Genotype-environment interactions in the susceptibility of the common carp, Cyprinus carpio, to Ichthyophthirius multifiliis infections

Half-sib groups of common carp, Cyprinus carpio, with different scale patte rns, were reared together at 16 degrees C. When 1-year old, they were split into three batches and maintained at 16, 19, and 24 degrees C for at least 1 month. Fish were exposed to Ichthyophthirius multifiliis infections usin g either a temperate or tropical isolate of the parasite. Some 217 fish wer e successfully infected. Significant differences in infection levels due to temperature, time of infection, surface area of fish, and scale type were observed, Fully scaled carp were less susceptible to infection than other g roups. Genotype-environment interactions were evident. Except with linear m irror carp, infection levels were higher with the temperate strain. With in creasing temperature, there is a greater reduction in susceptibility for fu lly scaled carp than in scattered mirror carp. Infection levels with increa sing temperature increase with the tropical strain, but decrease with the t emperate strain, suggesting different parasite strains may exist. Differenc es between scale pattern genotypes in susceptibility to white spot may alte r with varying culture conditions. (C) 1999 Elsevier Science B.V. All right s reserved.