Association between environmental stress and epidermal papillomatosis of roach Rutilus rutilus

We studied the association between environmental stress and epidermal papillomatosis of roach Rutilus rutilus L. in Finnish waters using a 'matched pairs' design. Populations impacted by industrial and/or sewage effluents were compared to reference populations from pristine sites. We examined both the prevalence (proportion of diseased fish) and intensity (number of scales covered by tumors) of the disease. Results of Generalized Linear Mixed Models (GLMM) indicated that the risk of papillomatosis was 7.5 times higher in males than females, and increased 1.3 times for every 10 mm increment in fish length. We controlled for the possible effects of fish size, sex and temporal variation through sampling procedures and statistical analyses. Mean prevalence of epidermal papillomatosis was 16.6 and 5.8% in impact and reference populations, respectively (10 population pairs; nfish = 1714). Results of GLMM suggested that the risk of being diseased was 2.7 times higher in the impact than reference populations. Thus, the prevalence of epidermal papillomatosis in roach can be used as an indicator of environmental stress. Results of Linear Mixed Models indicated no difference in the intensity of the disease between impact and reference populations (5 population pairs; nfish = 73; mean+/-SE 10.7+/-1.8 and 11.7+/- 2.9 scales, respectively), although prevalence was higher in impact populations in those 5 population pairs. The possible relationship between environmental stress and intensity of epidermal papillomatosis in natural roach populations remains to be demonstrated.