Environmental degradation of the insect growth regulator methoprene. IX. Metabolism by bluegill fish

The accumulation, persistence, and metabolism and/or elimination of methoprene (isopropyl (2E, 4E)-11-methoxy-3,7,11-trimethyl-2,4-dodecadienoate) was examined for bluegill sunfish (Lepomis macrochirus) contained in a dynamic flow-through system and a model aquatic ecosystem. The fish in the dynamic flow-through system acquired moderate residues of largely unmetabolized methoprene when continuously exposed to about 30 times anticipated environmental levels of methoprene, but residues were rapidly eliminated (93–95%) within 2 weeks when fish were transferred to flowing uncontaminated water. When bluegill were treated with methoprene in a model aquatic ecosystem, fish showed a highly misleading accumulation of ¹⁴C residues since residual radioactivity was found almost exclusively in radiolabeled natural products, including cholesterol, free fatty acids, glycerides, and protein. Less than 0.1% of the total radioactivity in fish could be attributed to unmetabolized methoprene or its primary metabolites; thus, simple radioassay procedures in ecosystem studies can be severely compromised unless coupled with more sophisticated analytical techniques.