DEJEN , E. & F.A. SIBBING

Experimental Zoology Group, Wageningen Institute of Animal Sciences, Wageningen University, Marijkeweg 40, 6709 PG Wageningen, The Netherlands. E-mail: eshete.dejen@morf.edc.wag-ur.nl

Small pelagic barbs: The major resource in the ecosystem of Lake Tana (Ethiopia)

Barbus humilis and B. tanapelagius are two important pelagic fish in Lake Tana. These small barb species (< 10 cm FL) play a key role in the lake's ecosystem as a link between the zooplankton and the top predators, mostly large barbs (up to 80 cm FL).

This paper presents an overview on taxonomy, distribution, feeding, reproductive biology and the position of small barbs in the food web of Lake Tana. Of the four species of small barbs earlier reported for Lake Tana, only three appeared as valid species; one composed of two ecotypes. Gill netting and trawling data show that about half of the total catch by weight (kg) are B. humilis and B. tanapelagius. Preliminary data from surface and bottom gillnetting show that B. humilis (ca. 97%) dominates shallow inshore areas whereas B. tanapelagius (ca. 99%) dominates the deep offshore areas of the lake. At intermediate depths both species co-occur, B. humilis at the bottom (55 %) and B. tanapelagius at the surface (45%). B. tanapelagius is the most important prey species exploiting zooplankton. Detritus and zooplankton constitute the major part of the diet of B. humilis. It is concluded that B. tanapelagius and to some extent the benthopelagic B. humilis transfer energy from zooplankton to top level of the food web. B. humilis bridges the transfer of energy from detritus to top predators. Like other small pelagics from African lakes, both species have short lifecycle, high fecundity (until 3235 eggs per female of 7.5 cm fork length), smaller maturation size (5.5 - 6.0 cm), extended breeding season. Infection rate by tapeworm larvae Ligula intestinalis in B. tanapelagius was higher (10%) than in B. humilis (5%) and affected their reproductive strategies. Experiments are running to investigate if these small pelagic barbs are potential targets for fisheries, like "dagaa" in Lake Victoria.