Long-term monitoring reveals a consistent female-biased sex ratio in Pimelodus maculatus from the Upper Uruguay River Basin

Sex ratio is a crucial demographic parameter for the viability of natural populations, and it is commonly balanced in Neotropical freshwater fish species. This study investigated the sexual proportion of yellow-mandi Pimelodus maculatus in the Upper Uruguay River basin, southern Brazil, between 2000 and 2019. Fish were captured at different sites in a proportion of 2,018 females and 995 males. The total length ranged from 14 to 60 cm for females (31.8 ± 6.8 cm) and 13 to 45 cm for males (26.7 ± 13.6 cm). The frequency of females was higher from class 27 to 51 cm (P < 0.05), with a mean female:male sex ratio of 2.05:1, whereas a 1:1 sex ratio was found in fish shorter than 27 cm. The most important predictor explaining the dominance of females was the length, followed by year, site, and environment. Females are larger and live five years longer than males and they can become more abundant. This disparity in lifespan between females and males, the life history, and social and environmental factors may be associated with the female-biased sex ratio in P. maculatus population of the Uruguay River Basin.