Artificial maturation and reproduction of European silver eel: Development of oocytes during final maturation

Attempts on artificial maturation of European eel (Anguilla anguilla) have largely been unsuccessful. The moment of stimulation of final maturation and ovulation is mainly based on weight increase related to the hydration response of the oocytes, which, in the European eel, is irregular. In contrast to Japanese eel, European eels show wide individual variability and much slower response to hormonal stimulation. In this study, the oocyte development of wild European silver eels was followed during final maturation. We describe 7 developmental stages based on 6 parameters: transparency, position and visibility of the nucleus, diameter of the oocyte, and diameter and number of oil droplets. Together, these parameters describe unidirectional changes from immature to over-ripe eggs. The developmental status of the gonads can thus be determined from biopsies. Of 23 female eels, 14 ovulated and were stripped, and 9 gave eggs that could be fertilised. Oocytes mature asynchronously, but this seems to be an artefact since fertility dropped with every new generation. As the timing of ovulation is crucial for fertility of the eggs, our developmental index of oocytes may result in more successful maturation protocols.