An epidemiological study was carried out to investigate the possible aetiology of squamous cell carcinomas which occur in a population of rudd Scardinius erythrophthalmus (L.) from Lough Aderry in south-east Ireland. A total of 1343 rudd were sampled from Lough Aderry and 2 nearby small lakes in spring, summer, autumn, and winter over 2 yr, 1986 to 1988. Fish were weighed, measured, sexed, aged, and examined for lesions. Water quality parameters and natural radioactivity were assessed as possible influencing factors in the disease. The prevalence of squamous cell carcinoma was 6.1% overall, with no significant difference between the lakes, seasons or years. Both male and female fish were affected, of ages from 1+ to 5+ yr. No fish of 6+ or 7+ yr was found with tumours, indicating that the neoplasm caused premature death. The female:male sex ratio of rudd with tumours was higher than that of healthy rudd overall, suggesting that males are more susceptible to the neoplasm. Of the parasites observed, Posthodiplostomum cuticola was common, but rarely found in rudd with tumours. Sphaerospora sp.was also common, but not in sufficient densities for statistical inference, and Argulus sp. was present on 7 fish. No viral particles were found. Natural radiation levels in the vicinity of the lakes were low. The lakes sampled are in an agricultural catchment, and the waters were eutrophic. While rudd are tolerant, it is likely that environmental conditions were stressful at least some of the time, with possible consequences for the immunocompetence of the rudd. The aetiology of the carcinoma is still unknown, but the possibilities of carcinogenic compounds resulting from the high nutrient levels, or of carcinogenic algal toxins produced during algal blooms, should be examined.KEY WORDS: Squamous cell carcinoma · Rudd · Length · Age · Sex · Environmental parameters Resale or republication not permitted without written consent of the publisherDis Aquat Org 80: [145][146][147][148][149][150][151][152][153][154][155][156] 2008 be environmental rather than viral in aetiology. Chemical contaminants in the water have been suggested as associated with the elevated tumour incidence (Black et al. 1982, Malins et al. 1984. Brown et al. (1973) stated that the presence of water pollution may enhance carcinogenesis in fish.The epidemiology of the neoplasm in rudd in Lough Aderry and in 2 nearby lakes was investigated over a period of 25 mo. This paper (1) reports on the prevalence of the tumours in rudd from the 3 lakes, (2) examines the relationship between the prevalence of the neoplasm and the size, age and sex of the fish and the season, and (3) examines the possible roles of water quality, natural radiation and parasitic infection in the disease. Natural radiation was examined because of regional concerns about radiation levels following the Chernobyl disaster. MATERIALS AND METHODS Fish samples. Preliminary samples of rudd were taken from Lough Aderry (Irish Grid Reference [IGR]W9473) and from Ballyhonock Lake (IGR W997...