Helping fish navigate the Tees Barrage

The River Tees was a major salmon river in the 19th century but numbers declined due to pollution, urbanisation and industrialisation, principally in the lower river. Action was taken in the 1980s to try to address this and the river's stocks of salmon were in an early stage of recovery when the Tees Barrage was constructed in 1995.

However, the effect of the barrage is to temporarily delay migratory fish in their journey until conditions are suitable for them to pass through. During this delay fish are at more risk of being caught and eaten by seals. Only at high water are the fish able to move through the barrage more swiftly.

Possible solutions

A Denil-style fish pass was included in the original barrage construction and a further fish pass was added as part of the upgrade to the adjacent white water course in 2011. However, studies have shown that the main route for migratory fish crossing the barrage is over gate 1 at high tide, rather than the original fish pass. But we also know that salmon and sea trout use other routes, such as the navigation lock and the white water course.

Even though gate 1 is a route for fish passage, tidal cycles mean that this route is not available all of the time. So, with the support of the Environment Agency, we've been investigating options to extend the time available that fish can migrate over the barrage and make it easier for rapid ascent using all the routes available.

This study has identified a number of ways we could improve fish passage. The options include:

• replacing gate 1 with a smaller sluice and new fish pass
• removing the existing Denil fish pass and replacing it with a new type of pass
• fitting a slope or ‘nape' to the front edge of one of the gates
• making changes to the navigation lock to make it work a bit like a fish pass

Once further design work has been completed, we'll have several options that are technically possible, an assessment of the added benefit they may provide and an indicative cost. This will allow us to have an informed discussion on which option to take forward.

All of the options are expensive. We'll need to identify new sources of funding, such as the Heritage Lottery Fund, and submit funding bids, either on our own or in partnership with other organisations.

Current fish monitoring

A counter in the Denil fish pass counts the fish that use this route to move upstream of the barrage, as shown in the table below. The other routes used by the fish are not automatically or regularly recorded, but are monitored using sonar. Also, new fixings and clamps have been fitted above gate 1 and above the navigation lock, which means that future monitoring, hopefully during 2021, can take place more safely and for longer periods of time. This will provide a better indication of the number of fish that use these routes in comparison to the Denil fish pass.

Although we don't have exact data for the other routes, we do know from previous work in 2016 that 52% of fish used gate 1, 39% used the Denil fish pass and 9% the navigation lock. So the numbers in the table are a huge underestimate of the total number of fish passing upstream.

Encouraging the fish to use a range of different routes gives them a greater chance of successfully making it up the river and avoiding the seals. But this means more of the fish will be bypassing the counter at our Denil pass and we won't have as much data, so it's a difficult balance at the moment.

Using sonar

The sonar monitoring we do is called ARIS - Adaptive Resolution Imaging Sonar, and the cost of this work is shared by ourselves and the Environment Agency (EA).

ARIS monitoring involves using a special underwater camera that sends out high-frequency sonar pulses. These pulses bounce off the surrounding environment and return to the camera. Software then processes the resulting echolocation data into an image that can be viewed on a computer. The sound waves produced then allow EA fisheries specialists to determine the size of the fish captured on film. Sadly the restrictions around coronavirus prevented any ARIS monitoring from taking place in 2020.

Sounding the dinner bell

Predation of fish by seals is still a significant issue at the barrage. Although we know that seals predate salmon all the way up from the mouth of the estuary, the barrage is certainly a key feeding location for both grey seals and common seals.

In 2017 we hired an Acoustic Deterrent Device (ADD) to see if it would help to reduce fish predation by seals. The ADD emits loud, high-frequency sounds that are judged to be unpleasant to seals, to the extent that the seals would be expected to move away from the location of the device.

It was trialled for one month in the navigation lock. The results initially showed some promise but were still ambiguous. A longer, three-month trial was implemented in 2018. Results recorded that seals spent approximately twice as much time in the ADD survey area when the ADD was on compared to when it was off. The rate of fish predation was also approximately twice as high when the ADD was on.

It was observed anecdotally that the switching on of the ADD appeared to act as an attractant for the seals, creating a ‘dinner bell' effect. As a result of these findings, we decided it made no sense to continue to use ADDs at the barrage.

Seal surveys took place again in 2019 with a similar pattern of results to previous years. Grey seals were the dominant species but numbers were lower than in recent years. Seal predation of fish was at its lowest, possibly owing to the low numbers of migratory fish present in the river, as recorded by the fish counter. Due to the pandemic, very few seal surveys took place in 2020. The intention in future surveys is to record the location in the river where fish are being predated and to extend the surveys to other stretches of the river. Results of the analysis will be announced in due course.

Monitoring eel passage

Did you know that European eels are critically endangered? To help monitor elvers (young eels) on their journey along the River Tees, an elver pass was built into the Denil fish pass within the barrage. The monitoring work is carried out by the Tees Rivers Trust through a team of their local volunteers. The programme aims to increase understanding of the populations of eels that live in the Tees and see what factors might affect their migration upstream.

Between April and November 2019, volunteers carried out 34 weeks of monitoring. May recorded the highest capture of elvers, similar to the results in 2018 (briefly capturing the eels allows the volunteers to gather data on them before they are then released to continue their journey). Elver numbers were significantly higher in 2018 than 2019, and the highest and lowest temperatures recorded in the elver pass in 2019 were both 2 degrees Celsius lower than in 2018.

The data from the last two years doesn't provide any clear patterns on what factors might affect elver migration, but surveys in future years are being designed to explore this in more detail.

Thanks to our partners

All decisions about the Tees Barrage fish pass are made by the steering group, which comprises ourselves, the Angling Trust, Environment Agency, Salmon & Trout Conservation Trust, Tees Rivers Trust, Industries Nature Conservation Association, Tees White Water Course and local angling interests. We thank all the members of the group for their continuing support.