Tailwater fish of a Brazilian dam: abundance estimation and protection from turbine-induced mortality

Abstract: Aim To determine for Pimelodus maculatus, the fish most affected by the operation and maintenance (O&M) of the Amador Aguiar II Hydropower Dam, Araguari River, Brazil, (i) the most suitable fishing gear for its sampling, (ii) the seasonal variation in catches, (iii) the abiotic variables that most influence catches and (iv) the best period of the year to schedule O&M risky to the species. Methods We collected fish by hook-and-line, cast net, and gillnet in the first 300 m downstream of the dam every two months for three consecutive years. We analyzed the catches of P. maculatus and its temporal variation as a function of fishing gear type, year's season, dissolved oxygen, water temperature, water transparency, rainfall, turbine discharge, spillway discharge, and tailwater discharge. Results We captured 5,117 individuals of 32+ species. Pimelodus maculatus (52.6% of the total) was the most sampled species for all fishing gear types. Gillnet captured 70.2% of all P. maculatus, followed by hook-and-line (22.6%) and cast net (7.3%). The bycatch of gillnet (55.4%) was much higher than that of cast net (10.9%) and hook-and-line (7.6%). Temporal variation in the catch of P. maculatus by the three types of fishing gear showed synchrony. Between the two best fishing gear types for sampling P. maculatus, gillnet caught more individuals but caused more bycatch and fish death than hook-and-line. Season of the year and water temperature were the abiotic variables that most influenced temporal variation in the number of P. maculatus sampled. We sampled more individuals during the wet season when the water temperature was higher. Conclusions For any O&M activity that poses a risk of fish death, particularly turbine dewatering, we recommend scheduling it for the dry season when the catch of P. maculatus near the dam is lower. Additionally, we advise sampling fish in the tailwater before the O&M using gillnet or hook-and-line, with the latter preferred due to its lower bycatch and fish mortality.