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

3. Results

3.1 Toothed Adult Survey

RPSI values in toothed adults ranged from 0.02-0.31% inside Sullom Voe to 0.00-0.07% outside the voe at 'boundary' sites and in the waters of Yell Sound ( Table 1, Figure 2). The highest RPSI value ( Table 1, Figure 2) from the toothed adult populations of N. lapillus was found in the middle of Sullom Voe (site 11, Northward, 0.31%) and very low compared to historical values. The RPSI values from toothed adult populations in Yell Sound and at boundary sites close to the voe were at or close to zero (<0.01%) with the exception of site 13 (Skaw Taing) with VDSI 0.07. These low values of RPSI are generally associated with areas distant from sources of TBT (Bryan et al., 1986; Bailey and Davies, 1989).

All sites either showed a decrease in RPSI levels or a very small change from previous close to zero values (increases of 0.01% or less).

The VDSI values ( Table 1, Figure 3) follow a similar distribution to that seen in previous surveys. Populations within Sullom Voe (sites 7‑12) showed VDSI values of 1.25-2.73. Most of the populations outside the voe in Yell Sound (sites 1-5, 14-17, 19,22) showed low VDS indices (0.06-0.67) and boundary sites just outside the voe (6, 13, 18) had populations demonstrating intermediate levels of VDSI (0.62-1.6). Site 14 (Grunna Taing) had shown unusually high levels of imposex in 2009 (3.24) increasing from previous surveys. The current survey returned a result of 0.19, more in line with expectations for a site in Yell Sound removed from sources of TBT, that has been showing recovery in imposex levels over time.

At none of the sites within or outwith the Voe, were any toothed adult females ( Table 3) found to be sterile (reproductive tract blocked by vas deferens tissue, VDS Stage 5) and no females were found with a solid mass of egg capsules within the capsule gland (stage 6).

The imposex results show that populations at most sites within Sullom Voe still show clear signs of TBT exposure. At five sites (7, 8, 9, 11, 12) these levels are in excess of criteria values ( VDSI >2) used for marine assessment purposes including Water Framework Directive water body classification and OSPAR ecological quality objectives for the North Sea. Most of the sites outwith the Voe have very low levels of imposex indicating a continuing exposure to considerably less TBT than those sites within the Voe. There is still a general gradation in imposex from the low levels at the outer sites in Yell Sound with increasing degrees of imposex towards Sullom Voe.

In 2011, for the first time a new site was sampled, Sweinna Stack (22). The location of this site is intermediate between The Brough and Norther Geo and was included due to the absence of sufficient dogwhelks in the population at Norther Geo (20). The population sampled at this new site returned very low levels of imposex ( RPSI 0.00, VDSI 0.06) in line with its location, far removed from any sources of historical TBT contamination.

3.2 Un-toothed Adults, Sub-Adult and Juvenile Surveys

Untoothed adults (21-35 mm shell length), sub-adults (15-21 mm) and juveniles (10-15 mm) were sampled at 5 sites, 2 from within Sullom Voe (sites 9 and 12) and 3 from the Western shores of Yell Sound (sites 1, 3 and 5) ( Table 1, Figure 1; sites marked with a J). As for the survey of adult dogwhelks, the highest levels of imposex ( RPSI and VDSI) in these size classes were recorded from the two sites inside Sullom Voe, The Kames (site 12, e.g. 21‑26 mm, 0.23% RPSI, 1.7 VDSI) and Mavis Grind (site 9, e.g. 10-15 mm, 0.46% RPSI, 1.33 VDSI) ( Table 1, Figures 4 and 5).

Sites outwith Sullom Voe displayed a much lower level of imposex development in the smaller size classes. Untoothed adults, sub-adults and juveniles from Easterwick (1), Burgo Taing (3) and East of Ollaberry (5) demonstrate low RPSI (0.05% or less) associated with sites distant from sources of TBT (Bryan et al., 1986; Bailey and Davies, 1989). VDS in individuals of the smaller size classes from these sites outside of Sullom Voe was also low, at 1.00 (26-35 mm, site 3) or less. The levels of imposex indicators in the smaller size classes were generally lower than have been recorded in previous surveys, suggesting a continued improvement in the levels of exposure to TBT being experienced by dogwhelks at these five sites. The high levels of imposex observed in some size classes of young dogwhelks at Billia Skerry in 2009 were no longer evident in the 2011 survey.

At sites within Sullom Voe (9, 12) levels of VDSI experienced by juvenile and sub-adult dogwhelks were lower than that experienced by the adults. This may also be symptomatic of a recovery of populations, with reduced levels of imposex observed in younger individuals ( Table 1, Figure 5).

None of the untoothed adults, sub-adults or juveniles sampled showed VDS >4, implying an absence of sterile females from these size classes of the sampled populations.

3.3 Comparison of the 2004 Toothed Adult dogwhelk Survey with previous Survey Results (1987, 1990, 1991, 1993, 1995, 1997, 1999, 2001, 2004, 2007 and 2009)

A summary of the results of all surveys for sites 1-21 is given in Tables 3 and 4 and Figures 6 and 7. For investigation of temporal trends in imposex, the sites have been classified by location and the data obtained in each survey is graphically shown in Figure 8 for RPSI, Figure 9 for VDSI and Figure 10 for the incidence of sterile females, using the following site designations:

• outer sites in the Yell Sound (I: 3, 4, 5, 17; II: 1, 2, 14-16, 19, 20),
• boundary sites between Yell Sound and Sullom Voe (sites 6, 13, 18)
• sites within the Voe itself (sites 7-12, 21)

3.3.1 Comparison of the degree of imposex in toothed adults

The RPSI and VDSI values for the populations at sites in Sullom Voe (7-12) have generally decreased with time. There is some variability between surveys, however the overall pattern remains one of decreasing imposex with time for populations from the Voe sites. While RPSI values declined only slowly from 1993 to 2001, there has been a marked decrease in RPSI at all sites in Sullom Voe (7-12) between 2001 and 2009, with further smaller decline from already very low values in 2011. Changes in VDSI between 2001 and 2007 were smaller than the changes in RPSI, but since 2007 show a marked decrease that continues to 2011. In 2009, significantly, for the first time, all sites within Sullom Voe show VDSI <4.00, showing that populations appear to be no longer reproductively impaired. This remains the case in 2011.

VDSI has decreased since 2009 at all sites except 6 (from 0.59 to 1.44). RPSI has also decreased or remained at very lows levels since 2009 at every site surveyed and in all cases either remained at very low levels (increases of <0.01% at 3 sites) or decreased further.

3.3.2 Comparison of incidence of sterility in the females obtained during 1987-2011

The incidence of female sterility at each site from each sampling survey from 1987-2011 is shown in Figure 10. The proportion of sterile females at all sites has decreased to 0% for the first time in 2009 and this state is maintained for 2011.

3.4 Assessment of Imposex Data against OSPAR Assessment Criteria

In order to aid environmental assessments, the Oslo and Paris Commission ( OSPAR) have derived a set of biological effect assessment criteria for TBT, based on the development of imposex in gastropod species ( OSPAR, 2004). For dogwhelks, these criteria are based on VDSI, and the values chosen relate to effects on the reproductive capability of females in the populations and the effects expected from exposure to TBT concentrations in water equivalent to Environmantal Quality Standard ( EQS). The VDSI values used to discriminate 6 assessment classes (A-F) and the effects that these values relate to are given in Table 5. In 2011 a proposal to change the assessment of imposex data was proposed to OSPAR to allow more integration of assessment across other contaminants and effect types. This includes the derivation of Background Assessment Criteria ( BAC) at VDSI 0.3 and Environmental Assessment Criteria at VDSI 2. Failure of the EAC level may be used by the UK (and other OSPAR Contracting Parties) as an indicator for Descriptor 8 of the Marine Strategy Framework Directive ( MSFD).

The VDSI data from the 2011 survey were assessed against the criteria presented in Table 5 and the results are shown in Figure 11. In 2011 no sites showed a deterioration of assessment status. None of the sites within Sullom Voe change assessment class in 2011 and five sites within the voe remain at Class C (above the proposed EAC for imposex). Seven sites outside of Sullom Voe itself have improved status to assessment class A (Table 6).