ABSTRACT. East Africa's Lake Victoria provides resources and services to millions of people on the lake's shores and abroad. In particular, the lake's fisheries are an important source of protein, employment, and international economic connections for the whole region. Nonetheless, stock dynamics are poorly understood and currently unpredictable. Furthermore, fishery dynamics are intricately connected to other supporting services of the lake as well as to lakeshore societies and economies. Much research has been carried out piecemeal on different aspects of Lake Victoria's system; e.g., societies, biodiversity, fisheries, and eutrophication. However, to disentangle drivers and dynamics of change in this complex system, we need to put these pieces together and analyze the system as a whole. We did so by first building a qualitative model of the lake's social-ecological system. We then investigated the model system through a qualitative loop analysis, and finally examined effects of changes on the system state and structure. The model and its contextual analysis allowed us to investigate system-wide chain reactions resulting from disturbances. Importantly, we built a tool that can be used to analyze the cascading effects of management options and establish the requirements for their success. We found that high connectedness of the system at the exploitation level, through fisheries having multiple target stocks, can increase the stocks' vulnerability to exploitation but reduce society's vulnerability to variability in individual stocks. We describe how there are multiple pathways to any change in the system, which makes it difficult to identify the root cause of changes but also broadens the management toolkit. Also, we illustrate how nutrient enrichment is not a self-regulating process, and that explicit management is necessary to halt or reverse eutrophication. This model is simple and usable to assess system-wide effects of management policies, and can serve as a paving stone for future quantitative analyses of system dynamics at local scales.

Hydrological regimes are significant drivers of fisheries production in many African Lakes due to their influence on fish habitat and food availability, breeding success, and catchability. Lake Turkana, Kenya, will undergo substantial changes in hydrology due to water regulation and extraction along the Omo River in neighboring Ethiopia, which provides over 90% of its water. The objective of this study was to predict how the lake's fisheries, which provide an important livelihood and protein source in the region, will respond to hydrological change. While variations in fishing effort are poor predictors of fisheries catch in the lake, water levels and their fluctuations strongly influence fisheries production. Seasonal oscillations play a particularly important role, and with complete loss of these oscillations, the lake's predicted fisheries yield will decrease by over two thirds. The fishery is predicted to collapse at a lake level decline of 25 m, regardless of seasonal amplitude magnitude. The lake's total littoral habitat, where fisheries are currently concentrated, will increase in surface area with lake level declines of <25 m. However, the extent of productive, dynamic littoral habitat will decrease with dampening of the lake's seasonal oscillations. The most severe habitat loss will occur in the lake's Turkwel Sector, which hosts the region's highest human population densities, and North Sector, where inter‐tribal conflict over resources is common and likely to be exacerbated by lake level decline. The continued ecological functioning of Lake Turkana necessitates immediate efforts to develop and apply a water resource management plan rooted in science.

Background and Trophic Diversity StudyLake Turkana is an understudied desert lake shared by Kenya and Ethiopia. This system is at the precipice of large-scale changes in ecological function due to climate change and economic development along its major inflowing river, the Omo River. To anticipate response by the fish community to these changes, we quantified trophic diversity for seven ecological disparate species (Alestes baremose, Hydrocynus forskalli, Labeo horie, Lates niloticus, Oreochromis niloticus, Synodontis schall, and Tilapia zillii) using stable isotopes. Based on their marked morphological differentiation, we postulated that dietary niches of these species would be similar in size but show little overlap. The degree of trophic diversity varied greatly among the species studied, refuting our hypothesis regarding dietary niche size. Oreochromis niloticus and L. niloticus had the highest trophic diversity and significantly larger dietary niches than T. zillii, A. baremose and H. forskalli. Low overlap among the dietary niches of the seven species, with the exception of the synodontid catfish S. schall, is consistent with our second hypothesis.Predicting Species’ VulnerabilityBreeding vulnerability was highest among those species with the lowest trophic diversity. We predict that in suffering two strikes against them, A. baremose, H. forskalli, T. zillii, and L. horie will be most affected by the highly altered Lake Turkana ecosystem and that O. niloticus, L. niloticus and S. schall will be least affected. Low vulnerability among O. niloticus and L. niloticus is promising for the future of the lake’s fishery, but the third most important fishery species (L. horie) will be highly vulnerable to impending ecosystem change. T. zillii should be treated as separate from O. niloticus in the fishery given higher sensitivity and a different ecological role. We see potential for expansion of the fishery for S. schall but don’t recommend the development of a fishery for A. baremose and H. forskalli.

This chapter addresses the methodologies for assessing the potential ecological effects of transgenic fish before they actually enter unconfined environments. A strategy for identifying the most important data is outlined, and methods for generating them are presented. The following are also discussed: types of information needed to accurately characterize potential receiving environments for the transgenic fish; phenotypic characteristics of transgenic fish that will influence their interaction with ecosystem processes and components; experimental approaches for assessing transgenic fish phenotypes and their ecological impact; and major sources of scientific uncertainty influencing empirical assessments.

The Lake Victoria ecosystem once hosted a diverse fish community dominated by a large species flock of haplochromine cichlids. Today this fish assemblage is highly altered by anthropogenic activities, with at least half of the indigenous species either extinct or very rare. The fishery has been reduced to three fishes of economic importance: introduced Nile Perch (Lates niloticus) and Nile Tilapia (Oreochromis niloticus), and Indigenous Minnow, Rastrineobola argentea, locally known as Omena. Decline in fish stocks in the lake is still continuing; however, in order to rejuvenate stocks therein, a fishing ban (closed season) was imposed in November, 2001 between 1 April and 31 July of each year through a Kenyan government gazette and legal notice (L.N.214/2003). The ban was not only intended to protect the Omena fishery but was also used to anchor efforts towards protecting and rejuvenating dwindling stocks in the lake. However, whether the Kenyan approach has made any gains for the region is still equivocal. It is on this basis that we reviewed existing information on both historical and current stock levels of R. argentea, and the closed fishery implication on the livelihoods of the riparian communities. Our review results indicate that Omena biomass in Lake Victoria has increased considerably from approximately 500,000 MT in 2001 to 911,328 MT in 2011. Rastrineobola argentea is now the most important fish stock by mass in Kenyan waters contributing almost 61.5% of the total fish landings (10,339.0 MT), with ex-vessel value of total fishers’ earnings of about Kshs. 207 million (approx. USD 2.5 million). In response to the emerging uncertainties regarding the closed season or ban, we recommend and advise for a balanced, win-win approach balancing the biological cycle of stocks against the socio-political and economic issues, in order to sustain the stocks and enable the fisher folks to eke their daily food and earnings.

The Lake Victoria ecosystem once hosted a diverse fish community dominated by a large species flock of haplochromine cichlids. Today this fish assemblage is highly altered by anthropogenic activities, with at least half of the indigenous species either extinct or very rare. The fauna and flora of the tropic's largest lake are still in flux, and little is known about even the most basic ecological questions, such as the source of carbon and nitrogen for its spectacular productivity. The actual food-web structure is difficult to determine with results based on traditional gut analysis. At the moment both scientists and conservationists are at odds as to whether there is any evidence of functional replacement or functional overlap amongst the resilient pelagic fishes of Lake Victoria. This study used a static stable isotope model to investigate the source of carbon and the extent of the trophic overlap amongst three pelagic fishes, Rastrineobola argentea, Yssichromis laparogramma and Y. fusiformis in the Kenyan waters of Lake Victoria. The d 13 C of plants ranged from )8.8 to )24.6&, while fishes ranged from )18.6 to )24.5&, suggesting assimilation of mostly C 3 sources for the fish species. On the basis of the isotope model, it is evident that Yssichromis laparogramma, Yssichromis fusiformis, and Rastrineobola argentea obtained from 90 to 97.5%, with an average of 94.2%, of their food from the same trophic level. The very high degree of overlap in the effective trophic level of these three zooplanktivores has important ecological and conservation implications.

Stable isotope analysis was used to investigate the migratory status and to determine the relative contribution of allochthonous and autochthonous sources of carbon for the major riverine fish species (Barbus altianalis and Labeo victorianus) in rivers draining the Kenyan side of Lake Victoria. The two fish species derived carbon from both C4 and C3 plant sources, although L. victorianus exhibited less enriched isotopic carbon values. Fish samples from stations under direct influence of effluents from sugar factories exhibited enriched d 13 C signals. Assuming that this reflects carbon sourcing from riparian C4 plants, it suggests that carbon from terrestrial sources can be a major energy source in some rivers. This heavy carbon enrichment associated with sugar factories was spatially restricted and occurred in all seasons, implying that sub-populations of the two fish species are non-migratory. The large migratory populations of these two species, for which Lake Victoria was once famous, may be no more. # 2007 The Fisheries Society of the British Isles

The fish community of Lake Victoria, East Africa, has continued to exhibit an intriguing degree of resiliency despite stress from anthropogenic activities. Frequent environmental perturbations include wild fluctuations in fishing pressure, limnological conditions, and lake levels. Surprisingly, many of the endemic and other indigenous fishes have survived, and some have even increased in numbers in recent years. While a positive development, this resiliency is a red flag to scientists because we do not understand it, or know whether it can be expected to continue. Furthermore, besides issues of immediate human well-being, there remain grave concerns about long term resource sustainability. This paper explores possible reasons for ecological resilience in the Lake Victoria fish community, with a focus on omnivory and functional redundancy as possible explanations. The analysis used published data based on both traditional gut content analysis (GCA) and stable isotope analysis (SIA). Trophic plasticity is ubiquitous among the surviving fishes of Lake Victoria. This, combined with an overall simplification of the food web, contributed to the lake community's current resilience.