• biomagnification;
  • diet shifts;
  • mercury;
  • piscivory;
  • stable isotopes

The diet, habitat use and mercury concentration of the small fish species, the straight fin barb Barbus paludinosus, were studied in Lake Awassa, Ethiopia, for a period of 1 year from February 2003 to January 2004. Stable isotope signatures of nitrogen and carbon in different total length (LT) classes were used to determine trophic positions and organic carbon sources, respectively. Barbus paludinosus mainly occupied the protected benthic habitats (littoral and profundal) of the lake. The δ13C values were in the range from −24 to −19‰, indicating that the carbon source for B. paludinosus was benthic, as well. Small individuals (≤ 60 mm LT) mainly preyed upon ostracods, intermediate sizes (60–100 mm) on aquatic insects and gastropods, while a tiny cyprinodont fish Aplocheilichthys antinorii dominated the diet of large individuals (100–160 mm). The progressively increase in δ15N with increasing LT also indicated a diet shift towards piscivory in larger individuals. The mercury concentration ranging from 0·02 to 0·74 mg kg−1 wet mass (wm), was unexpectedly high in this small species, and was significantly positively related to LT, as well as to δ15N. Some large individuals had mercury concentrations < 0·1 mg kg−1 wm, and low δ15N, indicating substantial variations in diet between individuals of same size. The study suggests that other piscivorous species which include B. paludinosus in their diet may have a high mercury intake risk.