Study shows boost to fish population and species after fish passes opened on weirs along River Aire

Researchers from the University of Hull, funded by the Environment Agency, have revealed the effectiveness of fish passes at multiple weirs along the River Aire in Yorkshire.

The study revealed a huge boost for fish with species able to pass weirs faster and travel further along the river with up to 100 per cent more fish able to successfully swim past weirs – a huge boost for the rivers biodiversity and the population of a range of fish species.

The study began in 2018, as part of the Developing Natural Aire Project (DNAire) and involved studying a series of weirs along the River Aire to understand how passable these were to fish.

A weir is a small dam built across a river to control the upstream water level along sections of river that were used for barges to transport goods. These structures now hold heritage value and cannot be easily removed so effective solutions to allow passage for fish are required. Initial results from the study showed movements between key life stage habitats, including those for spawning, feeding, resting and nursery grounds, were restricted due to these structures.


The study found that only a small proportion of brown trout were able to ascend some weirs without fish passes (except for one very small weir), but even this hardy species struggled to pass larger weirs. When a few trout did finally ascend, they often had to make numerous attempts, and could only successfully climb under high flow conditions, when there was sufficient water depth coming over the weir – one ‘successful’ fish spent 90 days below a weir before managing to make the ascent.

Due to the large migratory movements these species undertake, and the technology used to monitor their movements, this investigation was also able to show fish were able to successfully ascend fish passage structures that had previously been installed within the study area.

In late 2020/early 2021, three fish passes were installed at Armley, Kirkstall and Saltaire, and fish were tagged and studied to see to what extent the structures allowed them to negotiate weirs.

The commissioned study demonstrated that fish could ascend these weirs faster and on a wider range of flows than prior to the fish pass installation. Overall, up to 100% of tagged brown trout were able to pass, taking them a maximum of just 3.28 days to make the ascent.

A 26% increase in the proportion of coarse fish tagged (such as chub and barbel) that were able to successfully navigate these obstacles was also reported, meaning many more fish were able to reach spawning, nursery and feeding habitats further up the catchment.

It is hoped that the passes on the River Aire will also enable the iconic Atlantic salmon to complete their mammoth migration from the sea to historic spawning habitat beyond Skipton and into the Yorkshire Dales, and help safeguard the endangered eel (which migrate to sea to spawn).

Dr Jamie Dodd, research lead at the University of Hull International Fisheries Institute, said: “The fragmentation of habitats is having a severe impact on species globally with over 1 million barriers in Europe alone, there is no surprise that migratory freshwater fish are one of the most threatened species on the planet. By studying both the before and after catchment-wide movements of migratory species on the River Aire, we were able to provide the Environment Agency with evidence on the improvements to connectivity these fish passes have had. The importance of understanding the behaviour of fish around migratory barriers and the effectiveness of the mitigation measures is vital if we are to help our fish populations recover.”

Dr Jamie Dodd
Dr Jamie Dodd, University of Hull International Fisheries Institute

Emma Calverley, Interim Director of Knowledge Exchange at the University of Hull, said: “This project is a fantastic example of knowledge exchange between our brilliant researchers at the University of Hull, the Environment Agency and the River Aire Trust. Our University strategy focuses on two of the most important challenges of our generation: environmental sustainability and social justice. The outcomes that this project has demonstrated on the environmental sustainability and ecology of our region are wonderful to see. Our work centres around people, place and partnership and this project is an excellent exemplar of research and knowledge exchange with all three at its heart.”

Thomas Somerville, DNAire Project Manager said: “It is fantastic to know that these fish passes in the upper River Aire are working as designed, and important fish species are rediscovering their key habitat.

“As well as reopening rivers to fish migration and protecting ecologically important and endangered species like salmon and eel, fish passes are an amazing opportunity to reconnect river-resident species and the local community.

“Over the coming years, we look forward to seeing a growing proportion of trout, chub, barbel and salmon run spawning journeys higher up the river and a recovery in their populations.”

Neil Trudgill, Environment Agency Fisheries Technical Specialist, said: “DNAire is a fantastic project and a great example of partnership in action between the Environment Agency and Aire Rivers Trust.

“The Aire has a fascinating history, and this important work has focussed on ensuring the sustainability of the river’s fragile coarse and trout populations as well as re-establishing populations of the iconic Atlantic salmon and eel.

“By opening up these barriers along the river, we can preserve our beloved fish species for future generations while reconnecting the community with their blue and green spaces.”

The Aire, like many other urban rivers, has suffered from over 200 years of industry. By 1825, 19 of the 32 fish species that once inhabited the river had disappeared along with the wildlife that relied on them.

The DNAire project revitalised the river to its pre-industrial glory, creating a haven for both wildlife and angling, boosting tourism and putting back what was once lost for the people and communities of West Yorkshire.

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