Seasonal Variation in Heavy Metal Content of Tongue Sole, Cynoglossus brownii and Croaker, Pseudotolithus typus from Lagos and Delta States, Nigeria

Aims: The aims of this study were to determine the levels of lead (Pb), Cadmium (Cd), Copper (Cu), Chromium (Cr) and Zinc (Zn) in two economically important Nigerian fish species, tongue sole, Cynoglossus brownii and Croaker, Pseudotolithus typus from Makoko and Koko in Lagos and Delta states respectively; compare the levels of these metals with recommended international tolerable levels thereby creating awareness on potential health risks of human consumption of these fish species.
Study Design: The study locations were purposively selected, being coastal locations where major industries and crude oil exploration facilities are located; and with a high probability of discharging effluents with high levels of heavy metals. Twelve samples each of both fish species were purchased from landing sites in Makoko (Lagos State) and Koko (Delta State) respectively and transported in ice boxes to the laboratories for subsequent analyses.
Place and Duration of Study: Department of Environmental Management and Toxicology, Federal University of Agriculture, Abeokuta (FUNAAB) between May, 2010 and April, 2011.
Methodology: Twelve samples each of tongue sole and croaker were collected bi-monthly from each sampling location in May, July and September, 2010 (Wet season) and November (2010) and January and March, 2011 (Dry season). Six samples each were analyzed for heavy metal contents using Atomic Absorption Spectrophotometry (AAS) after wet digestion of dried, milled samples of the fish muscles with 1:1 HNO3: H2O2.
Results: Metal contents (µg/g, dry weight) for tongue sole in Lagos and Delta states during the wet season (April – October) were: Pb (0.330 and 0.149), Cd (0.097 and 0.063), Cu (0.869 and 0.434), Cr (1.184 and 0.826), Zn (7.612 and 7.890); for croaker: Pb (0.163 and 0.045), Cd (0.115 and 0.029), Cu (0.990 and 0.694), Cr (1.008 and 1.038) and Zn (6.756 and 6.168). For the dry season (November – March) metal contents obtained were: Pb (0.534 and 0.527), Cd (0.373 and 0.410), Cu (0.437 and 0.556), Cr (0.834 and 0.983), Zn (6.764 and 9.343) for sole; and Pb (0.210 and 0.213), Cd (0.264 and 0.307), Cu (0.442 and 0.483), Cr (0.577 and 0.597) and Zn (4.783 and 5.924) for croaker. Significant (P = 0.05) variations were observed in metal content between sole and croaker and also between dry and wet seasons notably for Cd, Cr and Cu. Generally, metal concentrations were higher in sole than croaker; and Cadmium content was significantly (P = 0.05) higher in fish samples during wet season, while Chromium and Copper were significantly (P = 0.05) higher during the dry season. Cd and Cr levels in the test species were found to exceed recommended international tolerable levels.
Conclusion: Heavy metal contents varied among the fish species evaluated and also between seasons. During the dry season, chromium and copper contents of fish were significantly higher, while Cadmium content of fish was significantly higher during the wet season. Cadmium and Chromium contents exceeded international tolerable limits, thus continued consumption of these fish species could expose consumers to serious health risks arising from metal toxicity. A programme of continuous monitoring and control of the physical and chemical characteristics of the components and food products of Nigeria’s coastal zone is recommended as a management tool for this important ecological zone.