Histomorphological and morphometric characterization of Nile tilapia (Oreochromis niloticus) ovaries, with emphasis on processes and structures related to the ovarian regressing phase.

Understanding fish reproductive characteristics, specifically ovarian development and regression, is essential for optimizing the performance of commercially important species like Nile tilapia (Oreochromis niloticus). This study characterizes the histomorphological modifications associated with follicular atresia and post-ovulatory complexes (POCs) during the ovarian regressing phase. Genetically improved (GIFT-type) female fish were monitored over a 60-day reproductive management period, with sequential sampling (G0-G7, n = 20) performed before and after de-mating. Ovaries were evaluated via gonadosomatic index (GSI) and detailed histological analysis. Macroscopically, three ovarian patterns were identified: "final maturation," "post-spawning," and a previously undescribed "hardened" pattern. Significant differences in ovarian weight and GSI supported a multiple-batch spawning strategy, while histology confirmed asynchronous folliculogenesis. During regression, early-stage atresia and POCs predominated up to 21 days post-unmating. By days 28-35, ovarian functional recovery was assessed by observing later atretic stages and increased mature follicles. These findings reveal that ovarian remodeling occurs gradually through tissue reabsorption. Based on these data, we propose an evidence-based 60-day resting protocol for broodstock reintroduction to maximize gonadal recovery, reproductive performance, and sustainability in aquaculture systems.