DE GRAAF, M. 1, E. NENTWICH1, F.A. SIBBING1, M. MACHIELS2, A. LEMMA3 & T. WUDNEH4

1Experimental Zoology Group, Wageningen Institute of Animal Sciences, Wageningen University, Marijkeweg 40, 6709 PG Wageningen. E-mail: martin.degraaf@morf.edc.wag-ur.nl
2Fish Culture and Fisheries Group, Wageningen Institute of Animal Sciences, Wageningen University, The Netherlands
3Bureau of Agriculture, Amhara National Regional State, Bahar Dar, Ethiopia
4Ministry of Agriculture, Addis Ababa, Ethiopia

Decline of Barbus stocks in Lake Tana, Ethiopia: Is their reproductive success affected by fisheries?

Lake Tana, Ethiopia's largest lake, is situated at an altitude of 1800 m, 500 km north of Addis Ababa. It is the source of the (Blue) Nile and, as far as we know, it contains the only remaining species flock of cyprinid fish left in the world.

Before 1986, only a subsistence, reed boat fisheries exploited this resource in Lake Tana. Introduction of motorised fisheries after 1986 rapidly increased the annual catch. A recent experimental trawl program in the southern Bahar Dar gulf showed a dramatic decline in Barbus spp. stocks. Between 1991-1993 and 1999-2001 Barbus spp stocks decreased 60-75%. No major differences were found in abiotic parameters between the two periods. The most likely explanation for this drastic decrease in Barbus stocks is the rapid development of the unregulated motorised fisheries in Lake Tana, especially at suspected spawning sites of migrating Barbus.

For effective management, both the reproductive biology of the Barbus spp (length at maturity, spawning season, spawning sites) and fishery efforts (fishing areas, mesh size, CpUE) should be known. The aim of this study was to determine the interactions between the reproductive characteristics of Barbus and the characteristics of Lake Tana fisheries. This will facilitate the formation of clear management options. To better protect the Barbus species, fisheries regulations have to be implemented and controlled.

Detailed knowledge about reproductive biology of the Barbus spp will also provide further insight into the evolutionary processes underlying the cyprinid speciation events in Lake Tana.