Some possible factors leading to decline in fish species in M. Njiru,1∗ O.C. Mkumbo,2 and M. van der Knaap3 1Moi University, Department of Fisheries and Aquatic Sciences, PO Box 1125, Eldoret, Kenya 2Lake Victoria Fisheries Organisation (LVFO), PO Box 1625, Jinja, Uganda 3Maxillion Consultancy, PO Box 43, 6700 AA Wageningen, the Netherlands ∗Corresponding author: [email protected]; [email protected]

The decline in fish species in Lake Victoria is one of the largest documented losses of biodiversity in an ecosystem. The reduction in species in the lake was attributed to overexploitation through increased fishing capacity, use of illegal fishing gears and poor enforcement of regulations. Introduction of the predatory is blamed for the decline of the native species, especially the . The native tilapiines, esculentus and , declined due to hybridisation and competition with the introduced Oreochromis niloticus. Diversity loss in haplochromine cichlids has also been attributed to hybridisation caused by increased water turbidity, which reduces visibility in recognising conspecifics during breeding. Degradation of the environment through poor farming patterns and waste disposal has led to increased nutrients into the lake, in turn leading to changes in water quality, increased algal blooms and subsequent anoxia which led to frequent fish kills in the 1990s. However, recent resurgence of thought to be extinct, disputes the fact that extinction of several species occurred. Though not denying that a drastic reduction in the number of native species occurred, the much hyped extinction could be a result of a lack of adequate information on and ecology of the haplochromines as well.

Keywords: exotic introductions, ecological changes, hybridisation, haplochromines

Introduction and Witte, 1997; Goudswaard et al., 2002). Other important species included the native cyprinid, Lake Victoria in East Africa is the second Rastrineobola argentea (Pellegrin), Protopterus largest lake in the world by surface area, covering aethiopicus (Heckel), Bagrus docmak (Forskall),˚ 68 000 km2. It is shared by the riparian states of Clarias gariepinus (Burchell), various Barbus Kenya (6%), Uganda (43%) and Tanzania (51%) species, mormyrid species and Schilbe intermedius and has a mean depth of 40 m and a shoreline of (Ruppell).¨ Currently the Lake Victoria fishery is about 3 500 km. Until the 1970s, Lake Victoria dominated by Lates niloticus (L), R. argentea and supported a multi-species fishery dominated by the Oreochromis niloticus (L). The reduction in fish tilapiine cichlids (Graham) species in Lake Victoria is the largest documented and O. variabilis (Boulenger) and over 200 species loss of biodiversity caused by man in an ecosystem of haplochromine cichlids (Kudhongania and (Witte et al., 1999). This paper explores possible Cordone, 1974; Ogutu-Ohwayo, 1990; Goudswaard factors that led to the decline in fish species in Lake

3

Aquatic Ecosystem Health & Management, 13(1):3–10, 2010. Copyright C 2010 AEHMS. ISSN: 1463-4988 print / 1539-4077 online DOI: 10.1080/14634980903566253

Downloaded from http://read.dukeupress.edu/aehm/article-pdf/13/1/3/886628/3njiru.pdf by guest on 26 September 2021 4 Njiru et al. / Aquatic Ecosystem Health and Management 13 (2010) 3–10

Victoria and outlines possible management strate- around the lake grows at around 3% per annum gies to reduce further decline. Information was (Yongo et al., 2005; UNEP, 2006), rainfall is erratic compiled from published and unpublished literature. and agriculture is poorly developed, leaving the lake as the main source of livelihood for the surround- Factors leading to decline ing communities. In order to meet increased fish demand for food and export, the number of fishers, in species fishing crafts and gears has increased in the lake Commercialisation of the fishery over the years (Figure 2a). Fishers are increasingly using more efficient and illegal fishing gears in the Trends in catch show that up to the early 1980s, lake (Cowx et al., 2003; Njiru et al., 2006). The le- the Lake Victoria fishery was dominated by endemic gal gillnet mesh size for the lake fishery is 5 inches haplochromine cichlids (Figure 1). Following the (127 mm, stretched) or more, but gill nets with mesh explosion of Nile perch and commercialisation of smaller than 5 inches are still prevalent (Figure 2b). the fishery, there was a noticeable decline of other Banned gears like beach seines and monofilament species in the lake. The reduction in catches of the nets are still used in the lake (Figure 2b). Illegal fish- native species was attributed to intense exploitation ing gears like beach seines disrupt spawners, cap- (Kudhongania and Cordone, 1974; Ogutu-Ohwayo, ture immature fish, and destroy breeding areas, espe- 1990; Witte et al.,1999). The human population cially those of nest building cichlids and substratum.

b) Haplochromines Clarias Protopterus O. niloticus L. niloticus Others

100

80

60

40 Catch (%) 20

0 1970 1981 1989 2000 2005 Year

a) L. niloticus O. niloticus R. argentea Others 140 120 100 80 60 Catch (t) 40 20 0 73 75 77 79 81 83 85 87 89 91 93 95 97 99 01 03 Year Figure 1. Contribution of the major fish species caught by bottom trawl in Lake Victoria, Kenya. Others include Synodontis, Brycinus, Bagrus and Schible species.

Downloaded from http://read.dukeupress.edu/aehm/article-pdf/13/1/3/886628/3njiru.pdf by guest on 26 September 2021 Njiru et al. / Aquatic Ecosystem Health and Management 13 (2010) 3–10 5

Figure 2. Capacity trends in Lake Victoria, (a) fishers and fishing gears, (b) illegal fishing gears; Source: Frame survey, 2008.

Increased overexploitation could have affected the (Ogutu-Ohwayo, 1990; Mkumbo, 2002), which recruitment process by the capture of immature fish were abundant (Figure 3). The voracious predator subsequently leading to decline in catches. could have contributed to the decline in the native species. Recent catch statistics indicate a reduction Exotic introductions in Nile perch and a resurgence of the native species (Figure 1), revealing that Nile perch played a pivotal Nile perch role in the catch composition on the lake. Nile perch, Lates niloticus (L) was introduced in Tilapiines the 1950s and 1960s mainly to convert the small, bony, but abundant haplochromines, to suitable ta- To compensate for the decreasing catches of na- ble fish (Ogutu-Ohwayo, 1990; Goudswaard et al., tive tilapiines (O. variabilis and O. esculentus), in 2002). The contribution of haplochromines there- the 1950s, exotic tilapiines O. niloticus, O. leucos- after to the fish biomass in the lake decreased rapidly tictus (Trewavas), Tilapia zillii (Gervais) and Tilapia from >80% during the 1970s to <1% by the late rendalli (Boulenger) were introduced into the lake 1980s, whereas other native species were occasion- to fill the “empty niches” (Welcomme, 1967). The ally recorded in fish landings (Figure 1, Kudhon- (O. niloticus) presently dominates the gania and Cordone, 1974; Ogutu-Ohwayo, 1990; tilapiine fish landings, whereas most of the other Njiru et al., 2005). In its early existence in the lake, tilapiines are rarely caught in the lake. O. escu- Nile perch predominantly fed on haplochromines lentus which feeds almost entirely on diatoms and

Downloaded from http://read.dukeupress.edu/aehm/article-pdf/13/1/3/886628/3njiru.pdf by guest on 26 September 2021 6 Njiru et al. / Aquatic Ecosystem Health and Management 13 (2010) 3–10

Figure 3. Diet of L. niloticus in Lake Victoria a) 1968–1977, (b) 1988, (c) 1998–2000. Others include Schible, Brycinus, Clarias, etc. Haplos = haplochromines; adapted from Njiru et al., 2005.

O. variabilis on phytoplankton were out-competed ing in loss of diversity (Seehausen et al.,1997). by the more diversified feeder, O. niloticus Primary productivity has doubled, algal biomass (Welcomme, 1967). The Nile tilapia diet consists of has increased 8–10 fold accompanied by a shift a variety of food items including algae, fish and de- in algal species composition to a predominance tritus (Njiru et al., 2004). The algal composition in of cyanophytes brought about by abundance in the lake has also changed towards toxic and unpalat- phosphorous (Mugidde et al., 2005). Average val- able cyanobacteria (Lung’ayia et al., 2000), further ues of transparency decreased from 7.3–7.9 m diminishing the food base for the native tilapiines. for offshore stations and 1.3–1.5 m in Nyanza Female Nile tilapia produces between 905–7619 Gulf in 1927 (Worthington, 1930) to 1.1–1.4 m eggs and is more fecund than O. variabilis with and 0.6–1.7 m in the same areas respectively in 323–547 eggs and O. leucostictus with 99–950 eggs 1994/1995 (Lung’ayia et al., 2000). Laboratory (Lowe-McConnell, 1955; Njiru et al., 2006). The in- studies have shown that female haplochromines troduced Tilapia group, T. zillii and T. rendalli,may mate with heterospecific males when conspecific not have become established because they lay and males are not available (Crapon de Caprona and bring up their eggs on the lake bottom; the eggs and Fritzsch, 1984). Decrease in water transparency larval fish could have suffered high predation from has narrowed the light spectrum in Lake Victoria then abundant haplochromines (Lowe-McConnell, which changed the way male haplochromines per- 1955). The Oreochromine group, which are mouth ceive colours of females. Reduction of water clar- brooders, offer higher protection to their eggs and ity has caused loss of genetic and ecological dif- fingerlings compared to the substrate brooders of the ferentiation among haplochromine species and has Tilapia group, giving them a competitive advantage. contributed to loss of species diversity (Seehausen et al., 1997). Hybridisation of cichlids Tilapiines Haplochromines Tilapia are known to interbreed both under nat- Change in water transparency may have in- ural and artificial conditions (Welcomme, 1967). creased hybridisation of haplochromines result- Hybrids of O. variabilis x O. niloticus, and O.

Downloaded from http://read.dukeupress.edu/aehm/article-pdf/13/1/3/886628/3njiru.pdf by guest on 26 September 2021 Njiru et al. / Aquatic Ecosystem Health and Management 13 (2010) 3–10 7

esculentus x O. niloticus occurred under exper- Enforcement of regulations imental conditions. Studies in northern waters of Lake Victoria found such hybrids (Balirwa, Management of Lake Victoria by the three 1992). A characteristic feature of all hybrids is riparian states of Kenya, Uganda and Tanzania the dominance of O. niloticus morphological fea- is coordinated regionally by the Lake Victoria tures. Therefore for all practical purposes, the na- Fisheries Organization (LVFO) but implementation tive tilapias of O. variabilis and O. esculentus of agreed measures is through their respective Fish- have been swamped up and the common tilapia eries Departments/Divisions. Rules and regulations in Lake Victoria is some form of O. niloticus on how to conduct fishing in the lake to minimise (Balirwa, 1992). detrimental fishing practices are very explicit, their low compliance is attributed to laxity and weakness in their enforcement (Njiru et al., 2008). The De- partments/Divisions of fisheries do not have the ca- Ecological changes pacity to police the entire lake. There are incidences Water quality of corruption where fisheries officers are compro- mised to allow illegal fishing gears and methods Water quality in Lake Victoria has declined to continue to be used in their areas of jurisdiction. greatly in the past few decades, owing chiefly to This is manifested in the high numbers of illegal eutrophication arising from increased inflow of nu- fishing gears still operated on the lake (Figure 2b). trients into the lake (Lung’ayia et al., 2000; Mugidde et al., 2005). Increased input of nutrients is at- Management strategies tributed to changes in farming patterns such as de- forestation, monoculture cultivation up to the lake’s Capacity enforcement bank, which encourages soil erosion. Further nu- trient load is through entry of untreated industrial The majority of the native fishery of Lake and municipal waste water into the lake. Enrich- Victoria occurs in waters of 0–10 m deep which ment of the water has led to massive blooms of apparently is the most heavily fished area (Witte algae especially of the toxic blue-greens (Lung’ayia and Van Densen, 1995; Njiru et al.,2005, 2006). In et al., 2000; Mugidde et al., 2005). Collapse of order to sustain the fishery the ban on illegal fishing these blooms led to reduction in dissolved oxygen, gears and methods should be enforced. Licensing at times dipping below 1.9 mgl−1, a level which schemes and extension campaigns should be put is lethal even to the more tolerant fishes in place in anticipation of the need to restrict the (Mhlanga et al., 2006). Further, there has been a open access to the fisheries. The LVFO developed loss of about 30–50% of the oxygenated waters a RPOA – IUU (Regional Plan of Action on Illegal, volume in Lake Victoria since the 1960s, which Unreported and Unregulated Fishing) and RPOA- has reduced the fish habitat (Mugidde et al.,2005). Capacity to guide initiatives to eliminate fishing Low dissolved oxygen concentrations and probably illegalities and to control fishing effort. A 200 m phytotoxins contributed to occasional fish mortal- from the shore no-fishing zone can be adopted ity observed in the Nyanza Gulf of Lake Victoria to reduce fishing in the shallow nursery and the (Ochumba, 1990). Clearance of fringing swamps breeding areas of the lake. The zone will be easier around the lake has reduced their buffering ca- to monitor than discrete areas in the lake created as pacity (Mugidde et al., 2005). Reduced marginal water parks either as breeding or nursery grounds. vegetation has impacted negatively on recruitment The policies and regulations governing Lake and survival of most Lake Victoria fish which de- Victoria resources were different in each country pend on the fringing zones during their early stages (Ntiba et al., 2001; Van der Knaap et al., 2002; of development (Njiru et al., 2006). For example, Njiru et al., 2005) but substantial efforts for harmo- the disappearance of water lilies and other aquatic nization have been going on under LVFO (LVFO, weeds reduced the nursery grounds for O. esculen- 2004; 2007). For, example, the use of monofila- tus, while decline of plants such as Potamogeton ments was banned in Tanzania and Kenya but al- pectinatus and Ceratophylum demersum favoured lowed in Uganda but during the LVFO Council of by T. zillii drastically reduced its feeding niche Ministers Session in February 2009, a joint commu- (Welcomme, 1967). nique´ was signed to ban monofilament nets in the

Downloaded from http://read.dukeupress.edu/aehm/article-pdf/13/1/3/886628/3njiru.pdf by guest on 26 September 2021 8 Njiru et al. / Aquatic Ecosystem Health and Management 13 (2010) 3–10

three partner States. The same session harmonized pal treatment systems to include tertiary treatment the minimum mesh size for R. argentea to be 10 mm of waste using constructed wetlands can reduce nu- instead of 5 mm nets. However, fishing for R. argen- trient flow into the lake. Laws deterring effluent tea is prohibited only in Kenya between 1st April– discharges should be reviewed and weakness in en- 31st August. So far, the minimum mesh size of gill- forcement strengthened. Public sensitization should nets in Lake Victoria is 5 inches but there should be be heightened to curb pollution. an agreement regarding minimum mesh size for cer- tain fishes such as Brycinus, Synodontis and Schilbe Co-management species which are relatively small in size. Though the regulations of all the three countries are similar, A monitoring, control and surveillance scheme the penalties are very different. There is therefore properly implemented should be linked into an urgent need to harmonize all the regulations and community based management approaches because penalties in the Lake Victoria fishery and treat the government enforcement has not previously worked lake as one ecosystem. properly (Ntiba et al., 2001; Njiru et al., 2005). A study conducted by SEDAWOG (1999) found Alternative livelihoods that beach communities formed patrol units to monitor their fishing grounds and as a measure Even with the best management systems in place, to keep off thieves and illegal gears. These were the supply of fish within the lake basin available to very effective. Recently Lake Victoria Fisheries the local community will be insufficient to meet the Organization (LVFO) through a EU funded project ever growing demand due to a rapidly increasing (IFMP) has mobilized the formation/reformation population (Abila, 2000; Yongoet al., 2005). Kenya, of 1069 community-based structures along the Uganda and Tanzania have a combined aquaculture lakeside referred to as Beach Management Units annual production of less than 10 000 mt which is (BMUs). Each of the BMUs is responsible for a less than 0.02% of the global production (Mushi part of the lake near their settlement. The BMUs et al., 2005). The possibility of aquaculture contri- have obtained legal status and are operating in bution is enormous because the East Africa region is collaboration with the Fisheries Officers and Local endowed with adequate photoperiod, water bodies, Goverments in managing the lake resources. It wetlands and suitable native aquaculture species is hoped that the BMUs formation will translate (Mushi et al., 2005). LVFOhas developed a Strategy to good fishing practices and conservation of the and Investment Plan to guide the development of resources of the lake and the ecosystem at large. aquaculture and to attract investors. Other economic activities such as ecotourism and horticulture would Funds reduce pressure on the Lake Victoria fisheries. Though Lake Victoria generates up to $500 Pollution million US annually from fish catches (Yongo et al., 2005), there are no substantial funds from Decline in water quality and the change in phyto- the riparian governments assigned for research plankton community in Lake Victoria towards domi- and management of the lake; most of the funds nance of toxic cyanobacteria in the past few decades, for these purposes originate from donors. Donor is chiefly due to eutrophication arising from increase funding is inconsistent and sometimes comes with of nutrients (Lung’ayia et al.,2000; Mugidde et al., specific objectives which might not be of immediate 2005). These nutrients originate from agricultural concern to the management of the lake. However, land, unregulated effluents from factories and un- the LVFO Council of Ministers has approved treated sewage. Reduction in nutrient emissions can creation of ‘Lake Victoria Endowment Fund’ aimed be through improved agricultural practices, such as at conserving and ensuring sustainability of the reduced clearance of vegetation to avert soil erosion Lake Victoria Fisheries. The revised Fisheries man- and transport of nutrient to the lake, reduced slash agement Plan for 2009–2014 includes a strategy to and burn vegetation as well as forest fires started de- form a Fisheries Trust Fund of which a percentage liberately to clear the forest for agriculture, which will be contributed from the taxes levied on fishing. increases phosphorous in the atmosphere (Mugidde This fund will then be used for management and et al., 2005). Upgrading the industrial and munici- demand driven research.

Downloaded from http://read.dukeupress.edu/aehm/article-pdf/13/1/3/886628/3njiru.pdf by guest on 26 September 2021 Njiru et al. / Aquatic Ecosystem Health and Management 13 (2010) 3–10 9

Conclusions and environment in the Kenyan waters of Lake Victoria. Freshwat. Biol. 43, 529–543. Various factors have acted singly or in conjunc- LVFO, 2004. LVFO Regional Plan of Action to Prevent, Deter tion in reduction of species in Lake Victoria. To and Eliminate Illegal, Unreported and Unregulated fishing on further arrest the decline, there is a need to treat Lake Victoria and its Basin. LVFO/FAO May 2004. ISBM: 9970-713-07-5. the lake as one ecosystem. Stakeholders including LVFO, 2007. LVFORegional Plan of Action for the Management various government ministries and NGOs working of the Fishing Capacity in Lake Victoria. LVFO/FAO March around the lake and its basin should work col- 2007. ISBM: 9970-713-13X. lectively with an aim to protect and conserve the Mhlanga, L., Day, J., Chimbari, M., Siziba, N., Cronberg, G., lake and its environment. The Nile perch definitely 2006. Observations on limnological conditions associated played a major role in the disappearance of the na- with a fish kill of Oreochromis niloticus in Lake Chivero following collapse of an algal bloom. Afri. J. Ecol. 44, 199– tive Haplochromine species but various other fac- 208. tors affected their demise also. It should be noted, Mkumbo, O. C., 2002. Assessment and management of Nile however, that the taxonomic research and descrip- perch (Lates niloticus L.) stocks in the Tanzanian waters of tion of the Haplochromine species flock was under- Lake Victoria. PhD Thesis, Hull University, UK. way when the Nile perch boom occurred. It will re- Mugidde, R., Gichuki, J., Rutagemwa, D., Ndawula, L., main unknown how many distinct species occurred Matovu, A., 2005. Status of water quality and implications in Lake Victoria before the Nile perch and how on fishery production In: The State of the Fisheries Re- sources of Lake Victoria and their Management, pp. 106– many became “extinct” after its establishment. With 112. LVFO Secretariat, Jinja, Uganda. ISBN:9970-713-10- the high fishing pressure exerted on the Nile perch, 2. some haplochromine species thought to be extinct Mushi, V. E., Oenga, D. N., Mwanja, W. W., 2005. Meeting in- have reappeared in catches. It is recommended that creasing demand for fish through development of aquaculture proper taxonomic studies be conducted in order for in the Lake Victoria Basin. In: The state of the fisheries Re- fisheries managers to have a complete inventory of sources of Lake Victoria and their Management, pp 159–165, the fish biodiversity in the lake. LVFO Secretariat, Jinja, Uganda. ISBN:9970-713-10-2. Njiru, M., Okeyo-Owuor, J. B., Muchiri, M., Cowx, I. G., 2004. Shift in feeding ecology of Nile tilapia, Oreochromis niloticus References (L.) in Lake Victoria, Kenya. Afr. J. Ecol. 41, 1–8. Njiru, M., Waithaka, E., Muchiri, M., Van der Knaap, M., Cowx Abila, R., 2000. The development of the Lake Victoria fishery: I. G., 2005. Exotic introductions to the fishery of Lake A boom or bane for food security? IUCN, Nairobi, Kenya. Victoria: What are the management options? Lakes Reser.: No. 8. Res. Manage. 10, 147–155. Balirwa, J. S., 1992. The evolution of the fishery of Oreochromis Njiru, M., Ojuok, J. E., Okeyo-Owuor, J. B., Muchiri, M., Ntiba, niloticus (Pisces: Cichlidae) in Lake Victoria. Hydrobiologia M. J., Cowx, I. G., 2006. Some biological aspects and life 232, 85–89. history strategies of Nile tilapia Oreochromis niloticus (L.) Cowx, I. G., Van der Knaap, M., Muhoozi, L. I., Othina, A., 2003. in Lake Victoria, Kenya. Afri. J. Ecol. 44, 1–8. Improving fishery catch statistics for Lake Victoria. Aquat. Njiru, M., Kazungu, J., Ngugi, C. C., Gichuki, J., Muhoozi, L., Ecosyst. Health Manage. 6(3), 299–310. 2008. An overview of the current status of Lake Victoria Crapon de Caprona, M. D., Fritzsch, B., 1984. Interspecific fer- fishery: Opportunities, challenges and management strate- tile hybrids of haplochromine Cichlidae (Teleostei) and their gies. Lakes Resev. Res. Manage 13, 1–12. possible importance for speciation. Nether. J. Zool. 4, 503– Ntiba, M. J., Kudoja, W. M., Mukasa, C. T., 2001. Management 538. issues in the Lake Victoria watershed. Lakes Reser: Res. Frame Survey, 2008. A status report on the frame surveys 2000 to Manage. 6, 211–216. 2005. Lake Victoria Fisheries Organization. Jinja, Uganda. Ochumba, P. B. O., 1990. Massive fish kills within the Nyanza Goudswaard, P. C., Witte F., 1997. The catfish fauna of Lake Gulf of Lake Victoria, Kenya. Hydrobiologia 208, 93– Victoria after the Nile perch upsurge. Env. Biol. Fish. 49, 99. 21–43. Ogutu-Ohwayo, R., 1990. The decline of the native fishes of Goudswaard, P.C., Witte F., Katunzi, E. F. B., 2002. The tilapiine lakes Victoria and Kyoga (East Africa) and the impact of fish stock of lake Victoria before and after the Nile perch introduced species, especially the Nile perch, Lates niloticus upsurge. J. Fish Biol. 60(4), 838–856. and the Nile tilapia, Oreochroms niloticus. Environ. Biol. Kudhongania, A. W., Cordone, A. J., 1974. Batho-spatial distri- Fish 27, 81–96. bution pattern and biomass estimation of the major demersal SEDAWOG (Socio-economic data working group), 1999. The fishes in Lake Victoria. Afr. J. Hydrobiol. Fish. 3, 15–31. survey of Lake Victoria’s fishers LVFRP/TECH/99/05. Jinja, Lowe-McConnell, R. H., 1955. The fecundity of tilapia species. Uganda. E. Afri. Agri. J. 11, 45–52. Seehausen, O., Van Alphen, J. J. M., Van Witte, F., 1997. Cich- Lung’ayia, H. B. O., M’Harzi, A., Tackx, M., Gichuki, J., lids diversity threatened by eutrophication that curbs sexual Symoens, J. J., 2000. Phytoplankton community structure selection. Science 277, 1808–11.

Downloaded from http://read.dukeupress.edu/aehm/article-pdf/13/1/3/886628/3njiru.pdf by guest on 26 September 2021 10 Njiru et al. / Aquatic Ecosystem Health and Management 13 (2010) 3–10

UNEP, 2006. Africa’s Lakes: Atlas of Our Changing En- Witte, F., Goudswaard, P. C., Katunzi, E. F. B., Mkumbo, O. C. vironment. Division of Early Warning and Assess- Seehausen, O.,Wanink, J. H., 1999. LakeVictoria’secological ment (DEWA). United Nations Environment Programme changes and their relationships with the riparian societies. In: (UNEP), Nairobi, Kenya. Earthprint.com ISBN: 92 807 H. Kawanabe., G. W.Coulter, A. C. Roosevelt (Eds.), Ancient 2694. lakes: Their cultural and biological diversity, pp. 189–202. Van der Knaap, M., Ntiba, M. J., Cowx, I. G., 2002. Key elements Kenobi Productions, Belgium. of fisheries management on Lake Victoria. Aquat. Ecosyst. Worthington, E. B., 1930. Observations on the temperature Health Manage. 5, 245–254. hydrogen-ion concentration and other physical conditions Welcomme, R. L., 1967. Observations on the biology of the of Victoria and Albert Nyanza. Int. Rev. Ges. Hydrobiol. introduced species of Tilapia in Lake Victoria. Rev. Zool. Hydrogr. 24, 328–357. Bot. Afr.76, 249–279. Yongo,E., Keizire, B. B., Mbilinyi, H. G., 2005. Socio-economic Witte, F., Van Densen, W. L. T. , 1995. Fish Stocks and Fisheries impacts of fish trade. In: The state of the fisheries Resources of Lake Victoria. A Handbook for Field Observations. Samara of Lake Victoria and their Management, pp. 132–142. LVFO Publishing, Cardigan, Britain. Secretariat, Jinja, Uganda. ISBN: 9970-713-10-2.

Downloaded from http://read.dukeupress.edu/aehm/article-pdf/13/1/3/886628/3njiru.pdf by guest on 26 September 2021