Trends in Mute Swan Blood Lead Levels: Evidence of Grit Reducing Lead Poisoning

Lead from spent gunshot and discarded anglers' weights continues to pose a threat to Mute Swans (Cygnus olor) in Europe. In many parts of the world, lead is still widely used in fishing and hunting, and in areas where it is banned previous losses still pose a threat. We collected 590 blood samples from ringed Mute Swans between October and January over six winters: 1983-84 to 1986-87 inclusive, 1994-95 and 1999-2000. Most samples were collected from swans wintering at Cork Lough, Ireland, an urban site where coarse fishing is popular, while a smaller number were collected from swans at rural sites within a 50 km radius of Cork Lough. The median blood lead levels in swans at Cork Lough declined from 36.2 μg/100 ml in 1983-84 to 12.1 μg/100 ml in 1999-2000. There was also a significant decline in the proportion of birds suffering from acute lead poisoning (>40 μg/100 ml of whole blood), from 60% in the winter of 1985-86 to 2% in the winter of 1994-95. A significant decline in the median blood lead level of Mute Swans in rural areas, from 49.7 μg/100 ml in 1984-85 to 7.7 μg/100 ml in 1999-2000, was also noted. The decline in the median blood lead level and in the proportion of birds suffering from acute lead poisoning is most likely explained by a combination of factors, including the provision of grit at Cork Lough.