The recovery of populations of dogwhelks suffering from imposex in the Firth of Forth 1987–1997/98

The impact of tributyltin (TBT) contamination on dogwhelk (Nucella lapillus) populations was assessed at 33 sites in the Firth of Forth, UK, during spring 1997 and summer 1998. The sex ratio of the animals; the ratio of juveniles to adults; the degree of imposex, as determined by the relative penis size index (RPSI); and the total tin concentration in a sample of dogwhelk tissue were used as measurements of TBT impact on dogwhelk populations. These data were compared with data from a similar survey carried out at the same sites in the Firth of Forth in 1987 (Bailey, S.K., Davies, I.M., 1988. Tributyltin contamination in the Firth of Forth (1975–1987). Science of the Total Environment 76, 185–192.) before restrictions were introduced on the use of TBT. The results demonstrate a general recovery in dogwhelk populations from the impact of TBT at the majority of the sites studied, although at six of the sites, the RPSI value remains high enough to suggest breeding problems in the population. It is suggested that large vessels are responsible for localised cases of imposex within the Firth of Forth.