Mercury Accumulation in Great Skuas Catharacta skua of Known Age and Sex, and Its Effects Upon Breeding and Survival

Abstract

(1) Great skuas Catharacta skua Brunnich have among the highest tissue mercury concentrations of British seabirds, and many of the birds breeding on Foula, Shetland in 1988 and 1989 had concentrations well in excess of those known to affect reproduction adversely in some terrestrial bird species. Comparison of mercury concentrations in feathers of great skuas collected before 1900 and in 1988-89 indicates a significant increase during this century, which is assumed to be due to anthropogenic activities (Thompson 1989). (2) Mercury in muscle tissues of great skuas was entirely methyl mercury, whereas approximately half the mercury in liver and kidney tissue was inorganic. Feather mercury concentrations correlated with total mercury concentrations in soft tissues of birds culled during incubation. (3) Mercury concentrations in feathers of adult great skuas on Foula, Shetland were found to be higher than those in feathers of chicks, but were independent of adult age and sex. (4) There was no evidence of accumulation of inorganic mercury in soft tissues with age. This suggests that dietary variation and specialization are more important than age as determinants of mercury concentrations in this species. (5) There was no relationship between diet during the breeding season and feather mercury concentration, either comparing mercury concentrations in different years covering a period of major change in diet at the colony, or comparing different birds with markedly different diets in the same year. Great skuas disperse widely during the winter, and differences in exposure to mercury during this period may be more important than differences in diet during the breeding season. (6) Despite evidence of increased mercury pollution (Thompson 1989), there was no evidence of a relationship between the mercury concentrations of individuals and their breeding performance or survival.