​Key​ Message

​​The threshold values for sediment and imposex used in this core indicator are not yet commonly agreed in HELCOM. The threshold values are included as test threshold values for the purposes of the mid-2017 '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 the tributyltin (TBT) and its breakdown products dibutyltin (DBT) and monobutyltin (MBT) in the Baltic Sea; in water, biota (mussels, marine gastropods) and sediments. Imposex effects of TBT on marine gastropods are used as another source of data on TBT. 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 2015 to evaluate the assessment period 2011 - 2015.

key message fig 1.png 

Key message figure 1: Status assessment results based on evaluation of the TBT concentrations in sediment and biological effects in marine gastropods (VDS). The assessment is carried out using Scale 4 HELCOM assessment units (defined in the HELCOM Monitoring and Assessment Strategy Annex 4).Click to enlarge.

Data on TBT induced effects of imposex (as Vas deference Sequence index – VDSI) biological effects in marine gastropods are available from Denmark and Sweden (webtool: dome.ices.dk/HELCOMHZ2016/main.html). Data was extracted from the HELCOM COMBINE database for TBT in water and biota.

The confidence of the indicator evaluation is high in the Kattegat and Belt Sea/The sound area since the data on TBT concentrations and imposex bivalves, sediments and 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 is downwards, as would be expected as the main source of TBT is in antifouling paints which has been banned, but sediments are still a source for 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, 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 forming was found on 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, so an integrated risk assessment is 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 th​​e 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 Frame​work Directive Tributyltin is listed as no. 30 on the priority substances list in directive 2013/39/EU ​ ​


Cite this indi​​cator

HELCOM (2017) TBT and imposex. HELCOM core indicator report. Online. [Date Viewed], [Web link].

ISSN: 2343-2543​

Downloa​​d full indicator report

HOLAS II component - Core indicator report – web-based version July 2017 (pdf)