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The Nile tilapia, Oreochromis niloticus, is a tropical fish species of commercial importance in both aquaculture and in the wild. It plays a great role in human nutrition and food security. River Ruiru is one of the rivers faced with pollution from nearby industries. It is inhabited by various species of fish such as tilapia and common carp, which, may be harvested by surrounding communities for food. High levels of heavy metals disrupt normal reproductive process in fish. Besides, it has been reported that edible fish contaminated with heavy metals has deleterious effects on the health of humans and other animals that consume them. There is no documented information on the association between heavy metals and the reproductive biology of Oreochromis niloticus in River Ruiru. This study was aimed at assessing the level of heavy metals in fish ovaries and their association with selected reproductive parameters in the reproductive cycle of O. niloticus. Fish samples were collected monthly, for 8 months, from the downstream and upstream sections. Gonadosomatic index (GSI), serum 17β-estradiol (E2) levels and levels of heavy metals lead, cadmium, copper, iron and zinc in ovaries were determined. The levels of the heavy metals were measured using Atomic Absorption Spectrophotometer. The level of E2 was analyzed using Enzyme-Linked Immunosorbent Assay. There was no significant difference in the GSI between the upstream and the downstream sites (t=0.82, p=0.416). Similarly, there was no significant difference in the levels of E2 between the downstream and the upstream sampling sections. In the downstream, the levels of lead and iron in fish ovaries were significantly higher compared to the upstream (lead: t = 3.36, p = 0.002; iron: t = 4.920, p=0.001). The results showed that levels of heavy metals did not associate with the selected reproductive parameters in the Nile tilapia, along River Ruiru. Levels of lead and cadmium were above allowable concentrations for fish consumption when compared to WHO levels. The study recommends that the Ministry of Environment and Natural Resources should put measures in place to stop discharging raw effluents into River Ruiru.
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