Seals in London? Sightings 'quite common' say scientists Scientists have carried out the first large-scale survey of grey and harbour seals in the Thames estuary. They found there are more than 700 seals living along the river, which they say is an indication of improved water quality. The BBC's Steph McGovern says the Thames estuary is not the only area where seals can be spotted - one of London's best-known markets has a regular visitor.