Realistic nitrate concentrations diminish reproductive indicators in , an endemic species in critical endangered status.

Goodeinae is a subfamily of critically endangered fish native to central Mexico. Populations of , a species belonging to this subfamily, have significantly decreased in the past two decades. A previous study showed that is sensitive to acute nitrate-nitrogen (NO-N) exposure, leading to noticeable changes in both behavioral and histopathological bioindicators. The aim herein was to determine the vulnerability of to NO-N exposure at realistic concentrations registered in freshwater ecosystems in central Mexico where the species was historically reported. Offspring of were chronically exposed during 60 days to concentrations of 5, 10 and 20 mg NO-N/L, with 2 mg NO-N/L used as the reference value (control). Survival rate, feeding behavior, aquatic surface respiration, body growth, scaled mass index, immature red blood cells, as well as histopathological changes in branchial, hepatic and gonadal tissues were evaluated. Additionally, this study analyzed water quality in freshwater ecosystems where presently persists. The results showed decreased survival as NO-N concentration increased, as well as increased feeding latency, aquatic surface respiration and histological damage in the gills and liver. These organs showed differential sex-dependent responses to NO-N exposure; females were more sensitive than males. In the ovaries, a decreased density of stage III oocytes was associated with increased NO-N concentrations. No changes were observed in body growth and number of immature red blood cells. Concentrations recorded in the three freshwater ecosystems that inhabit were below 2 mg NO-N/L. Together, the results could explain why the species has disappeared from more contaminated freshwater ecosystems where NO-N levels exceed 5 mg/L. Moreover, the study warns about the risks of increasing NO-N concentrations in the current sites where the species lives.