Species composition, abundance, and spatial distribution of rotifer and crustacean zooplankton were studied in Lake Ziway from late April to early July 2004. A total of 49 rotifer species was recorded, with Anuraeopsis fissa, Brachionus angularis, Filinia novaezealandiae, and Trichocerca ruttneri being numerically dominant. Variation in abundance was extremely high, ranging from 2 to 1000+ individuals per litre. There was no significant difference in the distribution of rotifer species between inshore and offshore regions. Crustacean species richness was low, with only five cladoceran and three copepod species occurring in the open water. Moina micrura and Diaphanosoma excisum dominated the cladoceran community, whereas Thermocyclops decipiens was the dominant copepod. Although numerically dominant (75%), rotifers accounted for less than 30% of mean total zooplankton biomass. Peak abundance of crustaceans was observed in May and June, following the onset of the rainy season and increased phytoplankton production. Variation in the spatial distribution of crustacean species was neither observed horizontally between inshore and offshore areas nor vertically in the highly turbid and wind exposed deeper part of the lake. On the other hand, Moina micrura varied significantly in size between inshore and offshore areas. Adult M. micrura dominated offshore, whereas juveniles were more abundant inshore, suggesting a predominantly littoral selective predation on large and adult crustaceans by fish.
Mass mortalities of fish sporadically occur throughout the globe and often caused by single or multiple factors posing difficulties on the search for plausible explanations. Following the selective mass mortality of Oreochromis niloticus in two of the Bishoftu Crater Lakes: Babogaya, and Hora-Arsedi between April and May of 2013, we carried out a snapshot survey on the limnology of Lakes Babogaya and Hora-Arsedi including the histopathological and bacteriological examinations of O. niloticus fish. Complete mixing of the lake water and the subsequent anoxic condition was primarily hypothesized as the likely cause. However, vertical, profile of water temperatures and dissolved oxygen concentrations revealed that both lakes were stratified and the epilimnion was well oxygenated (> 8.9 mg L -1 ). All key water quality parameters were within the optimum range for O. niloticus culture except ammonia which was well above the lethal level (2.1 mg L -1 ) in Babogaya and stressive sublethal level of 0.1mg L -1 in Hora-Arsedi. Live fish samples showed clinical symptoms such as caudal skin lesion and injured gills. Postmortem examinations revealed hemorrhagic shock, liver and kidney damage with Aeromonas sp. and parasitic infections detected. Therefore we concluded that direct ammonia toxicity have caused the mortalities in Lake Babogaya, whilst extended sublethal ammonia exposure with subsequent stress and gill damage predisposed the fish to bacterial and parasitic infection that led to mass mortalities in Lake Hora-Arsedi.
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