Assessment of Heavy Metal Contamination and Its Impact on Reproductive Parameters in Nile Tilapia (Oreochromis niloticus) in River Ruiru, Kenya

River Ruiru originates from the slopes of the Aberdare Ranges, Kenya and flows through several coffee plantations and then through Ruiru Town. River Ruiru is affected by rural and urban effluents, as well as agricultural and industrial effluents which make the river polluted with heavy metals. River Ruiru is however inhabited by various species of fish such as tilapia and the common carp, which, may be harvested by surrounding communities for food. Fish can be contaminated by heavy metals. This study was aimed at assessing the physicochemical characteristics of River Ruiru and the level of heavy metals in fish ovaries and their association with selected reproductive parameters in the reproductive cycle of Oreochromis niloticus. Excessive levels of heavy metals in fish can impair gonadal development through suppression of synthesis of 17 β -estradiol. They can also suppress gamete development. 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. High levels of heavy metals disrupt normal reproductive processes in fish. Besides, it has been reported that edible fish contaminated with heavy metals have deleterious effects on the health of humans and other animals that consume them The physicochemical characteristics of the water were determined at each sampling site during each session of sampling water. These were turbidity, temperature, pH, electrical conductivity and dissolved oxygen. Measurements were taken during morning hours throughout the study period to avoid temporal variations during the day. The high electrical conductivity in the upstream, high pH values of water both in the upstream and downstream sampling, and the varied levels of dissolved oxygen in both upstream and downstream, varied turbidity levels observed in different months throughout the year and low temperature both in upstream and downstream sampling sites were observed. Despite the presence of heavy metals that have contributed to the present physicochemical status of Ruiru River, this ecosystem still supports aquatic life of interest to man. Fish samples were collected monthly, for 8 months, from the downstream and upstream sections. Gonadosomatic Index (GSI), serum 17 β -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 an 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 recommended 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.