USING OSMOTIC PUMPS TO INDUCE SEXUAL MATURATION IN MALES AND FEMALES OF EUROPEAN EEL Anguilla anguilla

Introduction

The European eel (Anguilla anguilla) is a high commercial valued species for aquaculture. Nowadays, the control of the reproduction and the ability to produce eels seems to be the unique sustainable solution to reduce the pressure on  eel natural populations (Kagawa et al., 2009; 2013) . Traditionally, the European eel has been induced to sexual maturation by hormonal injections (Asturiano, 2020) , but the weekly injections require repetitive handling, which causes stress to the fish . In this sense, the study of new  methods  must be a powerful tool to optimize the sexual maturation protocols in this species.  The goals of this study were  i)  to validate osmotic pumps as a system to induce the sexual maturation in both sexes and ii) to evaluate the production and quality of gametes both from males and females.

 Material and methods

Eels were moved to our facilities in the  Universitat Politècnica de València, Spain. Regarding males, the eels were divided i n  Control, OP-100 and OP-200  groups  (n=10 per group). The c ontrol group received  weekly injections of hormonal treatment (hCGrec , 1.5 IU/g fish)  for 15 weeks. The induction of sexual maturation of osmotic pumps (OP) groups was carried out by implanted hCGrec -loaded osmotic pumps (ALZET 1004 and ALZET 2006, respectively) . Sperm samples were collected weekly by stripping and  several sperm parameters were assessed using software CASA. Biometric parameters such as fin color and gonadosomatic index were evaluated

 Concerning females, the eels were divided in Control (n=11) and OP-2ML4 group (n=10). The control group received weekly injections carp pituitary extract (CPE; 20 mg/kg fish) for 15-20 weeks. The induction of sexual maturation of OP-2ML4 was carried out by CPE-loaded osmotic pump ALZET 2ML4. The females were weekly monitoring and  samples of oocytes were collected using a cannula.  The ripe females  were carried out to spawning using DHP injections. The eggs were collected by stripping and some fertilization trials were done. Biometric parameters such as gonadosomatic index were  also evaluated.

Results and discussion

Concerning males, outcomes from study showed that the implantation of osmotic pumps ALZET 1004 and ALZET 2006 were able to induce the sexual maturation and gamete production in European eel males (Fig. 1A).  In that sense , OP groups reached gonadosomatic index values of 2.23 ± 0.81 (OP-100) and 3.06 ± 1.43 (OP-200), also showing a fin colour darker throughout the maturation treatment.

Regarding gamete production, while the Control group generated 90% of spermiating males from 8th week, and sperm samples from this group showed high quality (>70% of motility) during the peak of the spermiation period (12-15th week) , the Osmotic Pumps groups (OP-100 and OP-200) did not show enough values of volume (Fig. 1B), density (Fig. 1C), and motility (Fig. 1D).

 Concerning females, the use of osmotic pumps also became as an effective method to induce the sexual maturation. The 80% of females of OP-2ML4 group reached the sexual maturation and the 50% showed an unusual early  maturation between 4th and 10th week (Fig. 2). Several females were successfully induced to spawning, reaching embryos stage.

 Therefore, this study showed that the use of osmotic pumps was able to induce sexual maturation, spermiation and ovulation on European eel. In the case of males, results showed that  osmotic-controlled release delivery system have not still enough potential to produce acceptable values of sperm quality parameters for aquaculture purposes. However, in European eel females the osmotic pumps were able to generate spawning females so this method could become and viable alternative for eel hatchery procedures.

Bibliography

Asturiano, J.F., 2020. Improvements on the Reproductive Control of the European Eel, in: Yoshida, M., Asturiano, J. F. (Eds.), Reproduction in Aquatic Animals: From Basic Biology to Aquaculture Technology. pp. 293–320.

 Kagawa, et al . 2013.  Using osmotic pumps to deliver hormones to induce sexual maturation of female Japanese eels, Anguilla japonica . Aquaculture 388–391, 30–34.

Kagawa et al .  2009. Effects of continuous administration of hCG , salmon pituitary extract, and gonadotropin-releasing hormone using osmotic pumps on induction of sexual maturation in male Japanese eel, Anguilla japonica . Aquaculture 296, 117–122.