Persistence of DDT in Crayfish in a Natural Environment

DDT residues were measured by gas chromatography in crayfish collected in streams sprayed a single time but in various past years, in streams sprayed two and three times, and in streams never sprayed. Residues in the year of treatment ranged from 0.5 to 2.5 ppm. By 2 yr after treatment, residues had declined to about 0.1 ppm but remained near this level through 9 yr after treatment. Grayfish in unsprayed streams carries uniformly low residues, 0.009—0.014 ppm. Prolonged contamination in crayfish in sprayed watersheds apparently derives from highly persistent soil residues. Ecological significance of these residue levels is discussed.