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In the last decades, the European eel Anguilla anguilla has suffered a drastic decrease in the recruitment in most areas of their distribution range, leading the species to be included as critically endangered in the IUCN list. This situation, together with the high commercial importance of the species, evidence the need of taking actions such as development of reproduction in captivity and control of fisheries based on the complexity of their life cycle. One of the most interesting tools for its use in conservation biology is the sperm cryopreservation, which presents several advantages for this species such as the synchronization of gametes, selection of genetic lines or cryobanking. However, the development of cryopreservation protocols necessarily requires good quality sperm, and it is also needed an accurate method to assess sperm quality both pre- and post-cryopreservation. On this last matter, fish sperm motility is considered one of the best quality biomarkers for sperm quality assessment in fish, and it can be evaluated subjectively or objectively using computer assisted sperm analysis (CASA-Mot) systems. First, an experiment was conducted to evaluate the precision and accuracy of both methods for assessing sperm motility: the subjective method and the objective technique using CASA-Mot system. Moreover, it was tested whether the degree of expertise of the technicians in the case of the subjective method, has an effect on the accuracy of the motility estimation, and therefore there is an influence of the laboratory staff on the sperm motility assessment. Here we concluded that both the method and the technician expertise were key factors in order to accurately assess sperm motility in European eel, so the use of CASA-Mot together with qualified stuff is required to obtain reliable results. Secondly, and alternative methods for European eel males maturation was evaluated by testing two different hormonal treatments: OVI, a recombinant ?-choriogonadotropin; and VET, a human chorionic gonadotropin purified from female urine. After choosing the best hormonal treatment, the effect of three different doses was evaluated aiming for best performance and lowest cost on the treatment. The results of this experiment pointed at OVI as the best hormonal treatment in terms on sperm quantity and quality in most of the weeks of treatment, and at a weekly dose of 1.5 IU/g fish, which also provide the greatest profitability, obtaining high quality sperm at a lower price. In a third experiment, and using the knowledge acquired in the two first experiments (using the OVI hormonal treatment and CASA-Mot to assess sperm quality), a series of experiments were conducted to standardize the European eel sperm cryopreservation protocols available at the moment (using DMSO or methanol as cryoprotectant). The results indicated that the protocol using methanol was the best of them two in terms of sperm motility and velocity, sperm viability and preservation of DNA integrity. Following this last standardized method, a fourth experiment was conducted aiming for improvement of the protocol in terms of volume (larger volumes) and sperm quality outcome. Moreover, a simple protocol for short-term storage was developed to complement the options to preserve sperm for different time periods. Of all the tested storing conditions, 1/50 dilutions at 4 oC showed the best results, maintaining the motility compared to control for 3 days, and some sperm motility (12%) was still observed after 7 days. From the cryopreservation experiment, it was possible to scale up the cryopreserved volumes to 2 and 5 mL without losing sperm quality compared to lower volumes. Moreover, the protocol was further improved by supplementing the protocol with egg yolk as an additive, obtaining the highest cryopreserved sperm motilities (over 50%) ever reported in European eel.

Improvement of techniques for sperm evaluation and cryobanking in European eel.