Lead poisoning in Mute Swans Cygnus olor in England
Lead poisoning due to the ingestion of anglers’ lead weights has been the major cause of death of Mute Swans in England. Various measures have been taken to reduce the problem, including the introduction of legislation prohibiting the sale and use oflead weights used in ‘coarse-angling’ (fishing for non-game species). This study reports on changes in the incidence of lead poisoning in relation to these measures. Lead poisoning was monitored by three methods: post-mortem exam ination of swans found dead, veterinary diagnosis of rescued swans and blood lead analysis of live swans. Dead swans from throughout England were examined at the Sutton Bonington Veterinary Investigation Centre. A more detailed study into lead poisoning amongst the population of swans in the Thames valley in Southern England was initiated in 1979. The proportion of deaths due to lead poisoning amongst swans from throughout England dropped from 50% in 1980/81 to 40% in the mid-1980s, when alternatives to lead were introduced. In 1987 there was a further reduction to 30% which corresponded to the introduction of the ban on the sale and use of lead weights. The number of cases of lead poisoning amongst swans rescued from the River Thames and adjacent waters has dropped from a peak of 107 in 1984 to 25 in 1988. There has also been an annual reduction in the blood lead levels of swans on the River Thames at Windsor, from a median of 127 μg/100ml in 1984 to 22 μg/100ml in 1987. Changes in the seasonal pattern of lead poisoning amongst swans in the Thames area during 1987 and 1988 corresponded to predictions made on the basis that swans were ingesting mainly recently used weights. There was no longer a peak in lead poisoning during the summer months and the incidence throughout the year dropped to levels only recorded during the coarse-angling closeseason in previous years. Reasons for the continuing occurrence of lead poisoning are discussed.
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