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In order to investigate potential links between marlne pollution and fish diseases, an ep~zootiological 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 2 2 yr old were 14.3%r and 2.8% respectively. Also notable was the presence of neoplastic 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 feeding arcas. 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-toyear variation showed little correspondence 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 with condition factor of the fish or with concentrations of contaminants in sediments or tissues. However, disease preval e n c e ~ at different sites showed a strong positive correlation with fishing activity (possibly indicating an effect of damage by fishing gear) 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 February-April a t offshore sites indicated that disease preval e n c e ~ 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 resulting from 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 explaln some aspects of the spatial pattern of liver neoplasms, whereas i t was less important in the case of epidermal diseases, which develop more rapidly. On the basis of the findings of this study, the possible 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 con~plex, and it may be that effects of pollution interact with those of other factors to produce observed, spatial patterns.

diseases of aquatic organisms

Diseases of flounder Platichthys flesus in Dutch coastal and estuarine waters, with particular reference to environmental stress factors. II. Liver histopathology