In-Situ Investigation of Copepod Predators of Ichthyophthirius multifiliis Theronts from Fish-Farming Pond

<i>Ichthyophthirius multifiliis</i>, a parasitic ciliate, causes "white spot disease" in freshwater fish and poses a significant threat to global freshwater aquaculture. Eliminating the free-swimming theront stage from the aquaculture environment is a critical measure for controlling <i>I. multifiliis</i> infections. The natural predator of <i>I. multifiliis</i> theronts in fish-farming ponds were identified using fluorescent dye-labelled live theronts and quantitative PCR; meanwhile, the zooplankton community composition in the positive ponds of <i>I. multifiliis</i> detected by quantitative PCR were analyzed by eDNA metabarcoding assay. The results revealed predation on theronts by cyclopoid copepods, including <i>Cyclops vicinus</i>, <i>Thermocyclops taihokuensis</i>, <i>Cyclops</i> sp., <i>Thermocyclops</i> sp., <i>Eucyclops</i> sp., and <i>Mesocyclops</i> sp. from the in-situ predation aquatic ecosystem, and among these copepods, <i>C. vicinus</i> was identified as a natural dominant predator of <i>I. multifiliis</i>. This study provides a scientific basis for further exploration and utilization of natural predators to enhance sustainable and environmentally friendly control strategies against <i>I. multifiliis</i>.