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Status and Trends in the Levels of Imposex in Marine Gastropods (TBT in Shellfish)

D8 - Concentrations of Contaminants

D8.2 - Effects of contaminants

Following bans on tributyltin in antifouling paints there has been a marked improvement in the reproductive condition of marine snails over the assessment period 2010–2015.

Area Assessed

Printable Summary


Antifouling paints are widely used on marine vessels to prevent the growth of marine organisms on the hull. In the 1980s, antifouling paint containing tributyltin (TBT) was used to prevent the attachment of algal slimes and other organisms. By the mid-1980s, the cause of poor growth in oyster stocks was identified as TBT in antifouling paints used on small craft operating in waters near the commercial shellfish beds.

TBT is toxic to many marine organisms at very low concentrations and is unequivocally linked to reduced reproductive performance in several mollusc species.

The OSPAR Hazardous Substances Strategy has the ultimate aim of achieving concentrations in the marine environment close to zero for man-made synthetic substances. Since the mid-1980s, a range of national and international measures has resulted in the phasing out of paints containing TBT in the OSPAR Maritime Area. In 2008, a global ban on TBT in antifouling systems on large vessels came into effect.

Following TBT exposure, some female marine snails (gastropods) develop male sexual characteristics. This is termed ‘imposex’. An OSPAR indicator has been developed to measure the extent of imposex within the OSPAR Maritime Area using the Vas Deferens Sequence (VDS). Although TBT ultimately affects many organisms, marine gastropods such as the dog whelk (Figure 1), are among the most sensitive, making this an ideal species for monitoring.

OSPAR’s Ecological Quality Objective for the North Sea is to reduce the level of occurrence of imposex in dog whelk and other marine gastropods.

Figure 1: Tributyltin affects many creatures, but marine gastropods, such as the dog whelk are among the most sensitive. © Martin Talbot

Antifouling paints are widely used on vessels of all sizes to prevent the growth of marine organisms. Historically, antifouling paints were primarily based on the use of copper, creating higher, toxic, concentrations close to the hull and so preventing the attachment of organisms. Around the beginning of the 1980s, a more effective component began to be used, tributyltin (TBT). This compound proved extremely effective at preventing the attachment of algal slimes, which are usually the first organisms to attach and which then provide a coating to which other organisms can attach. By the mid-1980s, oyster growers in France and the United Kingdom were becoming extremely concerned about poor growth in their stocks. Cultured Pacific oysters (Crassostrea gigas), in particular, were misshapen and contained little meat, so were not marketable. Eventually, the cause was traced to the use of TBT in antifouling paints applied mainly to small craft used in estuaries and moored in marinas, close to the commercial shellfish beds.

Exposure to TBT at even very low concentrations is now known to harm many marine organisms (e.g. Law et al., 2012). TBT is unequivocally linked to reduced reproductive performance in several molluscan species, with some female marine snails (gastropods) developing male sex characteristics in response to TBT exposure; this is termed ‘imposex’. TBT ultimately affects many organisms, but the particular sensitivity of marine gastropods makes them an important indicator species, and changes in their levels of imposex can provide an early warning of changes as a result of TBT in the marine ecosystem. Since 2005 a range of national and international measures have resulted in the phasing out of paints containing TBT in the OSPAR Maritime Area, and their use on vessels, in aquaculture and on underwater structures. A global ban on TBT in antifouling systems on large vessels came into effect in 2008. Together, these measures address the main TBT-related pressures on the marine environment. Assessment criteria in the form of background assessment criteria (BAC) and Environmental Assessment Criteria (EAC) have been established by OSPAR for imposex measurement in a range of gastropods, representing the species most to TBT.

In assessing a suite of marine contaminants both ‘relative’ and ‘absolute’ aspects have been analysed:

  • ‘trend assessment’ or spatial distribution assessment to focus on relative differences and changes on spatial and temporal scales – provides information about the rates of change and whether contamination is widespread or confined to specific locations; and
  • the ‘status’ assessment of the significance of the (risk of) pollution, defined as the status where chemicals, are at a hazardous level, usually require assessment criteria that take account of the possible severity of the impacts and hence require criteria that take account of the natural conditions (background concentrations) and the contaminants’ ecotoxicology. For example, Environmental Assessment Criteria (EAC) are tools in this type of assessment.

In assessing data generated through the Co-ordinated Environmental Monitoring Programme (CEMP), OSPAR states that the primary assessment value used in the assessment of contaminant concentrations in sediment and biota “corresponds to the achievement, or failure to achieve, statutory targets or policy objectives for contaminants in these matrices” (OSPAR publication 2009-461). These assessment criteria were specifically compiled for the assessment of CEMP data on hazardous substances contributing to the QSR 2010 (OSPAR 2009a,b). Their use was considered an interim solution for the purposes of the QSR 2010 and were to be used until more appropriate assessment methodology could be agreed upon and implemented. These ‘interim’ criteria have also been used in the annual CEMP assessments since 2010, and will continue to be used until OSPAR agrees on the adoption of improved assessment criteria and subject to the conditions set out in the agreement.

OSPAR IA 2017 Indicator Assessment values are not to be considered as equivalent to proposed European Union Marine Strategy Framework Directive (MSFD) criteria threshold values. However, they can be used for the purposes of their MSFD obligations by those Contracting Parties that wish to do so.

Provenance and limitations of BACs

Background assessment criteria (BACs) have been developed by OSPAR to test whether measured concentrations are near background levels for naturally occurring substances and close to zero for man-made substances, the ultimate aim of the OSPAR Hazardous Substances Strategy. Mean concentrations significantly below the BAC are said to be ‘near background’ (for naturally occurring concentrations). BACs are statistical tools defined in relation to the background concentrations or low concentrations, which enable statistical testing of whether measured concentrations could be considered to be near background levels.

Background concentrations (BC) are assessment tools intended to represent the concentrations of certain hazardous substances that would be expected in the North-East Atlantic if certain industrial developments had not happened. They represent the concentrations of those substances at ‘remote’ sites, or in ‘pristine’ conditions based on contemporary or historical data respectively, in the absence of significant mineralisation and/or oceanographic influences. In this way, they relate to the background values referred to in the OSPAR Hazardous Substances Strategy. BCs for artificial substances should be regarded as zero. It is recognised that natural processes such as geological variability or upwelling of oceanic waters near the coast may lead to significant variations in background concentrations of contaminants, for example trace metals. The natural variability of background concentrations should be taken into account in the interpretation of CEMP data, and local conditions should be taken into account when assessing the significance of any exceedance.

Low concentrations (LC) are values used to assist the derivation of BACs where there have been difficulties in assembling a dataset on concentrations in remote or pristine areas from which to derive background concentrations. LCs were prepared on the basis of datasets from areas that could generally be considered remote but which could not be guaranteed to be free from influence from long-range atmospheric transport of contaminants. LCs have also been used to assess concentrations in sediments from Spain due to the specific bulk composition of sediments from the coasts of the Iberian Peninsula. It is recognised that natural background concentrations may be lower than LCs and that they may not be directly applicable across the entire OSPAR Maritime Area.

BACs are calculated according to the method set out in Section 4 of the CEMP Assessment Manual (OSPAR, 2008b). The outcome of this method is that, on the basis of what is known about variability in observations, there is a 90% probability that the observed mean concentration will be below the BAC when the true mean concentration is at the BC. Where this is the case, the true concentrations can be regarded as ‘near background’ (for naturally occurring substances) or ‘close to zero’ (for man-made substances).

BACs are calculated on the basis of variability within the CEMP dataset currently available through databases held by the ICES Data Centre and will be refined by the relevant assessment group as further CEMP monitoring data are collected.

Provenance and limitations of EAC

Environmental assessment criteria (EAC) were developed by OSPAR and ICES for assessing the ecological significance of sediment and biota concentrations. Some EACs were specifically compiled for assessing CEMP data on hazardous substances contributing to the QSR 2010 (OSPAR Agreement 2009-2, OSPAR 2009b). The EAC do not represent target values or legal standards under the OSPAR Convention and should not be used as such. The EACs were set such that hazardous substance concentrations in sediment and biota below the EAC should not cause chronic effects in sensitive marine species, nor should concentrations present an unacceptable risk to the environment and its living resources. However, the risk of secondary poisoning is not always considered. EAC continue to be developed by OSPAR for use in data assessments.

As concentrations below EAC are considered to present no significant risk to the environment, in most cases EAC are considered analogous to the Environmental Quality Standards applied to concentrations of contaminants in water or biota, for example under the European Union (EU) Water Framework Directive (EU, 2000).

Caution should be exercised in using these generic environmental assessment criteria in specific situations. Their use does not preclude the use of common sense and expert judgement when assessing environmental effects and/or the potential for them to occur. Furthermore, the EAC do not take into account specific long-term biological effects such as carcinogenicity, genotoxicity and reproductive disruption due to hormone imbalances, and do not include combination toxicology.

Countries submit monitoring data to ICES DOME. This imposex assessment was based on data from the ICES database from 2010 to 2015.

Methods for analyses of imposex trends and status

Tributyltin (TBT) is on the OSPAR List of Chemicals for Priority Action (OSPAR, 2004). Monitoring of TBT concentrations in sediment, and its biological effects, are mandatory elements of CEMP (OSPAR, 2010). Thus, TBT differs from the indicators for other compounds, which are only defined as concentrations of the harmful compound in the animal's tissues. Technical Annex 3 to the JAMP Guidelines (OSPAR, 2008a) sets out the guidance for monitoring TBT-specific biological effects (imposex/intersex) in the gastropod species Nucella lapillus, Nassarius reticulatus, Buccinum undatum, Neptunea antiqua and Littorina littorea. At present, only Nucella lapillus (dog whelk) and Nassarius reticulatus (netted dog whelk) are monitored for assessment of status and temporal trends (Table a) using the methods described here and in the contaminants online assessment tool ( In addition, Ocenebra erinaceus (European sting winkle) is monitored in two locations in the Northern Bay of Biscay; these are only monitored for trend assessment, as BAC or EAC are not available for this species for assessment of status.

Imposex is measured using the Vas Deferens Sequence index (VDS), a seven-stage measurement based on the degree of penis and vas deferens (a male sex organ) in females (Table b). VDS = 0 indicates normal genitals, VDS = 5, and VDS = 6 indicates that the female is incapable of reproducing.

Table b: Stages of imposex development in females and associated Vas Deferens Sequence index (VDS) scores
Characteristics of female genitalsVas deferens Sequence (VDS)Nucella lapillusNassarius reticulatus
No signs of imposex can be seen0
Vas deferens is evident at the site of the vulva1
A small penis is evident behind the right eye tentacle2
Vas deferens has developed from the penis but does not connect with the vulva3
Vas deferens is continuous4
Vas deferens tissue proliferates over the vulva opening, making the female incapable of breeding5
Egg capsules cannot be released and form a solid mass within the capsule gland6
BAC – VDS Index (VDS index = mean VDS)0.3
EAC – VDS Index20.3

Meta-analysis of imposex status and trends

The meta-analyses summarise status and temporal trends for each assessment area, based on monitoring site-wise estimates. The meta-analyses only consider:

  • Baseline and representative monitoring sites (impacted monitoring sites, i.e., those close to a point sources, have been omitted because they are ‘unrepresentative’ of contaminant levels at the regional scale). The monitoring sites that were considered to be impacted were the monitoring sites close to the Sullom Voe Oil Terminal (Shetland, United Kingdom) and one monitoring site in Mallaig Harbour (Scotland, United Kingdom); and
  • Assessment area with at least three monitoring sites  with good geographic spread (the minimum number that can be considered to provide an evidence base at the OSPAR regional level).

Imposex status

Two assessment criteria are used to assess the status of imposex in snails: BACs and EAC.

Mean values significantly below the BAC are said to be near background, and values below the EAC indicate no chronic effects of TBT on snails. Both criteria are available for Nucella lapillus, while only EAC is available for Nassarius reticulatus(Table c).

Table c: Criteria for imposex assessment measure per species. VDS, Vas Deferens Sequence index (see Table b); BAC, Background Assessment Concentration; EAC, Environmental Assessment Criteria
Assessment criteria (limits of VDS from Table b)
SpeciesLatin nameBACEAC
Dog whelkNucella lapillus0.32.0
Red whelkNeptunea antiqua0.32.0
Netted dog whelkNassarius reticulatus0.3

Time series of imposex measurements are assessed for status against the EAC if: there are at least three years of data, and there is at least one year with data in the period 2009–2014.

The status of each time series is summarised by two values: (1) the ratio between the estimated mean VDS in the final monitoring year and the EAC (this is modelled on the square root scale to better satisfy the distributional assumptions, but is presented on the original scale for interpretation), and (2) the ratio between the estimated mean VDS in the final monitoring year and the BAC.

Subsequently, these values were summarised by assessment area using meta-analysis. Assessment area status relative to the EAC is assessed by fitting the following linear mixed model by restricted maximum likelihood (McCullagh and Nelder, 1989):

  • Response: status (sqrt(mean VDS / EAC));
  • Fixed model: OSPAR contaminants assessment area
  • Random model: status estimation variation + residual variation.

In the case of the ratio of VDS to EAC, there was a large spread among Iberian Sea monitoring sites. This has two causes. First, some sites monitor Nucella (EAC = 2), while others monitor Nassarius (EAC = 0.3), with the higher (poorer) estimates of status being those for Nassarius (i.e. with the lower EAC). Thus, the two species give a different indication of status, suggesting that the EAC for the two are not properly aligned relative to the actual impact on the animal’s health. This issue has not been addressed; it also affects the Skagerrak, but to a lesser degree. Second, the time series in the Iberian Sea are short and only pooled data have been submitted. Therefore, confidence intervals based on individuals are not possible).

Imposex temporal trends

The ICES database contains a mixture of time series, some with individual measurements only, some with pooled measurements (i.e. data on several individuals combined into one number), and some with a mixture of individual and pooled measurements. In all cases a temporal trend was not fitted if the most recent data were from 2008 or before.

For the time series containing individual measurements only, the trend in each is summarised by the estimated odds ratio of the VDS of an individual whelk being above the EAC in a given year relative to the previous year. Values of 1 indicate no trend; i.e. the odds of an individual being above the EAC in a given year are the same as in the year before. Values <1 indicate that the odds of being above the EAC in a given year are lower than in the year before, so there is a decline in the level of imposex. Conversely, values >1 indicate an increase in the level of imposex. (Odds ratio, rather than log odds ratio, was used because a few time series with extreme trends have less influence on the odds ratio scale). Time series with individual measurements were assessed for trends if there were at least three years of data (not necessarily consecutive years).

For the time series containing pooled measurements only, a linear trend line was fitted if there were at least four years of data. The trend was then transformed to make comparison among series possible.

For time series that contained a mixture of individual and pooled measurements, the time series was truncated to use the individual-measurement method if there were enough data (i.e., three years); otherwise, a linear trend was fitted.

Regional trends are assessed by fitting the following linear mixed model by restricted maximum likelihood (McCullagh and Nelder, 1989):

  • response: trend (odds ratio);
  • fixed model: OSPAR contaminants assessment area;
  • random model: trend estimation variation + residual variation.

The fixed model means that a separate trend is estimated for each OSPAR contaminants assessment area. The random model has two terms:

  • trend estimation variation, i.e., the variation in the trend estimates from the analysis of the original time series, assumed known and fixed; and
  • residual variation, i.e. the variation that cannot be explained by any of the fixed effects or the other random effects.

General guidelines for assessment in CEMP

Methods for data screening, treatment of quality assurance information, temporal trend assessment and assessment against criteria used previously by CEMP are described in the CEMP Assessment Manual (OSPAR, 2008b) and in the help files for the contaminants online assessment tool.

Criteria used to assess environmental concentrations of hazardous substances are set out in the OSPAR agreement on CEMP Assessment Criteria for the QSR 2010 (OSPAR, 2009b). The derivation of these criteria for hazardous substances is discussed in a Background Document on CEMP Assessment Criteria for the QSR 2010 (OSPAR, 2009a, 2011). These criteria reflect a two-stage process in which data are compared to concentrations yielding limited risk of biological effects (EAC), and then against BCs or zero, expressed as BACs. The latter reflects the OSPAR Hazardous Substances Strategy, in that concentrations should be at or close to background levels for naturally occurring substances (and zero for man-made substances).

Location of monitoring sites

Imposex is currently monitored at more than 200 sites (Table d and Figure a) on up to three marine gastropod species.

Figure a: Monitoring sites used to assess imposex by OSPAR contaminants assessment areas (white lines) determined by hydrogeographic principles and expert knowledge not OSPAR internal boundaries

Monitoring sites were selected using a range of approaches, although there is an emphasis on monitoring sites which are in, or near, harbours, ports and marinas.

Differences in methodology used for the IA 2017 compared to the QSR 2010

For the IA 2017, a meta-analysis is used to synthesise the individual time series results and provide an assessment of status and temporal trends at the assessment area level. Meta-analyses take into account both the estimate of status or trend in each time series and the uncertainty in that estimate. They provide a more objective regional assessment than was possible in the QSR 2010, where a simple tabulation of the trend and status at each monitoring site was presented.


Imposex, measured as VDS, is currently monitored at more than 200 sites in the OSPAR Maritime Area, on up to three marine gastropod species. At the majority of sites monitored, imposex levels (VDS) are below the level at which harmful effects are first expected to occur. These levels are known as Environmental Assessment Criteria (EAC). In six of the ten OSPAR contaminants assessment areas, where there were sufficient data for assessment (over the period 2010–2015), levels of imposex in the three species monitored are significantly below the EACs for each species (Figure 2). In three assessment areas (Skagerrak and Kattegat, Celtic Sea and Northern Bay of Biscay) levels are at the EAC and in the Iberian Sea levels are more than five times in excess of the EAC.

In none of the assessment areas was imposex at levels close to background, i.e. they were not significantly below the Background Assessment Criteria (BAC).

Temporal trends in imposex were analysed at 174 sites using the VDS. Improvement in imposex was detected at 48% of sites, worsening imposex at 0% of locations, and there was no statistically significant change in imposex at 52% of sites (2010–2015). The percentage of improvement in imposex was lowest in the Irish and Scottish West Coasts. Dog whelks are the most common monitoring species, and temporal trends in dog whelk imposex were assessed at 157 (of 174) sites and showed significant improvement at 74% of sites.

When assessed at an assessment area scale, overall improvement relative to the EAC is evident for the nine assessment areas assessed (Figure 3).

Figure 2: Mean Vas Deferens Sequence (VDS) (2010–2015) in three species of gastropods in each OSPAR contaminants assessment area relative to the Environmental Assessment Criteria

(EAC) (with 95% upper confidence limits) where value of 1 means that the mean concentration equals the EAC. Green: Statistically significantly (p <0.05) below the EAC. Red: Not statistically significantly below the EAC

Figure 3: Temporal trends in imposex (Vas Deferens Sequence, VDS) in gastropods (2010–2015) in each OSPAR contaminants assessment area;

mean concentration is statistically significant (p <0.05) decreasing (downward triangle). 95% confidence limits (lines). The Barents Sea has too few monitoring sites for a regional assessment

There is high confidence in the assessment and sampling methodology and high confidence in the data used.

Regional Assessment Results

The focus of the assessment was on the antifouling component tributyltin (TBT), and the effects of the ban on TBT with regard to TBT concentrations in the marine environment and the biological effects on marine snails (imposex). As TBT is phased-out use of copper in antifouling products may increase.

The site count where the VDS for the three gastropod species has been determined for the assessment period is shown in Table d. Data for VDS are not necessarily reported every year for each monitoring site. This assessment is based on the results for three gastropod species (dog whelk Nucella lapillus, netted dog whelk Nassarius reticulatus,European sting winkle Ocenebra erinaceus) from more than 200 sites distributed in ten of the 13assessment areasof the Arctic Waters, Greater North Sea, Celtic Seas, Bay of Biscay and Iberian Coast.

Over half of the sites were located in the Greater North Sea and less the 1% in Arctic Waters. The Celtic Seas, and Bay of Biscay and Iberian Coast provided 25% and 23% of sites, respectively. With so few sites (only two) in Arctic Waters, no overall assessment could be made for this region. It should be noted that in the Bay of Biscay and Iberian Coast there are no sites below 42°N, i.e. along the coast of Portugal and the Gulf of Cadiz. The results in this region are representative of the northern half of this region.Assessmentscould only be made for ten of the 13areaswhen considering current status, and only nine of theassessment areaswhen considering temporal trends.

The percentage of improvement in imposex was lowest in the Irish and Scottish West Coasts (Table e).

Six of the ten assessment areas had an overall VDS significantly below the EAC (Figure 2). These were in the Norwegian trench, Northern and Southern North Sea, Irish and Scottish west coasts and Irish Sea. The overall VDS was significantly higher than the EAC in only one assessment area; the Iberian Sea. This could be partly due to more frequent sampling of netted dog whelk compared to the more frequent sampling of dog whelk in the other assessment areas; the two species give two perceptions of status. No area’s overall assessment was significantly below the BAC. There is a downward trend in VDS in all eight assessment areas (Figure 3).

Individual Time Series Results per Monitoring site

A summary of individual time series results at monitoring sites across the OSPAR Maritime Area for imposex (VDS) in gastropods is presented here In summary, in 96 out of 209 time series, mean VDS in shellfish are above the EAC. None of the 174 monitoring sites show an increase in VDS over the assessment period (2010–2015). It should be noted that not all individual time series results are included in the regional assessments (see number of monitoring sites used in each OSPAR region and contaminants assessment area in Table d), due to the criteria set out in the assessment methods.

Confidence Assessment

There is high confidence in the quality of the data used for this assessment. The data have been collected over many years with established sampling methodologies. There is sufficient temporal and spatial coverage and no significant data gaps in the areas assessed over the relevant time periods. Although synthesis of monitoring sites data for assessment areas scale uses new methods they are based on established and internationally recognised protocols for monitoring and assessment per monitoring site, therefore there is also high confidence in the methodology.


Followingactions taken to reduce, minimise or ban TBT use within individual countries, the European Union or globally,imposex is decreasing significantly.Compared to the QSR 2010, levels of imposex have markedly improved. In most assessment areas, imposex induced by TBT is at or below the level at which harmful effects are first expected to occur and there is also evidence of significant downward temporal trends in the severity of imposex in all areas assessed. Nevertheless, some areas are still subject to high imposex levels. Although levels of imposex are reducing, imposex is not yet at natural background levels in any of the areas assessed.

Ongoing measurement of imposex in marine gastropods is an effective tool for monitoring a contaminant-specific pollution effect. Imposex will continue to provide a good indicator for TBT pollution and will help in identifying illegal use of stocks of TBT-containing antifouling paints or losses of TBT from dockyards, marinas and vessel maintenance activities. Monitoring will identify whether there is any decrease in imposex at sites where imposex levels are not currently declining.

The CEMP assessment measures progress towards the OSPAR objective of having concentrations of hazardous substances at background levels, or close to zero, by 2020. Concentrations of the antifouling agent tributyltin (TBT) and biological effects in marine gastropods resulting from its use have decreased following the ban on the use of TBT on small craft in 1987 and on all ships in 2008. The biological effect Vas Deferens Sequence Index decreased significantly in 48% of time series. However imposex levels at a number of sites (47%) are not significantly below the EAC. The reason for this is not clear.

The OSPAR web-based tool ( enables assessment of temporal trends in the marine environment. The outcomes track progress towards the OSPAR aim of reaching levels of contaminants not giving rise to pollution effects and the cessation of discharges, emissions and losses of hazardous substances by 2020. This indicator assessment report will be produced and updated regularly in the CEMP assessment processes.

Although reduced impact of tributyltin (TBT) is evident and indicates the positive impact of implementing internationally agreed legislation, the flux of TBT from bottom sediments or the illegal use of TBT warrant continued monitoring of TBT in the marine environment.

Knowledge Gaps

There could be concern about the potential for environmental harm associated with the substitute chemicals used to replace tributyltin (TBT) in antifouling paints.

The use of copper-based paints, in some cases with the addition of other chemicals, should be monitored to avoid adverse consequences of use of substitute chemicals, i.e. imposex measured as the Vas Deferens Sequence (VDS). TBT present in historically contaminated sediments could be remobilised and enter the water column, representing a potentially long-term issue. Impacts of illegal use of TBT should not be discounted.

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