Food and feeding habits of juvenile and adult Nile tilapia, Oreochromis niloticus (L.)(Pisces: Cichlidae) in Lake Ziway, Ethiopia

Oreochromis niloticus (Nile tilapia) is an important fish in the ecology of tropical and subtropical aquatic ecosystems. The fish is commercially the most important fish species in Ethiopian water bodies including Lake Ziway. In recent years, the fish community structure and ecosystem dynamics of Lake Ziway have changed, but no studies have been carried out to assess whether the fishes have altered their diets in the lake. This study investigated the food and feeding habits of Nile tilapia from April to August 2017 in Lake Ziway, Ethiopia. A total of 365 Nile tilapia specimens (170 adults and 195 juveniles) were collected ranging from 2.5 to 30 cm TL and 0.5 to 459.7 g tw. Adult fish samples were obtained from the catches of fishermen, while the juveniles were collected from three sampling sites which were located in the shallow part of the lake. Guts of 165 (85%) juveniles and 115 (73.5%) adults that contained food items in their stomachs were analyzed using the frequency of occurrence and volumetric methods. Volumetrically, the major diets of juveniles were zooplankton (33.79%), phytoplankton (25.44%), insect (18.69%), and detritus (14.02%) while the diet of adults were mainly macrophytes (36.2%) followed by phytoplankton (34.36%) and detritus (18.41%). Nile tilapia, which was a phytoplanktivorous fish, has now consumed predominately macrophytes even though phytoplankton is the second dominant food item. Juvenile Nile tilapia depends on zooplankton and insect larvae. The study demonstrates that juveniles mainly fed on animal-based food items whereas the adult fed primarily on macrophytes followed by phytoplankton food items. We have discussed the possible causes of macrophytes as a major diet of adult Nile tilapia. This study contributes to the sustainable utilization of Nile tilapia and for the development of the aquaculture industry