Elevated Temperature Accelerates Ovarian Maturation without Somatic Costs in Astyanax lacustris (Characiformes, Characidae)

Introduction: Temperature is a major environmental regulator of fish reproduction, influencing endocrine processes and gonadal development. Although elevated temperatures may accelerate sexual maturation, their effects on energy allocation between somatic growth and reproduction remain poorly understood in Neotropical species. Astyanax lacustris, a commercially and ecologically relevant Neotropical characid, was used as a model to investigate the effects of chronic thermal exposure on ovarian maturation and somatic growth. Methods: Female Astyanax lacustris reared from the juvenile stage at 26 °C (n = 10, control) and 32 °C (n = 10, treatment) were sampled at 100 days post-hatching (dph). Total body mass (Wt) and gonad mass (Wg) were recorded, and ovaries were removed, processed for histology, and examined under light microscopy. The gonadosomatic index (GSI) was calculated as GSI = (Wg/Wt) x 100. Results: At 26 °C, females exhibited a mean body length of 8.52 ± 0.69 cm and body mass of 9.06 ± 2.99 g, distributed across the Immature (20%; GSI = 0.37 ± 0.23%), Developing (20%; GSI = 0.51 ± 0.37%), and Spawning capable (60%; GSI = 4.78 ± 3.29%) phases. At 32 °C, all females were classified as Spawning capable (100%), with a mean body length of 8.43 ± 0.65 cm, body mass of 9.06 ± 2.17 g, and GSI of 9.76 ± 5.43%. No significant differences were detected between temperatures in body length or body mass (Student’s t-test, p = 0.763 and p = 0.999, respectively), whereas GSI was significantly higher at 32 °C (Mann–Whitney test, p = 0.007). Conclusion: Compared to 26 ºC, chronic exposure to 32 ºC accelerated ovarian maturation without affecting somatic growth at 100 dph. These findings suggest that prolonged exposure to 32 ºC can impact the reproductive development of A. lacustris, highlighting a potential benefit for fish farming production, although further analysis is required to verify the viability of the oocytes and offspring.