Shad Monitoring

In 2017, the Environment Agency led the first ever coordinated monitoring programme of the shad migration on the River Severn. In 2022, we monitored the first shad run in a completely reconnected River Severn.

Find out all about our scientific monitoring programme below.

How have we monitored the shad?

Counting shad at Upper Lode Weir

At Upper Lode Weir in Tewkesbury, Gloucestershire, shad are able to pass over the weir using a notch next to the riverbank. We recruit volunteers to help us count the shad seen passing through the notch. The number of shad counted in the spring is used to estimate the migrating population.

Acoustic tagging

Tagging is a useful method to track the behaviour of individual shad during their migration. Tiny tags are carefully inserted into the fish, and the shad are released to continue their migration. When shad pass different receivers along the river, we receive a ‘ping’ from the tag and the time that shad passed by is recorded. We can use this to see how far a shad has migrated upstream, and track its unique journey in the river.

PIT tagging

The other tag we use to track migrating shad are Passive Integrated Transponders (PIT tags). These are tracking tags which provide a location when fish swim through a special antenna, or PIT tag loop. We have several PIT tag loops in Diglis Fish pass, and the data from these provides information about the efficiency of the fish pass, as well as shad movement through the pass.

Spawning monitoring

Shad make a very distinctive splashing sound when spawning. It is important for our project to gather information as to where shad spawning is taking place. We use night-time spawning walks and AudioMoth devices to listen out for spawning activity.


eDNA is a method of testing an environment for genetic material. Every shad releases DNA into the environment that can be picked up via eDNA sampling. This helps us identify how far up the river shad have managed to migrate.

Diglis Fish Window

At Diglis Fish Pass in Worcester, we have built an underwater viewing gallery. The gallery contains a 2.5 by 2 metre window which looks into the fish pass, allowing us to see the fish swimming past. Scientific monitoring cameras record footage of the fish swimming through the pass.

2017 Shad Run Monitoring

Gathering data for our first year of monitoring

We monitored shad from where they entered freshwater in Gloucester, past the first weir in Tewkesbury, right up to the weirs that are currently a major impediment to their migration: Diglis Weir at Worcester and Powick Weir on the River Teme.

An estimated run of over 10,000, and possible as high as 15,000 shad was recorded migrating over Upper Lode Weir in Tewkesbury. This may sound a big number, but it actually represents a tiny proportion of the original population of shad that used to run the river Severn before the construction of navigation weirs in the mid 1800’s.

It was however very encouraging to show there was a sufficient residual population of shad to recolonise the upper reaches of the river. Over the next 5 years, we worked to install fish passes past the four navigation weirs from Worcester to Stourport.

By providing shad access to their historic spawning grounds, the aim is to achieve a thriving population once again.  This will significantly contribute to the Severn Estuary Special Area of Conservation achieving a favourable status for shad designation and becoming the only significant population in the UK.

New technologies were used to count and track shad

The science underlying a project on this scale must be rigorous. Specialists tested independent counting techniques on the lower river, including hydro acoustics, automatic counter, continuous filming and timed visual count.

Additionally, the first ever acoustic tracking of shad in the UK was attempted with a batch of 25 tagged fish ultimately being released.  The tracks obtained from a network of 30+ receivers will give previously unknown insight into their behaviour.  The tracking results confirmed that tagged shad were blocked by both Diglis and Powick Weir.  What was unexpected was the huge distance these fish travelled during their time in river, many making multiple trips up and downstream before ultimately returning to the sea.  The shad generally spent less than a month in the river in total and are well deserving of their common name “the May fish”

Genetic testing of both fish eggs captured downstream of Powick weir and DNA found in the environmental collected in water samples (eDNA), and scale- reading also provide important information about the return-immigration patterns of the fish. Specialist film-makers recorded the first images of shad under water on the Severn as well as the first images of the night time spawning of shad.  And this intriguing footage was broadcast as part of the on BBC Midlands contribution to Springwatch.

2017 Citizen Science

Volunteers dedicated many hours to help at the riverside – monitoring shad movements and counting the numbers of shad that can be clearly observed as they pass through a notch in the weir at Upper Lode Weir in Tewkesbury.  This important contribution of volunteers is another exciting aspect of our project, allowing members of the public to contribute towards our scientific monitoring.

Allis Shad

Our team were also thrilled to discover that the larger and even rarer cousin of the twaite shad – the allis shad on the River.  They were captured on film passing through the notch at Upper Lode Weir.  This hopefully means there is still a tiny population surviving in the river, which will equally benefit from the passage improvements being delivered as part of Unlocking the Severn.

2018 Shad Run Monitoring

Developing techniques in our second tagging year

During the 2018 migration, we used the knowledge we had gained in 2017 to build on the programme.  We had proved that shad could withstand the tagging process and so tagged more in 2018 – a total of 84 individuals. This we believe represents the largest tagging study of shad ever attempted in Europe.

The acoustic tags are used under Home Office Licence and will transmit for up to 3 years, giving the potential to capture data from 3 further annual spawning migrations. Twaite shad will spawn up to 5 times over their life and live for 8-10 years. Shad for tagging were either captured using a large trap or through angling, at two locations in the lower river before being released to continue their journey.

All 84 tagged shad were shown to migrate upstream; some swam upstream more directly, others demonstrated a roaming behaviour, swimming up and down parts of the river many times.  None of the tagged shad managed to ascend the next weirs at Diglis or Powick.  This provided further confirmation that both structures represent significant barriers to migration.

The average (mean) length of time that shad spent on the river was 17 days, but some shad spent up to 41 days on the river before returning to the estuary.  65 of the shad were tracked back to the estuary – showing that at least 77% of the shad survive their foray up the river to spawn and return to the sea, hopefully to return in 2019.

2018 Citizen Science

More volunteers than ever were able to join us at the riverside and we therefore logged an observation for every day of the shad run.  From these observations we calculated a minimum run estimate of shad counted passing over Upper Lode weir at 6998.

This is significantly down on the run in 2017, but not necessarily of concern, as large year-on-year variations in run size have been anecdotally reported going back over the years in the historical record.

In 2018 we saw only 1 migration peak whereas the year before there had been 3 distinct runs coinciding with the larger tides.  The compressed run in 2018 may well be due to lower water temperatures early in the run a result of the colder than average winter and late snow.  This resulted in a delay to the start of the shad migration by 12 days compared with 2017.

Nocturnal Spawning

18 shad spawning sites were identified through night-time spawning observation.  The fish create a characteristic spawning display – the male and female fish swim around in a tight circle, as if chasing each other’s tails.  It is quite a noisy activity, with a lot of splashing!  This means that shad spawning can be identified both visually and acoustically from the characteristic sound of the spawning behaviour.

Counting Techniques

In 2018 a camera filming at the notch and a resistivity counter were also used to record the run at Upper Lode.  The resistivity counter automatically detects fish passage by measuring a change in the bulk resistance of the water as fish swim across an array of electrodes that span the stream.  The Fish Counter records the time a fish swims past, the size of the fish, and it can also differentiate between upstream and downstream passage. The results of this counter have now been validated and are comparable with the citizen science counts suggesting a total of 6694 shad passed over through the notch at Upper Lode Weir in May 2018.

Shad eggs and environmental DNA (eDNA) samples were also collected this year for further analysis by Bournemouth University to confirm distribution and hybridisation rates between the two shad species (twaite and allis).  This will be reported in 2019.

One of the advantages of working collaboratively is that sometimes you can discover unexpected results…

Ten of our shad tagged in the River Severn in 2018 were recorded on third party acoustic receivers in the sea off the coast of North Devon between July and November.  These receivers are positioned at the mouth of the Taw-Torridge Estuary and were installed by Plymouth University to study tagged bass.  Apart from the odd sea angling record this is some of the first evidence on where our shad go during the rest of the year when they are at sea. This has shown that despite our acoustic tags slowing their ping rate to just one ping every ten minutes to preserve the battery life outside of the shad run: it is still possible to locate our fish in the sea.

This leads to a host of new collaborative opportunities in 2019 to work with other that undertake research in the Severn Estuary.  We are currently investigating opportunities to install a series of receivers in the Severn Estuary to hopefully find out more about our shad at sea.  Long term this additional research will help in protecting this rare fish as it migrates far beyond the river of its birth.

2021 Shad Run Monitoring

A tricky year for monitoring

During the 2021 shad run season, unseasonal weather created a challenge for our shad monitoring programme. We started with a dry and cool April – there was just enough warmth in late April for the first shad to appear at Upper Lode Weir in Tewkesbury.  Then, the weather turned and, by some statistics, May became one of the coldest on record for the last 60 years.  Low temperatures were also accompanied by persistent rain. The river levels rose up to high levels, over-topping the weir at Upper Lode and the notch where we normally observe shad passing over.

By the time the river levels finally came back down to normal levels the shad run was nearly over.  However, every cloud has a silver lining, and these extreme events do provide us a chance to learn about how shad react to changing environments.  Although we don’t have clear shad counting observations for the full run, we can gain new scientific insights to help build a richer picture of these fascinating fish and how they behave under different conditions.

Two complete fish passes

Despite the complications caused by the weather, 2021 was an exciting year of monitoring for Unlocking the Severn. After the completion of Bevere fish pass in September 2020 (pictured), and Diglis fish pass in April 2021, this was the first year shad would be able to migrate upstream of Worcester.

Works at Holt and Lincomb fish passes were still ongoing further upstream, however the team were looking forward to seeing whether pioneer shad would head upstream of Diglis this year.

A window into the River Severn

In May 2021, construction of an underwater viewing gallery was completed at Diglis fish pass. This exciting new space, just a 15 minute walk from Worcester city centre, contains a 2.5 by 2 metre viewing window, which looks into the upstream end of Diglis fish pass. During the spring and summer, all fish are directed past the window, giving a fantastic opportunity to see migrating fish.

As well as being an incredible opportunity for visitors to see the shad and other River Severn wildlife, the viewing gallery provided a useful tool for scientific monitoring. Motion-detecting cameras were set up in the gallery, to capture footage of fish swimming past the window. The footage was then reviewed by Environmental Agency scientists, to give a count of the number of shad passing through Diglis fish pass during the spring migration.

The first confirmed shad – a historic moment for the ecology of the River Severn – went through on the 9th of May. Monitoring cameras recorded 623 shad swimming through Diglis fish pass in 2021, from the 26th April until the end of June.

2021 monitoring techniques

Monitoring techniques used in 2021 included citizen science counting at Upper Lode, acoustic tagging, PIT tagging, eDNA sampling, spawning monitoring, and counts from the underwater viewing gallery.

Despite gaps in some of the data, caused by unfavourable weather conditions, we still managed to gather some good insights this year. Observations from the underwater viewing gallery showed Diglis fish pass to be a great success; 77 shad were tagged in late-April; and new PhD student, Mark Yeldham, began downloading data from the 50+ acoustic receivers along the River Severn.

Read the 2021 monitoring results in detail by clicking on the button below.

2022 Shad Run Monitoring

A historic year for the twaite shad!

In April 2022, works at our final fish pass were completed, meaning that the River Severn was now fully unlocked in time for the shad run. For the first time in nearly 180 years, shad had access to their historic spawning grounds.

Although we were anticipating that it would take time for shad to once again reclaim former spawning habitat, the team were keen to collect plenty of data from the first shad run in a reconnected river.

A loop the loop with joy

The first shad of 2022 went through Diglis fish pass on 21st April. It even did a little loop the loop for us on camera! What a great way to kick off the shad run this celebratory year.

We recorded 757 shad through Diglis fish pass between 21st April and 25th June, an increase of 22% on the previous year.

PIT tag data showed that on average, shad took around 35 minutes (the median figure) to swim through the fish pass, but individual variation is vast. In fact, the speediest shad raced through this 100-metre structure in less than 3 minutes from start to finish!

Acoustic tagging

This year a record number of shad were tagged. Scientists from Bournemouth and Hull Universities and the Environment Agency tagged 100 shad with acoustic and PIT tags, and a further 150 shad with just PIT tags.

This year, from the 100 acoustically tagged fish, shad were recorded travelling as far upstream as the fourth and final fish pass at Lincomb weir. Shad had not had access to this area of river for around 35 generations of shad, so we were relying on pioneer fish, in their first year of spawning, to venture further than before. We were thrilled to have confirmation that shad had already successfully navigated at least the first three fish passes.

Spawning monitoring

Despite our staff and volunteers not hearing any spawning activity during the night-time walks, we were hopeful that the AudioMoth devices, placed along the river, might have detected the distinctive splashing sounds.

We captured evidence of shad spawning in newly reconnected habitat in the vicinity of all the new fish passes.

Benefitting a whole host of River Severn wildlife

As well as recording record numbers of shad this year, we observed plenty of other species making use of easier passage at Diglis fish pass.

By July 2022, we had recorded 25 different species of fish using the pass!

This includes other rare and endangered species, such as salmon, eel and lamprey. And in fact over 1000 sea lamprey were recorded through Diglis fish pass by 25th June. The video opposite shows a sea lamprey suckered to the smaller viewing window.

We also noticed the fish pass providing a habitat for other organisms such as snails, freshwater mussels, and even hosting a grey wagtail nest beneath the terrace. Our YouTube subscribers have also been delighted by videos of otters ricocheting past Diglis Fish Window on their trips up and down the river.

See more videos from Diglis Fish Window here.

You can read the initial monitoring results from the 2022 shad run using the link below.

Shad Symposium 2022

In May 2022 (perfect timing for the Severn Shad Run!), Unlocking the Severn hosted a scientific conference all about the shad. The Shad Symposium welcomed over 100 delegates from 10 different countries across Europe.

The aim was to share research and findings from different river restoration and shad conservation projects from across the continent. The Unlocking the Severn team opened the conference with the latest from our scientific monitoring programme and the news of the completion of our fish passes on the River Severn. We then heard from a variety of speakers, working on other projects across Europe.

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