Hydropower Operation and Fish Conservation: Challenges of a Changing Energy Matrix

Brazil’s electricity matrix has long been dominated by hydropower. Recently, however, the rapid expansion of intermittent renewable sources, solar and wind, has reshaped the national energy landscape. In 2024, these sources accounted for 91.13% of the country’s electricity expansion, now representing around 25% of total installed capacity. This transition has altered the role of hydropower, which increasingly operates as a balancing resource to compensate for the variability of solar and wind generation, leading to significant intraday variations in turbine discharge, especially during periods of low solar generation and high system load. Such altered flow regimes can affect aquatic ecosystems, especially fish populations. This study evaluates whether the increasing share of solar and wind energy, relative to hydropower, may have adverse effects on fish. It examines how changes in energy mix influence hydropower operations and how Cemig monitors and mitigates resulting impacts. Nearly two decades of environmental monitoring data, maintained by Cemig GT, reveals an increase in in daily startup/shutdown cycles, low-load operations, synchronous condenser reversals, and zerogeneration periods. These conditions raise the risk of fish death due to mechanical trauma, barotrauma, and stranding. Case studies from three Cemig hydropower plants, demonstrate that zero or reduced generation can lead to fish stranding and greater exposure to hazardous zones, while frequent generator start-ups create additional risks from sudden pressure changes. In response, Cemig has intensified environmental monitoring and implemented restrictive operational protocols and structural mitigation measures, adjusted based on fish presence. The study highlights the need for adaptive management of hydropower operations to reconcile energy transition goals with effective protection of aquatic ecosystems.