An ambitious scheme is bringing life back to the heavily industrialised River Trent
Report by Jo Caird
Salmon used to be plentiful in the River Trent. So plentiful, in fact, that in the 1800s, Nottingham apprentices had a clause added to their contracts to specify that their dietary allowance would include salmon not more than twice a week. It was a similar story for lamprey and eels, migratory species that also used to swim up and down the Trent in huge numbers en route to their spawning grounds.
While we’re unlikely to see 21st-century Nottingham workers taking industrial action to limit their salmon intake any time soon, the opening later this year of the country’s biggest fish pass, at Holme Sluices in Colwick, will be a significant step in restoring the Trent catchment to its former glory as a habitat for migratory fish.
The Colwick Fish Pass (CFP) is an £8.5 million project by the Environment Agency (EA) to allow these species to bypass Holme Sluices, a vital flood risk management structure for the city of Nottingham and the largest barrier to the natural migration of fish on the Trent. Measuring 200 metres long, six metres deep and 6.5 metres wide, the CFP is divided into 21 ascending chambers into which water will flow through narrow slots. Fish of all species swimming upstream to lay their eggs in the gravel riverbeds of Trent tributaries such as the Dove and the Derwent will be able to pass through these slots and rest in the chamber above before continuing on.
‘There are all sorts of issues with salmon – exploitation at sea, predation, disease – but we’re fixing one here on the Trent,’ says Simon Ward, a fisheries technical specialist with the EA.
Holme Sluices was built in the 1950s, but the decline of migratory fish in the Trent catchment dates back to the Industrial Revolution, when large weirs were first built to open up the river for trade. Some fish were able to overcome these barriers – whether by leaping over or wriggling through what were often imperfect constructions of stone or wood – but these tenacious individuals were then beset by a further challenge: appallingly high levels of water pollution, both chemical and thermal.
‘I know we’ve got a pollution scandal going on at the moment, but on the Trent, when you look back into history, today is nothing,’ says Tim Jacklin, a Wild Trout Trust conservation officer with decades of experience in the Trent catchment. Pollution from the industrial centres of Stoke-on-Trent and Birmingham poured downriver until it reached the 80-kilometre tideway, where it was washed to and fro, with disastrous effects.
‘Salmon were made extinct by pollution, but they were already on the decline because of the lack of access to their spawning areas from all the barriers,’ Jacklin continues. Historical data on migratory fish are patchy, but net catch records for the Trent are illustrative of the decline: from around 3,000 salmon a year in the 1880s to just six in 1960. Water quality improved during the second half of the 20th century, enough that salmon started to bounce back. Small numbers were recorded in the lower Trent in the 1970s, ’80s and ’90s, and the population was bolstered by reintroductions between 1998 and 2013. The salmon population there is now regarded as self-sustaining, says Jacklin.
However, man-made barriers remain problematic, explains Scott McKenzie, senior catchment manager at the Trent Rivers Trust. The weirs on the lower Trent, including Holme Sluices, are only passable by migratory fish in very specific flow conditions and this ‘knackers the rest of the Trent. The Dove and the Derwent are stunning rivers and they should be really good salmon rivers – the gravel’s there and the habitat’s there. It’s just these weirs. All migratory fish are getting to that point and getting stuck.’
The new pass at Holme Sluices in Colwick will make a big difference, says Ward. ‘The improvements to the spawning in the Derwent and the Dove and all those tributaries will increase manyfold because we can get fish past Nottingham very quickly. If there’s more energy when they reach the spawning grounds, they can produce more eggs and the spawning will be more efficient, more productive.’
The CFP has been a long time in the making. Ward had his eye on a fish pass at Colwick back in the 1970s, when he first became aware of salmon returning to the Trent. ‘I was quite surprised to see them. It was the first time they’d been seen in the river for many, many decades,’ he recalls. ‘We figured something needed to be done but we didn’t have the resources.’ Fast forward more than 40 years and Ward, now on the cusp of retirement, is delighted to be overseeing the completion of a project that finally solves a problem that he first encountered at the start of his career.
The EA began conducting feasibility studies for the fish pass back in 2013, but it wasn’t until January 2022 that construction began because of the sheer scale and complexity of the project. There are the location constraints: diverting water through the fish pass must not lower the flow through either the canoe slalom course at the adjoining Holme Pierrepont National Watersports Centre or the turbines of a local hydropower operation; the EA also needs maintenance access to Holme Sluices. Then there are the factors relating to the migrating fish themselves: different species need varying flow rates depending on their anatomy and swimming ability.
It all requires technology, infrastructure and therefore expense well beyond what reinstating fish passage on a less complicated bit of river might entail. Jacklin gives the example of Gothard weir on the River Dove, which the Wild Trout Trust removed in September 2018 at a cost of just £15,000.
The CFP is a significant achievement but it’s not a silver bullet. To open up the lower river fully, the EA is working with local and national partners, including the Trent Rivers Trust, on the Trent Gateway, a project that aims to install fish passes at all seven of the remaining large weirs on the lower Trent. Two of those fish passes – at Averham and Thrumpton – are in the early stages of development.
The final five weirs are owned by the Canal & River Trust (CRT), which is hoping to install hydropower schemes with fish passes designed into them. These will ‘not only remove barriers for fish but also bring other important benefits, such as carbon reduction and generating green, local energy’, says CRT communication and campaigns manager Stephen Hardy. Work on one of them might start this year or next; the others, however, are likely to be much longer-term projects, thanks to complications around the financing of such schemes following the removal of the government’s feed-in tariff scheme in 2019.
For McKenzie, the pace is unacceptably slow. ‘There’s no urgency,’ he says. ‘In a state where we’ve got a massive biodiversity crisis, [reinstating fish passage is] an obvious step to do. The decline of a lot of freshwater species isn’t solely down to [these weirs]. There’s still a lot of work to be done around figuring out why these populations have declined so dramatically. However, we know this is a major factor. So why aren’t we doing more about it?’
‘A lot of people get frustrated by the timescale on this,’ says Ward. ‘The thing is that the river was messed up over probably a couple of hundred years and we’re not going to fix it in five or six years. All I can say is that there are plans and there’s determination to get this done.’
In an ideal world, says McKenzie, all the weirs would simply be removed. This would be beneficial not just for migratory fish but for the entire freshwater ecosystem and beyond. ‘When a weir is in place, it’s not just affecting that locality,’ he explains. ‘Depending on the size of the weir, it can back up the water for kilometres. It essentially ponds a river. If you want good insect life and good fish communities, you need that flow of water, which you just don’t get in and around weirs.’
However, he’s well aware that removing the Trent weirs is a non-starter. If nothing else, doing so would require repealing the act of parliament that protects the right of navigation on the river. However, McKenzie is adamant that fish passes are an option that should be pursued with no further delay.
Artificial barriers aren’t just a problem on the Trent, of course. This is a nationwide issue; according to a 2019 assessment by scientists at Swansea University, there’s at least one man-made barrier for every 1.5 kilometres of stream in the UK, with 97 per cent of our rivers fragmented by artificial barriers.
‘A lot of people see them as just nice features. It’s trying to find that balance between preserving that industrial heritage but also accepting the fact that they are incredibly damaging,’ says McKenzie. ‘The worst things for a river system are redundant weirs.’
The good news is that once fish passage has been reinstated – whether by installing a fish pass or removing a weir altogether – the results are immediate. ‘You remove it, water levels drop, the gravel’s clean. You can literally see the gravel starting to clean up. Fish are on them within days. It’s spectacular,’ says McKenzie. ‘This is instant impact.’
Undoing the negative impacts on our rivers of more than 200 years of industrial development, flood management and power generation won’t be easy. But every weir removed or fish pass installed – whether huge-scale schemes like that at Colwick or simpler projects taking place up and down the UK – is a step in the right direction.
‘Doing any barrier in isolation is always worth it because it’s a multiplier effect,’ says Ward. ‘You get a huge impact by getting fish up to the next barrier.’
