Shad Monitoring - Unlocking the SevernUnlocking the Severn

2017 shad run 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 a very encouraging to show there is a sufficient residual population of shad to recolonise the upper reaches of the river when, over the coming years, we install fish passes past the four navigation weirs from Worcester to Stourport. By providing shad access to their historic spawning grounds, we aim 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, and 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, which we intend to scale-up to other observable aspects of shad behaviour during the course of our project.

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

During the 2018 migration, we used the knowledge we’d 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 individual. 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 necessarly of concern as large year-on-year variations in run size have been ancodotally 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.

Shad at Sea

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.