Feeding habits of Cyprinus carpio was studied in Lake Koka, Ethiopia, in April and May (dry months) and July and August (wet months), 2011. The objective of the study was to identify the diet composition, seasonal variation in diet and ontogenetic dietary shift. Gut contents of 435 fish were analyzed using frequency of occurrence and volumetric analysis. In frequency of occurrence method the number of gut samples was expressed as a percentage of all non-empty stomachs examined while in volumetric method the volume of each food category was expressed as a percentage of the total volume of the gut contents. Detritus, insects and macrophytes were the dominant food categories occurring in 97.0%, 85.2% and 53.3% of the guts and comprising 39.8%, 36.4% and 12.4% of the total volume of food items, respectively. The remaining food categories were of low importance in the diet. The frequency of occurrence and volumetric contributions of the different food categories of C. carpio significantly varied (U-test, p<0.05) during the dry and wet seasons. During the dry season, insects and detritus were important food categories, occurring in 94.4% and 98.6 of the guts and comprising 42.3% and 36.1% of the total volume of food, respectively. During the wet season, detritus was the most important food category. It occurred in 93.9% of the guts and accounted for 48.8% of the total volume of food. Insects and macrophytes were also important during the wet season, occurring in 68.2% and 74.3% of the guts and comprising 23.6% and 22.6% of the total volume of food, respectively. The contributions of animal origin food categories were important in the diet of juveniles whereas, food of plant origin were more important in the diet of adults. However, there were no significant variations in the diet of individuals in the different size classes. Based on the results, it was concluded that C. carpio was omnivorous in its feeding habits in Lake Koka with considerable seasonal variation but insignificant onto genetic change in its diet.

A 96 h static toxicity bioassay was carried out to examine fish responses and to determine the median lethal concentration (LC 50 ) of Balanites aegyptiaca stem bark extract on adults of Brycinus nurse, Labeobarbus bynni and Labeobarbus intermedius. Experimental fish were exposed to piscicide plant extract of 0.0 (control), 15.0, 17.5, 20.0, 22.5, and 25.0 mgL -1 . Fish exposed to these extracts except the control showed symptoms of toxicity including darting, agitated swimming, air gulping, loss of sensitivity and knockdown before death. These responses were much frequent and faster in L. bynni and L. intermedius. The 96 h LC 50 values for the different test fishes were 18.99, 20.72 and 20.72 mg L -1 for L . bynni, L. intermedius and B. nurse, respectively. Based on the present investigation, we can conclude that the application of B. aegyptiaca extract causes lethal toxic effects on different fishes even at low concentrations and hence, indiscriminate use of the plant for fishing should be discouraged and regulated in order to protect fish biodiversity lose in the Alitash National Park area.

Riverine ecosystems contribute considerable fisheries production. However, in Ethiopia, there is a lack of comprehensive biological studies on such ecosystems. This study assessed the feeding habits of commercially important fishes of the Ayima and Gelegu Rivers. Fishes' guts were sampled seasonally from April 2017 to November 2018 using gill nets, cast nets and electrofishing. Four hundred and forty‐nine stomachs were analysed. Niche breadth and overlap indices were estimated using Levin and Schoener indices, respectively. The data indicated that Clarias gariepinus (Burchell, 1822), Bagrus docmak (Forsskål, 1775) and Hydrocynus forskahlii (Cuvier, 1819) mostly preyed on fish, whereas Heterotis niloticus (Cuvier, 1829) and Labeobarbus bynni (Forsskål, 1775) consumed detritus and macrophytes. H. niloticus, L. bynni and H. forskahlii were generalist feeders. There was a slight ontogenetic dietary shift observed in some predator fishes. In smaller sized L. bynni (6–20 cm, TL), insects and ostracods were important preys and their contribution differed significantly (p < 0.05). In H. forskahlii, the volumetric contribution of bivalves and detritus differed significantly amongst the different size classes. Generally, resource availability had little effect on the feeding behaviour of most fishes. This implies that fishes in floodplain rivers can forage any available prey without preference and accustom to a general feeding strategy to form biologically insignificant dietary overlap.

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