Feeding efficiency of predatory copepod on Ichthyophthirius multifiliis theront and natural dietary composition in fish pond

Ichthyophthirius multifiliis is a globally distributed, highly pathogenic ciliate of freshwater fish and threatens aquaculture worldwide. Biological control is an attractive alternative to traditional chemical methods, which face several limitations of efficacy and safety in fish farming. This study evaluated the predation efficiency of natural predators Cyclops vicinus and Thermocyclops taihokuensis on I. multifiliis theronts through indoor experiments, and determined the natural diet and dietary selectivity of C. vicinus in a fish pond using 18S rDNA metabarcoding. The results demonstrated that the two copepod predators clearly reduced theronts across concentrations of 10.0–166.7 cell/mL, with feeding rates ranging from 4.64 to 68.14 and 3.53–50.96 cell/ind/h under light, and 4.23–57.74 and 3.70–47.79 cell/ind/h under dark, respectively. Feeding rates of both predators increased with rising theront densities, and were concordant with Holling type II functional response curves. Light exposure significantly enhanced the feeding rate of C. vicinus on theronts under the experimental concentrations ( P < 0.05), while no effect for T. taihokuensis . Comparative analyses showed that C. vicinus demonstrated significantly higher feeding rates than T. taihokuensis ( P < 0.05), highlighting its superior predatory capability on theronts. Furthermore, 18S rDNA metabarcoding analysis of C. vicinus in situ dietary composition unveiled 34 eukaryotic taxa, confirming its generalist omnivorous diets, and Ivlev’s electivity index indicated positive selection for diatoms (E = 0.76), green algae (E = 0.31), rotifers (E = 0.87), and ciliates (E = 0.34). In conclusion, the findings suggest that C. vicinus could be used as a biocontrol agent against I. multifiliis infection in aquaculture. • Cyclops vicinus and Thermocyclops taihokuensis can strongly reduce theront abundance. • C. vicinus has a predatory advantage over T. taihokuensis for theronts . • C. vicinus is an omnivorous predator and indicate positive selection for ciliates. • C. vicinus has potential as a biological control of I. multifiliis in aquaculture.