Assessing the bioaccumulative impact of four heavy metals on the endocrine system of Tilapia Rendalli in the Kafue river

Abstract

The term, “heavy metals” (HMs) refers to a group of metals with atomic density greater than 4g/cm3. HM contamination is a major environmental problem worldwide today, especially in growing cities of developing countries mainly due to pollution. High levels of HMs can cause health problems in an organism, despite their various uses. Their biotoxic effects come into play once accumulation levels in organisms go beyond maximum permissible limits. Thus, this study aimed at assessing the bioaccumulative impact of cadmium (Cd), copper (Cu), nickel (Ni) and Lead (Pb) on the endocrine system of Tilapia rendalli fish in Kafue River. Water samples were collected in 2 x 500 mL polypropylene bottles, refrigerated at about 8 oC, then put at room temperature before filtering for analysis. Tissues (blood, gills, liver and muscle) were extracted per fish from 93 samples collected from upper, middle and lower sites of the river; KUP, ITT and SH respectively. Gills, livers and muscles were cleaned in distilled water, oven dried at 110 oC, weighed and transferred into digestion flasks. 10 mL of 55 % nitric acid and 5 mL of 70 % perchloric acid (ANALAR) were added into digestion flasks. Samples were digested at about 200 – 250 oC on a hot plate to a transparent solution after disappearance of initial brown fumes. White fumes indicated completion of digestion. Solutions were cooled and diluted to 20 mL using distilled water. The samples were assayed for Cd, Cu, Ni and Pb using Flame Atomic Absorption Spectrophotometric method (FAAS), Varian AAS 220 instrument. Blood samples were thawed at room temperature, centrifuged at 2016 x g (3000 rpm) for 10 minutes to collect supernatant serum. Serum was assayed for Estradiol (E2) and Testosterone (T) using the ELISA method and concentrations of unknowns determined from standard curves by interpolation using Graphpad prism 6.01. Hormone levels varied significantly among the fish samples. This difference was significantly related to variations in HM levels in fish tissues from the respective study sites. Highest mean levels of T (13.58±2.8 ng/mL) and E2 (774.33±66.98 pg/mL) were measured in SH while lowest mean levels of T (5.78±0.69 ng/mL) and E2 (63.75±45.39 pg/mL) were measured in ITT. The study further showed that low levels of Cd, Cu, and Ni in SH correlated significantly to high hormone levels recorded in fish at SH while high levels of Cu in livers and muscles, 496.73 ±184.96 mg/kg and 43.68±18.32 mg/kg respectively; recorded in ITT correlated negatively to low hormone levels in fish. Study also observed a positive correlation between hormone levels and Pb concentrations in tissues, indicating biphasic mechanisms of Pb and other metals on E2 and T levels in fish. Therefore, HM bioaccumulation affects expression levels of sex hormones in fish and this may be extrapolated to humans who are major consumers of fish along trophic levels. Since HM levels were high in internal organs than in muscle (flesh), high public awareness should be emphasized on the risk benefit of consuming fish with internal organs from Kafue River.