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.

We studied the effects of environmental driving factors (maximum depth, visibility, oxygen, temperature, and prey densities) on the distribution and diet composition of Nile perch (Lates niloticus) in south-east Lake Victoria from 2009 to 2011. We tested the hypotheses that (i) Nile perch distribution is regulated by the same environmental factors on a local scale (Mwanza Gulf) and on a regional scale (Mwanza Gulf, Speke Gulf and the open lake in Sengerema district), and (ii) driving factors act differently on different Nile perch size classes. Fish were sampled with gillnets. Nile perch densities were highest in the shallow part of the Mwanza Gulf and during the wet seasons, mainly caused by high densities of juveniles. The environmental driving factors explained Nile perch distributions on both regional and local scales in a similar way, often showing non-linear relationships. Maximum depth and temperature were the best predictors of Nile perch densities. Prey densities of shrimp and haplochromines did not strongly affect Nile perch distributions, but did explain Nile perch diet on a local and regional scale. We conclude that abiotic variables drive Nile perch distributions more strongly than prey densities and that feeding takes place opportunistically.

Operational and environmental factors limited available resource space of gillnet and longline fishers targeting Nile perch in the Speke gulf and open lake of southern Lake Victoria and drove their encounter rates with patches of fish resulting in gear specific distributional patterns. Catch-rate patterns were similar by region and gear: large (>50cm) Nile-perch densities increased over distance from homeport and deeper in the water column while small Nile perch (<50cm) densities decreased. Effects of season, (setting) depth and region were present but small and obscured by high variation in daily catch-rates and individual fisher strategies. Both fisheries distributed themselves over the size-productivity spectrum of Nile perch but reacted differently to patterns in size distribution of Nile perch: gillnetters focused more on numbers of productive juveniles between 30-60cm at on average 5km distance (59min travel time) from homeport and longliners on larger sized 40-80cm Nile perch deeper in the water column at 7km (108min). Sampled fishers likely were representative of most of the Nile perch fisheries. If so, this means that fishing pressure is mainly exerted on nearshore lake areas, and more lightly fished offshore areas then may act as a refuge for adult Nile perch. Total catch-rates by gear were generally equalized over the resource space, increasing slightly with distance from homeport, according to ideal free distribution predictions. Nile perch fishers on Lake Victoria appear to distribute themselves according to the underlying productivity distribution of the resource within the constraints of their available resource space.