The effect of hypoxia on sex hormones in an African cichlid Pseudocrenilabrus multicolor victoriae

Human activities increase the occurrence of aquatic hypoxia (low dissolved oxygen) globally. In fishes, short term hypoxia impairs multiple stages of reproduction (e.g., behavior, hormones, development), but no studies have investigated a species that lives and reproduces under hypoxia. This study examines the effects of hypoxia on sex hormones in the mouth brooding African cichlid Pseudocrenilabrus multicolor victoriae. Non-invasive measures of testosterone and estradiol levels in females were collected across the reproductive cycle in the laboratory, and at the time of capture in the field. In the laboratory, hormone levels were higher during pre-brooding (T = 1.06, E2 = 1.62 pg/mL/h) than brooding (T = 0.61, E2 = 0.34 pg/mL/h) or post-brooding (T = 0.53, E2 = 0.51 pg/mL/h) phases, but did not differ between hypoxic (1.2 ± 0.0 mg/L) and normoxic (7.3 ± 0.1 mg/L) populations. In the field, females were sampled from one low and one high oxygen population in two regions in Uganda (Mpanga River, Nabugabo Region). In both regions, hypoxic populations exhibited higher levels of testosterone than well-oxygenated populations, although there was no population level difference in estradiol levels. Hypoxic sites were also characterized by a higher testosterone/estradiol ratio and a lower proportion of brooding females. These results provide field evidence of hypoxia-mediated endocrine disruption in a fish species that experiences lifelong hypoxia.