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Key Message

The threshold values for sediment and imposex in this core indicator are not yet commonly agreed.

The threshold values are included as test threshold values for the purposes of the 2018 'State of the Baltic Sea' report, and the results are to be considered as intermediate.


This core indicator evaluates the status of the marine environment based on concentrations of tributyltin (TBT) and its breakdown products dibutyltin (DBT) and monobutyltin (MBT) in the Baltic Sea.


key message figure 1.png

Key message figure 1. Status assessment results based on evaluation of the TBT concentrations in sediment and water, and biological effects in biota - marine gastropods (VDS). One-Out-All-Out (OOAO) method (main figure), in sediment (upper insert), in biota (middle insert), and in water (lower insert). The assessment is carried out using Scale 4 HELCOM assessment units (defined in the HELCOM Monitoring and Assessment Strategy Annex 4). Click here to access interactive maps at the HELCOM Map and Data Service: TBT and imposex.


Monitoring is carried out in water, biota (marine gastropods) and sediments. Imposex effects of TBT on marine gastropods are used as another source of data on TBT, as a sensitive biological effect caused by organotin. Good status is achieved when the concentrations of TBT are below the threshold value. The indicator presents a status evaluation using all data available until 2016 to evaluate the assessment period 2011 - 2016. Some stations (data series) in the Kattegat, The Sound and the Great Belt achieve the good status (see results and confidence section) but only for imposex, and all sediments and water assessments fail their respective thresholds; for water mainly due to pre-2015 detection limits above the AA-EQS. When considering all available data using the One-Out-All-Out method the sediment status (fail status) in the southern Kattegat override the achieve imposex status (Key message figure 1).


Data on TBT induced effects of imposex (as Vas deference Sequence – VDS) biological effects in marine gastropods are available from Denmark and Sweden (see Results and confidence section). Data was extracted from the HELCOM COMBINE database for TBT in sediment and water. Sediment data was available from Denmark, Sweden, Germany and Lithuania, water data was available from Lithuania and Poland.

The confidence of the indicator evaluation is high in the Kattegat and Belt Sea/The sound area since the data on TBT concentrations in bivalves, sediments and imposex in marine gastropods is spatially adequate and time series are available for several stations. In the rest of the Baltic Sea, confidence is low due to very limited data availability.

All significant trends are downwards, as would be expected since the main source of TBT is in antifouling paints which have been banned, but sediments still represent a potential source of TBT in harbours and shipping lanes, which can be re-suspended during storms.

The indicator is applicable in the waters of all countries bordering the Baltic Sea.


Relevance of the core indicator

Organotin, and in particular TBT, has been shown to be very toxic to marine life, resulting in changes in oyster shells and interfering with the marine gastropods female reproductive organ, an effect known as imposex, causing sterility in some sensitive species. TBT is bioaccumulated by marine organisms causing harmful effects that mainly depend on the level of its final concentration in the tissues. Mussels are not able to degrade TBT by de-butylation, as fish and some marine gastropods are. Levels can be high in top-predators (Strand et al, 2005; Law et al, 2012). That is why the concentrations of TBT, especially those of importance to ecosystem or human health, have to be monitored. EQS values have been set for water AA-EQS at 0.2 ng/l and MAC-EQS at 1.5 ng/l, but the detection limit of analytical methods is up to 1 ng/l.

TBT and triphenyltin (TPT) were introduced in antifouling paints in the 1960s, but soon after, effects on growth and shell formation were found in French oyster grounds, and shortly after, also the effect on marine gastropods reproductive system was discovered. This led to a ban on use of these paints on pleasure boats, and eventually followed up by a total ban on TBT in antifouling paints (782/2003/EC (EC, 2003)) effective from 2008 (OSPAR, 2014). 

Some concern over TBT in fish for human consumption exists, EFSA set a group TDI of 0.25 μg/kg for TBT, DBT, TPT and dioktyltin (DOT) (EFSA, 2004), noting that also pesticides, plastic additives and food contact substances can contain organotin, an integrated risk assessment is therefore needed to take into account all of these sources. Based on the fish intake and concentration levels in fish, it was not expected TDI in general to be exceeded, except perhaps for consumption of fish caught in harbour areas or other very polluted sites.


Policy relevance of the core indicator

BSAP segment and objectivesMSFD Descriptor and criteria
Primary link

Hazardous substances

  • Concentration of hazardous substances close to natural levels.
  • Healthy wildlife.

D8 Concentrations of contaminants

D8C1 Within coastal and territorial and beyond territorial waters, the concentration of contaminants do not exceed the threshold values.

Secondary link

Hazardous substances

  • Fish safe to eat.

D9 Contaminants in fish and seafood

D9C1 The level of contaminants in edible tissues (muscle, liver, roe, flesh or other soft parts, as appropriate) of seafood (including fish, crustaceans, molluscs, echinoderms, seaweed and other marine plants) caught or harvested in the wild (excluding fin-fish from mariculture) does not exceed the threshold values.

Other relevant legislation: For some Contracting Parties also Water Framework Directive Tributyltin is listed as no. 30 on the priority substances list in directive 2013/39/EU.


Cite this indicator

HELCOM (2018) TBT and imposex. HELCOM core indicator report. Online. [Date Viewed], [Web link].

ISSN: 2343-2543


Download full indicator report

Tributyltin TBT and imposex HELCOM core indicator 2018 (pdf)