One of the UK’s rarest fish is getting the chance to return to historic breeding grounds on the River Severn which have been cut off by weirs for 170 years.

And as part of conservation efforts to help the endangered twaite shad, citizen scientists are also being sought to head out in the dead of night to monitor the distinctive sounds of their spawning.

The Unlocking The Severn project by the Canal & River Trust, Severn Rivers Trust, Environment Agency and Natural England is in the process of reopening 150 miles (241km) of the river for the fish by creating routes around four major weirs.

Two large fish passes alongside Victorian weirs at Diglis and Bevere, near Worcester, have been completed as part of the conservation project, the largest of its kind in Europe.

It means that this month the twaite shad – traditionally known as the May fish because of the timing of its migration on to the river from the sea to spawn – will be able to swim past the weirs for the first time in nearly two centuries.

The twaite shad migration was once a well-known feature of the River Severn as hundreds of thousands of fish migrated upriver to spawn, the team behind the project said.

But the Industrial Revolution saw the construction of weirs, which helped barge traffic on the river but blocked the path of migratory fish.

The shad, which could no longer reach their natural spawning grounds in the upper reaches of the river, saw numbers crash until only a small population remained migrating to the Severn each year, unable to travel beyond Worcester.

It’s never too late to give nature the chance to recover

The fish passes, which allow the shad to swim upriver of the city once again, will also help other species such as salmon, lamprey and eels.

The conservation project for the twaite shad is also seeking volunteers to head out at night to track their spawning around Worcester and Tewkesbury on nights in May when the fish are on the river.

The shad prefer to spawn in flowing water over gravel beds at night, in noisy “bull” events with lots of splashing as the male and female swim in a tight circle, as if chasing each other, as the eggs and milt, or semen, are released.

Although the events take place under cover of darkness, the distinctive sound can be recorded from the river bank to estimate the number of spawnings in different locations.

Good spawning conditions can result in a peak year for new shad, which can live for eight to 10 years and return to the river up to five times to spawn, so the monitoring could give an early indication of the prospect for the population in coming years.

Jason Leach, programme director at the Canal & River Trust, said: “It’s never too late to give nature the chance to recover.

“And our project’s night-time riverside spawning vigils are a fitting way to begin recording the recovery of the fish affected so badly when our predecessors inadvertently caused a big problem for migratory fish by the building of the weirs such a long time ago.

“We hope lots of volunteers will be inspired to join us to witness and record the spring shad spawning phenomenon.”

Charles Crundwell, senior technical specialist fisheries at the Environment Agency, said the habitat restoration project will have huge benefits for the twaite shad.

“Shad spawning is a nocturnal sign of spring on the River Severn. This year, with the help of volunteers, we’re excited to add acoustic monitoring to our efforts and our night-time monitoring will give us an early indication of the prospects for the shad population in years to come,” he said.

Other volunteers in the conservation project count shad seen migrating during the day from the riverside to estimate the size of the run.

People who want to get involved can also help count shad online when video clips are released on https://www.unlockingthesevern.co.uk/in early May.