Beginning in the mid-1980s Lake Victoria experienced severe eutrophication and it was suggested that deteriorating water quality might lead to a collapse of its fisheries. A series of lake-wide surveys carried out 1999-2001 and 2005-2009 revealed that the temperature of the lake had risen by > 1 °C since 1927, with more rapid warming of the deeper waters reducing the thermal gradient in the water column and thus weakening stratification and the extent and severity of deoxygenation. The chlorophyll a concentrations in open water decreased since the 1980s, while Secchi disc visibility increased, indicating a reduced severity of algal blooms. Chlorophyll a was higher and Secchi disc visibility lower in inshore waters but there has been no deterioration in these areas since the 1980s. The conductivity remained unchanged, although it was about 50% greater in the semi-enclosed Nyanza Gulf than in the open lake. The water quality of the lake has therefore improved considerably despite the fact than concentrations of plant nutrients have not decreased and the reasons why this may be the case are discussed. IntroductionLake Victoria is the world's second largest freshwater lake (area = 68,800 km 2 , mean depth = 35 m) and it supports one of the world's largest inland fisheries, yielding almost one million tons per annum (LVFO, 2009a). It has attracted much interest in recent years because of the extraordinary ecological changes that occurred in it over the last three decades, comparable in their magnitude to those in newly created man-made lakes. These changes were triggered in the mid-1980s by an introduced predatory fish, the Nile perch Lates niloticus (L.), and its subsequent destruction of the endemic haplochromine cichlids. These fishes, which formed a species flock with some 500 described forms (WITTE et al., 2007), accounted for over 80% of the fish biomass in the lake (KUDHONGANIA and CORDONE, 1974) but they were small fish thought to be of little commercial value. Nile perch were therefore introduced with the specific intention of converting haplochromines into a more * Corresponding author 210 L. SITOKI et al.

Understanding fluctuations in the abundance and distribution of fishery resources over space and time is fundamental in order to address concerns about their sustainability and the basis for their management. Stock assessment studies were conducted on Lake Victoria from 1997 to 2001, with the aim to assess the status of the stocks, in particular of Nile perch. This paper summarises the findings for the Tanzanian part of the lake and makes reference to the Kenyan and Ugandan national waters for comparison. Both fisheries independent data from bottom trawl surveys and fisheries dependent data from catch assessment and frame surveys were used. Current exploitation levels and practices are analysed and linked to growth, mortality and reproductive characteristics of the stock. Abundance estimates and distribution patterns are discussed in relation to the exploitation levels, and key threats to the sustainability of the resources highlighted. Options are provided for a sustainable management of the Nile perch fishery. The 2000 frame survey revealed an intensive fishing effort. In addition, the Nile perch fishery was found to depend largely on juvenile fish. The size at first maturity was at 54.3 cm TL (1.6 yr) and 76.7 cm TL (2.5 yr), for males and females, respectively. Over 80% of the commercial catch was below the size at first maturity for males and 99% below that for females. Bottom trawl data (88% juveniles) suggest good recruitment to the stock, but yield per recruit modelling indicates unsustainable exploitation tendencies. The current annual yield (estimated at 138 324 ± 6 229 t) is well above the estimated sustainable level (109 000 t). It is recommended to reduce the exploitation rate by 50% and to increase the size at first capture. Co-management is considered to be the most effective option to implement monitoring, control and surveillance strategies.

The impacts of global warming have been reported from several deep lakes in the African Rift Valley and in each thermal gradients within the water column have increased thus strengthening already existing oxyclines, below which the water is permanently anoxic^1,2,3,4^. The temperature of Lake Victoria rose by 0.9°C between 1960 and 1990 raising fears that thermal stability would increase resulting in more extensive and severe anoxia in the deeper waters^5^. This is of concern because of the eutrophication of the lake, which began in the 1960s,^6,7^ and led to dense blooms of sometimes toxic cyanobacteria, increased deoxygenation of the bottom waters, and fish kills in some parts of the lake^8,9,10^. Here we show that thermal gradients in the water column have weakened over the last decade and that deoxygenation of deeper waters is less pronounced than expected. Since 1927 the temperature of the deeper waters has risen by 1.3°C compared to only 1.0°C in the surface layers, thereby decreasing thermal and density differentials in the water column. This contradicts the view that eutrophication would increase deoxygenation of the water column perhaps to the point where fish production could not be sustained^11^. Our results suggest that the impacts of global warming on tropical lakes are likely to highly variable and may not, in the short term at least, be uniformly detrimental.

This was an investigation to characterize seasonal and spatial distribution of particulate organic matter (POM) in the surface water of Lake Victoria at Speke Gulf, by using carbon-13 and nitrogen-15 isotopic signatures. Speke Gulf is under the influence of terrestrial inflow through river Simiyu. The POM from the lake surface water had higher C:N ratio values decreasing towards the deeper lake water, an indication of decreasing nitrogen content relative to carbonic materials. Samples away from the river mouth provided C:N ratios within the Redfield ratio range (C:N:P; 106:16:1) indicating materials of phytoplanktonic origin. The POM isotopes composition indicated a maximum δ 13 C value of −21.2‰ close to the river mouth and lowest value (~ −25‰) away from the river influence. The δ 15 N maximum (14.7‰) was observed near the river mouth with lower values (5.8‰) found in deeper waters away from the river mouth. Overall, there was a general decrease of 13 C and δ 15 N values from shallow water to deeper water. There was higher δ 15 N value dispersion (std. Dev. ±3.4‰) compared to δ 13 C values with std. Dev.±1.2‰. These values also varied with seasons, a phenomenon showing significant influence of land run off on the lake elemental distribution, carbon and nitrogen in particular.

The biodiversity and taxonomy of colonial volvocine green algae are important in ancient lakes in tropical regions. However, few taxonomic studies of these algae have been conducted in African ancient lakes. Here, we describe two species of colonial volvocine green algae in cultures originating from water samples from Lake Victoria, an ancient lake in Africa. One was identified as an undescribed morphological species of Eudorina; E. compacta sp. nov. This new species can be distinguished from other Eudorina species by its compactly arranged vegetative cells that form a hollow ellipsoidal colony. The other was identified as Colemanosphaera charkowiensis. The genus Colemanosphaera is new to Africa.