Evidence for this was obtained from Copister Broch where remains of both adults and chicks were found. It is likely that the predator responsible is the otter, of which there is much evidence on the Broch and also on Linga where predation was high. Although the numbers of Tysties ringed was not as large as had been hoped, methods of finding nests and extracting chicks have been developed and we expect to be able to improve the totals in future years. Copister Broch, Yell Snd. The new Atlas of breeding birds in Britain and Ireland 1988-91 .
Populations can die out where badly affected.
It is known to occur in over 30 gastropod species including the common dogwhelk Nucella lapillus. This compound is a constituent of organotin anti-foulant paints used until 1987 as a biocide on ships hulls and in aquaculture. The maximun shell length of each dogwhelk is measured. Size class frequency distribution histograms allow us to monitor the changing proportions of each population class. In badly affect areas, the proportion of juveniles is low, as recruitment fails.