Assessing the Bioaccumulative Impact of Four Heavy Metals on the Endocrine System of Tilapia rendalli Fish Species in the Kafue River

Abstract

Some heavy metals (HMs) are of biological importance in animal life while others are important trace elements for plant growth and in higher animals. Despite various uses, their biotoxic effects arise once accumulation levels in animal bodies go beyond maximum permissible limits. Heavy Metal contamination is an environmental problem worldwide. This study aimed at assessing the bioaccumulative impact of Cd, Cu, Ni and Pb on the endocrine system of Tilapia rendalli fish in Kafue River, Zambia. Water samples collected in replicate, were stored at 8°C in polypropylene bottles. Samples were filtered at room temperature for analysis. Blood, gills, liver and muscles were extracted per fish sample collected from upper, middle and lower site of the river; KUP, ITT and SH respectively. Gills, livers and muscles were cleaned, oven dried at 110°C, weighed and digested using 55% nitric acid and 70% perchloric acid (ANALAR) at about 200 – 250°C on a hot plate to a transparent solution after disappearance of initial brown fumes. Solutions were cooled and diluted using distilled water. The samples were assayed for the metals using FAAS. Blood samples were thawed at room temperature, centrifuged to collect supernatant serum. Serum was assayed for Estradiol (E2) and Testosterone (T) using ELISA method. Hormone levels varied significantly among fish samples. This difference was related to variations of HM levels in fish tissues from respective 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 levels of T (5.78±0.69 ng/mL) and E2 (63.75±45.39 pg/mL) were measured in ITT. The study showed that low levels of Cd, Cu, and Ni in SH correlated significantly to high hormone levels, 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. A positive correlation between hormone levels and Pb concentrations in tissues was observed. Therefore, HM bioaccumulation may affect expression levels of sex hormones in fish as shown in this study. Since the levels of HM were high in internal organs than in muscle (flesh), fish of Kafue River may be safe for consumption, but may pose a health risk if consumed together with internal organs.