SummaryMercury in fish tissue is an important measure of water and sediment quality. for mercury to increase in concentration in fish tissue, it must be available in the environment (water and/or sediment) and methylated, usually by bacteria in hypoxic/anoxic conditions. General Information about this IndicatorWhat is it?: This is primarily a legacy effect of mining activities in the 19th century. Mercury in eroded soils and benthic sediments can be methylated under appropriate aquatic environmental conditions and end up bioaccumulated in fish tissue. High enough concentrations are present in many waterways and wetlands to harm piscivorous birds and mammals, including people.