Seals going swimmingly in the greater Thames estuary

The river was once biologically dead but seals are back and a population survey will help guard against threats from disease and dredging

"It's a good news story," says zoologist Anna-Christina Cucknell, as she watches seals glide smoothly through the water, their dark eyes watchful as their heads swivel like periscopes. "In the 1950s, the Thames was declared biologically dead. But the seals are coming back."

Cucknell will lead a land, air and sea survey of the seals in the greater Thames estuary which begins on Monday, including the harbour seals she is watching in the mouth of the river Stour, a short boat trip from Ramsgate marina.