Biofouling of nets is a primary source of cryptocaryoniasis outbreaks in cage cultures

Cryptocaryoniasis is a severe disease of marine cage culture fish, caused by the protozoan Cryptocaryon irritans. However, the sources of C. irritans infection in cage culture farms are still unclear. Previous studies confirmed many cases of biological fouling on the nets of the culture cages. Therefore, we speculate that fouling may be a key infection source of the cryptocaryoniosis outbreaks. This study evaluated whether tomonts develop the dormant stage at different seawater depths and in the sludge. The results showed that the seabed and sludge tomonts exhibited low division rates on the first 3 days after entering the reproductive stage. However, no significant differences in the hatching rate of seabed and sludge tomonts were observed in comparison to the shallow water layer on the 6th day after entering the reproductive stage. The nets with different biological fouling biomass were placed at the base of a tank containing diseased fish. We established that the tomonts could adhere to the net clothing and that the adhesion rate positively correlated with the fouling biomass. In addition, we determined the number of theronts in the seawater at the base of the net cages containing diseased fish in a cage culture farm by a qPCR to detect the DNA of theronts. The number of theronts in the seawater near the bottom of cages with diseased fish was significantly higher than that in other areas. In general, net biofouling is a vital source of C. irritans infection in cage cultures. Our findings provide a theoretical basis for the prevention and control cryptocaryoniasis in the cage culture farms.