Major difference in mercury concentrations of the African big barb, Barbus intermedius (R.) due to shifts in trophic position

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 mg center dot kg(-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.