River of life: zoo’s yearly count finds seals thriving on Thames

Hundreds of dozing seals show how much cleaner the river is since it was declared dead in the 1950s

This is a sushi conveyor belt," says the boat's skipper, Stuart Barnes, as we watch the customers, dozens of harbour seals slumbering on sandbanks at the mouth of the Thames estuary, a 15-minute ride from Ramsgate marina.

August is moulting season, when seals shed their coats and grow new ones, spending much of their day on the sandbanks as a result. This makes it a good time for scientists to count them, with the Zoological Society of London (ZSL) running its three-day annual seal survey, using boats and a specially chartered light aircraft to get a view from above.