A Study of the Nile Perch, an Introduced Predator, in the Kavirondo Gulf Lake Victoria: The Report of the Oxford University Nile Perch Project 1983
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Publisher Oxford University
Download date 30/09/2021 04:29:52
Link to Item http://hdl.handle.net/1834/7239 A Study of the Nile Perch, an Introduced Predator, in the Kavirondo Gulf Lake Victoria.
The Report of the Oxford University Nile Perch Project 1983
ed to: The Gecaga Institute. Kenya National Museums. Kenya Marine and Fisheri·es Research Institute, ~ i
CON TEN TST S
IntroductionIntroduction 1
BACKGROUND INFORMATIONINFORMATION
Lake VictoriaVictoria 33 The Indigenous and IntroducedIntroduced Fishes of Lake Victoria 55
THE RESULTS OF THE 1983 FIEL~ INVESTIGATIONINVESTIGATION
Materials and Methods 99
Length Weivht Relationships 1111
Age and Growth 1414
Breeding 1919
Food and Feeding 2626
AN ANALYSIS OF THE ECOLOGICAL CHANGES, PRODUCEDPRODUCED BY LATES, IN THE KAVIRONDO GULFGULF
An Analysis of the ecological chan~es, produced by Lates in the Kavirondo Gulf 4747
" Conclusions 5858 Summary 6161
Acknowledgements 6464
References 6565
Appendix 6767
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,_ , __ ., .__.----J. 1
INTRODUCTION
This project is an investigation of the changes that the Nile Perch (Lates niloticus (L.), Pisces: Centropomidae), an
introduced predator, has wrought in the ecology of the Kavirondo GUlf, Lake Victoria, East Africa.
It involved a review of the past ecological investigations carried out on Lake Victoria, by the East African Freshwater
Fisheries Research Organisation since 1949 (Reported in Ann Rep
E Afr Freshw Fish Res Org 1949-1974), and, more recently by the
Kenya Marine and Fisheries Research Institute. In addition, previous work on Lates was surveyed including
Gee (1969), Hamblyn (1966), Hopson (1972), Hopson, McLeod, Harbott and Ogari (1981), Okedi (1970), Kenchington (1939) and
~ Holden (1967).
A ten week field investigation was made to collect data on the present ecology of Lates in the Kavirondo Gulf.
During this time I lived and worked with Luo fishermen. It
is thanks to their hospitality and cooperation that this part of
the study was possible.
The project report is divided into three sections.
1 BACKGROUND INFORMATION
Here, briefly, I will introduce Lake Victoria and its indig enous, largely endemic fish fauna. Having in this way provided context for the subsequent study,
I will bring the reader up to date on the subject of the recently
introduced Nile Perch.
_-J 2
2 THE RESULTS OF THE 1983 FIELD INVESTIGATION
Inthis section I will describe the methods I used in the
field investigation, and present the results I obtained.
I discuss these results, in relation to earlier work on
Lates, in Lake Victoria and elsewhere. 3 AN ANALYSIS OF THE ECOLOGICAL CHANGES, PRODUCED BY LATES, IN THE KAVIRONDO GULF.
Past and present ecological data are considered and the
ecological changes, produced by Lates are discussed and
summarised.
e 3
LAKE VICTORIA
Lake Victoria, Africa's largest lake, covers some 69000 km 2
and stretches approximately 400 km in length and 320 km in breadth. It is relatively shallow, much of it being less than 20 m
deep, with a maximum depth of 84 m.
20% of its water budget enters by rivers, principally the Nzoia and the Mara in the East and the Kagera and the Katanga in
the West. The remaining 80% falls as rain on the lake surface. 90% of the annual intake is lost in evaporation. The remainder
leaves as the Nile in the north (Beauchamp 1964). Temporary stratification may occur in the period January
May and this results in a considerable reduction in dissolved
oxygen levels in the deeper areas (Fish, 1957; TaIling, 1963). • The temperature of the lake shows little variation about the mean of 24°C.
It has a low total ionic concentration with little or no seasonal variation (TaIling & TaIling, 1965).
Much of the lake bed below 50 m is covered with a thick
layer of organic mud. This bottom type is also common in shallower
areas, particularly in sheltered regions, although scattered
patches of harder mud, sand or shingle are also found.
The Lakes edge varies from deeply indented bays, especially in the North and South where the shoreline has a drowned appear ance to the straight exposed coast, of the west.
Much of the shoreline is fringed with papyrus (Cyperus papyrus), which may extend into large swamps, particularly in
bays. Sandy beaches and rocky, or cliffed exposures are less
common shoreline habitats.
? 4 4
The climate is dominated by two rainy seasons between October December and March-May. Due to the prevailing south east trade
winds these are most pronounced in the northern half of the Lake. Lake Victoria was formed some 750,000 years ago (see Doornkamp & Temple; 1966 for evidence) when warping of the earth's crust back-ponded the ancient east-west flowing drainage pattern. The tectOnic activity probably spread from the south, forming
a series of dendritic lakes, similar in form to the present day lake Kioga, which became confluent as the warping progressed
northwards.
Five raised beaches at 65, 35, 18, 12 and 3 M above the present lake suggest there has been a stepped drop to the present
level.
The present outflow to the north is thought to have been • formed about 25000 years ago (Bishop & Trendall, 1967). Kendall (1969) using lake sediment cores, produced a climatic
history for the last 15000 years. The picture is not one of
stability. Alternating wet and dry periods produced large changes in ionic composition as the Lake varied between an open and
closed drainage system. 5""
THE FISHES OF LAKE VICTORIA
The fishes of Lake Victoria have their origins in the ancient
east-west flowing rivers, which their ancestors inhabited before
the tectonic backwater of Lake Victoria was created.
THE CICHLIDS
The most astonishing event during the ensuing 750000 years
was the evolution of the Haplochromine species flock.
This closely related group contains an unknown number of
species, somewhere in excess of 200 (see Greenwood, 1974 and 1981).
In Lake Victoria these fish made up over 80% of the Lakes
fish biomass and exhibited a great range of trophic specialisations,
... utilising every major food source, with the possible exception
of zoo plankton.
For this reason, and because of the importance of the twotwo indigenous Tilapiines, Sarotherodon esculenta and S variabilis
to the commercial fishery, Lake Victoria has been termed a cichlid
lake.
THE NON-CICHLIDS
Compared to the Cichlids the 38 species of Non-Cichlids,
representing 20 genera and 11 families, have been very slow to
evolve habits andstructures to exploit the lacustrine environmentl
Many of them stick to their ancestral habits in various ways.
In the lake they utilise the habitats and foods most similar to
rr 6 6
those found in riverine conditions, (Corbet, 1961) and they still migrate up rivers to spawn (Whitehead 1958).
As a result these Non-Cichlids constitute le$s than 15% of the Lakes biomass (Bergstrand & Cordone, 1971) and most of this 10%) is contributed by the two siluroids Bagrus doc mac and Oarias mossambicus, which feed mainly on the Haplochrcmines,
and range over the entire lake. Bagrus docmac, Synodontis victoriae, the Elephant snout fish
Mormyrus kannume and the pelagic cyprinid Engraulicypris argenteus, are known, or suspected, to breed in the lake, and to
make fUll use of its resources. As a result they are four of the more abundant Non-Cichlids.
Excepting the lungfish Protopterus aethiopicus, the rest of the lakes Non-Cichlids, could, in 1970, be considered minor players
\,I in the lakes bionomics.
However, the present scarcity (1970) of anadromous fish such as Labeo victorianus, Barbus altianalis and Shilbe mystus,
could well be a result of man's activities since they are
extremely vulnerable to overfishing at river-mouths. They may once have been more abundant.
THE INTRODUCED FISHES
During the 1950s several species of Tilapiine fish were
introduced including Sarotherodon leucostictus, S.niloticus and
Tilapia zilli.
The logic behind the introductions was that these fish
would consume submerged macrophytes, a resource that was 7
being underused by the indigenous fauna.
In fact only T. zilli is particularly prone to eating
macrophytes. In the Kavirondo Gulf S. niloticus has almost entirely
replaced the indigenous, and other introduced Tilapiines.
THE NILE PERCH
The First Introduction
The idea that lakes containing large numbers of small
Haplochromines, would benefit, as far as man was concerned, from
the introduction of Lates niloticus was mooted by Graham (1924),
Worthington (1932) and by Belgian scientists (see Anderson (1961).
The notion was that Lates would control "Les indesirables
\I Hap1ochromis" as the Belgians put it, and turn the Lakes in question into flourishing Lates fisheries.
The arguments for and against the introduction of Lates
into Lake Victoria are put forward by Fryer (1960) and Anderson
(1961).
Before an official decision was reached fishermen began to catch Lates in northern Lake Victoria. A certain mystery still
surrounds their introduction.
Subsequently stockings were made, in both Ugandan and
Kenyan waters (see Gee (1963».
The Spread of Lates in Lake Victoria
Their spread around the lake, and data on their feeding and breeding ecology were documented by Hamblyn (1960 & 61), Gee (1963 1964 1966 1967 1969) and Okedi (1970) ------
'8
The spread was, at first, down the eastern shore, a fact
perhaps correlated with the clockwise surface currents in the
Northern part of the lake and the pelagic habit of larval Lates.
During the 1960s , although the population spread, it remained small. In 1972 KUdhongania and Cordone state "The bionomic contribution of Lates to the Ichthyofauna is stil.l insignificant." This state of affairs was not to last, at least in the
Kavirondo. During the past five years there has been a huge increase
in the Lates population~population y this is reflected by the commercial landings shown in Fig 1.
o o o 0 EST IMATED LANDINGS OF l/1rr1 LL.J Lates' niloticus Z Z o FROM KENYAN I- 0 ~ o LAKE VICTORIA W O ...... 0 a::N 1974 - 1982 ~ :E ~O o o :I: o w..- I o 74 757S 76 77 78 79 BDBO 81 82 YEA R Fig 1I The estimated commercial catch of LaLatestes frfromom Kenyan LLakeake Victoria. From 1974-1982 in metric tonnes. Data~ata provided by the KenxKen~aa Marine and Fisheries Research Institute. --~_._--Marine and Fisheries Research Institute. ~------_. -.'-'-'--~'--'-"'-"--- - .•••__• __ ._•• •••.•. _••..0 •.- .-._,_._,•.----_._..•..••------.---.------_._._..~--.-_.~--_ ...... ___- • • .______.. '.'-_.0.__.0. The present project was undertaken to study, and report on, the way in which this population explosion has influenced the ecology of the Gulf. 9 MATERIALS AND METHODS Duration and Date of Study The material on which this study is based was collected between the 14th July and 3rd September 1983. The Study Site The main study site was Ulugi beach, Northern Rusinga Island, South Nyanza Province, Kenya. Data were also collected around the Chamerungo Islands, Mbita and Utajo beach (see map below). u 10 Methods of fish capture Most fish under 15 cm total length were taken in mosquito seines ( 10 mm stretched mesh), operated at night, by local fishermen fishing for Engraulicypris argenteus, a small pelagic cyprinid. This technique involved the use of floating pressure lamps to concentrate E. argenteus. Fish over 15 cm, up to 168 cm, were caught in beach seines operated by local fishermen during daylight. These were fished with lead ropes of approximately 500 m. The wings had a stretched mesh of 2.5 inches and the bag 1.5 inches. The nets were typically 300 m long and 2.3 m deep and weighted to fish on the bottom. DataData Collected 11 From 897 specimens of Lates the following data were collected. " a)a) Total length to the nearest mm. b) Total weight in grammes.grammes. c) Sex and state of maturity.maturity. d)d) Stomachs were stored in 5% formalin and their contents analysed by the method described in the food and feeding section. 22 From 3976 specimens of Lates landed at Ulugi Beach, the following data were collected between 16th August and 3rd September. a)a) Total length to the nearest 0.5 cm. (To establish the size frequency of the catch and estimate that of the population). 3 From 60 specimens, between 20 cm and 106 cm, scales and otoliths were collected. 11"11' LENGTH-WEIGHT RELATIONSHIPS The relationship between log weight and log length is shown in Fig 2. 5,5 7cm-16cm LogW=LogO·02040+2·8LogLLogW=LogO-02040+2-8LogL .... 16cm-49cm Log W=LogO·01690+2-9LogL 49cm-100cm ~ W=LogO'OOO96 +364LogL 4 1 :3 I-I-- I: L:J .. 3 -L.LJLLJ 3 L:J 2 0 .-I.-..J j " / oOL--~~...... -.--~~-~------r-I ' o 1 2 LOG LENGTH{L) Fig 2 log weight is plotted against log length. Fish under 60 em total length are grouped in 1 em length classes, those over 60 em in 2 ernem length classes. The points fall about a series of three straight lines, the regressions for these, together with the length of fish to which they apply are given in Fig 2. The relationship for fish over 100 emern is not clear, these fish tending to gain weight less quickly than those between 49 em and 100 em. 12 Discussion A triphasic length-weight relationship for Lates in Lake Victoria was found by Gee (1969) with break points at 24 cm and 51 cm. The reason for these breakpoints is not clear and it should be noted that Hopson (1972) did not observe discontinuities of this sort when studying Lates on Lake Chad. Where they are observed they may be correlated with endo crinological changes associated with sexual development. The 16 cm break point may be correlated with the onset of sexual development (the first macroscopically detectable signs of maturation are visible at 25 cm among males) The 49 cm break point may relate to the onset of sexual ~ maturity (the first mature fish was a male of 50 cm). Previous workers (Gee 1969; Hopson 1972) have compared the weight-length relationship of Lates from various waters in terms of the condition factor--factor- where condition factor (CF) 100 x WEIGHT LENGTH However, when growth is not allometric (when b in Weight = b Constant x Lengthb does not equal 3) these figures can be mis leading. For example, inspection of Gee's 1969 paper indicates that the CF of Lates from Lake Victoria varies from 1.27 to 1.63 depending on the size of the fish. An average condition factor is therefore, strongly influenced by the size/frequency dis tribution of fish sampled. In order to be useful condition factors should be expressed in strict conjunction with the length of the fish to which they apply, unless growth is isometric. 1313 The length/weight relationships of Lates from lakes Chad " (Hopson 1972), Turkana (Hopson 1981) and Victoria (present study) are compared in Fig 3. •• 181e 1 1Lake _Victoria 16 -~>- 14 l.::J Lakes ~ 12 I/Lak~Chad+ t- TurkanaTIJrkana ::I::I: 10 3 L.':Jl.::J Y=-(}0125XY=.(}012SX 8 -LLJI.LJ 3 6 4 ... 2 0 0 20 40 60 80 100 LENGTH-CM (Xl Fig 3 The weight-length relationships of Lates in Lakes Victoria, Turkana and Chad. In Lakes Turkana and Chad growth is isometric and the regression on the figure relates weight (Y) to length (X). Lates from Lakes Turkana and Chad show almost identical relationships. This is approximated by fish from Lake Victoria until a length of 60 em is reached. Above this size they become progressively heavier than indigenous Lates. The present length/weight relationship for Lates in Lake Victoria is not significantly different from that calculated during the 1960s8 (Gee 1969). 14 AGE AND GROWTH Two approaches were made in an effort to calculate the growth rates of Lates in Lake Victoria. 1. Scales and Otoliths were collected from fish over the size range 20 em - 106 em. At present the otoliths are being examined usin~ techniques described by ~nnella (1973) in an attempt to identify daily and lunar periodicities. If this proves possible the age, and back calculated age~length relationships of individual Lates will be determined. 2 A size-frequency distribution was composed and analysed. This is shown in Fig 4 • .. . 80 IN SET: MOSQUITO SEINE :c 50 l/) 50 300300 ~j~ MAIN GRAPH: BEACH - 60 SEINE '" -u.. 1 200 50 LL 100 u-a 40 IJLJ'oo J l - 0 I 1010 20 30 ~ 30 uJ co ~ 20 ::::J Z 10 I ~ 0 J ~ ~lrli~nm.,n 00 10 20 ·30"30 4040 50 60 70 80 90 100 110 120 130 SSIZE I Z E CLASS-CM Fig 4 The size-frequency distribution of Lates eau~ht in mosquito seines (main histogram). 1 cm length classes are plotted against frequency. The fish in the inset were caufi:t 0:1 the night of Aug-Jst ~6t:. TheTh.e rest were captured between 16th1 6t h .';ugus:Augus: a::da~d 3rd SeptemSep(er;;b-2::"..or ber. Every fish was measured from each ha~l. 1515 ;1 A common problem with data of this kind is that gear selectselect- ivity and.ana variat.ionvariation of habitat preference with size (a known known ,j .'.1 " phenomenon in small Lates) can produce artificial peaks. ,I i However Fig 5 shows that the size-frequency distribution of juvenile Lates, eaten by larger Lates, is very similar in form ~ ~ to that collected in the mosquito seine. The fact that these these .'t larger Lates were caught 200 - 500 m offshore suggests that the I 8 cm and 24 cm peaks reflect the real population structure in f r, the area. area. f 360 IN MOSUUnOMOSQ.UlTO SEINE 280 ~ CATCHES :c III 200 VI -120 LL. 40 LL. ... LL. 0 0 0 ex: """'co 20 i ..... IN Lates STOl"'iACHS L 16 ::::l z 12 B 4 !,! 0 I 0 4 8 12 16 20 24 28 j: r SSIZE I Z E CLAss·eMCLASS-CM I' fl:1 i Fig 5 ThisTh is figure compares the size-frequency distrdistrib~tion_.ofibu.t ion.. of i small Lates Obtained ~y two independent sampline methods. The top histogram shows the distribution in mosquito seines cpera:edopera:cd from the shore at night. The botto~ histogram shows the distribution in the stomachs .... of largerlar~er Lates caught 200 - 500 m offshore. 1b It seems most likely that these peaks correspond to periods of intense spawning activity. The temperature of the Lake varies very little and is an almost constant 23 - 25 C, but there are two major seasonal events in the twice-yearly rainy seasons (Sep/Dec and Mar/May). It would seem probable that this periodic intense spawning is the result of Lates breeding in synchrony with this rainfall pattern. Supportive evidence for this suggestion comes from Lake Turkana where, in the absence of any real rainy season Lates niloticus spawns all year round. If this assumption is valid the ages of fish in the 8 cm and 24 cm peaks can be calculated by counting backwards, from the date of fish capture, to the previous rainy seasons. This is illustrated in Fig 6, and gives ages of 4 months ± 1, months for the 8 cm peak and 9, months ± 1, months for the 24 cm peak. w c:Jo...r->LJzcno::cr:>:Z--.J~o...l-> :::)uJLJOW«W~~WLJO Fig 6 Showing the period of data collection in relation to the two previous rainy seasons. Time runs from left to right. The periods during which the two peaks, at 8 cm and 24 cm, were probably spawned are indicated. The age is calculated by counting from these periods to the time of data collection. / // 17 Discussion Very few age/growth studies are available for comparison. Hopson(1972) aged Lates in Lake Chad by countin~ annual winter checks on scales, but the constant temperature of equatorial lakes and the resulting absence of seasonal checks on the scales limits the use of this method. Again Hopson (1981) noted that in Lake Turkana, where the temperature fluctuates little, as in Lake Victoria, the female Lates matured at lengths 35% greater than those in Lake Chad. He then assumed that females in Turkana and Chad matured at the same age, and adapted the age/length relationship he had estab lished for Lates in Lake Chad to model the growth of those in Lake Turkana.Turkana . ... This procedure might seem questionable in view of the fact that while male and female Lates grow at the same rate (Hopson 1972) male Lates in Lake Turkana mature at a size only 22% greater than those in Lake Chad, not, as with the females 35%. However, female Lates in Lake Victoria mature at the same length as those in Lake Turkana (as do the males). The age/length relationship of Lates in Lakes Chad and Turkana (using both 22% and 35% transformations of Lake Chad data) are shown in Fig 7. 18 160 140 • ~120. 120 ~ ~100u I 100 ::I:::I: t-t 80 l.:Jl.:J Z 60 UJ -J 40 Points 1+ 2 20 Calculated for present study .fI---_---.------.--_----.---.---~__._-~____.- o IC " ii' ii' , , oo 24 36 48 60 72 84 96 108 120 134 146 AGE INI N MONTHS ... Fig 7 Showing the age/length relationships of Lates in Lakes Chad and Turkana (using both 22% and 35% transformations of Lake Chad data). Points 1 and 2 with ranges of ± l! months are plotted for Lake Victoria (present study). The points from the presen~ study closely match the 35% conversion curve produced by Hopson for Lake Turkana. This supports his assumption that the size of ~males at first maturity is a reliable measure of growth rate. Until further data are analysed it seems reasonable to take the age/length relationship calculated by Hopson as applicable to the growth rate of Lates in Lake Victoria. 1 19 BREEDING The short duration of the present study made it impossible to thoroughly investigate the breeding of Lates in Lake Victoria. The data that were collected are presented below. The sex and maturity of all Lates examined was assessed using criteria developed by Hopson (1972) adapted to suit the present investigation. The stages of gonad development recognised are tabulated below in table 1. TABLE 1 Male '" Stage Criteria 1 1 immature Testes a pair of thin transparent sacs running along the dorsal wall of the body cavity. Sexes indistinguishable macroscopically. 2 2 early Testes semi transpar~nt and flattened. developing 3 3 late Testes white/milky. More or less flattened. developing No milt exudes when cut. 4 4 mature/ Testes white, firm + triangular in cross resting section. Small amount of milt when cut. S S mature Testes white, soft + trianeular with well rounded ed~p.!=:. rn"";,....,1'1~ ..-.:.,-&------.. I I 2020 Female Stage Criteria I immature Ovaries a pair of transparent sacs running along the dorsal wall of the body cavity. Sexes indistinguishable macroscopically. 2 resting Ovary clear-pinkish. Smooth firm ++ cylindrical. Cells colourless + transparent.transparent. 3 early Ovary pinkish. Smooth firm + cylindrical.cylindrical. maturing Small ova <0.2<~2 rom in a transparent matrixmatrix of follicular cells.cells. Ova smaller volume than matrix.matrix. 4 late Ovary pinkish - yellow. Pear shaped in crosscross maturing section. Opaque yellow ova between 0.2 andand .. 0.5 rom in a transparent matrix of follicular cells. Ova larger volume than matrix. 5 mature Ovary yellow + soft. Yellow ova 0.5 - 0.75 rom in diameter are visible through super-super ficial membrane. Ova tightly packed. Little or no follicular matrix present. 6 ripe/ As above, but ova extrude from vent when running pressure is applied from pectoral fin to vent. 7 spent Ovaries loose + flabby containing torn follicular tissue rich in blood. A fewfew scattered residual stage 5 ova. 'J' # 2121 The percenta~e of male and female Lates in various stages of maturity are tabulated against size class below in Table 2. Table 2 MALE FEMALE Length No. fish - S1a2E (X) No. Fish Stage %) 2 3 4 5 6 2 3 4 5 6 25-30 1 100 - - - ------30-35 3 66 30 ------;.. --- - 35-40 5 80 20 ------l"" - - 40-45 9 89 11 - - .-. 4-4 100 - ;.. - - 45-50 12 75 25 - - - 2 100 - - - - 50~55 11 81 - 9 9 - 4 100 ------55-60 17 35 29 18 12 6 8 100 ------60-65 27 22 8 19 12 38 1 100 - - - - 65-70 33 6 3 16 5 '70 1 100 I ------70-75 52 2 4 23 8 63 3 100 ------75-80 58 - 2 14 5 79 7 85 15 ------80-85 37 - 3 5 16 76 12 42 17 - 41 - 85-90 34 - 3 12 9 76 21 33 10 10 47 - 90-95 16 - - 6 6 88 37 .. 24 11 11 44 - 95-100 2 1~ " 95-100 2 - - - 1* 52 19 17 12 42 - 100-105 2 1 l' 1~ - - 47 26 11 11 44 - 105-110 ------34 29 18 15 38 - 110-115 ------21 19 15 19 47 - >115 I ------23 22 - 13 65 - The proportion of the male and female population at stages 5 " 6 is graphed in Fig. 8. For males this is equivalent to the proportion that have spawned,-or are capable of spawning. But females revert to stage 2 after spawning (Hopson 1912) so that Fig. 8 underestimates the proportion that have spawned. <), ""~" .' It 2222 100 100 wu.J Lake Victoria MALES ~ 80 I « Fig 8 Showing the proportion of the male and female poppop- u1ationulation in stages 5 and 6 (mature) against size class (for Lake Victoria) . Sex ratios (number of females per 100 males) " During the present investigation males outnumbered females in all size classes under 90 cm. The sex ratio is plotted against size in Fig 9. 240 Lak e VictoriaVidoria 200 o :160 « o0 ~--~~~-~---~1 ~ 40 50 60 70 80 90 100 110 120 Fig 9 Showing the sex ratio (Number of females per 100 males) plotted against size class (for Lake Victoria). The 1:1 ratio is marked on the figure and corresponds to 100 females per 100 males. ; 23 23 Hopson (1972 1981) and Gee (1963) observed similar skewed ratios. The sex ratio and proportion of males and females at stage 5 and 6 (mature) in Lake Chad are plotted against size class in Figs 10 and 11 for comparison. 100100 ~a::\.U 1 Lake [had[had ::::> BO ~t-« / MALES L 60 ~ ~ 40 40 z ~ FEMALES t:::LJ 20 j / / 0::: L.LJ\.U 0- 0.. 0 40 50 60 70 80 90 100 S IIZ Z E C[ LAS S-[-( M Fig 10 Showing the proportion of the male and female poppop- ulation in stages 5 and 6 (mature) against size class (for Lake Chad) • 240 Lake [had(had 200 • o :=160:160 males) plotted against size class (for Lake Chad). The 1:1 ratio is marked on the figure and corresponds to 100 females per 100 males. Hopson considered the paucity of females the result of sex differences in habitat preference, females preferring shallower water,prior to maturation, thanCb rrales and provides some evidence to support this. ::. 24 During the present investigation the sex ratios were ten .; times as skewed (3 females per 100 males) as in Hopson's Lake Chad study. In addition fish were all caught from the shore. The virtual absence of females cannot therefore be explained by them inhabiting shallower waters. If, lakewide, the sex ratio approximates 100 in all size classes then females under 40 cm must be restricted to a different habitat type from males. In Fig 12 the sex ratio of fish at Ulugi Beach has been superimposed on the size frequency distribution. 400 :::I: 300 =MALE V) .. --:: FEMALE -u... <> =IMMATURE. .. u...200 o0 I> 0:: u.J CO 100 :I: <> ::::> z o0 o0 20 40 60 80 100 120 SSIZE I Z E CLASS-CM Fig 12 shows the sex ratio superimposed on the size frequency distributon of the catch at Ulugi Beach. The contribution of immature, male and female fish to each size class is shown. The above figure shows that both the abRnlllTl'>ab!=:nlllT'::> ""mhon".."no...... 25 25 It is possible that females over 60 cm leave some unknown habitat to mix with the male population, althoueh I have no evidence, other than Hopson's findings (1972 1981) for suggesting this. An alternative is that Lates indulges in protandry. The rarity of small females could then be explained, as could the absence of large males (whose absence has been noted by other workers on Lates). Although a sex change may seem unlikely, P.H. Greenwood (Pers comm) tells me that this phenomenon is found amongst the Serranids, which are quite closely related to Lates. Dr G. Fryer recently sent me a paper on "The Atya-like .... Shrimps of the Indo-Pacific region"region'! (Chace 1983). These animals are in some locations protandrous, and the relationship between size-frequency distributions of male and female shrimps closely resemble those found for Lates in the present study. This aspect of Lates biology is not resolved in Lake Victoria. The 'unanswered questions are important ones. It is an area well worth further investigation.investigation . .. 26 FOOD AND FEEDING HABITS HABITS Methods Methods Assessing the importance of food types. types. When assessing the importance of food items in a fishes diet a volumetric method is most satisfactory (Hynes 1950) as this most accurately reflects the food value of various items relative to one another. For this reason and to facilitate comparison, the method used by Hopson (1972) while studying Lates in Lake Chad was adopted. Stomachs were awarded an index of fullness from 0 - 5 on criteria tabulated below (Table 3). TABLE 3 .. Index Fullness 0 No recognisable contents - empty. empty. 1 Recognisable contents 25% full. full. 2 >>25%25% full < 50% full. full. 3 >50% full < 75% full. full. 4 >75% fullfull<< 100% full. full. 5 Distended. Distended. The dominant food type by volume was then awarded points equalling the index of fullness. Any subsidiary food types were awarded one point. The importance of a food type can then be expressed as the , percentage given by - 27 27 Number of points contributed by the food item in question x 100 Total number of points awarded Ascertaining the length of prey Prey fish (usually juvenile Lates) were often in an advanced state of digestion. In order to determine the total length of these, the relationship between the size of certain persistent bones, and total fish length were calculated, using juvenile Lates caught by seineing. These relationships are presented in the appendix. Determining digestive rates In order to determine digestive rates common food items were categorised into digestive stages. .. The digestive stages used for the freshwater prawn, Caridina nilotica, were based upon the condition of the body to which the easily recognisable eyes were attached. These are tabulated overleaf (Table 4). 2828 TABLE 44 Stage Criteria 0 No noticeable digestion has occurred and Very freshly the abdomen is curled under the body. ingested 1 The prawn has uncurled. The exoskeleton is Freshly still rigid and the abdomen has a .... ingested pronounced hump. 2 The exoskeleton is soft and the body floppy. .... Early The adominal hump is not presentpresent.• digested 3 The abdomen has been lost. The eyes are Late attached to a portion of carapace including Digested recognisable "whiskers" (remains of rostrum + antennae) . 4 The eyes are separate from any recognisable Very late exoskeleton fragments. They may be digested separate or joined into pairs by membranous tissue Fish prey were also categorised into digestive stages, but because of the small numbers examined the classification was not useful in analysis and is omitted here.here • • ~ GIl ~ ::tl CD en ~ ~ ~ ("I("I- ::r en CD + TABLE (5) Showing the importance of various food types against Size Class. The figures given given f-I. 9 ll) are , '0 ::l of the total points awarded to that Size Class. Class. 0 ll) 11 ~ Average index of fullness is also given. given. ("I("I- '<: ll) en ::l f-I. (') en CD Size Class 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 7 0 H, 0 79 32 ~ 35 :a> 19 24 16 17 Z1 :;B J4 53 64 49 $ 53 54 126 Number of fish 79 66 35 :a> < The importance of Caridina compared to fish, and to other, invertebrate food items is presented in Fig 13. 100 - 100 I f- r- Caridina nitoti ca rrmr~ - Caridina nitoti ca 80 Ilf- 80 1 -I- r-- 60 - L- 40 I- l- V>~r- 20 ~.... z: ~ --.z: ~ 0 h n --.J 100 All fish ~ ~ 80BO ~L.r-rI- LL. .... o~ 60 -- u.J L.:J .... ,...... ,- ,.... -- ~ 40 rl.-- :z u.J - LJ 20 ...... I--Jl- 0::: UJ 0... o rrlI Other invertebrates r 20 -fl~ oO+--l-....L...... l-J--.t=-~..L..+-J.~I--+-..J...+-l------oo 20 40 60 80 100 }100 SSIZE I Z E C LAS S- CCM M Fig 13 Showing the importance of Caridina relative to fish and other invertebrates, plotted against size class. The importance is expressed as percentage of total points. ~ 31 The prawn Caridina nilotica Early in the present investigation it became clear that the prawn Caridina nilotica was a major food item in the diet of Lates. For this reason I decided to make a detailed study of the feeding relationship of Lates with Caridina. This investigation was directed towards finding minimum and maximum daily prawn intakes, and estimating the minimum daily ration of Lates in relation to size. Attaching a time scale to the digestive stages In order to find the time taken for freshly ingested prawns to reach the recognised digestive stages the day was divided into three hour intervals. All prawns examined during the ... investigation were pooled into the period when the fish, from .. which they were collected, were caught.caught . Two separate pools were used, one for fish under 50 cm and one for fish over 50 cm. The contribution of the different digestive stages were calculated as a percentage - Number of prawns in pool in stage X x 100 Total number of prawns in pool The results of this analysis are shown in Figs 14 and 15. 95% confidence limits are atta~hed. ~ 32 32 ;, ~ 40 ~ ~ STAGE 0 V) 20 w ~L:J 0 !,.-' . ~ .. .L~AA ;:f 0 vi wW 60 ]SSTAGESTAGES 0 1 > ~ t--t- ., w~V1 40 L:J~ ~ ~., o 20 V1V) ::> ~ B o 0+------o .B ~ex ~ BO ~ STAGSTAGESES 2 3 z:z ~ - 60 V1V) .. .. z ~ 40 ex .. CL u... 20 o c w~ 0 l.-T~-----~4_------- c L:J « , , , ~40 , , , , , w , , , , L.J , , , , 2 .... , 5ffi 200 , , .... CL .... , o ISTAQESTAGE 4 " D 24 3 6 9 12 15 18 21 24 THREE HOUR TIME PERIODS Fig 14 Showing the percentage of prawns in various digestive stages, in relation to three hour time periods, for Lates under 50cm. 95% confidence limits are attached • .. 33 33 • KMFltl LIBRAR~LltlRAR~ '" V) wW 20 L.:J l. STAGE 0 '*'- ~ o0 JSTAGE 0 - 1. ""'- A V) V) 60 l..LJ > ~ 40 v1 w L.:J 8~ 20 0 ~TAGESSTAGES 0+1 "1 B V) o0 => a0 ~ 0:: 10 0 ~ z 80 ~ V) z:~ 60 33; <{ '" 0::~ 40 a...... LL 6a 20 ~w 0 STAGES 2+3 c ~ 0 hTAGES ~ 2 z: 20a l..LJL.LJ LJ aolSTAGESTAGE 4 ~ ~l.~D0 0:: wW a... 24 3 6 9 12 15 18 21 24 THREE HOURHOU R TIME PERIODS Fig 15 Showing the percentage of prawns in various digestive stages, in relation to three hour time periods, for Lates over 50 cm. 95% confidence limits are attached. 34 34 Lates under 50 cm The two peaks in the proportion of freshly ingested Caridina (Fig 14 b) indicate that small Lates feed most in tensively during morning and late afternoon. The absence of any stage 0a prawns in Lates captured after dark suggests they do not feed on Caridina at night. These two peaks of freshly ingested prawns occur 3 and 6 6 hours before peaks in the proportion of digested prawns (Fig(Fig 14 c). c). The time taken for a freshly ingested prawn to reach reach stages 2 + 3 is, therefore, between 3 and 6 hours. hours. The proportion of very digested prawns (stage 4) peaks during darkness. The form of the peak suggests that prawns take between 3 and 6 hours to pass from stages 2 + 3 to stage 4. The absence of stage 4 prawns in the morning hours demon strates that by this time all prawns ingested the previous day have been fully digested. Lates over 50 cm Feeding peaks are not so obvious in larger Lates (Fig 15). I would suggest that these fish cannot capture prawns more efficiently than smaller Lates, an idea substantiated by the fact that the actual number of stage a0 prawns was no greater in large fish than in smaller ones, suggesting a similar foraging rate. However, the food requirements, and stomach capacity ofof fish over 50cm50 cm requires,and allows them to ingest, larger larger numbers of prawns than their smaller brethren. brethren. It may be for these reasons that feeding intensity is not peaked but sustained . •• 35 Examination of Fig 15 b + cC suggests that, as in smaller fish, prawns progress from stages 0 + 1 to 2 + 3 in approximately 3 - 6 hours. It is likely that the time taken for prawns to progress from stages 2 + 3 to stage 4 is also similar, and that all prawns ingested in one day have been digested by the following morning. Estimating minimum + maximum daily prawn consumption The number of prawns per fish based on the number of fish containing this food item is shown inFig 16 A. The number of prawns per fish, based on the total number of fish examined is shown in Fig 16 B. 280 l A 240 V'l200 z ... 3:60 20 2: LdllillWlb; h 0 0 20 40 60 80 100 )100 SIZES I Z E CLASS-CM Fig 16 Showing AA- - the average number of prawns per fish, based on the number of fish containing this food item, against size class. BB- - The average number of prawns per fish based on the total number of fish examined, against size class. if 36 Examination of these figures indicates that, when feeding on prawns, larger fish tend to consume more than smaller ones, but tend to feed on prawns less frequently. I have shown that all prawns consumed in one day have been digested by the morning of the next. The average number of prawns per fish, based on the total number of fish examined, can, therefore be taken as a minimum figure for daily consumption. This is because all prawns counted in a fish were consumed on the day of its capture, which prevented the fish from eating more prawns that day. The maximum number of prawns found in anyone fish is plotted against size class in Fig 17. These figures can be taken as the maximum possible daily prawn intake. 1000 I t/) FOR Lates (7(km (7(km u:: 1 1 0: 800 MAX=02"LENGTH h: ~ ~ 600 ~ ~ as 400 asco ~ z=> ?5 200 ~x <: ~ 0 J Hl n 0 20 4[) 60 80 100 )100 SIZE CLASS-eM Fig 17 Showing the maximum number of prawns per fish against size class. Up to 70 cm length this maximum number can be modelled by the exponential. 2 Maximum number = 0.2 x Length • of prawns 31-3r Above 70 cm Lates are unable to feed heavily on prawns. ~ I suggest this is because the gill raker spacing, which increases ... in a linear fashion becomes too large. Prawns will then be expelled through the gill rakers with the exhalent flow during feeding. Converting numbers of prawns consumed into grammes Using the relationship developed by . McLeod (1981) First, the average total length of 20 mm, established for prawns consumed by Lates in Lake Victoria, is converted into carapace length using - TL = 1.51 + 2.13 CL To give an average carapace length of 7.88 mm. This is converted to weight using - -44 Log W = Log 5.26 x 10-10 + 2.39 Log CL. To give an average weight of 0.073 g per prawn. Using the maximum + minimum estimates for numbers consumed the max + min consumption by weight can be calculated and is ... tabulated against size class in Table 6 following.following . TABLE (6) Size Cla.. 4~ CH ~ - 10 - 1~ - 20 - 2~ - 30 - 3~ - 40 - ...,. Pravn ConaUIDp1:1on 0.7 2.3 4.4 7.7 J1.3 H.O 20.5 26.3 (Gr_.> III H1n Pravn 4.2 Con.\llllP1:1on 0.2 0.2 0.4 2.9 4.7 6.3 6.7 (Gr...... l I S1z. Cla.. _ 7~ 8~ CH 4~ - ~O - ~~ - 60 - 6~ - 70 - 80 - IIax Pr..,. 40 48.2 57 66.1 24.9 4.0 5.8 Conlump1:1on 32.9 (Gr_.> Hin Pravn 0 conawn":?tion 2.2 6.3 1.3 3.3 2.7 0.7 0.2 0 <: (Gr_.> 38 If Fig 13 is consulted it is apparent that prawns form approximately 80% of the diet of Lates under 45 cm. For fish of this size range the minimum daily prawn intake could be considered 80% of the minimum daily meal. Fig 18 shows the estimated minimum daily meal (grammes of prawns ~ 0.8) plotted against standard length (Total length - ~ 1.18) for Lates under 45 cm. Values determined by Hamblyn (1966) for tank fed Lates and by Chilvers and Gee (1973) for Bagrus docmae in Lake Victoria are plotted for comparison. 1·'·2 2 10 ~ ~ 0·8 o £3 0·6 .. L::L =>~ 8' ~ 04 -.I'-' oa n')~ l...J >::t;::t .. VIl....J 0.2 t» VI 0·2 t»11- lLJlL.J 9' ~ 0 'itT' '/II « ...&4.,. Tank fed Lates IHamblyn,19661 e= Bagrus docmac IChilvers+Gee.1973lIChilvers+Gee,1973l Fig 18 showing the estimated minimum daily meal for Lates under under 45 cm plotted against standard length. The regression for a line,line, fitted to these points by eye, is given on the figure, Grammes Grammes per day = Y, standard length = x. X. Otl.er data are given for comparison as described.described. '" 39 39 The regression line on the figure can be converted to relate minimum daily meal to total length as - Log YY--= - 4.7 + 3.143 Log X. Where Y = Minimum daily meal. and X = Total length. The values calculated from this regression are approximately 50% of those found by Hamb1yn (op cit) and Chi1vers and Gee (op cit). The reason for this is probably because the mean capture time of ] Lates in the present study was around midday. As a result many fish involved in the above calculations had not completed feeding when they were captured. .. I have shown that prawn feeding occurred during both morning and afternoon. If feeding intensities were comparable over .. these two periods a mean capture time of 12.00 would under estimate the daily meal by a factor of 2. If this is a reasonable argument the calculated regression can be used as an estimate of 50% of the minimum daily meal, suggesting both Bagrus and Lates have similar rates of food intake. I suspect prawns fluctuated in importance as a food item, perhaps as a result of their breeding cycles during which their activity varied greatly. On certain overcast nights large numbers would congregate around the floating pressure lamps used to attract Engrau1icypris. This activity was not obviously correlated with the lunar cycle. " iii 40 <) Fish Fish became increasingly important in the diet as Lates increased in size, comprising more than 50% of the diet of Lates over 70 cm in length. The contribution of various prey species to the total fish intake is shown in Fig. 19. o Lates niloticus 40 20 o0 +------l=:L..I-L....-L-l--I-~..J_.J....I...l...J~1 'r''' """"""1 _ Vl t- ~z 20 Engraulicypris t- 20 Jl ~o arge!Li.&arge!u.s all a.. ~ 0 1---.- _ o 0 dJ 0O'------4-D..J.... D 1---.---_ --l~« IHaplochromineHaplochromine I-f- 10 ~ ~ 010 I--_,....J:::l-l-+--.....L.J.-.J-J-+-- 011 II 01 _ L1.. 10 Bagrus docmacdoc mac .. L1.. 10 o 0 t-----I --J.-L..,..II _ ~ Unidentified ccichlidichlid L:J o .. ;:! o I Ib 0 Z L.LJ a::~LJ 20 u.J a.. °l'l,rqql'=l"l""",Of-l-lc....L..,t:::l...4-.L-+-J....J,....o:=I-l-.;....L..l....L..J-..L..+-L.,--- 20 Potamon niloticus oOt-:--.--,...-l='-:-r--I" p ...... I I>---l-...J..-+- (I I'...... --- 10 01Ob-I--I:->...L..l=:o..,.j...L..l=1~.j....J.....,o=L-l- :~'~II:~_I __--- o0~...... =~,.;....::.,.:...... j:::J....,-.J...,J::J....,.....-~--~-- Gasijop§da : oo 20 40 ffJ 80 100 )100 SIZES I Z E [L AAS S S- [[M M Fig. 19 shows the importance of various fish species (and invertebrates) in the diet of Late~ against size-class. Juvenile Lates are by far the dominant prey fish (most unidentified fish were probably juvenile Lates in an advanced stage of digestion)digestion). . ., 41 41 In the 897 stomachs examined ten cichlids one clariid and one spec~en of Bagruswere identified. Engraulicypris, the second most important prey fish, may have been underestimated as it is rapidly digested and has no persistent identifiable bones. Prey size/Predator Size Relationships. Fig. 20 is a scatter graph of prey size (all juvenile Lates) against predator size. . ::£ ~ 20 ..!!' is fj"' • IP ..'" L , Q.; LJ.L.J 1515 'J~ I lot r;v LULLJ ~-:e! N N ~...... ~ ."• Vl - 1010 • • = . ••• • 'It:~&i· " " • > > II... ,."".. LULLJ ~ a! • ;• •«. .:M a:: a:: : ~ .' " ... ~ .... . Q.Q.. t ! ~ •" tI .'... 5 • . .'"" o o o 20 40 60 00 100 )100 SSIZE I Z E [LCL'ASS-CM'A S S-[ M Fig. 20 showing the scatter of prey size (all juvenile Lates) against predator size. 25% and 30% relationships are • givengiven. . 42 42 Lates is able to consume prey up to 25-30% of its total length (see Gee (1969) Okedi (1970) Hopson (1972)). In this study a similar limit is observed. There is no correlation between increasing predator size and increasing average prey size. In fact the size-frequency distribution of juvenile Lates taken from large~ Lates' stomachs closely resembles that from mosquito seine catches. This is shown below in Fig. 21. 360 IN MOSQUITO SEINE 280 CATCCATCHESHES :C 200 V1 -120 u..u... u.. 40 u... 0 o0 0 a::c:: L.LJl.LJ .. a:l 20 IN Lates SSTD:'1ACHSTO:'1ACHS L 16 • :=> • :zz 12 8 4 0 0 4 8 12 16 20 24 28 SSIZE12 E CLAS S-CS'C M Fig. 21 showing the size-frequency distribution of small Lates from mosquito seine catches (top) and larger Lates' stomachs (bottom). This suggests that large Lates are selecting prey at random from among the population of small Lates. Lates under 30 cm select prey from the short end of the distribution, a result of their inability to handle prey greater than 30% of their own length. 43 ~nvertebrate Food Other Than Caridina The importance of invertebrates other than Caridina is shown in Fig. 19. Large Lates occasionally gorged on Potaman niloticus, the freshwater crab. Individuals taken ranged from 3-7 cm carapace width. Anisoptera larvae were taken by Lates of all sizes. I suspect they may have fluctuated in importance But data was not collected over a long enough period to demonstrate this. Gastropods were occasionally found, these may have been ingested accidentally as empty shells were very common in the study area. .. However Hopson (1972) showed that Lates in lake Chad did feed on gastropods, actively selecting living molluscs from a •.. background of empty shells.shells . ... 44 4 The Question of Empty Stomachs a During the present investigation it was very striking that large Lates almost invariably had empty stomachs. This is illustrated in Fig. 22. l/l l/l U.J 5 Z --l --l => 4 LL LL 0 3 X U.J 2 Cl Z - 1 > « 0 0 20 40 60 80 100 100 1 )100 l/l 80 t-t => l.:J 60 r-r t- .. 6=c.. 40 :L uJ ... aJ 20 O"tro ~ o0 Ii' , o 0 20 40 60 80 100 )100 SIZES I Z E CLASS-CM Fig. 22 showing the average index of fullness against size class (Top). And percentage of fish examined with empty stomachs against size class (Bottom). Although this phenomenon has been observed by other workers (Gee 1963 1969) the present study found a much higher incidence than was observed in Lake Chad by Hopson (1972) or Lake Victoria during the late 1960s (Okedi 1970). 4545 • Using the regression for minimum daily meal it is calculated that a 100 cm 18 kg Lates would require at least five 8 cm prey per day. The 126 Lates examined, that were over 100 cm had, on average 0.06 fish per stomach. A probable explanation for this observation is that large Lates fed at night and had digested their meal by the start of fishing operations at Ulugi Beach, Unfortunately, I was not able to sample at night to test this idea. Discussion During the 1960s the Lates in Lake Victoria were almost entirely piscivorous (Gee 1969, Harnblyn 1960, Okedi 1970) with Haplochromine fishes comprising more than 80% of the prey . .. As I have shown the feeding habits in the present study area u are very different from those observed by earlier workers. This change will be discussed in detail later. The present feeding habits of Lates in Lake Victoria bear many similarities to those observed in Lake Chad by Hopson (1972). He found fish steadily replaced the freshwater prawn Macrobranchium niloticum as the dominant food item for growing Lates. Lates of 60 cm derived 50% of their food from fish and those of 100 cm were almost 100% piscivorous. A principle difference in Hopson's findings is that the fish component was comprised of seven commonly occurring species, and did not include significant numbers of juvenile Lat~~. '" 46 46 In both Lake Albert and Lake Turkana Lates niloticus coexist with one other species: Lates macropthalmus and Lates longis pinus respectively. In these lakes L. nicoticus inhabits the inshore areas and is almost entirely piscivorous, whereas the smaller congener, which inhabits the offshore region, feeds principally on the freshwater prawn Caridina and/or Macrobrachium (see Holden 1967, Hopson, McLeod, Harbott and Ogari 1981). In these two lakes the two species split the food resources which, elsewhere, in Lake Chad and Lake Victoria, are utilised by one species, Lates niloticus. " uu •• 47 AN ANALYSIS OF THE ECOLOGICAL CHANGES PRODUCED BY LATES IN THE KAVIRONDO GULF Lates has interacted with other fish of the Gulf in at least three ways. As a predator, as a competitor, and as a source of food. In this section I want to consider the first two of these interactions, and summarise the changes they have produced inin thethe Gulf's ecology. Lates as a Predator The data collected during the 1960s by Hamb1yn (1961), Gee (1963) and Okedi (1970) on Lates feeding habits in Lake Victoria provide information on early predator-prey interactions. Data from Gee (1963) is given below in table 7. .... TABLE 7 Prey Species No of Prey % % Total No of Lates Mormyriidae spp. 31 20% 21 Alestes spp. 5 3~ 4 Engraulicypris 7 4% 6 Barbus spp 3 2% 1 Clarias spp 1 1 o~ 1 Cichlidae spp 34 ) 15 15 ) Haplochromine 58 ) 70% 24 24 ) Tilapiine 18 ) 8 8 Unidentified 16 They indicate that, during the early 1960s Lates predated mainly on cichlids, but also on small mormyrids, and to a lesser extent, other taxa . • 48 Lates and the small Non-Cichlids It is hard to assess the impact Lates predation has had on the populations of small fish such as the Marmyrids, Characids, and Barbus species, because few reliable data on their past and present populations exist. The absence of these fish from Lates stomachs at Ulugi Beach, and numerous conversations with Luo fishermen suggest that these small species are far rarer now than a few years ago. Many small Non-Cichlids are known to favour habitats in the lake which most closely resemble riverine conditions (Corbet 1961). These areas, often rocky, or hard bottomed, with steeply sloping bottom and wind generated currents are also favoured by Lates. ... Sharing this habitat preference with Lates must have made these small and juvenile Non-Cichlids particularly vulnerable to predation. Lates and the Haplochromines The most striking result of the recent population explosion of Lates has been the virtual disappearance of the previously very abundant Haplochromines. The collapse of the Haplochromine stocks is well illustrated by data on the populations of larger fish, collected by the Kenya Marine and Fisheries Research Institute (KMFRI) between 1977 and 1981, using a research trawl boat. Data relevant to the Haplochromine stocks are presented in Fig 23. 4!-, •• ------. _.. -- - --....,.-.- - - 49 49 80 . Lates niloticus 70 0:: u.J CD 60 ~ ~ :z SO50 >-> CD I :r: 40 w I-I «w 30 20 10 :r: w 0::« 0 u.Ju.J VIVI Haplochromines u.J 40 ~ II 0:: U- 30 . 0 QJ l? 20 bQ .. 10 0 ... 1977 -i978'i978 1979 1980 1981 Y E A R Fig 23 Showing the percentage, by number, of Haplochromines and Lates in KMFRI trawl catches between 1977 and 1981. ~ased on a total of 29,000 fish. Mainga (Pers corom) head of the Kisumu Research Lab, considers that Haplochromines have been reduced from over 80% of the fish biomass to less than 2%. 50 50 Lates and the Tilapiines The impact that Lates predation would have on the Tilapiine fishes was of much concern, both before and after its appearance in Lake Victoria. Examination of the KMFRI trawl data, and the catch statistics do not reveal any obvious decline up to 1981. However, according to my host at Ulugi Beach his catch in the late 1970s was almost entirely Tilapiine, with catches of 50 - 100 fish per day being commonplace. Though he now fishes over exactly the same ground, with the same nets (although they are now reinforced by a stout bag) the Tilapiines have all but disappeared. During the 8 weeks of my stay the six - twelve hauls of the beach seine per day rarely caught a single Tilapiine, or, any other fish, apart - from Lates. During the past five years the abundance of Lates has increased enormously. They are now his sole commercial catch, one to two hundred fish, weighing between one and two thousand kilos being a typical day's return. These observations suggest there may be a connection between the disappearance of Tilapiine fishes and the recent population explosion of Lates.Lates . .. 5151 Lates and the Lungfish Protopterus aethiopicus The absence of a fish species from Gee's data (Table 7) does not mean that the species in question escaped ~jgnificantpredation Lates is a very opportunistic feeder. Its food intake depends on prey abundance and prey size. These two variables depend on season and location. In certain areas, at certain times of year certain species may suffer particularly heavy predation. For example Protopterus, formerly a common fish has suffered a drastic fall in numbers. It is known that the young of this species may be subject to intense predation by Lates as they leave their papyrus nurseries (Obea and Mainga Pers comm). It is also possible that concentrations of young Of anadromous fish suffer heavy predation as they descend the rivers and enter the lake. Lates as a competitor At one time or another Lates probably consumes every species of fish in the lake. As a result it is impossible to separate the effects of competition from predation. However, members of the Haplocromine species flock were the main food of the lakes piscivores. Lates has, as we have seen, reduced this previously superabundant resource to a shadow of its former importance. In doing so it has deprived these piscivores of their former mainstay. 52 Bagrus docmac and Clarias mossambicus The two major Non-Cichlid predators were the plentiful si1uroidssiluroids Bagrus docmac and ClariasC1arias mossambicus. Bagrus was an important and specialist predator on Haplochromines. Chilvers and Gee (1977) estimated it consumed 75% of the Haplochromine standing crop per annum. Clarias was more omnivorous but derived the bulk of its diet from Haplochromines (Corbet 1961). Fig 24 shows that these two siluroids,si1uroids, which in 1970 comprised 10% of the fish biomass, have, as a result of competition with, and predation by Lates, been reduced to a very low number. 0::: :r: as 20 Bagrus docmac LJL .. ~~ 10 UJ(/)> UJO) 0 o:::± 1977 1978 1979 1980 1981 W u..~ o Fig 24 showing the percentage by number of Bagrus and Clarias in KMFRI trawl catche~between 1977 and 1981. Based on a total of 29.000 fi~h . .... 53 Other Competitors The siluroid Shilbe mystus, Mormyrus macrocephalus, Protopterus, and large Barbus altianalis, also derived a considerable proportion of their diet from Haplochromines (Corbet 1961). All four species are less common now than in 1970, and competition with (as well as predation by) Lates may have contributed to their disappearance. illill .... 54 54 FoodFood Webs,Webs, TrophicTrophic DiversityDiversity andand NutrientNutrient CyclingCycling ByBy reducingreducing thethe previouslypreviously dominantdominant assemblageassemblage ofof HaploHaplo- chromines,chromines, andand theirtheir predators,predators, toto veryvery lowlow levelslevels LatesLates hashas fundamentallyfundamentally alteredaltered thethe original,original, CichlidCichlid dominated ecosystemecosystem ofof thethe Gulf. The The importanceimportance of thesethese changes are summarised in Fig 25. 1983 Lates nlloticus 1970 Haplochromine )80%)80% ~- ~ ":2. ~ ~ 1 )80% /1(r~ ('l. 't>~ "..l\, ~ UHa~lochrom,"U f!£::'I Gi:\ ~ ProtoptfrUS \J FigFig 25 shows the 1970, Cichlid dominated ecosystem on the left, andand thethe 1983, Lates dominated ecosystem on the right. The area of the pies is proportional toto biomass. TheThe 19701970 figuresfigures are basedbased onon KudhonganiaKudhongania (1972).(1972). TheThe 1983 figuresfigures onon a transformatontransformaton ofof 19701970 figuresfigures usingusing KMFRIKMFRI trawltrawl data.data. fl 5~ The slices of the Hap10chromine pie are not proportional to biomass. They are proportional to the number of described species, exhibiting various trophic specia1isations, that are known to inhabit depth less than 20 m (see Greenwood 1981). The 1983 pies for Hap10chromines, Protopterus, Bagrus and C1arias are maximum estimates. The real figures are almost certainly smaller. The 1983 pies for Ti1apiine and Non-Cich1id species are also optimistic. From evidence already given it is clear that their numbers have gone down severely since 1970. However, as definite figures are unavailable the 1970 biomass has been presented undiminished. The change is from a richly po1yspecific, trophica11y diverse system to a virtually monospecific, trophica11y restricted oo system, dominated by Lates and its predatory and competitive interactions. In some areas this loss of diversity is dramatically obvious. Comparison of beach seine catches at Kendu Bay in 1950, with the total catch record for Kendu Bay in 1982 illustrates this point overleaf (Table 8). 1950 data are from the Lake Victoria Fisheries Service Annual Report of 1950. The 1982 Data are from the 1982 Fisheries Annual Statistical Report, published by the Fisheries Department, NairobiNairobi. . .. 56 56 TABLE 8 Fish Species 1950 1983 No of Individuals Kilogrammes Haplochromines 650,000 Sarotherodon 30,000 esculenta Sarotherodon 3,000 variabilis Labeo 87,000 victorianus Alestes nurse 84,000 Barbus 21,000 radcliffi Synodontis spp 12,000 Bagrus docmac 6,000 Shilbe mystus 42,000 .. Clarias 2,000 4 mossambicus Mormyrus 400 kannume Lates 245,107 niloticus Sarotherodon 132 niloticus Table 8 An illustration of decreasing species diversity at Kendu Bay in the Kavirondo Gulf. Numbers of individuals caught in beach seines in 1950 are compared to the kg of various species landed by all methods in 1983. There is evidence that productivity in Lake Victoria is tightly controlled by rates of nutrient cycling (see Beauchamp 1967 and previous EAFFRO reports).reports) • ' . " ''- ---.,..~-li---:-"" c::::: ...... *. 51 ~ i. h The Haplochromines were previously the major elements in this process, making a major contribution to the lakes bionomics. It is questionable whether the longer lived, and trophically restricted Lates will cycle nutrients as rapidly as this multispecific system. This is a complex question but may prove to be an important one.one . .. ... 58 CONCLUSIONS From the above analysis it is clear that Lates now dominates the ecosystem of the Kavirondo Gulf. It also forms the backbone of the commercial and subsistence fishery, and in many locations is the only fish of any import whatsoever. The important question is, how will the present situation develop~ Two observations bear on this question. 1 Lates as a Cannibal The cannibalistic habits of large Lates is reducing " recruitment to the fishery. The present population explosion of Lates began in 1977 (see Fig 23). By 1980 these fish reached 60 cm, a size at which small fish became important in their diet. (see Fig 6 for age length relationships and Fig 19 for feeding habits-size relarelationships).t ionships) . By 1980 Haplochromines and other small Non-Cichlids, had been virtually eliminated from open water (see Fig 23). The most abundant available prey item for these growing Lates became juveniles of their own species, and from 1980 onwards these must have suffered increasingly heavy predation, as more and more Lates were recruited to size classes over 60 cm. " 59 At this point Fig 4 takes on a new suggestive meaning and is reproduced here for convenience. 80 IN SET: MOSQUITOMOSG.UITO SEINE :::I: 50 1/1 JIO MAIN GRAPH: BEACH ..... 60 ~~ SEINE u.. 1 100 50 u.. I JIIiL f 100 o0 40 dllllll~n I 01 0 10 10 30 a::: 30 uJ co ~:L 20 => z 10 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 SSIZE I Z E CLASS-CM The possible selectivity of the sampling method (Beach i" Seine) on which this graph is based, and unknown variables in the distribution of the various size classes of Lates around the lake, make any inferences drawn from this figure fairly speculative. On the other hand beach seining is one of the most unun- selective methods of sampling, being comparable to trawling with a 2! inch mesh net. This figure shows that fish under 65 cm are less common than those above. Although this would be impossible for a stable population it may represent a biological reality in the Kavirondo Gulf, reflecting heavy predation on juvenile Lates for the past three years My personal observations in markets, and on fishing beaches around the Gulf, and conversations with fishmongers tended .. to confirm this scenarioscenario. • 60 2 Heavy Fishing Pressure on Juvenile Lates The recent increase in the Lates population (and decreasing returns from other fisheries) has caused many fishermen to join the Lates fishery. This has been accompanied by a large increase in the number of beach seines in operation. These are ,~ften small meshed (I! - 2! inch stretched mesh) and catch large numbers of 0 year fish (fish smaller than 25 cm). This fishing practice adversely affects the fishery in two ways. a) By removing juvenile Lates future recruitment to the fishery is reduced. SmallLates by virtue of their light weight do not make a significant contribution to the fishery. It is not until they obtain a length of 50 - 60 cm that their contribution becomes important (personal observation). This corresponds to an age of 2 years. b) Removal of small Lates decreases the amount of food available to larger Lates. These juveniles are the major food for bigger fish, which in turn support the fishery. They provide the link between Caridina and the larger fish (which as stated earlier, are unable to feed on Caridina). Both of these observations suggest that the population and standing crop of Lates is likely to decrease, perhaps dramatically, over the next 3 - 5 years. It is questionable whether this reduction in Lates population will be accompanied by an increase in numbers of those fish which its activities have driven to low abundance.abundance . .. 6161 SUMMARY Present Lates ecology in the Kavirondo Gulf 1 The weight-length relationship for Lates in Lake Victoria can be represented by three regression equations. Length range of LatesLat~s Length (L)( L) weight (W) relationship 7 - 16 cm Log W = Log 0.02040 + 2.8 Log L 16 - 49 cm Log W = Log 0.01690 + 2.9 Log L 49 - 100 cm Log W = Log 0.00096 + 3.64 Log L 2 The growth rate of Lates in Lake Victoria is reasonablyreasonably represented by the curve produced for Lates niloticus in LakeLake Turkana (Hopson 1981).1981). 3 Male Lates in Lake Victoria first mature at 50 cm, femalesfemales at 80 cm.cm. 4 Males were significantly more common than females in sizesize classes under 80 cm.cm. Females were significantly more common than males in size classesclasses over 90 cm.cm. This is not due to differences in growth rate. It may be the result of sex differences in habitat preference or protandry (sex change). 5 The freshwater prawn Caridina nilotica was the major food of Lates under 70 cm. Fish (mainly juvenile Lates) became increasingly important as Lates increased in size, being the dominant food type of " 62 62 Lates over 70 cm. 6 An estimate for 50% of the daily meal relates grammes consumed daily (Y) to total fish length (X). by Log Y = - 4.7 + 3.143 Log X. 7 Adult Lates selected prey (juvenile Lates) at random from their size-frequency distribution. 8 Lates consumed Caridina during daylight. Evidence suggests that fish meals were taken mainly at night. The Ecological changes wrought by Lates in the Kavirondo Gulf 9 The present diet of Lates in the Kavirondo Gulf is now totally different from its composition in the 1960s .. Caridina and juvenile Lates have replaced a diverse diet of Cichlids and Non-Cichlid fishes. 11 Predation by Lates and/or competition with Lates has reduced the population size of all other fish in the Gulf. In some cases this has been dramatic. 12 Haplochromines, once 80% of the biomass now comprise <2%.<2% • .,. 6363 13 The Haplochromine predators Clarias mossambicus and Bagrus docmac, once 10% of the biomass now comprise <2%. 14 These activities of Lates have resulted in a large decrease in diversity, Lates itself now comprises >80% of the biomass. 15 The situation is in a dynamic state. Evidence suggests that a considerable fall in Lates population and biomass is likely over the next 3 - 5 years. 64 64 ACKNOWLEDGEMENTS One cf the most enjoyable aspects of this project was the contact I made with those working in the field of fish and fisheries biology. It is a pleasure to acknowledge them here. In England my tutor Dr P.L. Miller, my supervisor Dr M. Coe, scientific adviser Dr G. Fryer F.R.S., Dr P.H. Greenwood and Dr J.M. Gee, all provided assistance in the form of advice, information and infective enthusiasm. In Kenya my supervisors Professor M. Hyder and Mr A. MacKay, Mr Kuria and the University of Nairobi, provided material support and advice. I am particularly indebted to Mr O.M. Mainga B.Sc, who allowed me to use Kenya Marine and Fisheries Research Institute records and Reports. Mr A. Obea of the Kenya Fisheries Department far exceeded his call of duty and in doing so provided invaluable assistance to the field study. Mr Kefa Odundo, an expert fisherman, accepted me as a member of his family for 8 weeks and did everything possible to help. Finally I would like to thank the sponsors who contributed towards the funds of the field expedition, without which this would have been impossible. 6565 REFERENCES ANDERSON, A.M. 1961. Further observations concerning the proposedproposed introduction of Nile Perch into Lake Victoria.Victoria. E. Afr. agric. for J. 26 : 195-201. BEAUCHAMP, R.S.A. 1964. The Rift Vally lakes of AfricaAfrica Verh. Int. ver. Limnol. 15 : 91-9991-99 BERGSTRAND, E and CORDONE, A.J. 19701970 Exploratory bottom trawling in Lake Victoria.Victoria. Afr. J. Trop. Hydrobiol. Fish. 1970.1970. BISHOP & TRENDALL 1967 Q.J. Geol. soc. London. 122:122 : 385-420 CHACE, F.A. Jr. (1983) The Atya-like shrimps of the Indo-Pacific Region (Decapoda:Atyidae) Smithsonian Contributions to Zoology 1-54. CHILVERS, R.M. and GEE, J.M. 1974. The food of Bagrus docmac (Forsk) (Pisces: Siluriformes) and its relationShip with Haplochramis Hilgendorf (Pisces: Cichlidae), in Lake Victoria, East Africa. J. Fish. Biol.6 : 483-505. CORBET, P.S. 1961 The food of non-cichlid fishes in the Lake VictoriaVictoria basin with remarks on their evolution and adaptation to the lacustrinelacustrine conditions.conditions. Proc. zool. Soc. Lond., 136: 1-101.1-101. DOORNKAMP, J.C. & TEMPLE P.H., 1966 Surface drainage and tectonictectonic instability in parts of southern Uganda.Uganda. Geogrl.J. 132 : 238-252.238-252. FISH, G.R. Same aspects of the respiration of six species of fishfish from Uganda.Uganda. J. expo Biol. 33 : 186-195. FRYER. G. 1960 concerning the Introduction of the Nile Perch into LakeLake Victoria.Victoria. E.Afr. agric. J. 26 :: GEE, J.M. 1964. Nile Perch Investigations.Investigations. Ann. Rep. E. Afr. freshw. Fish. Res. Org. 1962/1963 : 14-2414-24 GEE, J.M. 1965a The spread of the Nile Perch (lates niloticus) inin East Africa with comparable biological notes.notes. J.appl. Ecol. 2 : 407-408.407-408. GEE J.M. 1965b. Nile Perch Investigations.Investigations. Ann. Rep. E. Afr. Freshw. Fish. Res. Org 1954 13-17.13-17. GEE J.M. 1966. Nile Perch Studies. Ibid 1965 56-6256-62 GEE. J.M. 1967. Nile Perch Investigations. Ibid 1966 : 10.10. GEE. J.M. 1968. Nile Perch Investigations. Ibid 1967 : 1313 GEE, J.M. 1969. A comparison of certain aspects of the biology ofof Lates niloticus (Linne) in same east African lakes.lakes. Rev. Zool. Bot. Afr. 80 : 244-262.244-262. 66 66 GRAHAM. M. 1929. The Victoria Nyanza and its fisheries. A report on on the fishing survey of Lake Victoria. 1927-28 London : Crown Agents for for the Colonies.Colonies. GREENWOOD. P.H. 1966. The fishes of Uganda. Kampala: Uganda Society. Society. GREENWOOD. P.H. 1974. The Cichlid fishes of Lake Victoria, East East Africa: The biology and Evolution of a species flock. flock. Bull. Br. Mus. nat. Hist. (zool) suppl 6. 6. GREENWOOD P.H. 1981. The Haplochromine fishes of the east African African lakeslakes Kraus International publications. Munchen. Munchen. HAMBLYN., E.L. 1960a Preliminary notes on Lates from Buhaka Lake Lake Albert.Albert. Ann. Rep. E. Afr. Freshw. Fish. Res. Org 1959 : 30-51. 30-51. HAMBLYN, E.L. 1960b The Nile Perch Project. Project. Ibid. 1960 : 26-32.26-32. HAMBLYN, E.EL. 1960c The Food and Feeding behaviour of the Nile Nile Perch. Ibid 1960 : 33-34.33-34. HAMBLYN, E.L. 1962. Nile Perch Investigations. Ibid 1961 :: 23-28. 23-28. HAMBLYN. E.L. 1966. Some notes on the Nile Perch (Lates niloticus) in in the role of predator in fish farm ponds. Ibid 1965 : 14-17. 14-17. HAMBLYN. E.L. 1966. The food and feeding habits of the Nile Perch. Perch. Lates niloticus (Linne) (Pisces: centropomidae) centropomidae) Rev. Zool. Bot. Afr. 74 : 1-28.1-28. HOLDEN, M.J. 1967. The systematics of genus Lates (Teleostei (Teleostei CCentropomidae)entropomidae) in Lake Albert. East Africa. Africa. J. Zool. Lond. 151 : 329 - 342 HOPSON.A.J. 1972. A study of the Nile Perch in Lake Chad. Chad. Overseas research publication No.19. Foreign & Commonwealth Office. Office. Overseas Development Administration.Administration. HOPSON. A.J. McLEOD. A.A.Q.R. HARBOTT, B.J. OGARI, J.T.N. 1981. The The biology of perciform fishes in Lake Turnkana. A report on the the findings of the Lake Turnkana project 1972 - 1975. Vol.5. Chapter 11. 11. HYNES. H.B.N. 1950. The food of the freshwater sticklebacks sticklebacks (Gasterosteus aculeatus and Pygosteus pungitius) with a review of the the methods used in studies of the food of fishes. fishes. J. animo Ecol. 19 : 35-58. KENCHINGTON.F.E. 1939. Observations on the Nile Perch (Lates niloticus) in the Sudan. Proc. Zool. soc. Lond (ser A). 109 : 157-168. 6767 KENDALL 1969. An ecological history of the Lake Victoria basin.basin. Ecol. Monogr 39 : 121-76.121-76. KUDHONGANIA, W.A. 1972. Past trends and resent research on thethe fisheries of lake Victoria in relation to possible futurefuture developments.developments. Afr.J. Trop. Hyrobiol. Fish. Special Issue No.2. 93-106.93-106. KUDHONGANIA.A.W. and CORDONE. 1974 Bentho-spatial distribution patternpattern & biomass estimate of the major demersal fish in Lake Victoria.Victoria. Ibid.1971.Ibid.1971. McLEOD. A.A.Q.R. 1982. A study of the prawns Caridina nilotica (Roux)(Roux) and Macrobranchium niloticum (Roux) in Lake Turkana.Turkana. A report on the findings of the Lake Turkana Project. 1972-1975.1972-1975. Overseas Development Administration. Vol.1. chapter 4.4. OKEDI, J. 1971. Nile Perch Investigations.Investigations. Ann. Rep. E.Afr. freshw. Fish. Res. Org 1970.1970. PANELLA. 1974. Otolith growth patterns: an aid in age determinationdetermination in temperate and tropical fish.fish. Ageing of Fish. Unwin Brothers Ltd. Edited'by T.B. Bagenel. 28-40.28-40. TALLING. J.F. 1963. Origin of stratification in an African rift lake. limnol. Oceanogr. 8 : 68-78. TAlLING. J.F. and TALLING I.B. 1965. The chemical composition ofof African Lake waters.waters. Int. Revue. Ges. Hydrobiol. Hydrogr. 50 : 421-463.421-463. WHITEHEAD, P.J.P. 1959. The anadromous fishes of lake Victoria.Victoria. Revue. Zool. Bot. Afr.54 : 329-363.329-363. WORTHINGTON. E.B. 1929 Report of the fishing survey of Lakes AlbertAlbert and Kyoga.Kyoga. London: Crown Agents.Agents. 68 68 APPENDIX 1. Total len~th of Lates plotted on X axis. Length of spinal colunn of. Lates on y ayis. Length of ~'pinal Colurm - em ...... o o o ...... o • • • 69 69 APPENDIX 2. Total len~th of Lates plotted on on X axis. Length of eleithrum of Lates on y axis. axis. Lenqth of Cleithrum - ern. o ~ I-' 0 t"" ~ .....~ I (1 at: I-' CJl l / /