Factors affecting the intensity of epidermal papillomatosis in populations of roach, Rutilus rutilus (L.), estimated...

Studies of epidermal papillomatosis in fish populations have only rarely focused on the intensity of the disease, i.e. the number and size of papillomas. Furthermore, the methods used to evaluate the intensity of papillomatosis have not been standardized. We tested the reliability of a method based on counting of scales covered by papilloma tumours in roach, Rutilus rutilus (L). In addition, we studied the frequency distributions of the number of scales covered by papillomas within populations, evaluated the correlation between the prevalence and mean intensity of the disease among populations and examined the intensity of papillomatosis in roach with respect to sex and size of fish. Reliability of the scale coverage method was high. Therefore, the method could offer an effective way to determine the intensity of papillomatosis in fish species with large scales. The frequency distribution of the scale coverage of papillomas was highly aggregated within all populations studied. The mean intensity of papillomatosis increased with the size of the fish and was higher in males than in females. However, there was no correlation between the mean intensity and prevalence of the disease among the 19 roach populations studied.