Using seabirds to monitor mercury in marine environments: The validity of conversion ratios for tissue comparisons

The ‘7:3:1 rule’ for converting mercury concentrations between feather, liver and muscle tissues was evaluated by measuring total and methyl mercury levels in liver, muscle and body feathers of a range of seabird species. Mean mercury concentrations were used to calculate feather: liver (both total and methyl) and feather: muscle ratios, the results obtained being compared with the predicted values of 2.3 and 7.0. Feather: liver ratios were found to approximate to 2.3 when liver methyl mercury concentrations were considered, but elevated inorganic mercury concentrations in the livers of some species resulted in greatly reduced feather: liver ratios for total mercury. Feather: muscle ratios varied from 3.8 to 15.3. Factors likely to affect the value of feather: liver and feather: muscle mercury concentration ratios, such as the predominant form of mercury present in the liver tissue, sampling date relative to the stage of the moult sequence and types of feather used for analysis, are discussed and we emphasize that the 7:3:1 conversion ratio used by a number of authors should be treated with caution.