The African big barb (Barbus intermedius, R.) from Lake Awassa, Ethiopia is an important fish species, especially with the ongoing decline of the Nile tilapia (Oreochromis niloticus, L.) fishery. Their diet and habitat use was studied using stomach content analyses, stable nitrogen and carbon isotopes, and transect netting. Mercury biomagnification was also determined. The big barb was found to primarily exist in the littoral habitat, with molluscs being their predominant food item. The proportion of small fish (Barbus paludinosus, P.) in the big barb diet tended to vary somewhat with size, with the largest fish tending to have the most piscivorous diet. Mercury concentrations in the big barb ranged from 0.01 to 0.94 mgAEkg )1 , and were positively related with size. Fish transects and stable isotope analyses suggest that there may be two feeding forms of big barb in Lake Awassa, with some larger fish preying upon fish (and accumulating higher mercury concentrations). With the declining Nile tilapia fishery in Lake Awassa, the implication of fishermen focusing on large big barb, with its associated higher Hg burdens, is significant with human health ramifications.

ABSTRACT:The chemical composition of the effluent from the Awassa textile factory was quantified and its effects on chlorophyll-a concentration and fish fry were examined. The effluent contained high concentrations of toxic heavy metals, and concentrations of about 70% of all the elements measured were higher (by 10 to 100 times) in effluent treated in ponds (biological lagoons) than straight from the factory, presumably due to concentration by evaporation. Chlorophylla concentration in lake water samples containing 10% and 20% textile effluent from the ponds increased by 5 to 1400% and 7 to 4000%, respectively, whereas the changes in the control were -24% to 433%. About 18% and 53% of the fish fry died within 12 hr in 10% and 20% pond effluent in lake water, respectively. The highest mean mortality level was about 64% in the 20% effluent treatment at 24 hr. It was concluded that the effluent treatment ponds of the factory do not efficiently reduce the chemicals in the discharge. The observed effects of the "treated effluent" on phytoplankton biomass and fish fry indicate the undesirable effects the effluent will have on Lake Awassa. It is recommended that an alternate waste disposal system for the textile factory be set up, with regular monitoring of its effectiveness, so as to avoid undesirable long-term changes to the lake.

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