Association Between Heavy Metals And Selected Reproductive Parameters In The Nile Tilapia, Oreochromis Niloticus, Along River Ruiru, Kiambu County, Kenya

ABSTRACT

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. Industrial effluents, contaminated with pollutants such as heavy metals are released into water bodies such as streams and rivers, in most countries. River Ruiru is one of the rivers faced with pollution from nearby industries. It is inhabited by various species of fish such as catfish, 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. Oreochromis niloticus forms an important source of proteins in many communities. The inhabitants neighbouring River Ruiru, especially those of low socio-economic status obtain fish from the River. This study was aimed at assessing the current status of heavy metal pollution in River Ruiru and their association with selected reproductive parameters in the reproductive cycle of Oreochromis niloticus. Fish and water samples were collected monthly, for 8 months, from the downstream and upstream sections. Morphometric measurements, gonadosomatic index (GSI), serum 17β-estradiol (E2) levels, physicochemical parameters of the river and the levels of five heavy metals (lead, cadmium, copper, iron and zinc) in water and ovaries were determined. The levels of the heavy metals were measured using Atomic Absorption Spectrophotometer (AAS). The level of E2 was analyzed using Enzyme-Linked Immunosorbent Assay (ELISA). The difference in means of GSI, level of E2, fecundity, morphometric measurements of the fish from the downstream and the upstream sections of the river was calculated using a two sample t-test. The relationship between the level of heavy metals versus morphometric measurements; GSI; and the level of serum E2 was determined using Pearson moment correlation. One way analysis of variance (ANOVA) was used to test if there were significant differences in GSI and levels of E2 between different months. Water in both sampling sites was found to have equal levels of heavy metals implying that both sites were equally polluted. There was no significant difference in gonadosomatic index 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. The mean standard length of the mature tilapia, Oreochromis niloticus, in the upstream was significantly lower compared to the mean standard length in the downstream (t =2.87, p = 0.008). 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) due to their higher levels of accumulation. 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 both in water and fish ovaries were above allowable concentrations for fish consumption when compared to WHO and United States of Environmental Protection Agency (USEPA) recommended levels. The study recommends that the Ministry of Environment and Natural Resources (MENR) should put measures in place to stop discharging raw effluents into River Ruiru. Also, National Environment Management Authority (NEMA) and Department of Public Health of the County Government of Kiambu (DPHCGK) should hold campaigns for residents of Ruiru in order to safeguard their health by avoiding consumption of fish from the River.