Native oysters returned to the Thames by scientists as they hope for large-scale harvest for first time since Roman era

While now they are an expensive delicacy, long ago native oysters were a cheap source of protein dredged in their thousands from the fertile riverbed of the Thames.

Most would in current days think twice before eating anything fished out of the murky depths of the Capital's river, but now scientists are returning the bivalves to its estuaries in the hope of the large-scale oyster production we last saw in Roman times.

The oyster beds in the Thames were almost depleted by Victorian times, and now only 5 per cent of the shellfish remain, so the Zoological Society of London has started to put in place the region's  first Mother Oyster Sanctuary.

It was thought in Britain that oysters may even go extinct, and now native oysters can only be enjoyed from a handful of places, including Whitstable in Kent and Falmouth in Cornwall. Recovery of the species has been hindered by habitat loss, pollution and the introduction of diseases. Natural replenishment of their native grounds is so limited that human intervention is their only hope.

Scientists have begun to lay an oyster bed in the depths of the Thames estuary in Essex, using recycled shells from oysters bred in Mersea and sold in Borough Market, as well as cockleshells from the Thames cockle fleet.

Soon, adult female oysters, or "mother oysters", will be laid, and it is hoped they will quickly spawn.

Oyster farming has been recorded in Mersea since Roman times, with the shellfish forming a staple part of the British diet throughout history.  Many of London's famous oyster bars sprung up largely because of the fruits of the Thames.

The project is not only good news for British seafood lovers; oysters are very useful for an aquatic ecosystem as they create the conditions for other species to thrive – stabilising shorelines, filtering water and providing vital food and habitat for coastal wildlife. One adult oyster, for example, can filter more than 140 litres of water in a single day.

ZSL’s Senior Conservation Programme Manager for UK & Europe, Alison Debney, said: “It may not be glamorous work, but laying ‘mother oysters’ at the right time is vital to the success of the restoration programme, and therefore vital for the survival of this native British species.

“The coalition has since moved more than 25,000 native oysters to Essex estuaries, as well as ensuring that fishing in the area is prohibited until the oyster stocks have sufficiently recovered and are able to withstand sustainable harvesting.”

Dr Rebecca Korda, Senior Marine Advisor at Natural England explained: “We are thrilled to be part of this collaboration which sees an array of stakeholders coming together and showing incredible innovation and drive to work to restore the native oyster and the native oyster beds back to the Essex waters. This is a hugely exciting and important step in taking this work forward and we are delighted to continue to support and contribute to the project”.