Determination of a tissue and sediment threshold for tributyltin to protect prey species of juvenile salmonids listed under the US Endangered Species Act

1. The purpose of this report is to determine the concentrations of tributyltin in sediments that would be protective against adverse effects on prey species of salmonids listed under the US Endangered Species Act. 2. Two approaches for determining adverse sediment concentrations due to tributyltin (TBT) contamination are presented here. The first is the equilibrium partitioning (EqP) approach, which relies on a sediment-water partition coefficient and toxicological data for water exposures. The EqP approach utilizes the large water quality database that has been generated over the last two decades for TBT and provides strong evidence for adverse effects at low exposure concentrations. 3. The second approach involves determination of a TBT tissue residue that is considered harmful for most species, which is then used to predict the sediment concentration that would likely produce this adverse tissue concentration. 4. Both approaches are presented here because they generally support each other but based on the information presented below, and the inherent difficulty in measuring porewater concentrations, the tissue residue approach is the recommended method for determining adverse sediment concentrations. 5. Using this analysis, the protective sediment concentration for TBT proposed here is 6000 ng g organic carbon. Direct effects are not expected on salmonids at this sediment concentration because of their relatively short residence time in the estuary, general lack of interaction with sediment, and relatively high metabolic capacity. This concentration may ensure adequate abundance of salmonid prey species; however, it may not be low enough for the protection of sensitive benthic species. Published in 2002 by John Wiley & Sons, Ltd.