Diseases of flounder <i>Platichthys flesus</i> in Dutch coastal and estuarine waters, with particular reference to environmental stress factors. I. Epizootiology of gross lesions

In order to investigate potential links between marine pollution and fish diseases, an epizootiological study was conducted in The Netherlands during 1983-89. This study concentrated on grossly identifiable diseases of flounder Platichthys flesus.Flounder were found to be affected by the viral skin disease lymphocystis and by skin ulcers probably of bacterial origin. Overall prevalences of these 2 diseases in fish ç2 yr old were 14.3% and 2.8% respectively. Also notable was the presence ofneoplastic nodules in the livers of 1.0% of the population, prevalences rising steeply with age and locally attaining values of up to 30% in 6+ yr old fish. Most of the samples were collected in September when flounder are resident in inshore feedingareas. Using data from 9 sites, spatial and temporal (year-to-year) variation in disease occurrence was analysed statistically using log-linear models which incorporated possible effects of length, age and sex. Year-to-year variation showed littlecorrespondence among the 3 diseases, but their spatial distributions showed striking similarities. Lymphocystis and skin ulcers were associated in individual fish. The observed variation in disease prevalence showed no significant correlation withcondition factor of the fish or with concentrations of contaminants in sediments or tissues. However, disease prevalences at different sites showed a strong positive correlation with fishing activity (possibly indicating an effect of damage by fishinggear) and appeared also to be positively related to salinity. When only strictly marine sites were considered, a relationship with pollution could not be ruled out. Additional data collected in FebruaryñApril at offshore sites indicated that diseaseprevalences were generally higher at this time of the year, which corresponds to the spawning period of the populations studied. This trend was particularly pronounced for liver neoplasms, and might be partly related to a low condition factor resultingfrom spawning activities. In view of the different aetiologies of the 3 diseases, the similarities in spatial patterns indicate the existence of 1 general underlying mechanism of disease causation, perhaps acting through immunosuppressive effects.However, age-related migration appeared to explain some aspects of the spatial pattern of liver neoplasms, whereas it was less important in the case of epidermal diseases, which develop more rapidly. On the basis of the findings of this study, thepossible contribution of pollution to disease prevalence cannot be adequately assessed due to the interfering effects of other factors (salinity, fishing activity, migration and spawning). Disease causation appears to be complex, and it may be thateffects of pollution interact with those of other factors to produce observed spatial patterns.