Case study:Unlocking the Severn
Project overview
| Status | Complete |
|---|---|
| Project web site | http://www.unlockingthesevern.co.uk |
| Themes | Environmental flows and water resources, Fisheries, Habitat and biodiversity |
| Country | England |
| Main contact forename | Jason |
| Main contact surname | Leach |
| Main contact user ID | |
| Contact organisation | Canal and River Trust |
| Contact organisation web site | http://canalrivertrust.org.uk |
| Partner organisations | Environment Agency, Severn Rivers Trust, Natural England |
| Parent multi-site project | |
| This is a parent project encompassing the following projects |
No |
Project summary
Unlocking the Severn has been a once in a lifetime river restoration project, restoring connectivity for migratory fish on the UK’s longest river and engaging thousands of people with the fascinating natural, cultural, and industrial heritage of the Severn. The twaite shad were once seen migrating upstream in their hundreds of thousands each May but became separated from their natural spawning grounds by artificial river barriers, sending the population into chronic decline and confining them to lower reaches of the Severn, downstream of Diglis weir in Worcester.
Victorian engineering brought huge advancements to society during the nineteenth century through the construction of locks and weirs along the Severn to address the challenges of fluctuating river levels. This enabled the transportation of more goods to support the industrial revolution which was integral to economic growth and local communities at the time. However, weir construction had significant negative ecological impacts, specifically on migratory fish which were unable to navigate around or over the weirs. The shad in particular do not have the ability to jump over barriers and could no longer migrate freely upstream.
To reconnect this fragmented ecosystem Unlocking the Severn set out to undertake work on two major rivers at six sites which had been identified as the barriers to migration. These were the weirs at Diglis, Bevere, Holt, and Lincomb on the Severn and Powick and Knightsford on the Teme. The aim was to help restore the twaite shad population by significantly improving access to quality spawning and nursery habitat, re-establishing a total of 253km of its former natural range. It also set out to deliver a ground-breaking scientific monitoring programme to greatly broaden and improve knowledge and understanding of the twaite shad, and an ambitious plan to engage local communities in conservation, long-term protection, and sustainability of the fish and of rivers.
The key objectives of the project were to:
1) Restore upstream passage and access for the shad and other migratory species. Ambitious and complex engineering and construction of fish passes to reconnect migration routes with historic spawning habitat, restore connectivity and improve access upstream for all species of fish.
2) Monitor population and behaviour using best practice techniques and citizen science to increase knowledge and understanding of the twaite shad and prove successful upstream migratory passage through the new fish passes.
3) Disseminate monitoring results and findings including effective transnational exchange of best practice to help support others and aid population restoration of twaite shad elsewhere, including an internationally attended scientific conference and networking with other fisheries managers across Europe.
4) Raise Awareness through an extensive programme of community and public engagement activities to help increase local understanding of the historical, biological, and potential economic values of the shad and the wider river environment, and the wide-ranging benefits of conservation initiatives.
Unlocking the Severn was delivered by Canal & River Trust in partnership with Severn Rivers Trust, Environment Agency and Natural England and made possible with funding from the National Lottery Heritage Fund and the European Union LIFE programme.
Monitoring surveys and results
Monitoring of the shad was a key component for this project, it underpinned all the other work, informing the fish pass design, measures of success, public engagement, citizen science, and international knowledge exchange and collaboration.
Shad are a relatively poorly studied fish, this is because they have only limited commercial value, there is no aquaculture of them, they are relatively rare, and were notorious for being tricky to handle for scientific investigation. This project initially had to rely on the limited amount of published literature, much of which was from the similar American shad. As for the European shads there was even less, so the project quickly made links with the few European organisation interested in shad conservation and management, primarily in Portugal, France, Belgium and Ireland.
This project allowed probably the largest study into shad behaviour anywhere in Europe to this point. Most previous studies had looked at only specific components of shad lifecycle. This required a multi-discipline approach and many of the techniques used were novel or adapted from known techniques to the conditions of the river Severn.
Spawning Behaviour, Genetics & Environmental DNA:
One of the key outcomes of this project was to see an extension in the spawning area used by shad. The Severn Estuary Special Area of Conservation status report classed shad as in “unfavourable” condition due to the barriers to migration cutting off their historic spawning grounds, with distribution of spawning only about 20% of that historically used. Effectively the weirs at Powick on the river Teme and Diglis on the river Severn were the upper most limits of spawning migration.
Two studies were carried out which concluded that the habitat on the Teme and the river upstream of Stourport on the Severn remained largely unchanged from when shad last had free access back in the 19th Century. When these results were compared with the latest evidence on good spawning habitat it was concluded that if shad could reach this habitat, it would be highly suitable for the restoration of the species. It was also known that a large amount of the total shad run ends their upstream migration at either Powick or Diglis weirs, suggesting that there are fish motivated to continue their migration if possible.
In each year of the project, night-time spawning observations were made by staff of the project, citizen science volunteers and students. This revealed some unexpected results, which were later supported by the tracking data. That is the lower navigable river is widely used by shad for spawning, this goes against much of the scientific literature, although similar behaviour is also witnessed in Belgium and Ireland. What the project investigation could not show is the effectiveness of the spawning in this area as shad were effectively forced to use these areas because of the barriers to migration.
The most effective method of recording spawning activity turned out to be audio recording equipment (several types were trialled). These devices were left in suitable locations throughout the river and the recordings were then analysed after the event. This revealed that with all the fish passes open in 2022, shad spawning was recorded between each of them. This was a great result showing that once access was restored shad would use the newly opened habitat as expected. This result also supported the other independent monitoring provided by the tracking and eDNA results.
Genetics also played an important part of this project, with one PhD successfully completing their studies looking at hybridisations and developing a technique to collect eDNA samples to confirm the presence or absence of shad as they returned to the newly opened river sections.
Other studies have shown that the two species of shad (twaite and allis) readily hybridise if forced to use the same spawning areas. This has been seen elsewhere in Europe and proved to be the same on the river Severn with circa 30% of eggs sampled showing a significant degree of hybridisation. It will take years to see if this level declines as the two species are able to spatially diverge their spawning over subsequent generations as a long-term result of this project.
The development of the eDNA technique, with the water samples now able to be taken by citizen scientist has been pivotal in ground truthing some of the other monitoring. The results clearly showed the barriers at Powick and Diglis were significant, however like many things in nature it was not quite as clear cut. eDNA in 2018 showed that in higher flow conditions some shad can negotiate these barriers, particularly on the river Teme. Although DNA detection levels were considerably lower than those below these barriers suggesting it is only a fraction of the population that make it over. This is actually an encouraging result as it clearly showed that a proportion of the run would start the recolonisation of the newly opened area once the fish passes were constructed, as has been proved right.
Counts, Run Estimates & Citizen Science:
To measure the initial and long-term success of the project it was important to have some idea of the starting population. Historical records show that shad numbers going back over the centuries were once sufficient to support an important local and export fishery from the estuary all the way up to the Welsh borders which ceased immediately after the construction of the navigation weirs in the 1840’s. In recent times an assessment of the population has not routinely been undertaken.
Early in the project it became evident that shad could be observed running the notch in one of the lowest weirs at Upper Lode weir in Tewkesbury. Not only was this the perfect public engagement opportunity, but it also provided an opportunity to try a variety of monitoring techniques to quantify a run of shad that pass upstream of this structure. Techniques used included hydroacoustic, resistivity counters, direct observation and video recording.
These methods were evaluated against each other, and direct observation proved to be the best for the project in determining a relative inter year run estimate. It was also hugely popular with the dedicated citizen scientists who undertook most of the observations. The use of video recordings made available on the website allowed the general public, often many miles removed from the river, to engage in the project, helping us to further validate the data.
In all but the wet year of 2021 when the weir was submerged in flood water from extended periods a consistent run estimate has been produced in all years of the project. By using the acoustic tracking results, it has also been possible to scale this run estimate up to a whole river run to include the proportion of shad that complete their whole lifecycle downstream of this structure. This whole river run estimate has already been put to valuable use for Habitats Directive reporting, production of Habitats Risk Assessments and in support of a sensitive Public Inquiry where the Environment Agency was the regulatory authority.
Diglis Fish Pass Viewing Gallery Cameras:
Opened in time for the 2021 shad run, the viewing gallery at Diglis provided the project with another opportunity to count shad further upriver as they recolonised habitat unlocked for the first time in 180 years. A series of cameras were set up to record fish movement through the viewing window as they passed upstream. The hours of data collected was primarily analysed for shad, producing a passage rate of shad upstream as well as giving us valuable information on temporal movements to help inform the public tours at this facility. In 2021, 623 shad successful passed upstream and in 2022 this increased to 759 individuals. It is the offspring of these individuals that will kick start the restoration of shad in the Severn when they return as adults after 4-5 years.
25 fish species have been recorded using the pass (up to Feb 2023) so the benefits for the wider ecosystem are clear to see. Counts of other species have been limited to the other Severn Estuary Special Area of Conservation species, particularly salmon and sea lamprey. This has been particularly useful for local fisheries managers. Tagging & Tracking Shad
Acoustic tagging proved to be far more successful than ever hoped, and by the end of the project 331 shad had been successfully tagged, far exceeding any previous study of shad in Europe. This was made possible by the design and installation of a trap that fitted in the notch in Upper Lode weir.
These fish provide data on all aspects of the shads’ behaviour, including duration of freshwater spawning migration, behaviour and delay at barriers (upstream and downstream), run estimates, distribution, freshwater mortality, difference in behaviour of individual between years, seaward migration, migration through the Bristol Channel (in collaboration with Swansea University), and marine mortality.
PIT tags were fitted to all 545 shad. These tiny tags are detected by an energised external electrical field (PIT loop) which were placed in several notches of the Diglis fish pass to monitor migration through the pass for efficiency trials.
Lessons learnt
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Catchment and subcatchment
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Project background
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Reasons for river restoration
Measures
MonitoringHydromorphological quality elements
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Any other monitoring, e.g. social, economic
Monitoring documents
Additional documents and videos
Additional links and references
Supplementary InformationEdit Supplementary Information
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