Imposex is an occurrence in some snails, particularly dogwhelks and whelks where the genitalia of the female whelk becomes deformed, thereby hampering
fertilization. Evans points out science reports have established that the biocide TBT is a cause of this condition.For a number of years, it was believed that TBT was the sole cause of imposex in whelks and other
bivalves. This would make imposex an excellent biological indicator of TBT toxicity.
Recent studies have shown that TBT is not the sole cause of imposex in ALL species. Certain species have been shown to exhibit
imposex due to copper, triphenyltin, parasitic infestations and even bird feces. Some species were noted to exhibit imposex in the natural population of females, without any outside influences. These studies prove that
imposex is not a MEASURE of TBT but an INDICATOR of TBT in some species.
Reports claiming TBT has devastated whelk populations causing extinction in the North Sea UK emerge partly from the misuse of imposex as a measure of
TBT. Because certain species exhibit imposex naturally, the presence of imposex does not mean TBT caused the condition.
Evans documents the dangers of incorrect assumptions of biological indicators can have far reaching
consequences. The IMO has proposed a complete ban on TBT biocide based paint, in great part due to incorrect assumptions made on a report presented to the IMO claiming destruction of whelk populations due to TBT. The
report used a species that is not a biological indicator for TBT as its proof.
Flagrant misuse of biological indicators such as the North Sea Report of 1993 mean inaccurate science is presented to the public and used to make
inappropriate decisions.
Linda Logan, Parametrix, Inc. - Comparing risks to aquatic life from using tin-free biocides versus TBT in antifouling paints.
Logan's report questions comparisons
of tributyltin (TBT) antifouling paint to alternative tin-free antifouling paints. Are comparable exposures measured, are comparable toxicity sets looked at, and are comparable risk characterizations performed?
Logan notes
there is a great deal of scientific and other data on TBT tin based paints, however there is very little data available for alternative antifouling paints. For example comparisons are made between actute and chronic
toxicity. Acute toxicity measures the toxicity level ending in death of the test species, while chronic toxicity measures the toxicity level ending in deleterious effects on growth to the test species. Some acute
toxicity data has been reported for alternative biocides. However, there is almost no chronic toxicity data available to the public for alternative biocides.
Also missing for alternative biocide antifoulants include data on:
- Future potential release rates
- Species sensitivity
- Other risk characteristics such as - excess fuel consumption, and invasive species threats.
Logan tells us an accurate comparable risk assessment of any alternative to TBT must include all information including indirect risks, toxicity and more. Such a comparative risk assessment is needed to achieve interim IMO
goals until we can find products that have no effect on non target organisms.
Stewart Evans - Community groups show TBT based paints
Evans discussed the changes in levels of TBT in the
marine environments since TBT regulations were put into effect worldwide. Regulations were passed in 1982 (France) and 1987-1990 (US, UK, Australia, Canada, etc.).
In 1987 very high levels of TBT were found in the water
column, particularly in marinas. Evans says it was believed the main input of TBT was small marina vessels, hence regulations banned TBT on vessels under 25m in length.
Numerous studies showing a decline in the level of TBT
all over the marine environment prove the legislation passed in the late 80's has worked. Evans believes this proves we CAN manage a difficult substance such as TBT biocide in the environment.
However in 1993, the North Sea
Report came out with numerous charges that TBT was still causing damage in the marine environment (See earlier presentation on misuse of biological indicators). The local press used the report to make incorrect
sensationalistic statements, such as charging the whelk population was completely wiped out in the North Sea.
Evans' group went to the same sites tested in the North Sea report and re-tested. His group found stable
populations of abundant animals with greatly reduced imposex. His report was published in the Marine Pollution Bulletin yet "nobody listened."
Evans went back and performed another study. This time however
he enlisted the help of local community group members, many of which were environmental group members expecting to see dead whelks. The point of the second study was to bring community group involvement into the science
field. The community group observed the same findings of a strong whelk population, supporting Evans' earlier findings. There was no evidence of total sterility.
This time, a number of media organizations were
interested in the report. They must have felt the participation of the community group made the findings more noteworthy. Evans found considerably greater impact with the second report than his first report.
Evans feels the message is -
- the regulations for TBT worked.
- scientific literature data is NOT getting to the IMO arena.
- non scientific groups such as World Wildlife Fund are given higher credence by regulatory bodies such as IMO then professional peer reviewed scientific reports
