Tissue Residues of Diuron in Channel Catfish Ictalurus punctatus Exposed to the Algicide in Consecutive Years

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AbstractIn 1999, the United States Environmental Protection Agency granted an emergency exemption under Section 18 of the Federal Insecticide, Fungicide. and Rodenticide Act authorizing the use of the algicide diuron [3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea] to manage cyanobacterial off‐flavors in channel catfish Ictalurus punctcatus. Catfish may remain in ponds for more than one growing season and may therefore be exposed to more than one set of diuron treatments. This study was conducted to determine whether diuron residues in fish carry over from one year to the next and, if so, whether the cumulative exposure causes residues to exceed the tolerance level of 2.0 mg/kg in fillets. Catfish in earthen ponds were exposed to the label‐specified treatment protocol of nine consecutive weekly applications of diuron at 0.01 mg/L in the fall of one year and again the following spring. Diuron concentrations in fillets were below limits of detection (0.05 mg/kg) in all fish (N = 18) prior to the initial diuron treatment series. Immediately after the ninth weekly diuron application in the fall. tissue diuron levels averaged 0.353 mg/kg (pooled SEM = 0.036; range = 0.078–0.724 mg/kg; N= 18). On the day before the second set of diuron treatments 6 mo later. diuron concentrations were below limits of detection in all fish sampled. Immediately after the second set of nine weekly diuron applications, tissue concentrations averaged 0.127 mg/kg (pooled SEM = 0.015; range = not detected to 0.191 mg/kg; N = 18). This study showed that diuron residues in channel catfish were depleted within 2 to 4 mo after exposure to the chemical and there was no carryover of residues in fish from one year to the next. Maximum residue levels in catfish fillets were less than half the tolerance level of 2.0 mg/ kg.