LEAD POISONING IN MUTE SWANS (CYGNUS OLOR) IN IRELAND: RECENT CHANGES

Abstract

Studies over two decades have shown a marked trend towards a decrease in blood lead levels in the mute swan (Cygnus olor) in parts of Ireland. However, this study shows that a small percentage of some populations of mute swans in Co. Cork still have significantly elevated lead levels, causing some potentially sublethal effects to the health status of the swans. Overall, at both urban and rural sites the median blood lead levels in adults and cygnets of both sexes were below a threshold level of 1.21μmol l⁻¹, which is indicative of elevated lead. However, 27% of the population at Cork Lough had elevated lead levels, with the highest blood lead level being 27.66μmol l⁻¹, indicating that acute poisoning is still a problem in some individuals. Mute swans from rural sites generally showed no variation, and median blood lead levels were generally low. Lead levels reached a peak in October 2004 and again during the spring months of 2005. Swans may be more susceptible to lead poisoning during spring, as it follows the winter months when natural food is scarce and lead is easily absorbed across the gut after ingestion. Autumn peaks may be explained by an increase in fishing activity over the summer months. A positive association was found between elevated lead levels and packed cell volume—an indicator of health status. Overall, a decline in acute lead poisoning at urban and rural sites is apparent from this study. However, due to its continued and persistent use in the environment, lead still poses a threat at a sublethal level.