Widdows and Donkin (1991) described how reductions in SFG in Mytilus edulis in contaminated sites can be apportioned between specific pollutants. In a study in Bermuda Widdows et al. (1990) showed that the overall reduction in SFG of Mytilus edulis could be proportional such that, at the most contaminated sites, tributyl tin accounted for 21% and hydrocarbons for 74% of the observed effects.