of a grant from the International Development Research 
Centre, Ottawa, Canada in conjuction with the Government of Uganda. 
Their financial assistance is hi ghly acknowledged. 
REFERENCES 
Balirwa, J.S., 1989 : The effect of ecological changes in Lake Victoria 
on the present trophic characteristics of Oreochromis 
niloticus in relation to the species role as a stabilising 
factor of biomanipulation. International Symposium on 
Resource use and conserva tion of the African Great Lakes, 
Bujumbura, Burundi, 29 Nov. - 2nd Dec. 1989. 
Freeman, P. and Cranston, P.S., 1980: Catologue of the Diptera of the 
Afrotropical Region. British Museum (Natural History), 
London. P. 175 - 202. R.W. Crosskey (Ed.) 
Hecky, R.E. and Bugenyi, F.W.B., 1989: Hydrology and Chemistry of the 
East African Great Lakes and water quality issues: 
Problems and Solutions. Interna tional symposium on 
"Resource Use and Conservation of the African Great 
Lakes" Bujumbura, Burundi , 29th Nov - 2nd Dec., 1989 
Kudhongania, A.W. & A.J . Cordone, 1974: Batho-spatial distribution 
patterns and biomass estimate of the major demersal 
fishes in Lake Victoria. Afr. J. Trop. Hydrobiol. 
Fish. 3: 15 - 32. 
---------
~tion
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9 
Macdonald, w.w. 1956: on the Biology of 
Chaoborids and Chironomids in Lake Victoria 
and on the feeding habits of the "Elephant 
snout" fish (Mormyrus kannume , Forsk.). J. Anim . 
Ecol 25, 36-53. 
Mbahinzireki, G.B. 1991: Initial Results of the the benthic 
fauna studies in northern Lake Victoria. The 
second Regional workshop on Lake Victoria Fisheries 
Research Project, Kisumu, Kenya, 25-27 Sept., 1991. 
Mbahinzireki, G.B. 1991: Unpublished data. 
Ogutu-Ohwayo, R. 1990: The decline of the native fish species 
of Lakes Victoria and Kyoga (East Africa) and 
the impact of introduced species, especially the 
Nile perch, Lates niloticus and Ni le tilapia, 
Oreochromis niloticus. Environ. Biol.Fish.27 : 
81-96. 
Tjonneland, A. 1962: The nocturnal flight activity and the 
lunar rhythm of emergence in the African midges, 
Conochironomus acutistilus (Freeman). Contr. 
Fac. Sci. Univ. Coll. Addis Ababa Sere C. 
(Zool.), 1-5. 
• • • 
• • 
Table. 1 . 2Densities of Chironomid larvae (individuals/m ) from various stations during 
JAN. FEB. MAR. APR. MAY JUN. JUT,. AUG. OCT. NOV. 
1990 191 •..
­
NAP. 
GULF(12-17m) 
1991 781 • • 1991 1734 943 381 • 572 
1990 • • • • • • 4629 895 
• 
BUVUMA 
1991 • • 591 443 • 
PILKI­
1990 3391 ?10 
• 
BAY(10m) 1991 • • 1962 476 2686 • 
BUGAIA 
1990 • • • • 756 19 248 57 
(60-65m) 
1991 • 
-
2057 172 19 229 
-
114 
• No 
No 
sample taken 
specimen recovered in the 
,1990 
(60-65m) 57

1991
 29
 152
 19
38
 229
 76
 
19
 19
 
No Specimen recovered No sample taken 
." 


Distribution and abundance of two dipteran aquatic larvae in Northern Lake Victoria

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Distribution and abundance of two dipteran aquatic larvae in Northern Lake Victoria

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