Mercury speciation and accumulation in Bangladesh freshwater and anadromous fish

Total mercury concentration in the muscle of 417 fish of 12 common freshwater and four anadromous species from Bangladesh were low, varying from 2 to 430 ng/g fresh wt. Depending on Hg speciation, three types of accumulation mechanisms were defined. Type I covers the majority of species and describes a pattern widely accepted as 'normal', with increasing levels of organic (methyl) mercury with length (age), combined to a low and constant inorganic level. This accumulation pattern leads to a relative increase of the organic mercury fraction with age, eventually reaching 90-100% of organic mercury in full grown specimens. Type II is found in both planktivorous genera only and showed increasing levels of inorganic mercury combined to low and constant organic mercury levels, leading to a relative decrease in organic mercury fraction with age. This unexpected pattern was only reported in cases of some marine species where it seemed to be linked to demethylation mechanisms or regional influences on Hg levels. A third intermediate accumulation pattern with increasing concentrations of both the organic and the inorganic Hg fraction with age was found in one bottom dwelling species only. The implications of these observations for the accumulation mechanisms of mercury in fish are discussed.