Dominance of introduced Nile tilapia, Oreochromis niloticus (L.) in Lake Victoria: A case of changing biology and ecosystem Njiru M.1, J.E. Ojuok2, A. Getabu2, M. Muchiri1, I. G. Cowx3 and J.B. Okeyo-Owuor4 1
Fisheries Department, Moi University, P. O. Box 1125, Eldoret, Kenya. fax +254 56 530045, 2 Kenya Marine and Fisheries Research Institute (KMFRI), P.O. Box 1881, Kisumu, Kenya 3 4
University of Hull International Fisheries Institute, Hull, HU6 7RX, UK
School of Environmental Studies, Moi University, P.O. Box 3900, Eldoret, Kenya 1
[email protected]
Abstract Nile tilapia, Oreochromis niloticus together with other tilapiines of Oreochromis. leucosticus, Tilapia. zillii and Sarotherodon melanopleudra (=T. rendalii) were introduced into Lake Victoria between 1951 and 1962 to boost the then declining fishery. Only O. niloticus was able to establish leading further to reduction in endemic tilapiines of Oreochromis variabilis and Oreochromis esculentus. O. niloticus currently forms the third commercially important species after introduced Nile perch, Lates niloticus and a native cyprinid Rastrineobola argentea, whereas other tilapias are extinct or are occasionally caught in the lake. Information was collected from by bottom trawling and from published literature to ascertain possible factors leading to dominance of O. niloticus. Compared to other tilapiines, Nile tilapia is widely distributed, feeds on a variety of food items, grows to larger sizes, is highly fecund and can survive in a wide range of physical chemical parameters in the lake. These attributes could probably be the reason for its dominance over the tilapiine groups in the lake. Studies further show that ecology and biology of the O. niloticus has changed probably in response to changes occurring in the ecosystem. Management measures to sustain the fishery which include reduced fishing pressure, use of legal fishing methods, and control of environmental degradation have been discussed Key Words: Overfishing, management, pollution; tilapia; ecology Running title: Dominance of Nile tilapia in Lake Victoria
Introduction Nile tilapia, Oreochromis niloticus (L) is presently widely distributed in many parts of the word (Trewavas, 1983). It occurs together with Sarotherodon galilaeus (L) and Tilapia zillii (Gervais) throughout much of its natural range in Palestine, the Nile, and across West Africa, and Lakes Turkana, Edward, George, Tanganyika and Albert in East Africa (Lowe McConnell, 1958; Trewavas, 1983). The herbivorous tilapiine, the Nile tilapia, Oreochromis niloticus (L.) was introduced in Lake Victoria in 1950s and 1960s to boost the then declining fishery (Welcomme 1967; Ogutu-Ohwayo 1990a). Currently Nile tilapia is the most commercially important tilapiine in Lake Victoria (Cowx et al. 2003; Njiru et al., 2005). This is in sharp contrast with the native species of 1950s and 1960s
of Oreochromis esculentus (Graham) and Oreochromis variabilis (Boulenger) Today O. niloticus constitutes the third most important fishery in Lake Victoria, after Nile perch, Lates niloticus (L.) and a native cyprinind, Rastrineobola argentea (Pellegrin). Increase in O. niloticus is attributed to over fishing of endemic tilapiines thus reducing competition, while swamps clearance could have increased it spawning areas (Balirwa, 1998). Nile tilapia can also survive a wide range of pH, resists low levels of dissolved oxygen and feeds on a variety of food items (Balirwa, 1998; Njiru et al., 2004). The paper evaluates possible factors which have led to the dominance of introduced Nile tilapia in Lake Victoria.
Introduction of tilapiines in Lake Victoria The first introduction of O. niloticus into Lake Victoria probably occurred in the early 1950s (LoweMcConnell, 1958; Welcomme, 1966, 1967; Balirwa, 1992). The stocking in Kenya and Tanzania waters were between 1956 and 1958 with fry from Kajjansi fish ponds in Uganda. In Uganda, a massive introduction, involving tens of thousands of fry whose origin is not well documented stocking were carried out from Entebbe between 1961 and 1962. Apart from O. niloticus, other tilapiines introduced into Lake Victoria between 1951 and 1962 were Oreochromis leucosticus, T. zillii and Sarotherodon melanopleudra (=T. rendalii)- the last originating from Zambia (Welcomme 1966, 1967). The picture of introductions is somewhat confused as Trewavas (1983) suggests that O. leucosticus came originally, by accident, with T. zillii from Lake Albert. She also points that the lake was stocked with Lake Victoria subspecies of O. niloticus (= O. niloticus vulcani). The aim of introduction was to increase the declining native tilapiines and return the use of 5’’ (127 mm) mesh gill nets because O. niloticus grows to a large size. Nile tilapia started appearing in commercial catches in 1960 and the objective on introduction was not realised even by 1963 because the species constituted only 1 per cent of the commercial catch (Welcomme, 1966; Balirwa, 1992). However, from 1965, it started featuring prominently in the commercial and encouraged a return of 5’’ gillnet.
255
stomachs preserved in 4% formalin. In the laboratory, the stomachs were dissected material constituting algae were thoroughly mixed with a small volume of water. A sub sample was then taken using a teat pipette and placed in a Sedgwick rafter cell, which carries a volume of 1ml. The food items were enumerated under a compound microscope (1000x) and the contents identified to the lowest possible taxonomic level using plankton keys. Selectivity of food items by O. niloticus was estimated using the Ivlev's index of electivity (E) (Strauss, 1979):
Methodology Fish samples were collected monthly by bottom trawling (head rope 22.6 m, codend mesh size 24.5 mm) from the Kenyan waters of Lake Victoria (Figure 1) between June 1998 and December 2000. The sample sites were defined using a Global Positioning System (GPS), and depth (m) estimated by an echo sounder. Stations 1, 2, 3, 6, 10 and 11 were < 5 m deep, 4, 7 and 12 were between 5 and 10 m deep, stations 5 and 8 had depths > 10 m and station 9 was 20 m deep. Fish caught were weighed and biomass was estimated using the swept area method (Sparre and Venema ,1998). Ovaries used to estimate fecundity was preserved in Gilson’s fluid and fecundity was estimated from total counts of ova in the most advanced state of development.
(E) = (ri-pi)/ (ri +pi), where, ri = proportion of i food item in fish stomach, pi = proportion of food item i in the water sample. Published and unpublished data on tilapiines especially in Lake Victoria were also gathered and used in this paper.
The samples for diet analysis were collected in June 1998 to December 2000, in July 2004, March 2005, June and September 2005 by bottom trawling. Fish 0
Ugand a
0
34 30 ’ E
34 E UGANDA
o Si
ve Ri
Lake Vict oria
r R
.
\r oi Nz
Kenya
Tanzania
R la Ya
er iv
11
Yala Swamp 10
R. So nd uM ir iu R .N ya nd o
00
Kisumu 1
Asembo Bay 4
6
Open water
Nyanza Gulf 8 9
2
3
5
Kendu Bay R.A w ac
Luanda Gembe
7
Homa Bay
h Ke n du
00 3 0 ’ E
R. O l uc h
N
Ku ja
International boundary 0
R.
12
TA NZA N IA
10
20
30
Kilometres
Figure 1. Map of Lake Victoria, Kenya showing the sampling sites. Catch trends The mean (±SD) catches of O. niloticus ranged from 6.99 ± 2.16 to 40.6 ± 21.9 Kgha-1 at station 8 (10 m) and 9 (20 m) respectively (Figure 2a). In 1970s O. niloticus was caught in areas less than 10 m and catches of 5 kg per 30 minutes were very rare in 1970s (Kudhogania and Cordone, 1974). In this
survey tilapia was caught up to 20 m deep and up to 100 kg per 30 minutes haul was common. Comparison with previous bottom trawl surveys reveal that in the recent past catches of O. niloticus are on the rise (Figure 2b). Higher catches and movement of Nile tilapia into deeper waters could be due to changes in fish compositions in the lake. More than 60 % species of cichlids have drastically
256
reduced or become extinct in Lake Victoria (OgutuOhwayo, 1990a). Therefore, Nile tilapia experiences less competition for resources and could be expanding it ecological niche to occupy “empty’’ niches left by these cichlids.(Ogutu-Ohwayo 1990a). The spread of tilapia could also have been
enhanced by the declining stocks of predatory L. niloticus (Figure 2b) Catches of perch have been on the declined since the late 1990s due to overexploitation (Njiru et al., 2002; Cowx et al., 2003). This means predation pressure especially on the young tilapia is reducing.
a) 1998-2000
80
-1
Catches (kgha )
(20 m)
60
(8.5m) (3.4m)
40
(5.0m) (3.0m)
(3.0m) (3.5m)
(8.0m)
(4.0m)
20
(3.5m)
(10.0m)
0 1
2
3
4
5
6
7
8
9
10
11
12
Stations
b)
Haplochromines
Clarias
Protopterus
O. niloticus
Lates
100
Contribution ( %)
80 60 40 20 0 1970
1981
1989
2000
2005
Figure 2. Bottom trawl catches in Lake Victoria, Kenya. a) catch per station in 1998-2000, b) contribution (%) over time (Adapted from Njiru et al. 2005). Overfishing of native tilapiines Increase in O. niloticus is attributed to over fishing of endemic tilapiines thus reducing competition. The history of the fishing industry on Lake Victoria up to mid-sixties is given by Mann (1969). Even in absence of catch record, there was virtually no impact on the stocks by subsistence requirements up to the 1900s when the gill nets were introduced as a new fishing technique. Lake Victoria then contained large stocks of O. esculentus. With the introduction of gill net, coupled with the growth of urban centres and communications around the lake, the fishing industry assumed a commercial role. By 1916, the catch rate per net per night (c.p.n) ranged from 25-100 O. esculentus in a 5-inch (127 mm) net of about 45 meters (EAFFRO 1955/1956). The gill nets become so popular that by the mid-1920s the number of tilapia had declined to 5 c.p.n. Despite measures to reduce the trend the c.p.n dropped
from 3.1 in 1933-37 to 1.9 by 1945 and 1.2 in 1955. As O. esculentus was marketed by weight and not by size, this encouraged fishermen to fish with undersized gill nets. From 1956, gill nets of 4.5 inches (114 mm) mesh appeared (Balirwa, 1992). This resulted in increase in profits to fishermen and catches of O. esculentus. The smaller meshed nets of 3” (76 mm) became wide spear in the late 1950s and early 1960s. They mainly captured stocks of O. variabilis that grew to a smaller size (EAFFRO 1955/56). With increased fishing effort, O. variabilis declined. The introduction of T. zillii and O. leucostictus made a temporarly impact but was also overfished by the early sixties (Balirwa 1992). Diet Nile tilapia has more diversified diet and can be described as an opportunistic herbivore (Balirwa 1998; Njiru et al., 2004). The species previously
257
described as herbivorous (Trewavas 1983), has a very diverse diet that includes insects, algae, fish, molluscs and detritus (Figure 3). The endemic tilapia, O. esculentus feeds almost entirely on diatoms (Welcomme 1967; Opiyo 1994) and O. variabilis ingests mainly on phytoplankton (Trewavas 1983; Fishbase 2005), T. zillii ingests mostly macrophytes (Fishbase 2005). There has been a shift from predominance of diatoms and green algae in the 1960s to the present domination of blue greens (Lung'ayia et al., 2000). Blue green algae offer poor quality food and have toxic groups (Lung’ayia et al 2000). The increase in blue green algae is due to increase eutrophication in the lake
attributed to nutrient rich sewage effluents and agricultural runoffs (Lung’ayia et al., 2000). The replacement of diatoms is attributed to decline in silicon in the lake necessary for cell wall formation (Lung'ayia et al., 2000). Studies have further shown that Nile tilapia is able to digest blue green algae (Moriarty and Moriarty 1973). This study shown that when O. niloticus is feeding on algae it is able to actively select for the rare diatoms and green algae even when blue greens are more abundant species(Figure 4). The diversified feeding mode of O. niloticus probably gave the species higher survival rate in the changing Lake Victoria ecosystem.
100 2000
Contribution (%)
80
1983
60 40 20 0 Algae
Insects
Detritus
Molluscs
Fish
Other Inverts
Figure 3. Contribution of the major food items ingested by O. niloticus. Source: 1983(Trewavas 1983), 2000 (Njiru et al. 2005).
1
0 Oscillatoria
P. circumcreta
A. circinalis
-0.4
A. nyassensis
-0.2
N. acicularis
Ivlev's index (E)
0.2
M. glauca
0.4
Station B M. aeruginosa
0.6
S. cunningtonii
Station A
0.8
-0.6 -0.8
Figure 4. Selectivity of algae species by O. niloticus in Lake Victoria, Kenya. (M= Microcystis aeruginosa, M = Merismopedia, N=Nitzchia, A=Anabena, S= Synedra, P=Planktolynbya).
258
Tilapia
Others
Haplochromines El Nino Hyacinth controlled
Catch (mt)
20
Hyacinth explosion
15 10 5 0 76
78
80
82
84
86
88 90 Year
92
94
96
98
00
02
Figure 5. Annual catches of haplochromines, tilapia and other species mainly (Clarias, Protopterus) in Lake Victoria, Kenya. 1968-77
1988 Other fish 16%
Molluscs 2%
R. argentea 2%
Mollusc s Caridina Insects 2% 8% Haplos Other5% 3% fish Dagaa 2% 20%
Haplos 80%
Perch 60%
Perch Other fish 4% 2%
2004-2005
1998-2000 Perch 3% R. argentea 11%
Other fish 2%
Haplos
Caridina 43%
31%
Cardina 52%
Haplos 41% R. argentea 11%
Figure 6. Diet of O. niloticus in Lake Victoria. Note O. niloticus does not appear as major diet even when catches are high. Source: 1968-77 (Ogutu-Ohwayo, 1990a), 1988 (Ogari and Dadzie ,1988). Clearance of swamps The near shore of Lake Victoria has been influenced by human activity in the drainage basin such as clearance of swamps and marginal vegetation and nutrients transport to the lake via run-offs (Lung’ayia et al., 2000). The effect of swamp clearance could
have increased the spawning area for O. niloticus (Welcomme, 1966, 1967; Balirwa, 1998). The disappearance of water lilies and other aquatic weeds reduced the nursery grounds for O. esculentus and feeding niche of higher plants
259
material (Potamogeton and Ceretophylum) which were fed upon by T. zillii (Welcomme 1967).
from then abundant McConnell, 1955).
Water hyacinth
Growth and predation
During infestation of Lake Victoria by water hyacinth, O. niloticus catches increased dramatically (Figure 5). Water hyacinth provided feeding and breeding grounds O. niloticus (Balirwa, 1998; Njiru et al., 2004). Diet of Nile tilapia by then consisted of mostly insects which were found in association with the hyacinth (Njiru et al., 2004). Infestation of the lake by the hyacinth also led to a drastic reduction in beach seining, a common fishing method in shallow waters, allowing the fish time to reproduce and grow. The declining trend thereafter was attributed to reduction in hyacinth and over fishing by fishers who targeted the species. Under the hyacinth water oxygen levels could go as low as 0.01 mgl-1. Nile tilapia which has a higher tolerance to low oxygen concentrations (Kolding, 1993) excluded the predator Nile perch whose critical dissolved oxygen is 5 mgl-1 (Kaufman, 1992). Thus, localised deoxygenation could have played a role in tilapia increase by occupying areas that Nile perch could not.
Among all the tilapias Nile tilapia grows to the largest length and size (Fishbase 2005). The maximum size attained by Nile tilapia in Lake Victoria is 60 cm TL (Njiru 2003). Studies from Lake Naivasha, Kenya found O. leucostictus to attain a maximum length of 37 cm TL, T. zillii 30 cm TL (Ojuok personal communication). Maximum sizes obtained for the tilapiines are 4324g for O. niloticus, 2200g for O. esculentus and 300 g for T. zillii (Fishbase, 2005). Growing to bigger size could probably have reduced predation of O. niloticus by Nile perch. Following the establishment of Nile perch there was a drastic reduction in native icthyofaunal diversity (Ogutu-Ohwayo 1990a) and this could have promoted the spread of O. niloticus. Nile perch diet studies reveal very little O. niloticus is ingested by Nile perch (Figure 6). Both introduced species are able to coexist may be due to ecological separation which has evolved over long time in their native habitats in the Nile, and lakes Turkana, Edward, George, Tanganyika and Albert in East Africa.
Massive stocking and El Niño
haplochromines
(Lowe-
Hybridisation
The massive 1960s stocking off Entebbe followed by rising lake levels could have enhanced spread of O. niloticus (Balirwa, 1992). Under favourable conditions tilapiines may spawn at frequent intervals throughout their productive life (Lowe-McConnell, 1955; Balirwa, 1992, 1998). Heavy rains of 19601963 led to flooding of marginal areas of the lake and creation of new beaches and lagoons, offering more breeding and feeding areas (Balirwa, 1998). Recent increase in catches in 1998-1999 could possibly be linked to the effect of El Niño rains (Oct. 1997-March 1998) (Figure 5). The rains increased lake water volume, flooding adjacent lands and increased feeding and breeding areas, thus increasing recruitment.
Tilapia are known to interbreed both under natural and artificial conditions (Lowe-McConnell ,1958; Welcomme, 1967; Trewavas, 1983). Hybrids of O. variabilis x O. niloticus, and O. esculentus x O. niloticus hybrized under experimental conditions (Lowe-McConnell, 1958). Preliminary studies in northern waters found such hybrids (Balirwa, 1992). A characteristic feature of all hybrids is the dominance of an O. niloticus morphological features. Therefore for all practical purposes, the native tilapias of O. variabilis and O. esuclentus have been swamped up and the common tilapia in Lake Victoria is some form of O. niloticus (Balirwa, 1992; 1998).
Fecundity
Conclusion
Nile tilapia in Lake Victoria is more fecund and shows a well-defined reproductive strategy, which has probably contributed to its success in the lake. Lowe-McConnell (1955) reports that O. variabilis produces 323-547, O. variabilis, 324-1627, O. leucostictus, 99-950, O. niloticus 340-3706 and T. zillii 1000-7061 eggs in Lake Victoria. Recent studies show the fecundity of O. niloticus has increased from 864-6316 eggs for fish of 28-56 cm TL (Lung’ ayia, 1994), to 905- 7619 eggs for fish of 28-51 cm TL (Njiru et al., in press). Adult O. niloticus also competed with O. variabilis for breeding grounds. High survival rate of the O. niloticus could be probably be due to its eggs and young brooding behaviour. The lack of establishment by T. zillii and T. rendalii could be due to the fact that they lay and raises their young on the bottom substratum of the lake, and their eggs could suffered high predation
Several factors have been advanced on why O. niloticus has replaced and dominated the tilapia fishery of Lake Victoria. These factors include a wide distributed range, utilisation of variety of food items, big sizes and reduced predation, higher fecund and survival in a wide range of physical chemical parameters in the lake. However, recent studies have shown that the dominant tilapia is also under threat from fishing pressure (Njiru 2003; Njiru et al in press). The fish matures earlier (ripe male 21.0 cm TL, female 22.7 cm TL) than in 1990s when it was reported to mature at 35 cm TL (Getabu, 1992). Fecundity, growth (K), fishing mortality (F), exploitation rate (E) are increasing whereas asymptotic weight (W∞) and length (L∞) is declining (Table 1).
260
Table 1. Growth parameters (K yr-1, W∞ g, L∞ cm), mortality (Z, M, F yr-1) and exploitation (E) estimates of O. niloticus from Victoria (Source: Njiru 2003). _____________________________________________________________________ Parameter L∞ K Z M F E Data source W∞ ________________________________________________________________________________ 5930 64.60 0.25 0.82 0.54 0.28 0.34 1985-1986 (Getabu ,1992) 63.10 0.35 1.71 0.72 0.99 0.58 1989-1990 (Dache, 1994) 4534 59.50 0.66 2.42 1.07 1.34 0.55 1998-2000 (Njiru ,2003)
_____________________________________________________________________________ These changes in biological and population characteristics of O. niloticus could be tactics to maximize survival and reproductive success (Njiru et al. in press), possibly linked to a population response to overfishing (Cowx et al., 2003). The number of boats on the lake has increased from 4000 in 1950s to over 40 000 in 2000 supporting 12 000 and 120 000 fishers, respectively. There is also a substantial amount of illegal and undersized nets in the lake, which mostly target juveniles fishes all the species (Cowx et al., 2003). However, even under this pressure O. niloticus still grows to large sizes and maintains a good condition (close to 1) (Njiru et al,. in pressure). This implies that if proper management measures are instituted the fishery of O. niloticus can continue providing protein and income especially to the local communities around the lake who depend mostly entirely of the fishery of Lake Victoria.
2-3 inches were frequently encountered during the present study. Beach seining, although banned it is still prevalent in Lake Victoria. This scenario has led to intense exploitation of the O. niloticus. There is a need to involve community in management of the lake resources because fisheries management undertaken by the government through the Fisheries Department apparently has not succeeded. Communities are the first beneficiaries of the lake resources and if properly sensitized they can be willing to protect the resource. Alternative livelihoods, such as aquaculture and other agriculture activities should also be encouraged to reduce pressure on the fishery. There is lack of funding to do research and management of Lake Victoria fisheries. Most of the money got from the lake is not ploughed back, thus is need to advance a harmonized levy to be impose on the fishery and same should be used in management of the lake.
Recommendations
Acknowledgements
Management measures should include restriction on entry to the fishery which is open access by limiting and imposing licences on boats and gears. Ban on the undersized nets and other illegal fishing method should be enforced. The recommended gill mesh size for O. niloticus fishery is 5 inches (127 mm), but
The survey was funded by European Union Fisheries Research Project (Ref: ACP-RPR 227). Paper presentation was funded by EU-IFMP ((Ref: 8 ACP/POR/029). Our gratitude also goes to KMFRI and LVFO for their facilitation.
References Balirwa, J. S., 1992. The evolution of the fishery of Oreochromis niloticus (Pisces: Cichlidae) in Lake Victoria. Hydrobiologia 232, 85-89. Balirwa, J. S., 1998. Lake Victoria wetlands and the ecology of the Nile tilapia, Oreochromis niloticus (L) PhD thesis, University of Wageningen, A.A. Balkema, Rotterdam. Cowx I.G.., Van der Knaap M., Muhoozi, L.I and Othina A. (2003) Improving fishery catch statistics for Lake Victoria. J. Ecosys. Health Mgmt 6, 299-310. Dache S.A.O., 1994. Observations on the fisheries, growth and mortality of Oreochromis niloticus (tilapia) in Nyanza Gulf of Lake Victoria. In. E. Okemwa, E. Wakwabi & A. Getabu eds. Proceedings of the second EEC regional seminar on recent trends of research on Lake Victoria fisheries, 25-27 Sept. 1994, Kisumu, Kenya. ICIPE Press, Nairobi, Kenya, pp. 59-65. EAFFRO, 1955/56. East Africa Freshwater Fisheries Research Organisation, Jinja, Uganda. Annaul. Report. 23 pp. Getabu, A., 1992. Growth parameters and total mortality in Oreochromis niloticus (L) from Nyanza Gulf, Lake Victoria. Hydrobiologia 232, 91-97. Getabu A., 1994 Mortality rate, exploitation and recruitment in Oreochromis niloticus (L) in Nyanza Gulf of Lake Victoria, Kenya, pg 43-52 In: Okemwa E, Wakwabi E. and Getabu, A. eds. Proceedings of the second EEC regional seminar on recent trends of research on L.Victoria Fisheries 25-27 Sept.1994 Kisumu, Kenya. ICIPE Press. Kaufman L, 1992. Catastrophic change in species rich freshwater ecosystem: lessons of Lake Victoria. Bioscience 42, 846-858. Kolding J., 1993. Population dynamics and life-history styles of Nile tilapia, Oreochromis niloticus, in Ferguson’s Gulf, Lake Turkana, Kenya. Environmental Biology of Fishes 37, 25-46 Kudhogania A.W. and Cordone A.J., 1974. Batho-spatial distribution patterns and biomass estimation of the major demersal fishes in Lake Victoria. African Journal of Hydrobiology and Fisheries. 3, 15-31.
261
Lung’ ayia H.B.O., 1994. Some aspects of the reproductive biology of the Nile tilapia, Oreochromis niloticus (L.) in the Nyanza Gulf of Lake Victoria, Kenya. In. E. Okemwa., E. Wakwabi and A. Getabu eds. Recent Trends of Research on Lake Victoria Fisheries. ICIPE Science Press, Kenya. Lung'ayia, H.B.O., M'Harzi, A., Tackx, M., Gichuki, J., and Symoens, J.J., 2000. Phytoplankton community structure and environment in the Kenyan waters of Lake Victoria. Freshwater Biology 43: 529-543. Lowe-McConnell, R.H., 1955. The fecundity of tilapia species. East Africa Agriculture. Journal. 11, 45-52. Lowe-McConnell, R.H., 1958. Observations on the biology of Tilapia nilotica Linné in East Africa waters. Rev. Zool. Bot. Afri. 57, 130-172. Mann, M. J., 1969. A resume of the evolution of the Tilapia Fisheries of Lake Victoria up to year 1960. EAFFRO Ann. Rep. 355: 18-41. Moriarty C. M. and Moriarty D. J. W., 1973. Quantitative estimation of the daily ingestion of phytoplankton by Tilapia nilotica and Haplochromis nigrispinnis in Lake George, Uganda. Journal of Zoology 171: 15-23. Njiru, M., Othina, A., Getabu, A. Tweddle, D., & Cowx, I.G., 2002. ‘‘The invasion of water hyacinth, Eichhornia crassipes Solms (Mart.), a blessing to Lake Victoria fisheries’’. In. I.G. Cowx, ed. Management and Ecology of Lake and Resevoirs Fisheries. Fishing News Books, Blackwell Science, Oxford, UK. Njiru M., 2003. Ecology and population characteristics of Nile tilapia, Oreochromis niloticus (l.) In Lake victoria, Kenya. Phd Thesis, Moi University, Eldoret, Kenya. 199pp Njiru, M.,Okeyo-Owuor, J. B, Muchiri, M., & Cowx I.G., 2004. Shift in feeding ecology of Nile tilapia in Lake Victoria, Kenya. African Journal of Ecology 42, 163-170. Njiru, M., Waithaka, E., Muchiri, M., van Knaap, M. and Cowx, I.G., 2005. Exotic introductions to the fishery of Lake Victoria: What are the management options? Lakes & Reservoirs: Research and Management 10, 147Njiru M., Ojuok J.E., Okeyo-Owuor J.B., Muchiri M., Ntiba M.J.and Cowx I. G. (In press). Some biological aspects and life history strategies of Nile tilapia Oreochromis niloticus (L.) in Lake Victoria, Kenya. African Journal of Ecology Ogari J and Dadzie S., 1988. The food of the Nile perch, Lates niloticus (L.) after the disappearance of the haplochromine cichlids in the Nyanza Gulf of Lake Victoria (Kenya). Journal of Fish Biology 32, 571-577 Ogutu-Ohwayo R., 1990a. The decline of the native fishes of lakes Victoria and Kyoga (East Africa) and the impact of introduced species, especially the Nile perch, Lates niloticus and the Nile tilapia, Oreochromis niloticus. Env. Biol. Fish. 27, 81-96. Ogutu-Ohwayo, R., 1990b. Changes in the prey ingested and the variations in Nile perch and other fish stocks in Lake Kyoga and the northern waters of Lake Victoria (Uganda). Journal Fish Biology 37, 55–63. Opiyo S.V. and Dadzie S., 1994. Diet utilization in Oreochromis esculentus (Graham) in Lake Kanyaboli, Kenya. Fisheries management and Ecology 1, 79-90. Sparre P. and Venema S. C., 1998. Introduction to Tropical fish stock assessment. Manual Part 1. FAO Fisheries Technical Paper no 306 uss, R. E., 1979. Reliability estimates for Ivlev’s selectivity index, the forage ratio and a proposed linear index of food selection. Transactions of American Fisheries Society 108: 344-355. Trewavas E., 1983. Tilapiine species of the genera Sarotherodon, Oreochromis and Danakilia. London: British Museum (Natural History) Publications No 878 p Welcomme R. L., 1966. Recent changes in the stocks of Tilapia in Lake Victoria. Nature 21: 52-54 Welcomme, R. L., 1967. Observations on the biology of introduced species of Tilapia in Lake Victoria. Rev. Zool. Bot. Afri. 76, 249-279. Witte, F., & VAN Densen, W.L.T., 1995. Fish Stocks and Fisheries of Lake Victoria. A Handbook for Field Observations. Cardigan, Britain, Samara Publishing, Cardigan UK.
262
