In high concentrations, the compounds are toxic to many types of marine animals. The levels of the compounds typically used in boat paints today are lower than they were in boat paints manufactured a decade ago. High tissue levels of the compounds have recently been found in some marine animals, but there is no record of any of those animals dying in unusually large numbers recently. The compounds break down into harmless substances after a few months of exposure to water or air.
Recently an unusually high number of dolphins have been found dead of infectious diseases, and most of these had abnormally high tissue concentrations of certain compounds that, even in low concentrations, reduce dolphins' resistance to infection. The only source of these compounds in the dolphins' environment is boat paint. Ttherefore, since dolphins rid their bodies of the compounds rapidly once exposure ceases, their mortality rate should decline rapidly if such boat paints are banned.
Which of the following, if true, most strengthens the argument?