Impact of oxygen deficiency on the disease status of common dab <i>Limanda limanda</i>

An epidemiological survey of the fish diseases lymphocystis, epidermal papilloma and skin ulcers in common dab Limanda limanda L. was conducted in the southern Kattegat each year in May from 1984 to 1993. During the period of investigation, severe oxygen depletion occurred in late summer 1986 and 1988. After the oxygen deficiency in 1986, the occurrence of lymphocystis and epidermal papilloma increased and peaked in 1989 with prevalences of 14.7 and 3.3%, respectively. The prevalence of skin ulcers never exceeded 0.6%. The relative risk of contracting lymphocystis increased significantly from 1987 to 1991 compared with 1984 to 1986, the period prior to the severe oxygen depletion. A significant increase in the relative risk of contracting epidermal papilloma was observed from 1987 to 1990. Females were 3 times more likely to contract this disease than males. The relative risk of skin ulcers did not change significantly during the investigation period. The prevalence of lymphocystis and epidermal papilloma was negatively correlated with the minimum oxygen levels measured in August and September the previous year; this negative correlation was significant (p < 0.05) for lymphocystis in September, while not for epidermal papilloma (p < 0.1). The prevalence of lymphocystis and epidermal papilloma was significantly correlated (p < 0.01). No significant correlation was observed between stock density (expressed as catch per unit effort) and the diseases in question. It is probably the stress caused by the oxygen deficiency - especially the sublethal levels - that triggered the outbreak of the 2 viral diseases lymphocystis and epidermal papilloma.