This risk assessment report provides an evaluation of the risk from TBT to giant Pacific oysters and short-neck clams in Tokyo Bay.
TBT has been found to have adverse effects not only on organisms attached to vessel hulls, but also on marine organisms in general. Because TBT may also affect the environment and enter the human food chain, many countries have enforced regulations which have led to a decrease in TBT concentration in the aquatic environment.
However, as Cardwell et al. (1999) indicated, it is unclear whether this decrease in TBT concentrations has led to a reduction in risk sufficient to protect marine organisms.
In this risk assessment, Tokyo Bay was selected as the study area because it is one of the most congested maritime areas in Japan. It has an estimated annual traffic volume of 240,000 vessels. The short-neck clam (Ruditapes philippinarum), which is the main fishery resource in the Tokyo Bay area, and the giant Pacific oyster (Crassostrea gigas), which is very sensitive to TBT and an important species inhabiting the study area were selected as the assessment organisms. The risks posed by TBT to these organisms were quantitatively assessed for the past (1990), the present (2000), and the period before complete regulation (2007).
This risk assessment report does not address the TBT risk to marine mammals and humans.
The assessment endpoint was established at the level at which TBT has no effect on giant Pacific oysters and short-neck clams. These effects were “abnormal calcification” and “growth reduction” respectively.
The no observed effect concentration (NOEC) used for this assessment was estimated using previously published values of the least observed effect concentration (LOEC) for the giant Pacific oyster and the short-neck clam based on the LOEC/NOEC ratio of Mercenaria mercenaria – a closely related species. The estimated NOEC values were 1.0 ng/L and 4.1 ng/L, respectively, as a TBT group. The risk was assessed using the margin of exposure (MOE), which is the ratio of the NOEC to the estimated environmental concentration (EEC), as calculated by “the chemical fate model.”
Assessment results for the short-neck clams for 1990, suggested that growth reduction was occurring throughout the year in the Ara River estuary and during winter in other habitats. In the case of the giant Pacific oysters it is highly likely that abnormal calcification occurred throughout the year in all habitats.
While the risk of growth reduction in short-neck clams had decreased by 2000, the results for the giant Pacific oyster indicated that there were still some habitats where abnormal calcification could occur.
The risk assessment model predicted that both growth reduction in the short-neck clam and abnormal calcification in the giant Pacific oyster are highly unlikely in 2007.
As for the alternatives to TBT, some paints are now under development by various manufacturers but little information is yet available on any possible harmful effects. Therefore, the function, toxicity, bioaccumulative properties, economic trade-offs and environmental impacts of these TBT alternatives still need to be examined.
This Tributylin (TBT) Risk Assessment Document was produced by the Development of Chemical Substance Risk Assessment and the Risk Assessment Procedure Program by the New Energy and Industrial Technology Development Organization.