A survey of total mercury and methylmercury in edible fish and invertebrates from Azorean waters

Levels of total mercury and methylmercury were determined in invertebrates and edible fish collected during 1993–1994 in Sao Miguel Island in the Azores. Among the invertebrates, the highest mean concentrations of total mercury (0.864 and 1.265 μg g−1 dry weight, respectively) were found in the gills and midgut gland of the edible crab (Cancer pagurus). The proportion of total mercury present as the methylated form in invertebrates varied widely between species, and with trophic position. Apart from white seabream (Diplodus sargus cadenati) the mean total mercury concentrations found in muscle tissues of the fish studied were generally low, ranging between 0.043 and 0.371 μg g−1 wet wt. Extreme inter-individual variability in mercury concentrations was found in white seabream of intermediate size, levels ranging between 0.132 and 24.61 μg g−1 wet wt. The particularly elevated mercury concentrations found in certain individuals of white seabream may be derived from local hot spot area(s) with raised environmental mercury levels. Methylmercury was the major form of the element accumulated in fish, comprising on average more than 80% of the total mercury. The selenium concentration in white seabream was significantly correlated to the total mercury concentration, although molar ratios of the two elements varied markedly between individual fish.