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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. [ABSTRACT FROM AUTHOR]

The concentrations of total mercury (THg), stable isotopes of nitrogen (δ15N) and carbon (δ13C), and the diet of the African sharptooth catfish Clarias gariepinus in Lake Awassa, Ethiopia, were studied from January 2003 to February 2004. Values of the δ15N were used as an index of trophic position in four length groups and compared to actual stomach contents. The diet of C. Gariepinus within the length range of 201–600 mm L T mainly consisted of the small barb Barbus paludinosus, aquatic insects, and mollusks. The proportion of fish prey in the diet was 60% by volume, irrespective of fish size. The differences in δ15N values of individuals within and between length classes were less than 3‰, and were not significantly related to total length, showing the similarity in trophic niche of the different sizes, which corresponded to recorded stomach contents. Mercury concentrations were in the range of 0. 002–0. 154 mg kg−1 ww, and had no significant relationship to total length. Hence, even large specimens of C. Gariepinus have Hg values below the WHO threshold of 0. 2 mg kg−1 ww. The slope of the regression line between log [Hg] and δ15N was small, 0. 06, indicating the absence of trophic shifts and biomagnification of Hg in larger specimens in our samples. The low Hg concentrations in C. Gariepinus compared to the Hg concentrations in other piscivorous fish species in Lake Awassa, such as Barbus intermedius and B. Paludinosus, may be due to its dependence on invertebrate preys at small size, diet switching towards low Hg prey fish at larger size, and growth biodilution owing to higher growth rate. [Copyright &y& Elsevier]

This paper surveys some major questions on the evolution of eight piscivorous Barbus species (B. acutirostris, B. dainellii, B. gorguari, B. longissimus, B. megastoma, B. platydorsus, B.truttiformis, and B. macrophtalmus), which together with six other species compose the endemic hexaploid ‘large barb’ species flock of Lake Tana. Preliminary data suggest that the major prey species of these piscivores (up to 90 cm length), in contrast to previous reports, is not Barbus trispilopleura, but at least two other diploid ‘small barb’ species (< 10 cm). Data are collected from 1) the resource partitioning of the piscivores in the field, 2) aquarium experiments on the predator success in catching different prey species and size classes, 3) analyses on allometric growth and diet shifts in the field and 4) molecular genetic data, which will be combined to construct a hypothesis on the process of the evolution of these piscivorous barbs. Being the major target of commercial fisheries, knowledge on these piscivorous fish stocks and their prey species is required for aiming at sustainable fisheries. Preliminary data show a serious decline of fish stocks, most probably largely due to overexploiting their spawning grounds. [ABSTRACT FROM AUTHOR]