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Distribution and Association of Tilapine Unit Stocks in the Lake Victoria Catchment (Kenya)

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Distribution and Association of Tilapine Unit Stocks in the Lake Victoria Catchment (Kenya) Distribution and association of Tilapine unit stocks in the Lake Victoria catchment (Kenya) Item Type Proceedings Paper Authors Jembe, B.T.; Boera, P.N.; Okeyo Owuor, J.B. Citation Odada, Eric & Olago, Daniel O. (Ed.) Proceedings of the 11th World Lakes Conference: vol. 2. p. 210-216 Download date 27/09/2021 03:48:19 Link to Item http://hdl.handle.net/1834/2324 Distribution and association of Tilapine unit stocks in the Lake Victoria catchment (Kenya) Jembe B.T.1, Boera P. N.1 & Okeyo Owuor J.B2. 1Kenya Marine & Fisheries Research Institute, P.O. Box 1881, Kisumu. [email protected]; [email protected], [email protected] 2Moi University, School of Environmental Studies, Biological Sciences Division, P. O. Box 3900, Eldoret. Abstract associate sympatrically, parapatrically or allopatrically with other tilapiine species. The Lake Victoria Catchment (Kenya) harbours six Interbreeding amongst tilapiines has been reported species of tilapia. Two species Oreochromis variabilis (L) and Oreochromis esculentus (L) are endemic while four severally (Lowe, 1958; Jembe et. al., 1998) thus species Oreochromis niloticus, Oreochromis leucostictus, raising the possibility of a changing genetic status of Tilapia zillii(Gervais) and Tilapia rendalli(Gervais) were the different populations. The introductions of alien variously introduced in 1950s and early 1960s. The species were intended to cover gaps in the existence of six tilapia species in the same geographical ecological niches of Lake Victoria. O. niloticus and range has had significant ecological and economical O. leucostictus are omnivorous (Greenwood, 1966; implications. Witte and Van Densen, 1995) while Tilapia zillii is The study investigated the ecological impacts by herbivorous (Lowe, 1958). They were all introduced determining species diversity, distribution, association during the 1950’s to improve the fishery. The (allopatric, sympatric and parapatric) and habitata introduced tilapiines were once said to have similar preferences for the six-tilapia species. Shannon-wiener ecological requirements to native tilapiines (Fryer, index was used to determine diversity, while species 1961). O. niloticus seems to have competitive distribution was evaluated on the basis of 39 habitats advantage over all the other tilapiines and this (dams and satellite lakes) within the catchment. Species seems to have contributed to its success. It grows to associations were determined using Cole’s Cab index. The Shannon Weiner function values of 2.44-2.7 indicate a larger size, has faster growth rate, is more fecund, relatively high species diversity in 15% of 33 habitats has a longer life span and wider food spectrum, and where Tilapia were resident. is less habitat restricted (Lungayia, 1991). Further evidence on inter-specific competition is reported by Cole’s Cab indices showed significant relationships in O. Fryer (1961) between O. variabilis and T. zillii. With esculentus X O. variabilis, O.leucostictus X O. esculentus, O. niloticus X O. variabilis and O. niloticus X O. competition, the distribution and association of leucostictus at P < 0.05. Habitat preference for the six tilapiines in the various water bodies in Lake Victoria species is provided. is expected to conform to niche characteristics and optimal species combination. In these combinations, Key words: Association, Distribution, Diversity optimal utilization of resources is achieved through Introduction resource partitioning. Trophic representation in many other species aim to achieve this objective. In The tilapiine inventory of the lake Victoria catchment tilapiines, stocks of herbivores, omnivores and , Kenya, consists of four Oreochromis and two phytoplanktivores achieve the partitioning of Tilapia species. The existence of these tilapiine resources. While O. niloticus is an omnivore, T. zillii species in the Lake Victoria basin (Kenya) has and O. leucotictus are herbivores (Greenwood, significant ecological and economical implications. 1966, Witte and Van Densen, 1995). O.esculentus Two of the endemic tilapiines (O. esculentus and O. and O.variabilis are phytoplanktivores. The food of T variabilis), which once dominated the fishery, are .rendalli consists of macrophytes and detritus. now relegated to the periphery. Their impact on the ecology and fishery economy is now insignificant. Resultant demographic changes in tilapia Dominance of two introduced tilapiines populations have great consequences on catches, (Oreochromis niloticus and O. leucostictus, is now a genetics and sustainability. Between 2001 and 2003 reality, with O. niloticus contributing up-to 90% of the tilapiine distribution was sampled on eight occasions tilapiine catches (Fisheries statistics, 2003). and mapped. The study provided data on site characteristics and an assessment of species Establishment of viable populations by the six composition, spatial distribution, diversity and tilapiines within the Lake Victoria basin (K) is associations. Factors affecting distribution of the six unknown, yet it is appreciated that O. niloticus is tilapiines is also discussed. widespread and has swamped and dominated the other tilapiine species. Lake Victoria Environmental Materials and methods Management Project (LVEMP reports, 2000) Study area surveys have established that extanct populations of the endemic tilapiines still exist. These isolated The study area showing sampled dams and lakes is extanct populations living in different habitats given in figure 2. Their sizes, hydrological conditions 210 and origin vary. Sizes range from 6,000 m2, Sampled areas in Lake Victoria to determine (Kamasawa) to 25,000 m2 (Koketch) (LVEMP distribution of tilapiine species are also provided in reports, 2000). While seasonal streams drain some figure 2. The selection covered littoral areas, dams, the water budget of others is solely (including river mouths), deep-water areas (>5 m), dependent on surface run-offs. Some dams such as open water areas and sheltered bays. Other Kamasawa are replenished by springs. Most of the ecotones sampled included those with mud, gravel, dams are man-made with an exception of a few rocky or sandy substratum. Areas with rocky which are natural entrapments of water. Some of the outcrops were also included. dams are temporary and were not included in the Four satellite lakes were sampled in this study; sampling regime. Dams short-listed for this study Lakes Kanyaboli, Sare, Simbi and Nyamboyo. were 39 some of which are shown in figure 2 below. Figure 1. Map of Lake Victoria showing study area and sampled habitats. Sampling times during the study period. All proposed ecotones were sampled. Each of the 39 dams and 3 satellite lakes was sampled twice during the study period. Sampling Trawling was carried out in the deeper ecotones of was done once during the wet and once during the L.Victoria while gillnetting was done in shallow areas dry season so as to accommodate physico-chemical and rocky outcrops. Catches from fishermen were attributes. Lake Victoria sites were sampled four also assayed to accommodate composition missed out during the experimental gillnetting and trawling 211 exercises. While gillnetting was the main method of satellite lakes (Kanyaboli, Nyamboyo and Sare) had sampling dams and satellite lakes, seining was three species, O. Niloticus, Orechromis esculentus occasionally carried out whenever conditions and Tilapia zillii in varied combinations. Lake Simbi permitted, as was the case with Lake Victoria. had no fish. 32 of the 39 dams sampled were Identification tilapias was on a key by Greenwood inhabited by tilapiines. All the six species were (1966). Total length (TL) to the nearest 0.1 cm of variously represented. each fish was recorded. A note was also made of Of the 39 dams sampled, 32 were inhabited by aquatic and semi-aquatic plants for description and tilapiines, 5 were inhabited by non-tilapiines, while 3 comparisons of tilapiine habitats. had no fish. Three of the satellite lakes had tilapiines Data analysis and other fish taxa, while one (Lake Simbi) had no fish. All six tilapiine species variedly inhabited the The geographical location of each habitat sampled satellite lakes and dams. Lake Victoria had only two was identified using a (Geographic Positioning tilapiine species. The distribution patterns of other System (GPS) coordinates. Counts of individuals in taxa per habitat category revealed a total of 15 each species were used to tabulate diversity while families represented by 26 genera in the non-cichlid frequencies for each species were obtained from category. In the cichlid family category, the total occurrence. Spatial distribution for allopatric, genera were 19. sympatric and parapatric populations was determined by plotting their positions on a map. Spatial distribution of tilapiine species The occurrence of associated tilapiine species was Introduced species were represented by four analysed following the method of Mountford (1962) populations of T. zillii, seven of T. rendalli, sixteen of and expressed in the form of a three-dimensional O. leucostictus and eighteen of O. niloticus. diagram. Data for length frequency analysis was Populations of endemics were nine for O. esculentus pooled from trawl and gillnet catches. Length and sixteen of O. variabilis. The least commonly frequency distribution was used to express occurring tilapiine was T. zillii which occurred in community population structure in the various Lake Sare, Kalenjuok (4), Ochot(32) and Kitaru (33). habitats. Habitat evaluation was
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