by
Ahmen Yimam
Ministry of Agriculture
Addis Ababa
Ethiopia
Lake Abaya is the largest of the Ethiopian rift valley lakes. The area in which the lake lies is mainly volcanic in origin while the waters of the lake are permanently turbid due to a heavy colloidal suspension of ferric oxide. The high turbidity of the lake greatly reduces primary productivity and fish production is similarly reduced.
The fish fauna of Lake Abaya is essentially soudanic and is the most varied of the Ethiopian lakes. According to Paranzan (1941), who first conducted experimental fishing on the lake, L. Abaya contains 20 species belonging to 11 genera.
The fisheries of L. Abaya have been studied by Riedel (1962) and Schroder (1984) who reported that Lates niloticus was abundant in the lake and that the genera Bagrus, Barbus, Clarias and Labeo were also of commercial importance. Schroder (op. cit.) conducted stock investigations on the lake using echosounding techniques and a bottom trawl. The surveys showed that:
Both number of fish and number of species decreased with increase in depth.
Calm, shallow bays supported more fish at a given depth than steep, rocky coasts.
Heavy winds and rough conditions caused fish to move into deeper water.
The trawling data were used to estimate the standing stock of the lake. Stock densities were estimated to range from 2.5t/km2 in water 2.5m deep to 0.03t/km2 in water 10.0m deep and from these figures a total stock of 1149t was calculated for the whole lake. It was noted, however, that larger species such as Lates and Bagrus were almost certainly underestimated as their high swimming speed would enable them to escape the trawl. The actual catches of these species were therefore multiplied by five thus giving a revised estimate of 2,000t of which half is Lates and Bagrus.
At the time of Schroder's survey, the fishery on Lake Abaya was at a low level of development. No boats were in use, largely because the concentration of fish in the littoral zone enabled the fish stocks to be exploited with the aid of simple rafts. Nets were not used because the dense littoral vegetation prevented the use of seines and the presence of large Lates and numerous crocodiles and hippopotamus made it impossible to set gill nets. All fishing was by hook and line and both small species (Synodontis and Oreochromis) and large predators (Lates and Bagrus) were caught in this way. Schroder (op. cit.) estimated that only about 120t of Lates (about 12% of the standing stock) were captured each year. He recommended that the yield of Lates could be doubled and also proposed that a trap fishery be developed to exploit the substantial population of Mormyrus longirostris.
Reidel, D., 1962 Der Margheritensee (Sud-Abessinien). Zugleich ein Beitrag zur zkenntis der abessinischen Graben-Seen. Arch.Hydrobiol. 58:435–66.
Schroder, R., 1984 An attempt to estimate the fish stock and sustainable yield of Lake Ziway and Lake Abaya, Ethiopian Rift Valley. Arch.Hydrobiol./Suppl. 69(3):411–44.
