Lake Naivasha is an important freshwater resource for Kenya's foreign-currency-earning agriculture and tourism, and for water-supply. It has always experienced extensive water-level fluctuations as a consequence of irregular rainfall patterns that are affected by continental-scale climatic events, and its communities — particularly of aquatic plants — were adapted to these changes.

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Executive summaryIncreasing exploitation of natural resources, inappropriate land-use practices, and uncoordinated sectoral policies and development activities in lake basins impair the various important functions of lakes throughout the world. New and innovative approaches to the management of lakes and their basins are urgently needed to ensure that these precious freshwater ecosystems continue to deliver their services. Stakeholder participation lies at the heart of the new policy approaches to management of lakes and reservoirs. The stakeholders in lake basin management are individuals or representatives of a group who make use of, have an impact on, or are impacted by the issue of concern. Experience from case studies on 28 lake regions from throughout the world shows that local communities and non-governmental organizations (NGOs) are among those stakeholders that can significantly affect the outcome of management efforts in lake basins. The case studies clearly demonstrate that active community participation is vital to sustainable development in lake basin management. Effective participation of local communities in lake basin management depends on the degree of awareness among local communities of the important technical and social considerations. This is why the involvement of local communities needs to be accompanied by public awareness and information campaigns. Moreover, the involvement of local communities must be based on a sound understanding and appreciation of the local cultural beliefs, values and norms.Nevertheless, as the numerous examples from the case studies suggest, changes in peoples' attitudes toward environmental issues happen only after they realize the benefit. The power of community-level participation is evident when the outcomes of participation are clearly and directly linked to the improvement of livelihood of participating communities. Experience with rural water supply, sanitation projects and afforestation projects from the case studies proves that participation of local communities greatly improves the likelihood that project assets will be fully used and properly operated and maintained. Effective participation of local communities depends on social organization that establishes manageable groups within the community. Communities may lack knowledge on how to build community institutions that represent a community's diverse interest groups and the capacity to be involved in collective action.

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Summary 1. Since avoiding predation can compromise animal fitness, prey are expected to respond to different predator species with an intensity appropriate to the level of risk. In fresh waters, the threat of predation is typically assessed by chemical cues, in particular by odours released by either injured/disturbed conspecifics (conspecific alarm odour) or predators (predator odours). Here, we used the most widely distributed crayfish in the world, the invasive North American Procambarus clarkii, to investigate the relative effectiveness of odours emitted by fish predators compared with conspecific alarm odour. We also tested whether P. clarkii is able to discriminate between fish predators of which it has ‘experience’ (either recent, via introduction to the same water body, or old, by sharing a native range), as well as between fish predators that pose low or high risk. 2. The study was carried out on introduced populations of P. clarkii from two sites, characterised by different fish assemblages: the Malewa River (a tributary of Lake Naivasha, Kenya) and Lake Trasimeno (Italy). Laboratory experiments consisted of three sequential phases (‘water’, ‘food’ and ‘smell’ phases) and five treatments. Treatments differed in the odour presented during the smell phase, i.e. no odour (plain water) and odours from either injured conspecifics or three fish species per site. Crayfish from the Malewa River population were confronted with the odours of largemouth bass (Micropterus salmoides), common carp (Cyprinus carpio) and tilapia (Tilapia zillii) (all introduced to Lake Naivasha but absent from the Malewa River), and those from the Lake Trasimeno population with the odours of the introduced largemouth bass and carp and the native chub (Squalius cephalus). Largemouth bass is the only predator that imposes a high risk to crayfish, and it also shares its native range with P. clarkii. We analysed the time spent by crayfish feeding, in locomotion and in adopting a raised or lowered posture. A reduction in the time spent feeding and in locomotion, and an increase in the time spent in the lowered posture were considered to indicate alarm. 3. Crayfish from both populations responded with a more pronounced reduction in feeding to conspecific alarm odour rather than to predator odours. Crayfish from the Malewa River reacted with the same intensity to the odours of the three fish species tested, whereas, in Lake Trasimeno, the odour of largemouth bass was significantly more threatening than the odours of the other two species. 4. Procambarus clarkii seems to perceive a general fish odour that alerts it to possible predation risk without the need of either a direct recent experience or via sharing a common native range. However, where they coexist with fish, crayfish become able to distinguish among species, adapting the intensity of their response to the effective risk. Our results confirm the relatively high learning capacity of P. clarkii reported in previous studies and suggest the existence of mechanisms that make predator rec...

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Taxonomic composition, distribution, community structure and seasonality of Lake Naivasha zooplankton between 1978 and 1980 are described. The ecological status of the zooplankton in relation to prevailing ecological factors and the lakes fishery are discussed.