Distribution of such fish is usually from large refrigerated rooms (cold rooms), to fish mongers who buy from such sources for onward sale to the public. S. scrombrus is commonly sold in fish markets in Benin city. This study principally ascertained the concentrations of Zn, Cu, Pb, Cd and Cr in the scombriid fish species, from selected markets in Benin city, in order to evaluate their suitability for human consumption, against the backdrop that such markets are located along busy vehicular routes which may conceivably expose fish to atmospheric deposition of chemical elements such as heavy metals. Possible sources of Zn are batteries, refuse, pesticides, alloys, dyes, fossil fuels, electroplating and metallurgical processes while sources of Cu include paints, copper pipes, copper wires, fertilizers and pesticides. Sources of Pb are batteries, alloys, solders, fossil fuels, plastics and pesticides while sources of Cd include batteries, fossil fuels, fertilizers, plastics, alloys and paints. Some sources of Cr include paints, textiles, automobile parts, fertilizers and alloys.

This study determined heavy metal:-(lead (Pb), cadmium (Cd), copper (Cu), zinc (Zn) and iron (Fe) concentrations in selected fish species including: (Brycinus intermidus, Clarias gariepinus, Parachanna obscura, Ctenopoma kingsleyae, Hemichromis bimaculatus and Phractolaemus ansorgeii) and water from Orogodo River situated in Agbor, Delta State, Nigeria. Furthermore, the health risk exposure to humans who consume the fish and utilized the water in Orogodo River was assessed. The mean (±SD) concentration of the heavy metal in water ranged from 0±0 mg/l for Cd to 0.35±0.29 mg/l for Fe. The mean (±SD) concentration of metals in fish species ranged from 0.06±0.02 mg/kg for Cd in Brycinus intermidus to 187.90±89.59 mg/kg for Fe in Parachanna obscura. The Bioaccumulation Quotient (BQ) values ranged from 4.51 mg/kg for Pb in Hemichromis bimaculatus to 1297.13 mg/kg for Fe in Phractolemus ansorgei. These results imply that continuous intake of fish and water obtained from this river would have cumulative deleterious effect on aquatic organisms and man.

The residual levels of polychlorinated biphenyls (PCBs) in the Atlantic Mackerel, Scomber scrombrus (mean length 44.87 cm, mean weight 478.35 g) marketed in Asaba, Delta State, Nigeria, were determined by Gas Chromatographic (GC) technique in order to ascertain its suitability for human consumption. The summary statistics for the mean concentrations of detectable PCBs in S. scrombrus ranged from 0.04 mg/kg (PCB 153) to 0.65 mg/kg (PCB 126), with a total of 0.93 mg/kg while the mean concentrations of PCBs in S. scrombrus by market ranged from 0.02 mg/kg (PCB 153) at Cable market to 0.82 mg/kg (PCB 126) at Abraka market with no observed significant difference (p>0.05) in the mean concentrations of PCB 153 and PCB 169 in fish between markets. The toxicity quotient (TQ) values for PCBs in S. scrombrus ranged from 0.02 (PCB 153) to 0.33(PCB 126) while the estimated daily intake (EDI) values in mg/person/day for PCBs ranged from 0.00072 (PCB 138) to 0.026 (PCB 126). A toxic equivalency (TEQ) value of 0.0656 was recorded with PCB 126 having the highest Sub-TEQ value of 0.065 while the total PCB content in S. scrombrus according to market ranged from 0.67 mg/kg (Cable market) to 1.20 mg/kg (Abraka market). The observed PCB levels in fish fell below International threshold values indicating suitability for human consumption. However constant monitoring of this aquatic species for hazardous levels of PCBs was highly recommended.

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Fish stock assessments of aquatic ecosystems are relevant against the backdrop of over exploitation, fish species extinction and the inadequate management of aquatic resources especially in developing countries such as Nigeria. The study specifically examined the fish stock assessment and the potential fish yield of the Ikpoba reservoir in Benin City, Nigeria. A total of 903 fishes were captured from the reservoir with individual abundance ranging from 18 (1.99 %) for Epiplatys sexfasciatus to 286 (31.67 %) for Synodontis clarias. A total of 15 fish species belonging to 8 families were identified while the dominant fish species were S. clarias, Hemichromis fasciatus and Marcusenius psittacus. A total of 11 fish species were identified as rare in this study. The Ekiuwa station of the reservoir was the richest species-wise (d= 6.45) while the Midpoint station had the greatest general diversity of fish species (H= 1.42). There was a more even spread and distribution of fish species at the Midpoint station (E= 1.00) while the commonest fish species were most frequent at the Okhoro station (C= 1.62). In this study, the Condition Factor ranged from 1.00 for M. psittacus to 5.78 for Gnathonemus abadii. The potential fish yield index of the reservoir was low due to a low nutrient content and ranged from 17.7 in the dry months to 23.13 in the wet months. It was advocated that the Ikpoba reservoir must be continuously monitored in order to ensure that fish species composition and diversity do not reach critical limits.