Scottish Marine and Freshwater Science Volume 3 Number 5: SOTEAG Rocky Shore Monitoring Programme. TBT Contamination in Sullom Voe, Shetland. 2011 Dogwhelk Survey

4. Discussion

The higher RPSI and VDSI values of N. lapillus populations in Sullom Voe relative to those of the dogwhelk populations in Yell Sound indicate that the Sullom Voe populations continue to be more impacted by TBT released from around the oil terminal until 2008. However, the results of the current 2011 survey show a continued improvement of populations at sites within the Voe. The RPSI and VDSI measurements at sites within the Voe indicate that these populations have good reproductive potential, with no recorded sterile females in the populations. Indeed intertidal shoreline surveys are revealing that population abundance is increasing at previously heavily affected sites close to the terminal. In addition, the continued and increased presence of juvenile and sub-adult dogwhelks at The Kames (site 12) reflects the fact that sterile females have not been found at the site since 2007 (Moore and Gubbins 2012).

The degree of imposex in populations of dogwhelks from the boundary sites (13 and 18) is continuing to decline and these populations continue to have a good reproductive potential, with no sterile individuals found in the last two surveys. Site 6 however shows an increase in VDSI on previous values, but not sufficient to alter assessment class or suggest any significant impairement of reproductive capacity for the population.

Sites in Yell Sound, farthest from the terminal show degrees of imposex which continue to reflect those of sites distant from sources of TBT (close to background/zero) and all sites show a decline in VDSI. RPSI values at these sites were already close to zero and either remain so or show small further declines.

The most notable changes since 2009 are the continued marked decrease in VDSI at sites within the voe. Only five sites now remain as 'cause for concern' with imposex levels above international standards ( OSPAR Eco EcoQO, WFD classification tool, proposed OSPAR EAC).

Since the ban on TBT on large vessels in 2008, direct inputs to Sullom Voe from ship-based sources have ceased. The continued presence of imposex in dogwhelk populations is likely to be caused by a combination of continued exposure to TBT in sub-tidal sediments from historical contamination and older surviving dogwhelks expressing higher levels of imposex relevant to historical exposure levels (the condition is irreversible).

This series of surveys of dogwhelk imposex in Sullom Voe represent the longest consistent data set of biological effects of contaminants on marine organisms in the UK, and possibly over a much wider area as well. Monitoring should continue to chart the course of the recovery following the recent IMO ban on exposed TBT based paints on hulls of large vessels. It is recommended that the surveys continue to be carried out at an interval of every two - three years at least until all sites have shown a decline in imposex levels to below cause for concern levels ( VDSI 2).