The unspoken truth about impacted rivers: Consequences and implications of barriers for conservation of freshwater fish

To serve the expanding land, water and energy needs of a growing human population, rivers have been altered extensively over the last centuries to build hydropower stations, weirs and reservoirs mainly for electricity, water security and irrigation, making freshwater the most threatened of ecosystems.

Barriers come in many shapes and sizes, though most have had devastating effects on freshwater ecosystems by destroying habitats and critical spawning areas, hindering and delaying downstream and upstream passage for moving fish as well as leading to death and/or extinction for a range of species. While these effects have been investigated widely, there has been a focus on 1) large barriers, 2) the upstream passage of highly synchronized migratory species, 3) engineered solutions rather than removal, and 4) the effects at a local scale rather than catchment scale.

As such, this thesis focuses on all of the ‘unspoken’ truths about barriers. In the first manuscript ( MS I ), the loss of rheophilic habitat caused by barriers is highlighted. Stretches of river characterized by gravel-substrate, high-gradient, fast-flowing and highlyoxygenated water are inundated due to barriers, leading to slow-moving water and sedimentation, thus removing the most suitable habitat for salmonid spawning and early development.

Using examples from catchments in Denmark, we demonstrate that multiple barriers have cumulative effects in terms of vertical and horizontal habitat loss, causing between 21 and 40% habitat loss. This habitat is crucial for many rheophilic species such as brown trout (Salmo trutta) and Atlantic salmon (Salmo salar).

While both species are strong swimmers and are typically considered as highly synchronized migrants, recent evidence suggest that anadromous brown trout are highly variable in their migration timing. As such, migration timing was investigated in the second manuscript ( MS II ), where the assumption that all juvenile trout leave in the spring is questioned.

Our findings suggest that trout also migrate in the autumn, and that these fish should be considered just as much as their spring counterparts with regards to barrier management. For example, practitioners should consider opening spillways during certain periods in the autumn, as is often done in the spring, to allow free passage.

This is obviously not a perfect solution, but will certainly promote outmigration. A better solution to passage however is removal. In the third, fourth and fifth manuscripts ( MS III , MS IV and MS V ), the long-term effects of barrier removal are investigated. Manuscript III uses 30 years of data to investigate how removal has impacted the downstream and upstream spawning grounds and rearing areas, with focus on young of the year fish.

Manuscript IV uses data collected over the last 25 years throughout an entire river system (including its tributaries) to study the effects of a creative pseudo dam removal. Manuscript V investigates the effects of six weir removals at a full river scale, with focus on the smolt run. All three studies demonstrate the benefits of removal, which are observed almost immediately.

The habitats have returned to a more natural state, and the trout populations of river Gudenå (in the Vilholt region), river Kolding and river Villestrup are healthy. Lastly, the lessons learned from working with barriers, and especially the forgotten aspects and assumptions that render current practices ineffective, are discussed in the sixth manuscript ( MS VI ).

The results from this thesis are alarming when thinking of the overlooked effects of barriers (though perhaps not surprising), and yet positive when thinking of the effects that removing these barriers can have in reinstating lost habitats and fish communities. Management should seek to remove any barrier as a first option whenever possible, and use the results from this thesis as guidance to ensure that underlying assumptions on the way fish “should” behave and respond to barriers are questioned adequately.