Fish passes give endangered twaite shad chance to swim up Severn River and spawn

Return of one of one of Britain’s rarest fish confirmed after DNA found in water samples above fish passes
2022-11-15 14:40

For nearly two centuries, one of Britain’s rarest fish has been shut out of its spawning grounds by large weirs.

But the endangered twaite shad has now returned to its historic spawning habitat on the River Severn, thanks to four new fish passes that enable the migratory fish to negotiate weirs and swim up river to lay eggs.

The return of the twaite shad (Alosa fallax) was confirmed when scientists detected shad DNA in water samples from the river above all four fish passes. These reconnect 158 miles of the Severn’s habitat and provide safer passage for other declining migratory fish including salmon, eel and lamprey.

“It is thrilling to know that the fish passes are working and shad are rediscovering key habitat lost to them for so long – 35 generations for these fish,” said Charles Crundwell, senior technical fisheries specialist at the Environment Agency. “Over the coming years, we look forward to seeing a growing proportion of the shad run spawning higher up the river and a recovery in the population.”

Shad are a type of herring that migrate from the sea into freshwater every year to spawn. The silver fish use only four British rivers to spawn and historically the Severn was the most important, with hundreds of thousands of fish swimming up the Severn on their annual migration to their spawning grounds in the upper reaches of the river.

Their route was blocked in the 1840s by the building of large weirs for navigation – a common obstacle for migratory fish, with only 1% of rivers in England, Scotland and Wales free of artificial barriers. On average, there is at least one artificial barrier every 1.5km of river in Britain.

The Unlocking the Severn restoration project, led by the Canal & River Trust in partnership with the Severn Rivers Trust and government agencies, spent four years constructing fish passes so that migratory species such as the shad could swim around the weirs. The final pass was completed this year, with financial support from the National Lottery Heritage Fund and the European Union LIFE programme.

This summer, a range of monitoring techniques were deployed to see if the passes would help shad reach their ancestral spawning grounds.

Cameras filmed through an underwater window at the new fish pass built at Worcester, while 100 shad were fitted with minuscule tags that sent sonic pings to acoustic receivers along the river. Film footage revealed 725 shad had successfully used the fish pass at Worcester by midsummer alongside more than 940 sea lamprey, while the acoustic receivers in the river picked up some of the 100 tagged fish travelling as far upstream as Lincomb Weir, the site of the fourth and final fish pass near Stourport, north Worcestershire.

“This proved shad were making use of the first three fish passes – great news for their first migration through new habitat – but did any venture further?” said Lorna Pedersen from the Canal & River Trust.

She said: “Alongside the tagging programme, our volunteers took water samples for environmental DNA analysis – a cutting edge technique for biomonitoring rare and difficult-to-observe species. Samples taken in Stourport contained shad DNA, proving that shad had successfully navigated all four new fish passes and had the opportunity to explore the spawning habitats beyond for the first time in almost two centuries.”