Absorption and bioaccumulation of water-borne inorganic mercury in the fingerlings of grass carp, Ctenopharyngodon idella

A study was conducted to evaluate the absorption and accumulation of total mercury (Hg) in various tissues of fingerlings of grass carp, Ctenopharyngodon idella when exposed to lower doses of water-borne Hg. Single breed fingerlings (10-12 cm) of grass carp were obtained from a commercial fish seed hatchery. Six groups (in triplicate) of fish (40 each) were maintained at 22 degree C, pH 7.0, hardness 140 mg/l and DO 7.0 mg/l in 90 liters of water in glass tanks. Each group was exposed to a sub-lethal dose of inorganic Hg as 0.0 mug/l (control), 0.1 mug/l, 1.0 mug/l, 2.0 mug/l, 2.5 mug/l and 3.0 mug/l for six weeks. Fish sampling was done on day zero and weekly thereafter. Five fish from each tank (15/treatment) were sacrificed; various tissues were collected and prepared for analysis of total mercury using cold-vapor atomic absorption spectrophotometry. Fish tissues that were directly exposed to Hg, the absorption and accumulation was significantly high (P less than 0.05) in skin (including mucus) (1.68 plus minus 0.012 mug/g dry wt.), gills (1.64 plus minus 0.013 mug/g dry wt.) and eyes (1.45 plus minus 0.011 mug/g dry wt.) as compared to the tissues from control group. While the tissues that were not directly in contact with water containing Hg had accumulated it significantly high (P less than 0.05) in muscles (2.03 plus minus 0.012 mug/g dry wt.), intestine (1.39 plus minus 0.015 mug/g dry wt.) and liver (1.38 plus minus 0.013 mug/g dry wt.) as compared to tissues from zero treatment. This study clearly indicates that the fishes living in water bodies receiving industrial effluents and city waste water containing various toxicants particularly heavy metal ions, even at low concentration, absorb and accumulate these heavy metals in their various tissues like skin (including mucus), gills and intestine directly from water as well as along with the food during feeding. With the passage of time as the fishes grow, these metals accumulate in various internal organs like liver, muscles and intestine to a significantly high concentration not suitable for human consumption.