Limnological parameters, fish species composition and some biology of Oreochromis niloticus in Belbella Reservoir was studied during the period between July 2018 to June 2021 using different limnolgical kits and fishing gears. The result obtained for conductivity and pH as the mean ± SE values were 377 ± 5.7 μS cm-1 and 8.4 ± 0.7, respectively. Among the phytoplankton taxa, Blue green algae with 11 genus, followed by green algae and diatoms with six and four genus respectively. Rotifers were the most species-rich zooplankton group with seven genus. The second most important zooplankton group was the cladocerans. The copepods were, poorly represented, with only two genus, in the zooplankton community of Belbela reservoir. Oreochromis niloticus was the dominant fish species in the reservoir and Length-weight relationship of the fish was curvilinear, show isometric growth and statistically highly significant (P<0.05). Sex ratio was not significantly different from 1:1 for all sampling months. The 50% maturity length (L50) of O.niloticus was estimated to be 18 cm total length for females and 17.5 cm total length for males. The current gill net mesh size (10 cm) used in the reservoir was captured the fish with the range of length class between 16-20 cm that coincide with at their L50 of both sexes of the fish. Hence, care should be taken based on the length at first maturity that immature fish will not be affected by the waterbodies.
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