Further review of certain persistent organochlorine pesticides used in Great Britain.

  • Great Britain. Advisory Committee on Pesticides and Other Toxic Chemicals
Date:
1969
Catalogue details

Licence: Public Domain Mark

Credit: Further review of certain persistent organochlorine pesticides used in Great Britain. Source: Wellcome Collection.

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    under controlled laboratory conditions. A significant proportion of a fish population normally dies of disease or other natural causes each year but few dead fishes are seen. Nevertheless, individual instances have come to light in which chemicals under review have been found to affect populations in stated. localities. 16. We have not found evidence of recent widespread deaths amongst birds attributable to single or short-term intakes of the chemicals under review. Use of these pesticides as cereal seed dressings, particularly the carry over of autumn- prepared seed into spring sowing, has continued to cause casualties to wildlife locally. Some casualties in early summer also continue to occur in orchards. It has been a very difficult task to assess the importance of the residues found in the many specimens of birds that have been examined and especially the long- term significance of any behavioural or other effects in them. Predatory land birds contain, on average, larger residues than other species [Appendix V (iv)] and during recent years various field observations have been made on their _ populations and behaviour. As in our 1964 Review, therefore, we paid special attention to the evidence about them and in particular that concerned with observed changes in populations, in success in breeding, and in behaviour. 17. So far as populations are concerned, the evidence relating to the peregrine falcon and the sparrowhawk indicates that very marked declines commenced in about 1955 and continued until about 1963; the first signs of a recovery were seen in 1966. An enquiry relating to certain birds of prey in Britain which was undertaken in 1964 and which covered records over the period 1953 to 1963 indicated decreases in populations of the barn owl and kestrel. In the kestrel the decrease became most evident in about 1960 in eastern England where in some counties it had then become rare as a breeding species. As indicated elsewhere, the available evidence suggests that dieldrin was responsible for these reported changes. No marked changes were shown to have occurred in recent years in populations in Britain of the tawny owl and the golden eagle, or in the heron and great crested grebe which feed on freshwater fish. 18. Other kinds of change have been recorded, namely reduced thickness of eggshells, increased incidence of egg breaking by parent birds, and reduction in breeding success (though not necessarily sufficient to affect population size) in predatory birds. Within a period of about three years in the late 1940s, eggshells of peregrine falcons and sparrowhawks became thinner and have remained thinner ever since. There was a moderately close correspondence in time between these phenomena and the expansion in the uses of DDT, but this evidence is not precise enough to establish a causal relationship with certainty. Increased breakages of eggs were noted after 1950, but the year when these began is not known. Although the number of young reared decreased about that time, the population of peregrine falcons remained steady up to about 1955. The changes in eggshell thickness, in egg breakage, and in breeding success, in peregrines may have been caused by DDT (paragraph 52); but other factors which have not been investigated so fully may be relevant or even significant. Similar changes in eggshell thickness, egg breakage, and breeding success, have been recorded in the golden eagle. None of these has in fact affected the total population of eagles, but they might affect recovery from some other adverse factor. Breeding success has improved since the withdrawal of dieldrin sheep dips.
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