Epidemiologische Untersuchungen von Fischkrankheiten im Wattenmeer

Between January 1988 and December 1989, 33 stations in the German part of the Wadden Sea were sampled at 3-month-intervals or monthly (Eider and Elbe estuary) with commercial shrimp trawlers. The stations were placed along 7 transects from the coast to the open sea. The prevalence of externally visible diseases was quantified in 14 fish species. First priority was given to flounder (n = 34,072), second priority was given to smelt, dab, plaice, sole, cod, and whiting. Altogether, more than 124,000 fish were examined. Seasonal and geographical fluctuations in disease prevalence and its relation to host length were studied. Comprehensive data lists enable further comparisons with results from old and new surveys. Besides many formeriy reported diseases, a number of new types were found, like e.g. yellow pest of cod, a fibroma of hooknose and several less frequent tumour-like lesions. Yellow pest seems to be caused by an infection with Flexibacter bacteria. In the fibromas of hooknose, lentivirus-like particles were found. The other disease types seem to be of less harm for their hosts. Healing stages have been observed. In general, fish in the Wadden Sea seem to suffer more heavily from a number of highly pathogenic parasite species genera (like Lemaeocera, Pleistophora or Pseudoterranova), some of which occur at very high prevalences. There is evidence, that fish in the Wadden Sea are more seriously affected by diseaes and parasites than fish in many other shallow coastal areas. About 6 % of all fish of 12+ cm in the Wadden Sea suffer from externally visible diseases. This value increases with increasing fish length. In flounder, for example, the total disease prevalence is 8 % in 12+ cm fish, 15 % in 20+ cm fish, and 29 % in 30+ cm fish. In 1988-89, 7 % of the smelt, eel, and cod populations (12+ cm), and 2 % of the plaice and the sole populations (12+ cm) were affected. On average of all samples, the prevalence was in the range of 1-5 % in flatfish affected by lymphocystis, skin ulceration, and fin rot (dab also by papillomatosis), in eel affected by cauliflower disease, in cod affected by vertebral compression and yellow pest, and in smelt affected by spawning papillomatosis and shortening of the opercula. Additionally, 29 % of the smelt suffered from buccal granulomatosis. Regional centres in disease prevalences were the northernmost part of the Wadden Sea in case of spawning papillomatosis of smelt, the area between the estuaries of Elbe and Eider in case of vertebral compression of cod, and the Elbe estuary and its northern extension in case of cauliflower disease of eel. Yellow pest of eel and all types of flounder diseases occurred most frequently in the estuaries of Eider, Elbe and Weser. Buccal granulomatosis of smelt was found most frequently in the estuaries of the most seriously polluted rivers Elbe and Weser. The average total disease prevalence of 18 cm long flounder was 15 % in the inner parts of the Eider and the Elbe estuary. On average of all other stations it was only 5 %. This distribution was most prominent for skin ulceration. There seems to be a negative correlation between disease prevalence and salinity, resp. a positive correlation between disease prevalence and degree of tidal fluctuations in salinity. As both the tidal fluctuations in salinity and the degree of pollution are highest in the estuaries of Elbe and Weser; the effects of these two stress parameters on fish health cannot be determined on a basis of the presently available data. Further information may be obtained from an evaluation of data from parallel projects, where the levels of pollutants in the fish, reproduction success, lyzosomal stability and mixed-functions-oxidases have been studied. Regarding their availability, disease prevalence and migration behaviour, flounder and smelt are considered to be indicators with first priority, and cod, eel, and dab are considered to be indicators of second priority for a biological monitoring in the Wadden Sea. Regarding a pollution effect monitoring, the following types of external fish diseases are recommended as possible indicators: (1) Buccal granulomatosis of smelt; cauliflower disease of eel. These two diseases were found most frequently in the relatively heavily polluted estuary of the River Elbe. Regarding buccal granulomatosis, the relation between anthropogenic waste water introduction and a secondary fungal infection still has to be clarified. Regarding cauliflower disease, the data basis should be broadened before definite statements are possible. (2) Yellow pest of cod; skin ulceration, lymphocystis, and fin rot of flounder. These diseases were found most frequently in the inner parts of several estuaries. There is evidence that their prevalence is affected by tidal fluctuations in salinity as a natural stress parameter. An effect of anthropogenic pollutants or nutrients so far has not been demonstrated. It may be masked by more significant natural parameters. The effects of anthropogenic substances might be identified, if the effects of natural stress parameters could be quantified successfully. (3) Different types of skeletal anomalies; diseases of dab; epidemic types of tumours. This study did not give any evidence for an indicator function of one of these diseases. However, as previous studies make an indicator function likely, these diseases are recommended with third preference for further surveys.