Abst,"act Introduction Diversity and abundance of

Uganda Journal a/Agricultural Sciences, 2003, 8: 209 - 220 rSSN 1026-0919 Printed in Uganda © 2003 ationalAgricultural Re earch Organisation

Diversity and abundance of invertebrates in the Victoria basin lakes and their role in fishery production

L. Mweba::a-Ndawula, V. Kigglllldlt alld J¥. Pabire Galldhi Fisheries R ources Research Institute (FIRRI) P.O Box 343, Jinja. Uaanda.

Abst,"act A field study was carried out on zoplankton communities in the Victoria basin lakes (Victoria, Nabugabo lakes, Koki lakes and Wamala) between 1997 and 2001. The objective of the study was to determine the composition, distribution and abundance of zooplankton and their role in fishery production. Field sampling was carried out once every quarter in selected inshore and offshore areas of the lakes. Samples were taken with Nansen type nets 01'0.25 m mouth opening and -0-100 urn mesh opening and a 5-litre Schindler trap. A total of 50 species was recorded within the basin lakes. Rotifers had the ~ighest diversity with 13-22 species. Cyclopoids contained 7, Calanoids 2, and ladocerans 9 species. The most widely distributed species were TlIer/llocyclops lIeglectlls, Moilla micrura and several rotiferan species. Rotifers were numerically most abundant in lakes Mburo, Kaehera, Nabugabo and Wamala while copepods dominated in Vietoria-Sio, Kayugi and Kayanja. However, eopepods had the highest dry weight in all the basin lakes. In Lake Victoria, copepods dominated the communities both inshore and offshore, while rotifers were nearly non-existent offshore. A historical perspective showed higher abundance of Calanoids and Cladoccrans during the 1960s than in the 1990/2000 communities. These changes are believed to be associated with drastic changes in lake trophy and fish faunal composition over the last four decades. Higher zooplankton densities offshore occurred in the Victoria, Nabugabo and Karhera. Species diversity was higher offshol'e in Vicloria-Sio, Nabugabo and Wamala. Seasonal abundance in Lake Victoria showed peak abundance during May, coinciding with the onset ofthe annual mixing period. Copepods dominated in the diets of Rastrilleoboltl argelltea (Mukene) and larval Lates "itotiells (Mputa) while larval cichlids consumed a proportional mix of copepods, Cladocera and aquatic insect larvae. Lakes with high abundance of relatively large-bodied copepods, C1adocerans and aquatic insect larvae are rated high in fishery production than those dominated by small-bodied rotifers. The latter lakes are generally eutrophic (over-fertilised) water bodies, where the invertebrates' fish food environment is no longer favourable for fishery production. There is need to put in place measures to mitigate increasing eutrophication of water bodies within the Victoria basin area in order to ensure stable food environment for sustained fishery production.

Introduction Aquatic invertebrates include all organisms without a backbone. On the basis of size, these organisms are classified as micro-invertebrates or zooplankton (generally < 2mm body length and can not be seen with a naked eye) and macro-invertebrates or macro-benthos (generally> 2mm body length and can be seen with a

naked eye). Zooplankton are minute insect-like organisms living in the water column and are composed of mostly crustaceans in which the key groups are copepods and cladocerans (water fl as). Non-crustacean zooplankton includes rotifers and larvae ofchironomid' and chaoborids (Iakeflies). Macro-invertebrates are generally sediment-dwelling while others live among roots of aquatic macrophytes. They can be categorised

• 210

L. Mwebaza-Ndawula, et al.

as semi-aquatic (e.g. dragonflies and lakeflies), having aquatic developmental stages, but living as terrestrial organisms as adults. Others like gastropods (snails & limpets), pelecypods (bivalves) i.e. clams & mussels, Oligochactae (earth wonns), Hirudinae (leeches) and ostracods (seed shrimps) are exclusively aquatic. Aquatic invertebrates are of economic and biological importance. They, in general, occupy an important position in aquatic food chains, linking algal primary production to higher trophic levels culminating into fish production. All fish larvae, regardless ofspecies must feed on invertebrates particularly zooplankton as the first external food following hatching from eggs. Zooplankton therefore provide high nutrition food supply to fishes at a most critical time in their life history to enhance rapid growth and development prior to switching to other food resources. A number of fishes such as the pelagic cyprinid, Raslrineobola argenlea (Mukene) and several species of Haplochromines (Nkejje), which support important fisheries. rely entirely on a diet of zooplankton throughout tbeir life histories. The freshwater prawn, widely occurring in lakes Victoria and Kyoga are an important food source for thejuveniles and sub-adults of cOl~1mercial.y important Nile perch, Lales nitalieus. Some inverlebrates including Chironornids rotifcrs, oligochaetes etc are useful as bioindicators of water quality. In Lake Victoria, Caridina niloliea and molluscs are commercially harvested, processed and incorporated into domestic animal feeds Others such as leeches and Argulids are parasitic on fish and other a'luatic organisms.·· The gastropods Bulinus spp. and Biomphalaria spp are vectors of Schistosoma, which cause bilharzia in man. Other types are SC3yengers on dead organisms e.g. fish and even vegetation thus helping in cleaning of the water bodies. Past research has shown that invel1ebrate diversity and abU11dance is threatened by changes in water quality including eutrophication (over-fertilisation) and pollution (contamination). In Lake Victoria, increasin eutrophication over most of the last quarter of the twentieth century has coincided with drastic changes in relative abundance of' key zooplankton taxa. The proportions of calanoid copepods and Cladocera have decreased relative to that of cyclopoid copepods (Mwebaza-Ndawula 1994). In the past, only limited investigations linking aquatic invel1ebrates to the fisheries have been done in thc Victoria basin area. Corbet (1961) provided concise documentation of the food ofnon-cichlid fishes in lake Victoria including EJigraulicypris (= Raslrineobala) argentea, Mormyrids, Barbus species, some haplochromines etc either specialising in consumption of inveJ1ebratc or incorporating substantial proportions of invel1ebrates in their diets. Greenwood (1966) ha provided notes on the food of most fishes of Uganda. In a recent study, Mwebaza-NdawuJa (1998) gives a detailed description of the diet of and prey selectivity by R. argenlea in Lake Victoria.

Diversity and

Fig. 1. Zooplankton relative abundance: modern (inshore, offshore) and historical.

Table 1. Occurrence

Offshore. (BG) Dec-95

Species Harpacticoida Total (Spp) Cyclopoida

41% Eucyc/ops spp. Mesocyclops spp Thermocyclops emini Thermocyclops incisus Thermocyelops neglec, Thermocyelops decipie Thermocyelops ineisuo Thermocyclops oblong Tropocyelops con/innis Tropocyclops tenellus Total (spp)

As many invertebrate-eating fishes are commercially importantspecies, it is important to study the dynamics of these organisms and in particular to understand their contribution to fishely production. This paper presents resuhs of a study on the diversity and abundance of aquatic invertebrates in various water bodies in the Victoria lake basin and attempts to elaborate the relationship between invertebrate communities, ecosystem functioning and fish production. Study Area The study area covered selected water bodies within the Victoria lake basin namely, lakes Victoria, MbLlfO, Kachera, Wamala, Kayanja. Kayugi. Nabugabo and the Victoria NiJe (Figure J)

Calanoida

Thermodiaptomus galel

Tropodiaptomus stuhlan Total (spp)

Cladocera

Alana bukobensis Bosmina longirostns Ceriodaphnia comuta

Materials and methods

Chydorid spp. Daphnia longispina

Zooplankton were sampled at selected inshore and offshore sites with plankton nets of 50 - 100um mesh and 0.25 m mouth diameter. Three vertical hauls were taken to make a composite sample, which was preserved

Daphnia lumholtzi Daphnia lumholtz; 'monE

Diversity and abundance of invertebrates in the Victoria basin lakes and their role in fishery productton

abundance: :md historical.

211

Table 1. Occurrence of zooplankton species in the Victoria basin lakes Kayanja Kayugi Nabugabo Kachera Mburo Victoria WamalaKabaka's Lake Freq.

Of Species

occurence

Harpacticoida

D

):l

Total (Spp)

P 0

0

'2

P 0

0

1

0

P p

p

P p

p

Cyclopoida

41%

Eucyc/ops spp. Mesocyc/ops spp.

p

p

p

;13

5 ~- ,.~.

•

p p

Thermocyc/ops emini Thermocyclops incisus

p

Thermocyc/ops neg/ectus

p

p

p

p

p

p

p

p

P

+... ~

.. )l

.....

3

$ 7

p

Thermocyc/ops incisus

p

Thermocyc/ops oblongatus

p

Tropocyc/ops confinnis

p

Tropocyclops tenet/us

are ~ating fishes I is important to study : and in I arti ular to hery production. This on the diversity and tes in various water ;in and attempts to ;ween invertebrate ctioning and fish

J.t

p

Thermocyc/ops decipiens

Total (spp)

-

p p

3

4

4

2

p

1

p

2

9

2

5

Calanoida Thermodiaptomus ga/eboides

p

Tropodiaptomus stuh/amanni

p

Total (spp)

0

0

0

0

0

2

0

0

Cladocera

water bodies within kes Victoria, Mburo, ~i, abugabo and the

ethods elected in hore and of 50 - 100um mesh e vertical hauls were which was pr served

p

Alana bukobensis

Bosmina /ongirostris

p

Ceriodaphnia cornuta

p

p

p

p

3

p

P p

4

Chydorid spp.

p

Daphnia /ongispina

p

Daphnia /umholtzi

p

Daphnia /umholtzi 'monacha'

p

2

1

L Mwebaza-Ndawula. et 01.

212 Diaphanosoma excisum Moina micrura

p

Macrothrlx sp.

p

Total (spp)

2

p

p

p

p

p

Dive

p

p 2

5

2

1

p

p

p

5

p

p

p

8

p

P 6

10

4

1

2

Others Caridina

p

Chaobrid pupae

p

p

p

p

1 3

p

2

Chironomid larvae

p

p

Ostracods

p

p

Total (spp) Rotifera

0

2

1

1

4

Anureupsis fissa Asplanchna brightwelli

p

p

p

p

Brachionus angularls

p

p

p

p

p

p

Brachionus bidentatus Brachionus budapeslinensis

p

p

p

p

Brachionus quadridenlalus

p

p

Brachionus caudatus

2

I I

0

Num'

p

p

p

p

p

p

p

P p

p

2 7

p

P p

8

p

p

6

4

p

p

p

p

p

p

8

p

p

p

8

2

p

Brachionus dimidiatus Brachionus falcalus

2

p

Ascomorpha sp.

Brachionus calyciflorus

i'

Macro-u edge (from I areas (sedirn was demarc2 0.5 m. Mac machete and Grab was USt sampling poil Ilm-mesh-wa labeled bonle Sediment typ described ac

p p

p

p

p

Brachionus forficula

p p

Brachionus patulus

p

Brachionus plicatili:>

p

1

p

p

2

p

p

p

Collotheca sp.

p

Euclanis spp.

2

3

Filinia longh;ela

p

p

p

p

p

p

p

p

8

Filinia opoliensis

p

p

p

p

p

p

p

p

8

Hexathra spp.

p

p

p

p

p

Keralella cochlearis

p

p

p

Keratella tropica

p

p

p

Lecane bulla

p

p

Polyarlhra spp.

p

p

Polyarlhra vulgaris.

p

p

Synchaeta pectinata

p p

p

p

Figure 2. Der lakes (1997-2

p

p

p

p

p

6

p

p

p

8

p

p

p

p

p

p

p

p

p

6

P

p

P

-r' Cl

...Ex

8(

If)

BC

0

CD 'J}

c:

5

't

CD

4(

c: rn

2(

8

I])

5

~

p

p

p

p

p

p

p

p

p

p

p

8

p

p

p

p

p

p

p

8

p

14

10(

--.s

7

p

121

r

-

I

Synchaeta spp.

p

Trichocerca cylindrica

p

Trichocerca spp.

p

p

Total (spp)

13

14

18

16

16

26

P 21

Grand total nO.of spp.

18

17

30

23

20

48

34

in 4% sugar-formalin. The samples were taxonomically identified and the di fferent taxa counted under an inverted stereomicroseope at 40X magnification. Pelagic fishes were sampled (in Lake Victoria) with a small, canoe-towed, mid-water trawl net described by Mwebaza-Ndawula (1998). Samples were preserved in 10% fonnalin. Each fish was dissected, and the gut contents examined under a compound microscope. Prey

4

organisms were identified to broad taxonomic groups and counted. Zooplankton biomass was computed from available length-weight regressions (Botrell et al. 1976). Historical zooplankton and fish samples were obtained from field collections of Jack Tailing and the Fisheries Resources Research Institute (FIRRI) Museum respectively.

Figure 3. losh Thermodiaptom

Cladocera com]

Daphnia. Cerioc Chydorus and A Ascol71orpha. I. Filinia, Hexath Synchaeta and 7

Diversity and abundance of invertebrates in the Victoria basin lakes and their role in fishery production p

5

p

8

Macro-invertebrates were collected at the water edge (from macrophyte roots), inshore and offshore areas (sediments) in each lake. Selected macrophyte area was demarcated with a wooden quadrat of size 0.5 X 0.5 m. Macrophyte roots were dislodged using a machete and placed in labeled polythene bags. A Ponar Grab was Llsed to take three sediment hauls from each sampling point. The sediment was sieved through a 500 ~m-mesh-washing bag and the sample placed into a labeled bottle containing 5% formalin or 70% ethanol. Sediment type was detelmined by visual inspection and described according to textur~ In the laboratory,

4

2 1 3

2 2

organisms were sorted and taxonomically identified as far as possible.

Results Zooplankton composition, distribution and abundance Zooplankton communities of the Victoria basin Jakes were composed of cyclopoid and calanoid copepods, cladocerans, rotifers, aquatic insect larvae/pupae and water mites (Table J). Copepods were composed oflh genera: Thel7nocyclops. Mesoeyclops and Tropoeyclops (Cyclopo ids) ,

Numerical den~i!ies (Indiv.llol')

7

p

8

Biomass (uglm')

/

2 p

213

(

4 p

6 , ;,,'\I.fI1'

2 p

I )

8

I 1

p

\

c. ::~

, ; .. ,.., .....

~ I

8 ,-,,'

1

2

Figure 2. Densities (ind.lm lakes (1997-2000)

2

)

and Biomass (ug/m 2 ) of zooplankton taxa in the Victoria basin

2 3

p

8

p

8

6

p

p

6 8

.p

'=1

--

.....

120 100

r

;.;.

~:..

:.....jk·es Figure 4. Inshore-Offshore dis.tribution of diversity of zooplankton in the Victoria basin lakes

numerically the most dominant groups. Howcver. in tenns of biomass, cyclopolds had the highest relative abundance in all basin lakes (Fig.2). Kachera anti Nabugabo had significantly higher mean densities of zooplankton offshore while other lakes showed no clear Inshore-olTshon: trends in abundance (Fig. 3).

Zooplankton divcrsif)' Mean zooplankton species diversity varied between 7 and 13 species. Higher species diversity offshore was recorded In Wamala, Nabugabo and Victoria while the reverse trend occuned in Lake Mburo (Fig. 4). There were no clear inshore-offshore trends in diversity of organisms in Kachera, Kayanja and Kayugi.

o q

'-

:'

...

Fig. 6. Contribut in the diet of R. c and offshore (BC and 0 as in Figul

R ~~,~

II 1(I'.,

,':1 T SI/';)

I

D 52~;o

Figure 5A & B. Relative abundance and dry weight of different zooplankton taxa in samples from inshore (NG, 1995) and historical (off Dagusi Island, 1961) samples from Lake Victoria

Diversity of macl"' A horizontal gradit was apparent wlthil occurred inshore, majority oflakes (f supported higher d

Diversity and abundance of invertebrates

111

the Victoria basin lake and their role in fishery production

Historical cbanges in zooplankton relative abundance in Lake Victoria Relative abundance and biomass of different ta a from the Napoleon Gulf. ( 1995) and off Dagll i Island ( 1961) indicated remarkable changes betwe nth communities of the1960' and 1990s (Fig. SA and B) Cyclopoid copepods 'howed a substantial increas from 56% in

Off 1500 ind.lm!) oCCUlTed in the lakes Nabugabo and Mburo, while very low densities « 500 ind./m') occurred in Kachera, Kayugi and Kayanja. Roots of aquatic macrophyte generally supported lligher densities of organisms than the sediments (Fig. 9) although in some cases thes differences were not significantly different. Within macrophytes, densities of organisms were generally higher in papyrus roots than in other vegetation types (Table 3). Among sedimenlS, sandy/gravel sediments supported higher densities of orga isms than muddy bottoms (Table 4).

L. Mwebaza-Ndawula, et al.

216

C,chhd L't"~

Diversity

Chironomus sp. Tanypodinae

(n~2))

[1

c-----~,'""' 0

_So E: 1,5 -

10

P P

0.5

00

P P P P

P

P P P

P P P

P

P

P

P

P

P

P P P

P

P P

P P

P P

P

P P

P P

P P

Fig. 8 Inshore-c 250

200

'b x

r'....,tl 50 P P P

r,

.c ffilo.o '1'

2

50

P P P P

P P P

P P

P P P

P

P

P P P P

00

P

Fig. 9 Mean de macrophytes.

Diversity and abundance of invertebrates in the Victoria basin lakes and their role in fishery production

Chironomus sp. Tanypodinae

p p

p p

p

Palpomyia sp. Chaoborus sp.

p p

p p p p

p

Tabanids Syrphids(flower flies) p

Trichoptera Hirudinae Oligochaetes Hydracarina

p p

Ostracods

oticus and larval N, R. and 0 as in

p

p

p p p

p

p p p p

p

Conchostraca Water spider

P

P

No of Taxa

21

23

~,

p p

p

p

17

8

p p

p

p p p

p

p

p p

p p p p p

p p

p p p p

217

19

p p p p

p p p p p p

18

26

II

ual lakes of the II'S

40

5

Nla

)

)

Vic

p p p p

~ 2 ", L

n .

0'5

o

0 -1--'--"-'''"''--

)

p p

)

1-."

hA,o

Ldl~lo;::'s

Fig. 8 Inshore-offshore densities (no.m2) of sediment macroinvertebrates in Victoria basin lakes.

p )

::50

,..,

200

Q ).;

Sediment

~150 --,

) )

Utf_hare

n-

p p p

Inst10re

['l'J

15 1

p

o

p p p

.Veqelalion

,-,

E §

00

..1. 50

p ;)

tJeJ

p

Mro

p p

"Jlo

Vi' L.1i(~·,

Fig. 9 Mean densities (no.m 2 ) of invertebrates in sediment and roots of aquatic macrophytes macrophytes.

L. Mwebaza-Ndawula, el al.

218

Diversity an,

Table 3. Macroinvertebrate diversity and abundance: papyrus Vs other macrophytes

30.0

Lakes

Victoria-Nile

Kachera

Vegetation Total no./m 2 No. of taxa

Water hyacinth

Papyrus

Nile-cabbage

Papyrus

2628 24

4886 16

4136

11526 13

17

250

ro

@ 20D

~

Table 4 Macro invertebrate diversity and abundance: sandy Vs muddy bottom Lake

Bottom type

Mburo Nabugabo Victoria Kachera Kayugi Kayanja Wamala

No. of taxa

Sandy/gravel with little plant debris Woody substrate and sand grains Soft organic material and sandy Muddy/ boggy Muddy/ boggy Muddy/ boggy Muddy/ boggy

16 12 26 9

7 8 8

Mean density (no.lm 2 )

0 "ill -0

E

::J

150

1[I [I

Z

1613 2127 910 189 144 248 558

50 DO ~./.ro

Fig.11 Diversity of basin lakes

300

::r;o In Shore

~

200

Off Shore

n

'0 OJ i5

[I

.D

F S

Z

00

Lakes Fig. 10 Inshore-offshore distribution of macrobenthos taxa in the Victoria basin lakes.

sediments (Fig. 11). varied with macroph)

I

The Victoria basin an invertebrates especial Rotifera. Diplera. Ga 1&2) \Vidcly distribl cyclopoid cope-pods. exciSll/II and ill. micr larvae. Odonata an imp0l1ant groups. wb available as prey te organisms that are abu as the small·bodied ro fish food resource dl probably 10\\ calorifi, been found to be most as Mburo. Nabugabo ill such minute organiSI such as copepods. Cla( rate high in temlS of they are evidently not 6&7) Jnshore-otlshore tr abundance observed i and 9) can also be reI; would naturally tenc provide both shelter prey organisms.

Diversity and abundance of Invertebrates in the Victoria basin lak s and their role In fishery production 'ophytes

219

310 250

Papyrus

11526 13

1

ro

>< ro ......

Sedime

200

'+-

0 '-

n

0.)

n density (no./m 2 )

..0

E :::l

1613 2127

Vegeta

150

100

Z

910 189

5C1

144 248

no i

558

~ ~l)

~ra

f'll'!

l