Dogwhelks (Nucella lapillus) have been collected from sites throughout Sullom Voe, Shetland (where an oil terminal has been in operation for 17 years) and in the nearby Yell Sound, Shetland on five occasions from 1987 to 1995. The animals were examined for the effects of tributyltin (TBT) contamination in the form of imposex. The 1995 survey indicates that dogwhelks were completely absent from the terminal area, on the promontory of Calback Ness. The degree of imposex in toothed adults, quantified in terms of the Relative Penis Size Index (RPSI) and Vas Deferens Sequence Index (VDSI), was high throughout Sullom Voe (RPSIs 11.6–31.3%, mean 23.2%; VDSIs 4.25–5.17, mean 4.57). On average, 50% of the females examined from sites within the Voe were sterile due to blockage of their reproductive tracts with vas deferens tissue. Outside the Voe, in the well flushed waters of Yell Sound, both the RPSI and VDSI values were much lower (RPSIs 0.00–2.69%, mean 0.31%; VDSIs 0.00–3.82, mean 1.33). Comparison of the RPSI and VDSI values found in the present survey with previous surveys indicate that dogwhelks at some stations in the Yell Sound and Sullom Voe are gradually recovering from the impact of TBT contamination. Imposex has fallen from the maximum values observed in 1991 although this is yet to result in a clear improvement in juvenile production. The continued low numbers of juveniles indicate the potential for the population to survive.
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