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

Abstract

Diseases of flounderFlounder 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 t h e presence of neoplastic nodules in the livers of 1.0% of the population, prevalences rising steeply with a g e and locally attaining values of up to 3 0 % 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, a g e 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 at 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 it 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.

Push net fishing seems to be responsible for injuries and post fishing mortality in glass eel in the Vilaine estuary (France) in 2007

“…The fishing causes an increase in stress, direct injury and abrasion of the mucus protective layer, which might contribute to the occurrence of a wave of infection in the fish trader tanks. A similar strong positive correlation between skin ulcers and the viral skin disease lymphocystis and the fishing activity was found in the Dutch Wadden Sea (Vethaak and Jol, 1996). The authors speculated that flounders, usually caught as a bycatch and discarded after fishery suffered wounds and damage of the protective mucus layer, and were subsequently more vulnerable to infectious pathogens.…”

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

“…Information about the sampling methods and a detailed characterization of the sampling sites are given by Vethaak & Jol (1996).…”

“…These samples were taken at random withln deflned length classes (Tab1.e 1). At each site an attempt was ., S O made to sample 10 fish In each of 3 length 1 1987 35 10 10 10 5 2 1987 35 10 10 10 5 3 1987 25 0 10 10 5 4 1987 35 10 10 10 5 5 1987 35 10 10 10 5 6 1987 The methodology of the log-linear model is described at length by Vethaak & Jol (1996). Odds ratios were The length and sex of each fish were recorded.…”

“…It also presents data-from the same samplesaged 2+ yr, prevalences rising steeply with age and on an additional histopathological investigation of livers locally attaining values of 30%% in fish aged 6+ yr lacking nodules. The purpose of the latter investiga- (Vethaak & Jol 1996). An attempt was made to relate tion was to provide general background information prevalences of these lesions to environmental pollu.tion.…”

“…In conjunction with their but grow up and feed in inshore coastal waters, abundance, widespread distribution, and susceptibility estuaries and fresh water. After spawning in winter to disease, this tendency to remain faithful to the same feeding areas makes flounder suitable for studies of disease in relation to pollution (Vethaak 1992a, 'E-mail: vethaak@rikz.rws.minvenw.nl Vethaak & Jol 1996).…”