The public is being encouraged to record sightings of sturgeon to help the return of the critically endangered fish to UK waters.

European sturgeon, which can grow to up to six metres (20ft) long and live for 100 years, were once widespread in most large rivers across Europe and in Britain, wildlife experts said.

But historic overfishing, river barriers and pollution drove the migratory fish’s decline, with the last one recorded in UK waters in the Tywi in Carmarthenshire in 1993.

The first reintroduction projects in France and Germany have led to the “dinosaur fish” – which date back to the Jurassic period and survived the demise of the dinosaurs and other mass extinctions – starting to return to the south coast of England.

Sturgeon have survived multiple mass extinctions, but humans have driven these extraordinary fish to the brink of total disappearance

Sturgeon return to rivers in the summer months to spawn before disappearing back to sea where they feed on bottom-dwelling species such as worms and mussels.

The UK Sturgeon Alliance is attempting to reverse the decline of the migratory fish and bring it back to UK waters – including the River Severn – and sightings from the public will provide valuable information on the species.

A Save the Sturgeon website aims to raise awareness of the fish and allows users to log records of spotting the fish, which will be fed into a database to help the alliance understand its current and historic presence.

The alliance is formed of the Zoological Society of London, Blue Marine Foundation, Institute of Fisheries Management and the Severn Rivers Trust.

Alex Hubberstey, project co-ordinator from the Blue Marine Foundation, said: “Sturgeon have survived multiple mass extinctions, but humans have driven these extraordinary fish to the brink of total disappearance.

“My hope is that sturgeon will once again be a regular sight in our rivers and coasts.”

Sturgeon have been associated with our rivers, estuaries and coastal waters as far back as records extend

Steve Colclough, from the Institute of Fisheries Management, said: “With the help of many interested partners, the alliance has recently developed an incredible history of the sturgeon in England, Wales and Scotland since 1700, through newspaper reports and museum exhibits.

“This database together with other historical and archaeological information shows clearly that sturgeon have been associated with our rivers, estuaries and coastal waters as far back as records extend.

“As the fish return, modern reports from fishermen, anglers and members of the public are becoming a vital element in developing our understanding and shaping our future work.”

Ashley Deane, from the Severn Rivers Trust, said: “Through our catchment-wide conservation delivery we’re committed to continuing our development of river restoration projects to enhance the River Severn and its constituent tributaries for freshwater wildlife, particularly migratory fish species.

“Ensuring the future resilience of the catchment for the critically endangered sturgeon is imperative for the survival of this species.”

People can find out more about sturgeon or record sightings at savethesturgeon.com