Methylmercury accumulation in tissues and its effects on growth and appetite in captive great egrets

To test the hypothesis that fledging wading birds would be more at risk fro m mercury toxicosis than younger nestlings, captive great egret nestlings w ere maintained as controls or were dosed from 1- to 14-wk-old with 0.5 or 5 mg methylmercury chloride/kg wet weight in fish. Birds dosed with 5 mg/kg suffered from subacute toxicosis at wk 10-12. Growing feather concentration s were the most closely correlated with cumulative mercury consumed per wei ght. Blood concentrations of mercury increased more rapidly after 9 wk in a ll groups when feathers stopped growing. Total mercury accumulated in tissu es in concentrations in the following order: growing scapular feathers > po wderdown > mature scapular feathers > liver > kidney > blood > muscle > pan creas > brain > bile > fat > eye. The proportion of total mercury that was methylated depended upon tissue type and dose group. Selenium accumulated i n liver in direct proportion to liver mercury concentrations. After wk 9, a ppetite and weight index (weight/bill length) declined significantly in bot h dosed groups. At current exposure levels in the Everglades (Florida, USA) mercury deposited in rapidly growing feathers may protect nestlings from a dverse effects on growth until feathers cease growing.