a b s t r a c tSeasonal variations in the biomass (Chl a) and primary production ( 14 C-method) of phytoplankton were studied during 12 months of 2005 in the three Ethiopian Rift Valley Lakes (ERVL) Ziway, Awassa and Chamo. Chl a showed an average value of 40, 20, and 30 mg m À 3 for the three lakes, respectively. Integrated areal primary production for the total phytoplankton (g C m À 2 d À 1 ) varied 2-fold in the three lakes but on different levels, from 0.67-1.8 in L. Ziway, 1.8-4.6 in L. Awassa, and 1.0-2.6 in L. Chamo. The overall photosynthetic efficiency of utilizing photosynthetically active radiation by the phytoplankton on molar basis (mmol C mol of photons À 1 ) resulted in an average value of 1.4 for L. Ziway, 3.5 for L. Awassa and 1.6 for L. Chamo. Among the different factors regulating phytoplankton primary productivity, light penetration and nutrients were the most important in the three lakes. The seasonal variations of incident radiation (most values between 5 and 7 E m À 2 h À 1 ) and water temperature (most values between 22 and 24 1C) were small and unlikely to result in the marked differences in phytoplankton primary production. Although relative increase in nutrient concentrations occurred following the rainy periods, the major algal nutrients were either consistently low (nitrate and/or silicate) or high (phosphate and/or ammonium) and remained within a narrow range for most of the study period in all the three lakes. Consequently, phytoplankton biomass and primary production seem to be maintained more by nutrient regeneration or turnover (facilitated by high temperature) than by allochthonous nutrient input. This would be coupled with wind-induced mixing that would play an important role in determining hydrographic characteristics (water column structure) and the associated redistribution of nutrients and phytoplankton, the availability of light and subsequently the spatial (vertical) and temporal patterns of phytoplankton production in these three ERVL. Phytoplankton production (PP) is regarded as a good predictor of fish yield in lakes and seasonal measurements of PP is a prerequisite for good such estimates.

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We hypothesized that unusual deaths and illnesses in wild and domestic animals in lake areas of the Rift Valley south of Addis Ababa were caused by toxic cyanobacteria. In the first cyanotoxic analyses conducted in samples from Ethiopia, we found lakes Chamo, Abaya, Awassa, Chitu, Langano, Ziway, and Koka all had concentrations of microcystins (MC) ranging from trace to hazardous, whereas only traces less than limits of detection (LOD) of cylindrospermopsin (CYN) were found. In the December 2006 dry season we sampled the lakes for analyses of MC, CYN, species structures, and calculations of cyanobacteria biomass. We used the Utermöhl technique to analyse cyanobacterial biomass and monitored MC toxins using HPLC-DAD, LC-ESI-MS-MRM, and ELISA-test and CYN with HPLC-DAD and ELISA. The various toxicity tests coincided well. In 4 of the lakes (Chamo, Langano, Ziway, and Koka), the inter-lake range of total MC concentration was 1.3-48 µg L −1 ; in 3 (Abaya, Awassa, and Chitu), we found only traces of MC. Microcystis aeruginosa was the dominant species, with Microcystis panniformis, Anabaena spiroides, and Cylindrospermopsis spp. as subdominants. The MC concentration, especially in Lake Koka, exceeded levels for serious health hazards for humans, cattle, and wildlife.

Feeding habits of Cyprinus carpio was studied in Lake Koka, Ethiopia, in April and May (dry months) and July and August (wet months), 2011. The objective of the study was to identify the diet composition, seasonal variation in diet and ontogenetic dietary shift. Gut contents of 435 fish were analyzed using frequency of occurrence and volumetric analysis. In frequency of occurrence method the number of gut samples was expressed as a percentage of all non-empty stomachs examined while in volumetric method the volume of each food category was expressed as a percentage of the total volume of the gut contents. Detritus, insects and macrophytes were the dominant food categories occurring in 97.0%, 85.2% and 53.3% of the guts and comprising 39.8%, 36.4% and 12.4% of the total volume of food items, respectively. The remaining food categories were of low importance in the diet. The frequency of occurrence and volumetric contributions of the different food categories of C. carpio significantly varied (U-test, p<0.05) during the dry and wet seasons. During the dry season, insects and detritus were important food categories, occurring in 94.4% and 98.6 of the guts and comprising 42.3% and 36.1% of the total volume of food, respectively. During the wet season, detritus was the most important food category. It occurred in 93.9% of the guts and accounted for 48.8% of the total volume of food. Insects and macrophytes were also important during the wet season, occurring in 68.2% and 74.3% of the guts and comprising 23.6% and 22.6% of the total volume of food, respectively. The contributions of animal origin food categories were important in the diet of juveniles whereas, food of plant origin were more important in the diet of adults. However, there were no significant variations in the diet of individuals in the different size classes. Based on the results, it was concluded that C. carpio was omnivorous in its feeding habits in Lake Koka with considerable seasonal variation but insignificant onto genetic change in its diet.

The sustainability of a given fishery is a function of the number of sexually matured fish present in water. If there is intensive immature fishing, the population of fish reaching the stage of recruitment will decrease, which in turn results in lower yield and biomass. The present study was conducted to estimate the extent of immature fish harvesting by the commercial fishery of Lake Hawassa. Random samples of 962 Oreochromis niloticus and 672 Clarias gariepinus were taken from the fishermen's catch for two weeks from May 15 to 30, 2011, which was peak spawning season for both fish species. The maturity of the sampled fish was determined by visual examination of developmental stages of gonads based on their size, structure and the space they occupy in the body cavity of fish. Lengths at first sexual maturity of male and female C.gariepinus were55.9cm and 54.8cm, respectively and that of male and female O.niloticus were 20.8cm and 20.3cm, respectively. There was heavy immature fishing of C.gariepinus as high as 77.6% but immature fish harvesting was slight for O.niloticus (23.0%). Thus immature fishing of C.gariepinus should be stopped as soon as possible since only one fifth of the population has the chance to breed and replenish the stock before it is caught. Widening the currently used mesh size from 8cm to 10cm is recommended to avoid immature fishing.