This work is the modification of the first author's Ph.D. dissertation.

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The food and feeding habits of Oreochromis niloticus in Lake Shalla, Ethiopia was examined by stomach contents analysis. Analysis of stomach revealed that 226 (65.9%) fish contained different food items, while 117 (34.1%) had empty stomachs. Non-empty stomachs had a variety of food items with phytoplankton, zooplankton, insect, detritus, nematodes and fish scales. O. niloticus mainly feed on phytoplankton and had phytoplanktivorous feeding habit. Bacillariophyceae (Nitzschia, Anomoeoneis, Navicula and Melosira) were identified as the most desired phytoplankton item. The prey items of O. niloticus differed among size classes (ANOVA, p< 0.05). The main prey items were zooplankton by the smaller-size class, and phytoplankton in fish of the larger-size class. Seasonal variation in the diet composition was evident (t-test, p< 0.05) and zooplankton, chironomids, nematodes, fish scales and detritus were important during the wet season, while phytoplankton predominates during the dry season. This study concludes that the food items and feeding habits depending on availability as influenced by season and fish size O. niloticus in Lake Shalla.

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Morphometric relationships, condition factor (CF) and reproductive biology are significant tools in fish stock management, fish biology, physiology, conservation and ecology. Growth and reproductive strategy of Oreochromis niloticus were studied from 343 fish specimens collected from Lake Shala between January and December 2018. Fish samples ranged from 7.7 cm to 33.0 cm in total length (TL) and from 7.80 g to 708.21 g in total weight (TW) were collected using gillnets of 4, 6, 8, 10, and 12 mm mesh sizes. The length-weight relationship of O. niloticus was TW = 0.0104TL 3.19 , indicating positive allometric growth of the fish. The sex ratio (0.93:1) was insignificant from the ideal fish distribution of 1:1 (χ 2 = 0.47, p > 0.05). Mean CF for males, females and combined sexes was 1.04, 1.06 and 1.05, respectively and statistically insignificant (p > 0.05). The spawning peak occurred in July (rainy) and February (dry) periods, as defined by ripe females and the breeding season. Absolute mean fecundity was 806 eggs and correlated positively with TL and TW of the fish (p < 0.05) (F = 0.56TL 2.29 , R 2 = 0.93, p < 0.05; F = 18.83TW 0.67 , R 2 = 0.90, p < 0.05).The study provides the first detailed account of the morphometric relationships and reproductive biology of O. niloticus in Lake Shala, which can be used as baseline information for successive biological-based studies in Soda Lakes of Ethiopia.

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