International Journal of Fisheries and Aquatic Research

Authors
Yirga Enawgaw, Brook Lemma

Abstract
The aim of this work was to investigate seasonal variation in diet composition and, the ontogenetic dietary shift of O. niloticus for Lake Tinishu Abaya, Ethiopia. Into the bargain, some of the morpholgical parameters of the study fish was also measured. Fish samples were collected using gill nets of 6 cm, 8 cm, 10 cm, and 12 cm stretched mesh sizes. Beach seine was used to obtain fingerlings in the shallower part of the lake. Samples were collected from March to May (dry season) and July to September (wet season) in 2017. The relative importance of the different food items found in the stomach contents was determined using frequency of occurrence and volumetric method. In the study, a total of 428 fish samples from 2.5 to 30.9 cm in TL and 1.1 to 475 g in TW were used to determine length-weight relationships. The relationship between total length and the total weight of O. niloticus was curvilinear with a strong relationship (R²= 0.9848). The slope of the regression (b) was 2.9876 which was closer to the isometric growth value (b=3) of fish. Among the 428 stomach content used to analyze the diet composition, 55 (12.85%) were completely empty and 373 (87.15%) observed with one or more food items. phytoplankton, detritus, zooplankton, and macrophytes were the most important food items of O. niloticus in Lake Tinishu Abaya. Insects, nematodes, ostracods and fish scales made up a minor portion of the diet of O. niloticus. There was notable variation in the type of diet and the proportions consumed by O. niloticus in Lake Tinishu Abaya during the dry and wet seasons. Phytoplankton was extensively dominated the fish gut during the dry season, while zooplankton, macrophytes, and detritus was high from the total food bulk of the fish in the wet season. Comparing the two seasons, the contribution of Nematodes and Ostracods to the diet of O. niloticus was relatively higher during wet season than dry season. The reverse was true for insects and fish scale composition by which its occurrence was higher during the dry season than wet season. Ontogenetic diet shift was evident during the present study by means of the importance of phytoplankton, macrophytes and detritus increased with size whereas the importance of zooplankton, insects and other animal origin food declined with fish size. Ontogenetic diet shifts in the present study demonstrated that at the earlier stage, O. niloticus was omnivorous; its diet depends on zooplankton and phytoplankton in a high proportion. It shifts to herbivores as its size increase. At the adult stage, the diet is depended mostly on macrophytes, detritus, and phytoplankton, which are a plant origin of food. Thus, we concluded that the dietary habits of O. niloticus in Lake Tinishu Abaya is generally, the omnivorous type.