Mercury is the most toxic heavy metal in the aquatic ecosystems which originates both from natural and industrial resources and is ultimately deposited in sediments, as methyl mercury. Due to its high dependence on protein sulfhydryl groups, this metal is quickly transferred through the food chain and accumulated in organisms. The high toxicity of mercury and its biomagnification characteristic, necessitates the temporal and spatial environmental monitoring of mercury. Mullet (Liza auratus) and kutum (Rutillus frisii kutum) are two species that their habitat and feeding habits cause them to accumulate significant amounts of heavy metals in their tissues and this not only affects fish health but also human health as the main consumer. To determine the concentration of mercury contamination in the sediments and its accumulation in the muscle and liver of mullet and kutum, 12 stations were randomly assigned in the central coasts of southern Caspian Sea and the samples were obtained from sediment and fish species. Standard analytical methods used to determine mercury concentration in both sediments and fish. Results showed that in sediments, the average concentration of mercury is below the standard level (at all stations 0.15 µg of mercury/g of sediment) which represents the low levels of mercury contamination in the coastal province of Mazandaran . The average concentration of mercury in kutum muscle and liver, was respectively 0.112 and 0.123 µg/ g fresh weight (0.382 and 0.395 µg/g dry weight) and for mullet was 0.137and 0.162 µg/ g fresh weight (0.432 and 0.472 µg/ g dry weight), respectively. The difference in mercury concentrations observed in the two species, could possibly be attributed to the differences in ecological requirements, nutrition, metabolism and different habitats. In both species, mercury concentration was less than the allowable amount for human consumption (0.3, 0.5, 0.5 and 1 µg/g of wet weight) determined by USEPA, WHO, FAO and the FDA, respectively. The comparison between the different stations showed that fishes of west coast of province had higher concentration of mercury than the east coast. In order to assessing the human health risks of kutum and mullet consumption, the allowable level, the daily intake of mercury by humans and also HQ ( Hazard Quotient ) index were obtained. HQ Index for both species was about 0.4ercury, Caspian Sea , Sediment, Organic matter, Liza auratus, Rutillus frisii kutum, Risk assessment.
