Abstract

European eel, as well as some other species under the Genus Anguilla, is under rapid wild stock population decline, with over 90% of fish stocks already disappeared.¹ Even under these circumstances, eels are still fished for human consumption in many different countries, including Spain.

During the present project we aim to develop an assisted reproduction program for breeding this species in captivity closing the reproductive cycle, with the final goal of minimizing fishing pressure but also to be able to generate additional stocks for future reintroduction.

In a first step, Anguilla specific gonadotropins (FSH and LH) were developed through molecular biology techniques expressing the specific Anguilla hormonal sequences in specific culture cell lines. Hormones were then purified and concentrated for IM administration on each fish. A group of 10 females and 18 males were stimulated using different protocols combining Anguilla specific FSH and LH as well as human chorionic gonadotropin (HCG)² (Veterin Corion®, Divasa Farmavic Group, Barcelona, Spain) at different dose regimes. The animals were kept in salt water, at full darkness increasing water temperature from 15°C to 20°C during the whole stimulation process. During these period animals were closely monitored based on morphological changes (weight, eye enlargement, coelomic distention and skin color changes), gonadal development (through ultrasound, coelioscopic visualization and biopsy) and hormonal profiles (estradiol plasma levels).

After 6 weeks of HCG stimulation, males initiated sperm production, obtaining up to 5 ml of ejaculate from each eel through standard coelomic massage. Sperm quality was validated on the different individuals at different times and stimulation regimes.

In the case of the females, the ovary initiated follicular response to FSH from the very beginning of therapy, reaching maximum follicular diameter (around 800 µm) after 20 weeks of stimulation. LH was introduced in a set of females from the very beginning (together with the FSH), while in the second group LH was introduced once follicles reached maximum size and the germinal spot was observed to be ready to migrate to one pole of the follicle. Premature or late administration of ovulatory dosages³ of LH and 17,20β-dihydroxy-4-pregnen-3-one (DHP), were considered to be causing ovulation of unviable eggs, so close monitoring is decisive to stimulate follicular maturation and specially ovulation at the right moment.

At the moment of writing this abstract several females are just about to spawn, and the eggs will be in vitro fertilized⁴ to be transferred into a Kreisel tank to attempt egg hatch and larvae development.

We hope the quality of gametes obtained under this specific protocol as well as further research on new feeding techniques will render higher larvae survival and growth rates compared with some previous studies in other eel species.^5,6

* Presenting author

Literature Cited

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2.  Asturiano JF, Pérez L, Garzón DL, Peñaranda DS, Marco-Jiménez F, Martínez-Llorens S, Tomás A, Jover, M. 2005. Effect of different methods for the induction of spermiation on semen quality in European eel. Aquaculture Research. 36(15):1480–1487.

3.  Ohta H, Kagawa H, Tanaka H, Okuzawa K, Hirose K. 1996. Changes in fertilization and hatching rates with time after ovulation induced by 17, 20β-dihydroxy-4-pregnen-3-one in the Japanese eel, Anguilla japonica. Aquaculture. 139(3–4):291–301.

4.  Butts IA, Sørensen SR, Politis SN, Pitcher TE, Tomkiewicz J. 2014. Standardization of fertilization protocols for the European eel, Anguilla anguilla. Aquaculture. 426:9–13.

5.  Tanaka H, Kagawa H, Ohta H. 2001. Production of leptocephali of Japanese eel (Anguilla japonica) in captivity. Aquaculture. 201(1):51–60.

6.  Tanaka H, Kagawa H, Ohta H, Unuma T, Nomura K. 2003. The first production of glass eel in captivity: fish reproductive physiology facilitates great progress in aquaculture. Fish Physiology and Biochemistry. 28(1–4):493–497.