AbstractFish presence in tailraces is a remarkable management concern in operating hydroelectric power plants (HPP) in Brazilian rivers. Fish get blocked during upstream migration for spawning, and large shoals can last for days in the tailrace; however, the circumstances that attract fish throughout the year to this region are uncertain, especially during drought conditions. To unravel this uncertainty, we modeled the relationship between Pimelodus maculatus abundance and environmental and operational variables (N = 26) in monthly fishing campaigns at the Machadinho HPP (N = 18). A generalized additive model was fitted to fish abundance in which water conductivity, 30‐day accumulated precipitation, reservoir inflow, and seasonality explained 93% of the deviance. It was found that abundance decreased as water conductivity increased, but abundance was higher in summer. However, winter showed the highest abundance in the entire rainfall range with peaks between 130 and 150 mm and above 250 mm. An increase in reservoir inflow decreased fish abundance. During droughts, our findings suggest that the powerhouse foundations, particularly the draft tube, may serve as refugia to P. maculatus.