## 1 Introduction  
The aquaculture sector is increasingly recognised as a key contributor to global food security and nutrition in the 21 st century [1]. One of the most important features of aquaculture is to help reduce pressure on natural marine fish resources. In this respect, to ensure the reliable production of offspring to support commercial farming, closing the life cycle of captive fish is the basis for the development of successful aquaculture [2]. The European Eel, _Anguilla anguilla_ ([PERSON], 1758), is a species with high social, cultural, and economic value and also has great aquaculture potential. However, wild juveniles, known as 'glass eels', caught in targeted fisheries are the sole source of eel aquaculture. In the last five decades, the European eel has gone from being a significant resource for fisheries in Europe and North Africa to being threatened with extinction. The species has been subject to extensive scientific inquiry due to its enigmatic natural ecology and social and economic importance. The stock has declined severely in most of its distribution areas and the population is at a historical minimum and classified as \"critically endangered\" [3,4]. In view of the precarious state of the natural population, the European Commission recognised the urgent need to establish an action plan for the recovery of _A. anguilla_. Among the different actions there are the following: EC Reg. No. 1100/2007, which establishes measures for the recovery of the European eel stock and the inclusion in Annex B of EC Reg. 338/1997, which establishes the CITIES certificate requirement for marketing. Subsequently, the adoption of EU Regs. Nos. 2019/124 and 2023/194 prohibit any fishing activity during all stages of the life cycle of eels for a period of three months and six consecutive months, respectively, based on what each Member State determines. Additional protection measures relate to Directive 92/43/EEC and Directive 2000/60/EC, which aim, inter alia, to protect, conserve, and improve the aquatic environment in which eels spend part of their life cycle. _A. anguilla_ follows the same complex catadromous life cycle as other eels [5; 6; 7], during which it undergoes morphological and physiological changes in preparation for long migration and reproduction [8; 9]. During its life phase in inland waters, it undergoes a process known as silvering, a metamorphosis that initiates puberty and sets in motion the physiological and morphological changes necessary for reproductive migration at sea. Currently, sexually mature eels cannot be obtained in a controlled environment without hormonal treatment. For female eels, this means weekly repeated injection of carp (CPE) or salmon (SPE) [10] pituitary extract and a final injection of 17,20b-dihydroxy-4-pregnen-3-one (DHP) [11; 12; 13]; the routine for males is a protocol of weekly human chorionic gonadotropin (hCG) injections [14]. New protocols have been developed, and there is a continuous effort to optimize the induction of maturity and artificial reproduction [15; 16; 17; 18; 19]. Fish recruitment may be significantly affected by egg quality, as reproductive success depends on the quantity and quality of eggs produced at spawning. In recent years, efforts have been made to develop reliable parameters to identify good quality eggs. According to [PERSON] et al. [20], egg quality is defined as the ability of eggs to produce viable fry, while [PERSON] et al. [21] defines good quality eggs for aquaculture as those with low mortality during fertilisation, hatch, and first feed. Other than the morphometric parameters, the reproductive performance (quantity and quality of the eggs/s spermatozoa) is an important marker often used to determine the quality of the protocols used in artificial reproduction and aquaculture [16; 17; 22; 23] and to evaluate the quality of broodstock for the restocking efforts of the natural populations [14; 24; 25].