Moment biggest shoal of fish yet swim by Diglis fish pass

This video by Unlocking the Severn gave a glimpse into life in the River Severn as at least 90 fish in the shoal swim along.

A shoal refers to a large number of fish swimming together as a group for social reasons, whereas a school of fish refers to when fish swim coordinated together in a single direction.

Unlocking the Severn has filmed 23 species of fish using the pass, with dace, bleak, roach and occasionally huge Atlantic salmon, chub and barbel. 

Twaite shad themselves migrate onto the river from late April or early May and leave the river around June.

A spokesperson from Unlocking the Severn said: "The new fish pass at Diglis, is part of a conservation and river engagement project working to unlock the River Severn for people and wildlife. 

"Since they were installed on the River in early Victorian times, the navigation weirs have blocked the path of fish in the River Severn – particularly an endangered migratory fish called the twaite shad that travels from the sea in May to spawn far up the river. 

"In fact, all fish are affected by the barriers that weirs form which prevents them being able to range freely to habitat along the length of the river.  The fish pass built at Diglis is the biggest deep vertical slot fish pass in England and Wales and it has a unique underwater viewing window. 

"The window allows scientists to monitor migrating fish and also provides us with amazing glimpses of wild fish swimming through our city. "

Diglis Fish Window - Credit Barbara Evripidou

Fish passes are for helping fish that need to move up and down the river get around man-made blockages.

When asked what the most unusual thing seen at the fish pass so far has been so far, Unlocking the Severn confirmed a variety of unusual sitings.

The spokesperson continued: "River and sea lamprey are very rare and we’ve seen them comfortably cruising though. 

"Recently including one lamprey, also known as the ‘vampire fish’ hitching a ride on an Atlantic Salmon."

Volunteer guides are providing free public tours of Diglis Island and the fish pass, with tours for April currently fully booked.

To book future tickets, you can visit the website at www.unlockingthesevern.co.uk.