In recent years the impact that barriers across rivers are having on migratory species of fish has become an increasingly prominent issue. Barriers mean that anadromous species (those which spend part of their life in freshwater and part in saltwater) are physically blocked from completing their life cycle, putting their survival as a species at risk. Salmon, sea trout, allis and twaite shad all hatch from eggs in freshwater rivers before making their way to the sea for part of their lives and then returning to rivers to spawn and complete their life cycle. Silver eels follow the opposite life cycle, hatching in the Sargasso Sea in the North Atlantic Ocean and then swimming to rivers in Europe where they live for years before returning to saltwater and then migrating back to the Sargasso Sea to spawn and die. All of these species have declined in recent years due to the growing number of barriers which are being built across rivers, blocking their natural migratory routes and, in many cases, stopping the fish from spawning and reproducing.
Large hydroelectric dams which use the power of a flowing river to generate electricity are relatively rare in the UK. The vast majority of dams are relatively small and are made to manage water flow, prevent flooding and regulate rivers. However, many of the dams in Britain are very old, with some built during the Industrial Revolution. While the mills and factories which the dams once powered have long since disappeared, in many cases the dams remain, going mostly ignored by the public, but continuing to block rivers for migrating fish.
Professor Carlos Garcia de Leaniz is a member of Amber (Adaptive Management of Barriers in European Rivers), a project which aims to stop the fragmentation and blocking of Europe’s rivers. He told the Guardian in 2019 that they had counted 460,000 barriers built across rivers in Europe and expected this to reach 600,000 by the end of the project (1). While a large number of the structures which blocked rivers were dams, there were plenty of other types of barriers. These included fords (where a small section of a river is made artificially shallow to allow people or vehicles to cross), weirs (structures which allow water to pass over the top to regulate water flow), culverts (structures used to redirect the direction of a river) and sluice gates (which can be open or closed to control the amount of water passing through). All of these contributed to fragmenting and blocking rivers and preventing the passage of migrating fish.
Another scientific study published in the academic journal Nature put the number even higher at 1.2 million barriers across rivers in thirty-six European countries. This means that many European rivers had an average of one blockage per mile. Furthermore, the vast majority (68 per cent) of these barriers were two metres (6ft) or less in height, meaning they were either ignored when river surveys were carried out, or their impact on the river was underestimated due to their small size (2). The issue of small barriers being ignored or seen as insignificant was seen in 2022 when a study of the rivers Kennet and Pang in southern England was carried out. Researchers for the Action for the River Kennet project set out to establish the total number of barriers across a 160-kilometre (100-mile) stretch of both rivers. They discovered a total of 221 during the study, but seventy-eight of these had not been previously recorded (3).
The need to remove obstacles to fish migration is becoming increasingly apparent, not just in the UK, but across the world. Research published by the World Fish Migration Foundation in 2022 found that the number of migratory river fish had decreased by 76 per cent on a global basis since 1970, but in Europe, the figure was 93 per cent. It is feared that hundreds of species of fish could go extinct across the world in the coming decades, with the presence of dams and barriers being a major contributing factor (4).
But it is not just migratory fish which are affected by blockages to rivers. Nutrients are prevented from flowing from freshwater into the sea to the detriment of marine ecosystems, and fish which are adapted to live in fast-flowing water are forced into slow-moving water by the presence of dams. Predatory animals – such as birds in the UK, but other species such as wolves and bears elsewhere in the world – are also deprived of the migrating fish they prey on. Small freshwater aquatic creatures are also prevented from reaching the wide variety of fish and other creatures which prey on them. As Professor Garcia de Leaniz stated: “Flowing rivers are healthy rivers – and by flowing we don’t mean just water, we mean sediment, energy, nutrients and organisms” (1).
The issue of rivers being blocked by dams is not a new one. In fact, people were aware of it eight centuries ago. In the book The Unnatural History of the Sea, Callum Roberts describes that “a Scottish statute of 1214 required mill dams to have an opening sufficient to allow salmon clear passage and for all barrier nets to be lifted on Saturdays” (2). Furthermore, a clause of the Magna Carta, also from the 1200s, stated “All fish-weirs shall be removed from the Thames, the Medway, and throughout the whole of England, except on the sea coast” (6). Writer John Burnside explains that this was included as weirs and dams had been used to trap salmon and maximise catches for the king, but, it had become apparent that this was causing a decline in salmon stocks and “within a few years of Magna Carta the rivers were teeming with life. (6)”
In both the United States and Europe campaigns to remove both large and small dams have been growing. Plans have been made to demolish four dams which span the Klamath River which runs through Oregon and northern California. The project began in 2023, and is the largest dam removal scheme in North America. Similar projects have taken place in Europe. In 2019 the 150-metre wide (492ft), 4.5-metre high (15ft) Sindi Dam was removed from the Pärnu River in Estonia, along with a series of smaller dams, freeing more than 3,000 kilometres (1,864 miles) of the river from barriers. In 2019 the removal of the 37-metre (121ft) high Vezins Dam on the Sélune River in northern France began and was completed in late 2020, reconnecting the river with the English Channel (2). One of the largest projects in Britain in recent years has been the removal of the Bowston Weir on the River Kent in Cumbria. This three-metre tall (10ft) structure was demolished in 2022. It was a perfect example of an obsolete river barrier, having been built in 1874 to provide power for the Bowston Mill, which had shut down in the 1960s and had long since been demolished (9). The organisation Dam Removal Europe aims to restore Europe’s rivers by removing obsolete barriers and aims to connect individuals and organisations which can help achieve this goal. They state on their website that, as of summer 2023, there have been 6,223 barriers removed from rivers across twenty-three European nations (8).
Of course, not all dams and weirs should be removed. Some are needed to regulate water levels and prevent flooding, while the largest produce power through hydroelectricity – which currently accounts for around 2 per cent of the UK’s total energy production. However, fish passes (also known as fish ladders) can be incorporated into the design of such dams and weirs to allow migrator species to pass. They work by giving fish a route through or past the barrier, often by allowing them to swim or jump between small sectioned-off pools which run alongside the dam.
A number of fish passes have been built across barriers on the River Severn by the Unlocking the Severn project, a collaboration between the Canal and River Trust, Severn Rivers Trust, Natural England and the Environment Agency. One of these passes even includes a viewing window where migrating fish can be observed swimming through the pass. The project has been successful, as DNA analysis revealed in 2022 that shad, which had been absent from the river, were once again spawning in River Severn (10). Similarly, the River Thame Conservation Trust has built a fish pass on Chalgrove Brook in Oxfordshire. This will allow fish to migrate past an old mill structure which had previously blocked their progress and is part of a wider project to make the whole of Chalgrove Brook passable to fish (11).
The issue of barriers to fish migration appears to be one of the rarest things in fish conservation – an issue which, having been highlighted, is now seeing real and meaningful action being taken to resolve it. This is continuing today with larger, obsolete dams being demolished and smaller, previously unnoticed barriers, also being removed as the harm they cause has belatedly come to light. Furthermore, many countries across the world are united in the belief that fish stocks and aquatic ecosystems will be improved if redundant barriers to migration are removed and dams, weirs and sluices which are necessary are adapted to allow fish to pass through them. While there are many other threats facing the world’s freshwater and saltwater fish, positive developments such as the widespread consensus on removing barriers to fish migration should be celebrated.
References
1. ‘To save our fish, we must first find ways to unblock UK’s rivers,’ say scientists, The Guardian, 1 September, 2019.
2. Belletti, B., Garcia de Leaniz, C., Jones, J. et al. More than one million barriers fragment Europe’s rivers. Nature, 588, 436–441 (2020)
3. Kennet and Pang eels survey highlights river barriers, BBC News website, 19 March, 2022.
4. Migratory river fish populations plunge 76% in past 50 years, The Guardian, 27 July 2020.
5. Roberts, C. (2007), The Unnatural History of the Sea: The Past and Future of Humanity and Fishing, Gaia.
6. Magna Carta was good for humans – but even better for fish, The New Statesman, 28 May, 2017.
7. European rivers are littered with barricades, but a movement grows to remove them, National Geographic, 18 December, 2020.
8. Dam Removal Progress 2022, viewed 18 May, 2022.
9. Weir today, gone tomorrow: work starts to free Cumbrian river, The Guardian, 4 July, 2020.
10. Return of endangered twaite shad to spawning grounds celebrated, BBC News website, 1 May, 2023.
11. Project to restore fish population creates a bypass, BBC News website, 13 July, 2023.