Fipronil transformation in aquatic ecosystems and ecotoxic effects on fish populations

The phenylpyrazole insecticide fipronil (FIP) is a wide-range agrochemical that is extensively used in agricultural, veterinary, and household settings. It is oxidized, reduced, hydrolyzed, and/or photolyzed in water into -sulfone, −sulfide, −amide, −desulfinyl, or fipronil-(13C3, cyano-13C) metabolites having varying degrees of ecotoxicity. Temperature, pH, organic matter content, and other environmental factors influence its bioavailability and toxicity in water and sediments. Excessive applications have raised concerns about FIP's ecotoxicological consequences on aquatic non-target organisms. This article discusses the impact of FIP's biotransformation on non-target invertebrates and vertebrates in aquatic systems. Being a gamma-aminobutyric acid (GABA) inhibitor, it is an efficient disruptor of the central nervous system of target insects and non-target aquatic species (both invertebrates and vertebrates), including fish. Widely distributed hormone receptors and neurotransmitters (including acetylcholine, dopamine, and serotonin) in the central hypothalamic and peripheral endocrine systems of fish are sensitive to FIP and its metabolites. Multibiomarker studies have demonstrated numerous sublethal effects on various physiological responses like endocrine disruption, oxidative stress, growth, and reproductive impairment. These effects lead to alterations in behavioral responses, feeding efficiency, and spawning success, ultimately lowering the long-term viability of fish populations. This review also comprehends the potential risks of FIP exposures to fish populations by emphasizing the susceptibility, physiological impairments, and risk assessment. Bioaccumulation studies indicate FIP-accumulation in freshwater and marine food webs, posing direct/indirect risk to piscivores of higher trophic levels. Further studies must elucidate mechanisms of sub-lethal effects, long-term bioaccumulation, and chemical exposure-ecosystem interaction to enhance the understanding of population-level effects.