Seasonal fluctuations in the incidence of epidermal papillomas of the European eel Anguilla anguilla L.

The eels of the Lower Elbe (15–35 cm total length, freshwater age groups III–V) show a high incidence of epidermal papillomas. These neoplasms, situated mainly in the snout region, show a marked seasonal fluctuation. The incidence is low in spring and autumn (min. 2–3 %) and rises sharply in summer (max. 28 %). The size of the papillomas is also smaller during the colder seasons of the year (up to 300 mm3) reaching a maximum in August/ September (up to 900 mm3). The increase in growth of the tumours follows the rise in frequency with a delay of 3–4 weeks. These seasonal fluctuations of tumour growth are possibly related to the alteration of the water temperature and oxygen content.