DISEASES OF FLOUNDER PLATICHTHYS-FLESUS IN DUTCH COASTAL AND ESTUARINE WATERS, WITH PARTICULAR REFERENCE TO ENVIRONMENTAL-STRESS FACTORS .1. EPIZOOTIOLOGY OF GROSS LESIONS

In order to investigate potential links between marine pollution and f ish diseases, an epizootiological study was conducted in The Netherlan ds during 1983-89. This study concentrated on grossly identifiable dis eases of flounder Platichthys flesus. Flounder were found to be affect ed by the viral skin disease lymphocystis and by skin ulcers probably of bacterial origin. Overall prevalences of these 2 diseases in fish g reater than or equal to 2 yr old were 14.3% end 2.8% respectively. Als o notable was the presence of neoplastic nodules in the livers of 1.0% of the population, prevalences rising steeply with age and locally at taining values of up to 30% in 6+ yr old fish. Most of the samples wer e collected in September when flounder are resident in inshore feeding areas. Using data from 9 sites, spatial and temporal (year-to-year) v ariation in disease occurrence was analysed statistically using log-li near models which incorporated possible effects of length, age and sex . Year-to-year variation showed little correspondence among the 3 dise ases, but their spatial distributions showed striking similarities. Ly mphocystis and skin ulcers were associated in individual fish. The obs erved variation in disease prevalence showed no significant correlatio n with condition factor of the fish or with concentrations of contamin ants in sediments or tissues. However, disease prevalences at differen t sites showed a strong positive correlation with fishing activity (po ssibly indicating an effect of damage by fishing gear) and appeared al so to be positively related to salinity. When only strictly marine sit es were considered, a relationship With pollution could not be ruled o ut. Additional data collected in February-April at offshore sites indi cated that disease prevalences were generally higher at this time of t he year, which corresponds to the spawning period of the populations s tudied. This trend was particularly pronounced for liver neoplasms, an d might be partly related to a low condition factor resulting from spa wning activities. In view of the different aetiologies of the 3 diseas es, the similarities in spatial patterns indicate the existence of 1 g eneral underlying mechanism of disease causation, perhaps acting throu gh immunosuppressive effects. However, age-related migration appeared to explain some aspects of the spatial pattern of liver neoplasms, whe reas it was less important in the case of epidermal diseases, which de velop more rapidly. On the basis of the findings of this study, the po ssible contribution of pollution to disease prevalence cannot be adequ ately assessed due to the interfering effects of other factors (salini ty, fishing activity, migration and spawning). Disease causation appea rs to be complex, and it may be that effects of pollution interact wit h those of other factors to produce observed spatial patterns.