ABSTRACT

European eel is an important commercial fish in the European aquaculture, but the dramatic decrease in natural populations has caused the incorporation of this species in the CITES list (convention on the international trade in endangered species of wild fauna and flora). Considering this situation, more research on the eel reproductive control is necessary.
The present document contains 5 studies that can be divided in two principal parts: first, a description of the artificial eel maturation process, followed by a second part with the development of techniques for the sperm quality evaluation and sperm conservation. 
Once the optimal hormonal treatment was fixed in previous studies, we studied the evolution of the sperm quality through the spermiation period, trying to determine the weeks with higher percentage of samples with high quality sperm. To evaluate the sperm quality the following parameters were analyzed: percentage of live cells, motility and spermatozoa head morphometry. The percentage of live cells was measured by fluorescent staining techniques, while the measurement of head size was performed using a computer program (ASMA system), but this analysis presented problems due to the special spermatozoa head shape. To validate this technique it was necessary to compare the results obtained by ASMA system with those obtained by scanning electron microscopy.
Males receiving the same hormonal treatment show different individual responses in terms of gonad development and sperm quality. To understand which factors can influence in the gonad maturation and the sperm quality we made the first description of European eel male maturation process analyzing GnRHs and gonadotropin expression, 11-KT plasma levels, morphometric parameters and sperm quality in relation with the testis stage of development. 
To complete this work, new techniques for the short- and long-term storage of European eel sperm were developed. We have designed one medium that preserves the fresh sperm during several days, as well as a new method to cryopreserve the sperm immersed in liquid nitrogen.