SUMMARY

The production of eels is based on the capture of elvers, but the captures have not been increased so fast as the demand from the fish farms. The fall of the capture of elvers led to the search all over the world of new sources of supply, making of the reproduction in captivity one of the most important alternatives.

Due to the fact that the eels do not mature sexually in captivity there have been developed methods for the hormonal induction that achieve the production of important volumes of sperm of good quality (Pérez et al. 2000; Asturiano et al. 2004; Müller et al. 2004). This fact has motivated the interest to develop techniques for the compilation and appropriate managing of the sperm of this species. In this regard we decided to carry out a series of experiments with the purpose of developing freezing technologies and studies on the quality of the European eel sperm.

First experiment tried to optimize the treatments of hormonal induction of European eel males to obtain sperm. Five hormonal treatments with human chorionic gonadotropin (hCG) were assayed to induce the gonad maturation and the spermiation in eel males proceeding from fish farm. Treatment A, consisting in weekly doses of 1.5 IU hCG/g fish induced the highest percentage of spermiating males, a longer period of  spermiation and a higher volume of sperm. 

Secondly, cryopreservation protocols were studied using as a base extenders described for Japanese eel (K15, K30, TNK) and two more previously described for European eel (P1 and P2). Different motility analyses were carried out (pre and postcryopreservation) in the sperm samples frozen with these media: straight line velocity (VSL), curvilinear velocity (VCL), angular velocity (VAP), and spermatozoa beating cross frequency (BCF). The evaluation of these parameters was performed by computer-assisted sperm evaluation analysis (CASA). Sperm frozen in the freezing media TNK and P1, containing dimethylsulphoxide (DMSO) and lecithin, showed the best results on post-cryopreservation survival.

Once defined the extender with the best characteristics, several cryoprotectants were assayed (DMSO, acetamide, ethilenglycol, propanol, glycerol and methanol) with the purpose of defining the best for the European eel sperm. At the same time we evaluated the effect of the addition of foetal bovine serum (FBS) to the freezing media, as well as the sperm:freezing media dilution factor. The effect of these factors was evaluated by means of the comparison pre and post-cryopreservation of the percentage of mobile cells, the percentage of alive cells and its morphometry. Every mixture was formed by 75 µl of cryoprotectant, 425 µl extender P1 and 250 µl of sperm for a total volume of 750µl in relation 1:2 (v/v).

The best global result was obtained with DMSO mixed in P1 modified as extender, containing FBS, resulting in a significant increase of the sperm mobility percentage post-freezing.