The lack of tools for sex identification and assessment of gonadal development are hindering our ability to study the reproductive dysfunction of Arapaima gigas in captivity. This study initially aimed to validate a non-surgical endoscopy procedure to identify sex in juveniles and assess stage of ovary development in female broodstock under field operational conditions. Cannulation, assisted through the description of the genital anatomy, made ovarian biopsy possible to describe oocyte development from primary growth to pre-ovulation, providing a first classification scheme for oogenesis in the species including description of the micropyle morphology using scanning electron microscopy. Cannulation was also successfully performed without endoscopic guidance, which allowed monitoring of the ovarian development along the reproductive season together with profiling of plasma sex steroids (17β-oestradiol (E 2) and 11ketotestosterone (11-KT) in females and males respectively). The monitoring of our study population showed females paired with males in earthponds sexually matured and reached oocyte maturation during the spawning season. However, since no spawning was recorded, eggs had either been resorbed or released and not fertilized by the male. Plasma E 2 levels remained high in females, as expected in an asynchronous species during the spawning season with multiple batches of oocytes being recruited. Plasma 11-KT showed a tendency to decrease suggesting a male reproductive dysfunction or the end of the reproductive season with a lack of synchronisation between genders. In conclusion, endoscopy and cannulation are tools that can be promptly applied to aid sex identification, assessment of reproductive function and overall broodstock management in wild and captive stocks. These tools will greatly help future studies looking at the effects of environmental, social and hormonal cues on reproductive development with the aim to develop a spawning induction protocol for the species. Body of text The Pirarucu Arapaima gigas is a dioic and iteroparous species without evident sexual dimorphisms (Chu-Koo et al., 2009). Like salmonid and anguillid species, the ovary of A. gigas lacks an external capsule (gymnovarium), meaning mature oocytes are directly released into the coelomatic cavity before reaching the gonopore at spawning (Grier et al., 2009; Godinho et al., 2005; Colombo et al., 1984). However, the gonopore morphology and position have not been described, which hindered the practice of cannulation (Núñez et al., 2011; Chu-Koo et al., 2009). This is a major constraint to the study of reproductive function in the species for both wild and captive stocks (Torati et al., 2016; Núñez et al., 2011), as assessment of reproductive condition can only rely on sacrificing animals to sample gametes, which contrasts with conservation efforts. Consequently, data on gonadal development, oocyte maturation, ovulation, and spawning rhythms is still scarce (Godinho et al., 2005; Núñez and Duponchelle, 2009) as for data on spawning rhyth...