Impact of stability in prey supply to the stocks of the Nile perch in Lakes Victoria, Kyoga and Nabugabo
Item Type conference_item
Authors Ogutu-Ohwayo, R.
Download date 27/09/2021 00:52:51
Link to Item http://hdl.handle.net/1834/35651 .
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.1 LV
Importance of stability in. prey.' sti·PP~.y to the ~tocks of the Nile perch in Lakes VictorIa, Ky~ga and Nabugabo. ;
Richard Ogutu-Ohwayo, .. _-~ -'-'--- Uganda Freshwater Fisheries Research Organization,
P. O. Box 343, Jinja, Uganda.
Prepared for the'sixth session of the CIFA sub-committee for
development and management of the fisheries of Lake ,Victoria,
February 10th -14th, 1992; Jinja, Uganda.
Abstract
The food of the Nile perch has changed since its introduction
into lakes Victoria, Kyoga and Nabugabo .andstabilized on \ Caridina nilotica, Anisopteran nymphs, Rastrineobola argentea,
Nile perch juveniles, and tilapiines. For the Nile perch to
sustain pi.'oduction in these lakes , its is important that these
prey species are properly managed.
Introduction.
fisheries~of Considerable changeo have taken place in the • lakes Victoria, Kyoga and Nabugabo since Nile perch and several
tilapiines species were introduced into these lakes. Many of
these changes were caused by other factors such as overfishing
but some of them have been due to predation by Nile perch which
were introduced into Lake Kyoga in 1955.and 1956, Lake Nabugabo
in 1960 and 1963 and Lake Victoria in 1962 and 1963 (Gee 1964).
Stocks of the Nile perch started to increase rapidly in
1965, in Lake Kyoga, and in 1977, 1981 and 1983 respectively in
the Kenyan, Ugandan and Tanzanian regions of Lake Victoria. This III •
;.. -2 j
was followed by rapid increases in fish yield (Hughes, 1983;
Okaronon et aI, 1985; Gaudswaard and Witte, 1985). For instance
in Lake Kyoga, the quantity of fish landed increased over ten
fold from an annual catch of 4500 tons at the time of the
introductions to a record of 165,000 tons in 1977 of which about
40% were Nile perch. In Lake Victoria, the total quantity of
fish landed in the Ugandan, Kenyan and Tanzanian parts of the
lake increased between two and six fold. In the Ugandan section,
commercial catches increased from 17,000 tons in 1983 to 132,000
tons in 1989, in'the Kenyan part from 20,000 tons in 1977 to
123,000 tons in 1988 and in the Tanzanian part from 64,000 tons
in 1982 to 89,OQO tons in 1985. The Nile perch is also well
establishes in Lake Nabugabo and contributes the biggest part to
the fish landings.
The increases in the populations of the Nile perch have teen accompanied by ~ reduction and in some cases totalpisappearance II 'i of many of the n'ative species. The haplochromines, which were II 11 the mCist abundant fish species in Lake Victoria and formed the I: Ie bulk of the food of the Nile perch soon after its establishment
were depleted aJd rapidly from lakes Kyoga, Victoria and
I . . Nabugabo. This~has, resulted in a shift in prey selection to the extent ttlat Nile perch feeds even on members of its own kind. In
the mid-1980s, the stocks of the Nile perch in Lake Kyoga where
it was introduce9., earliest declined raising fears that the I predominantly Nile perch based fisheries of Lakes Victoria, Kyoga
and Nabugabo would, not sustained the level of production they had attained. \
The rapid increases in harvestable fish following
establishment of Nile perch also attracted many investors. The
number of fishing canoes in the Ugandan region of Lake Victoria -3
has increased from 3264 in 1971 to 8674 in 1990 (Weatherall 1972,
Tumwebaze & Coenen 1990). Many fish processing plants targeted
to fillet Nile perch have mushroomed around Lake Victoria.
The reduction in prey availability by Nile perch predation,
direct human exploitation of main prey species and high fishing pressure on Nile perch and those species that form its main prey have increased uncertainity about the future sustainability of the Nile perch based fishery and the long term viability of the investment in the industry. This paper discusses stability in prey supply as one of the major factors that will contribute to stability in Nile perch yield in Lakes Victoria, Kyoga and
Nabugabo.
Materials and Methods
The data discussed here were collected from: 1) Lake
Victoria, where Nile perch were introduced in 1961 and were well established around 1977, 2) Laka Kyoga where Nile perch were , I, I introduced in 1955 and were established by 1965, 3) Lake i,
Nabugabo where Nile perch were introduced in 1960 and are well " established and, 4) Lake Albert, the original and native habitat of the Nile perch stocks which were introduced into Lakes
Victoria, Kyoga and Nabugabo (Figure 1).
The types, sizes and numbers of prey that have been ingested by Nile perch in its original habitat of Lake Albert and those that have been ingested in the new habitats during various periods have been examined and compared. The data have been divided in Blocks on the following criteria:
Block AI: Lake Albert (1989 to 1991) - the original and
native habitat of Nile perch. .. -4
Block V1: Lake Victoria (1968 to 1977) - before the Nile perch
were well established and haplochromines were still abundant.
Block V2: Lake Victoria (1988 to 1991) - after the Nile perch
were well established and haplochromines had just been
depleted.
Block N1: Lake Nabugabo (1991) after Nile perch were well
established and haplochromines depleted.
Block K1: Lake Kyoga (1967 and 1968) - after the Nile perch were
well established and haplochromines had been
depleted.
Block K2: Lake Kyoga (1978 to 1980) - about ten years after K1.
Block K3: Lake Kyoga (1991) - about ten years after K2.
The type, length and number of prey ingested by Nile perch
changes with the le~gth of the predator (Worthington 1929,
Hamblyn 1966, Gee 1~69, Okedi 1971, Ogutu-Ohwayo 1965), The data in each Block wa~ therefore analysed by length class based on
Ii major shifts in pre~ selecticn as follows. I I! A: < 20 cm -Nilel'perch, feeding mainly on invertebrates.. I I B: 20 to 59 cm - Nile perch feeding on invertebrate and fish.
I c: 60 to 99 cm - ~ile perch feeding on many small fish • especially B. argentea in lakes Victoria, Kyoga and Nabugabo.
D: 100 cm and beycind Nile perch feeding mainly on one or two
large fish preJ especially Q. niloticus in lakes Victoria
and Kyoga. ,
\ What I would li~e to illustrate ie that prey selection by i
the Nile perch in the new habitats has changed and stabilized on specific types which ~re vital in sustaining its production. ,
...... -5
Results:
The type of prey
The relative importance of the major food items that have
been ingested by Nile perch in lakes Albert, Victoria, Kyoga,
Nabugabo is illustrated in Figures 2. The minor types of prey
and other materials found in the stomachs are given in Table 1.
In Lake Albert, C. nilotica, Haplochromines and Anisopteran
nymphs were the most important·prey in Nile perch of less than 20
cm length. Nile perch of 20 to 99 cm mainly ingested Alestes spp
and haplochromines and those of of more than 100 cm ingested Nile
perch juveniles, Alestes spp, Tilapiines, Polypterus, Bagrus spp
and Mormyrids.
\ In Lake Victoria, haplochromines were the dominant prey
among most sizes of Nile perch during the period 1968~1977. At that ;time, Clarijds also made a significant contribution~ to the
diet 9f Nile perch and other prey included Tilapiines, \ Pr0topterus aethiopicus, Mormyrids, and Synodontis spp and
mo lluscs (Tablo 1).
The major types of prey ingested by Nile perch in the
northern waters of Lake Victoria between 1988 and 1991
included;· Q. nilotica,· Anisoptera nymphs, E. argente·a and Nile
perch juveniles in fish of less than 60 cm. Nile perch o·f 60 to
99 cmmainly ingestedR. argenteaand Nile perch juveniles while
those of more than 100 cm ingested Nile perch juveniles, tilapiines, Mormyrids, ~. eathiopicus, Clariids, crabs and
molluscs.
The major types of prey ingested by Nile perch in Lake Kyoga
between 1967 and 1980 were .Q. nilotica and Anisopteran nymphs in Nile perch of less than 20 cm, Anisopteran nymphs and B. argent~a in fish of 20 to 59 cm, R. argentea and haplochromines in fish of -6
60 to 99 cm and tilapiines in those of more than 100 cm. The
data collected in 1991 showed a drammatic increase in haplochromine prey in Nile perch in Lake Kyoga. However,~.
nilotica, Anisopteran nymphs and R. argentea remained important. f, Haplochromines were particularly important in Nile perch of less than 100 cm beyond which Nile perch juveniles and tilapiines were
still the main prey. During the early years when haplochromines
were still abundant as indicated by the 1968-1977 data for Lake
Victoria (Fig. 2), haplochromines formed the main prey even in
Nile perch of more than 100 cm total length. The haplochromines
which are currently ingested by Nile perch in Lake Kyoga are,
however, of a much smaller size and it may be energetically
uneconomical for Nile perch of more than 100 cm to feed on them.
In Lake Nabugabo, Ephemeropterans (especially Povilla) and
Anisopteran nymphs were the main prey of Nile perch of less than i 20 cm (Table 1) . .Q. nilbtica which is very·important in Nile perch of this length CIJss in lakes Victoria, Kyoga and Alb~rt ,I I were absent and unlike in the other lakes, fish prey
I: comprising B. argentea ~nd tilapiines were prominent at a smaller I size. Nile perch of 20;to 99 cm fed mainly on B. argentea and tilapiines. However, a ;more detailed analysis showed that Nile
I perch of more than 80 cm depended almost solely on tilapiines..
It can, therefore be concluded that the most prominent prey items in the original. .and I native habitats of the Nile perch. are Caridina nilotica, Anisoptera nymphs, B. argentea, Alestes spp,
. haplochromine, tilapiine~ and Nile perch.
The relative importepce, of different types of prey to different length classes of the Nile perch is given in Figure 3.
Caridina nilotica was the most important prey in Nile perch of i less than 20 cm total length decreased in importance in fish of -7
20 to 59 cm, and was not ingested by larger Nile perch. When
compared between the different lakes and over different periods, ~. nilotica was absent in the diet of Nile perch in Lake
Victoria between 1968 to 1977. It was also absent among prey
ingested by Nile perch in Lake Nabugabo. When compared between the lakes, ~. nilotica is more important in Lake Victoria than
Kyoga and Albert. Whereas it is ingested only by Nile perch of less than 59 cm in lakes Albert and Kyoga, in Lake Victoria, C.
nilotica is ingested by Nile perch up to the 60 to 99 cm length
class..
Anisopteran nymphs starts appearing in Nile perch of less
than 20 cm, reached a peak in fish of 20 to 59 cm beyond which
they are not ingested. In the period 1967 to 1968, Anisopteran
',nymphs seemed to be more import:mt in Lake Kyoga than they are
presently. I Rastrineobola argentea, for lakes Victoria, Kyoga and !, 1 Nabugabo, and Ale3tes, for Lake Albert started to appear in the ,;,I diet in Nile perch below 20 crn length. E. argentea reaches a "I peak between 20 to 99 cm and are not ingested by larger Nile
perch. In Lake Albert, Alestes spp are, however, ingested by
Nile perch larger than 100 cm.
Haplochromine were the most important prey in all length
classes of Nile perch in Lake Victoria between 1968 and 1977.
Their importance decreased only slightly with length class of the
Nile perch.
Cannibalism was prominent in Nile perch of more than 20 cm.
It is however, more prominent in Lake Victoria than in the other
lakes suggesting that alternative prey are more scarse in Lake
Victoria than in the other lakes. -8 • ,I Tilapiines were, ingested by Nile perch of most length
classes but were mostly important in those of more than 100 cm.
The length and numbers of prey ingested
Predators choose prey on the basis of their size and
abundance. If the scenario emerging above suggests stability in
prey selection, then both the size and number of prey ingested by
Nile perch should be stable and follow similar pattern between
those habitats for which the types of prey are similar.
The average sizes and numbers of prey ingested by Nile perch
of different length classes for Blocks V1, V2, K2 and A1 are
illustrated in Figure 4. The length and the number of prey
ingested by Nile perch in Lake Victoria between 1968 and 1977
increased evenly over all length classes of the predqtor. By
1988 to 1991, the length of prey increased evenly only in Nile
I . :
perch of length classes less than 100cm and then incr~ased
suddenly sUggestin~ a sudden switch to larger prey. ~e number
,.'If, of prey ingested a~so increased evenly only in fish of less than 100 cm after whichLthere. was a sudden drop in 'the numbers of prey ingested. The pattern in prey size selection for Lake Victoria
I between 1988 and 1991, lake Kyoga between 1978 to 1980 is similar
to that of Lake Albert. This·provides additional evidence that
prey size selectioq by ,the nile perch in the new habitats has
stabilized.
Discussion
The types of prey ingested by the Nile perch in lakes I Victoria and Kyoga soon after its introduction comprised of
haplochromines and small mormyrids (Hamblyn, 1962, 1966; Gee,
1964, 1969). Earlier observations in Lake Albert (Worthington, ",
-9
1929; Hamblyn, 1961, 1966; Gee, 1964, 1969) show that Nile perch of less than 60 cm ingested ~. nilotica and Anisopteran nymphs,
and became piscivorous at larger sizes. During its piscivorous
phase, the predator mainly ingested the pelagic characid species
of Alestes and on Hydrocynus. The present observations show that
the types of prey ingested by Nile perch in Lake Albert have not
changed for over 50 years since Worthington's survey of the lake in 1928. The types of prey ingested by Nile perch in Lake Victoria by
1988 and in Lake Kyoga since 1967 have similarities to those in
other native habitas. In Lake Chad, prawns and small pelagic
characins Alestes dageti Blache and Microalestes acutidens
(Peters) were the most important prey in Nile perch up to 80 cm,
beyond which Hydrocynus spp. became the most important (Hopson, 1872). In Lake Turkana, a pelagic cyprinid, Engraulic~~
stellae, was an important prey of Nile perch. The type of prey
ingested by the Nile perch in Lake Kyoga, where it was introduced
and became established earlier than in Lake Victoria, initially changed from haplochromines to ~. nilotica, E. argentea,
tilapiines, and L. niloticus in a period of about 10 years from t~e time of its introduction (1956 to 1967). These types of prey ha"ve remained prominent for a period of over 20 years (1967 to
1990). The current composition of prey in Lake Kyoga is similar to that of its original habitat,where ~. nilotica and
Anisopteran nymphs are the important prey during juvenile and
sub-adult stages, and pelagic fish species and haplochromines
becoming more important in the sub-adult and adult stages. The
situation in Lakes Victoria and Nabugabo is showing a similar trend. -10
In lakes Victoria and Kyoga, Nile perch shifted to Q.
nilotica, Anisopteran nymphs, R. argentea, tilapiines, and Nile perch juveniles after haplochromines had been depleted. No C.
nilotica was found among stomach contents of Nile perch in Lake
Nabugabo and cannibalism was rare compared with the other lakes. Since ~. nilotica are very important in the diet of juvenile Nile
perch, their scarcity may affect the abundance of the species in
the lake. It also means that Nile perch would have to depend
more on the other prey species. The scarcity of cannibalism
could partly be due there being little food to support large
juvenile Nile perch stocks.
The length of prey ingested by Nile perch in lakes Victoria and Kyoga also follow a pattern similar to that in its native habitats of Lake Albert (this study) and Lake Chad (Hops9n
1972). Currently, the three dominant fish prey types of Nile perch comprising R. argentea, Nile perch itself and Nile tilapia are
,I also heavily commerci~ilY exploited. The stability of Nile perch " stocks will depend upon the level of exploitaion of these , species. It is already clear that human exploitation and Nile perch predation have reduced the overall size of R. argentea in Lake Victoria (Wandera, this volume). The stocks of Nile perch in Lake Kyoga also almost collapsed due to heavy exploitaion of juvenile perch using beachseines (Twongo 1986). It is therefore I important that exploitation of Nile perch prey are minimised if
the species has to sustain production.
I The relationship b~tween the Nile perch and its prey emphasise the recommendantion made earlier by Ogutu-Ohwayo (1985) for Lake Kyoga which proposed that exploitation of R. argentea in
Lake Kyoga should not be permitted in view of its importance as •
-11
food of Nile perch in the lake. I would like to extend this
recommendation to Lake Nabugabo especially in view of the factor
that Q. nilotica which is one major prey of Nile perch is absent
from the lake. It is further suggested that strict management of
Nile perch prey especially that of B. argentea in Lake Victoria
should be enforced.
Acknowledgements.
This work was done with the aid of a grant from the
International Development Research Centre IDRC, Ottawa, Canada.
References. It
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~~three lakes (Victoria. Kyoga and Albert). The areas la~:~ ") ..: sampled on each I are shaded with dark .colour.
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LENGTHClASS OF NILE PERCH
figure 1. The relative Importanceof prey Ingested~ Nile perch of A: lea than 20 em, B: 20 to 69em,C: 60 to 99 em, D: 100em and over in Lau. Victoc'l8,Nabugabo, Kross and Albert over different period..
•• 40 <20cm 20 to 59 an >=l00cm
l1apines100 1 o i,! '0" -~ [(~. 0..
u.. 100 1 ~ HapIochrocOOe.
o 0 I . l1li . , 1~---""- ~ 100 [( . o Rastrineobola * \- . ~. -- ..--_. J~ ~ . (orAJestes*)0, ., w > 100 1 i Misopl...., 1 .•. 1 ..
:~ .. w .... (I: ,.L..- 100 CaIidina ..1" . o 1. , l.. . , Vl V2 Nl Kl K2 K3A1 Vl V2 N1 Kl K2 K3Al Vl V2 N1 Kl K2 K3Al V1 V2 Nl Kl K2 K3Al
BlOCK
Figure 2. The re&ativeimportance of prey to differentlength classes of Nile perchin lakesVi(foria. Nabugabo. Kyoga and Albert over different periods.
_~J'.,~J II ... J.: ./ .' ' - _..... ~-'- .' - :,._..-. " ..~' .. ;._~:~
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. , . I . THE LENGTH AND NUMBERS OF PREY, BY CLASS , 40 ' 40 A1 A1 30 ' , 30
20 ! 20 / \ 10 ,I . 10 -~ 0 '. 0, :1 ' 40 II 40 K2. I K2 30 30 -E I, (,) 20~ , I, 20 -> > UJ 10:, 10 W a:: ' . a:: a.. I a.. 0 LL o " LL I 0 0 :c .40 . 40 a: I V2 . V2 W (!) 30 30 l:O Z ~. W 20 20 :::::> ...J z 10 / 10 0 - 0
40 40 V1 V1 30 30 20 20
10 10 ~ ~ 0 0 A 8 C 0 A 8 C D LENGTH CLASS OF NILE PERCH ., '..
q. .. III 'i ---,