New research shows how hydropower’s impact on fish can be reduced

By combining advanced flow modelling with detailed studies of fish behaviour, new research shows how hydropower can be adapted to reduce harm to fish and other aquatic organisms. The results provide practical tools for balancing climate benefits with biodiversity protection.

Hydropower plays a key role in the transition to a fossil-free energy system. At the same time, regulated rivers place significant pressure on ecosystems, particularly on fish whose upstream and downstream migration is disrupted. In her doctoral thesis, Lovisa Sjöstedt has investigated how water flows influence fish movement patterns and how this knowledge can be used to develop more environmentally considerate operating strategies.

“Hydropower is essential for a stable energy system, but it must work in harmony with nature. My research shows that we can understand how fish move in rivers and use that knowledge to reduce environmental impacts without compromising electricity production,” says Lovisa Sjöstedt, Doctoral student in Fluid Mechanics at Luleå University of Technology.

Small changes can have a big impact

When young salmon migrate towards the sea, they often pass large hydropower plants. Lovisa Sjöstedt’s research shows that their route through the river is strongly influenced by how the water flows. At higher flow rates, smolts tend to concentrate in the main channel, while lower flows result in a wider distribution across the riverbed. The study also shows that technical installations, such as booms designed to guide water towards fish passages, influence how smolts move and which routes they choose.

“By linking the actual movements of fish with detailed flow models, we have been able to show how small changes in flow can have a major impact on the fish’s ability to pass hydropower plants safely,” says Lovisa Sjöstedt.

Lovisa Sjöstedt, doctoral student in fluid mechanics at Luleå University of Technology.

Gradual flow changes reduce impacts

The research points to several concrete ways to reduce impacts on fish in regulated rivers. Avoiding rapid and large changes in water levels, especially during sensitive periods such as spawning and migration, can reduce risks. The results also show that more gradual planning of peak hydropower production can make a significant difference. In addition, flow control and the design of technical solutions, such as structures that guide fish towards safer passage routes, can be better adapted to natural fish behaviour.

“The results show that relatively small adjustments in how flows are managed can make a difference for fish survival. It is not about shutting down hydropower, but about adapting operations at critical times,” says Lovisa Sjöstedt.

Climate change increases vulnerability

The research also shows how climate change may intensify challenges in regulated rivers. Using historical data and two-dimensional models, extreme flow events were analysed in relation to the needs of grayling. The results show that large variations in water flow can have serious consequences, particularly when extreme events coincide with spawning periods.

Because many river stretches are heavily regulated and hydropower plants are often located close to one another, changes in flow also affect upstream areas. Rapid reductions in water levels can lead to fish becoming stranded. Based on models of so-called hydropeaking, she has been able to identify particularly critical zero-flow situations that should be avoided.

“This is about providing decision-makers with practical tools. With better flow planning, we can protect sensitive species while maintaining hydropower’s important role in the energy system,” says Lovisa Sjöstedt.

• Fluid mechanics