Diet composition, length-weight relationship and condition factor of Barbus species Rüipell, 1836 (Pisces: Cyprinidae) in Lake Awassa, Ethiopia.

Abstract

Length-weight relationship, condition factor and composition of the diet of Barbus sp. in Lake Awassa were studied from monthly samples taken between December 1987 and September 1988. Total length and total weight of the fish were curvilinearly related (b = 3.14). Fulton's as well as relative condition factors did not vary between months and between sexes. Barbus sp. in Lake Awassa was considered to be polyphagous, because its diet was composed of a diverse group of food items. Insect larvae (mainly chironomids), fruits, fish (mainly Oreochromis niloticus) and molluscs were the major food items of the fish whereas crustacean zooplankton, ostracods, nematodes and hydracarines were minor food items. Detritus and sand grains were also ingested by several individuals. The dominance of benthic food and the presence of sand in the gut indicate that the fish feeds mainly near the bottom. Although the fish is generally polyphagous, there were seasonal, individual and size-based differences in food and feeding habits. Thus, insects contributed more to the diet in the dry season whereas fruits, fish and molluscs contributed more in the wet season. This could probably reflect seasonal differences in availability between the food items. In addition, some individuals appeared to feed selectively on particular food items which could be a mechanism by which competition for food is minimised. Furthermore, individuals between 20.5 and 32.0 cm in length fed similar amounts of the major food items except that insects were taken at a relatively larger proportion. Fish larger than 31.5 cm appeared piscivorous. The degree of piscivory found in Barbus sp. in Lake Awassa was unexpectedly high, but as is the case in other cyprinids, it is believed to be due to the absence of specialised piscivores in the lake. The results of the study may also suggest that, as is the case in Lake Tana, the large Barbus of Lake Awassa may contain more than 1 morphotype, possibly more than one species. Thus, detailed taxonomic study is needed.