The charity says more and more of the seals are travelling further up the river, with one spotted as far west as Windsor.
It is thought there are about 3,500 seals living in the Thames and that they mostly stay around the estuary, east of London.
Marmalade was considered rare because there are only 500 harbour seals living in the river, compared with about 3,000 of the more common grey seals.
Gareth Richards from UK Seal Alliance says “seals are very transient, and will basically go to where there is good fish stocks”.
He says “seals can swim from anything between 60 and 80 miles a day”.
Because they are being spotted more frequently, Richards says he would like to see further protection of the animals in law.
Neither grey nor harbour seals are protected from intentional or reckless disturbance under the Wildlife and Countryside Act 1981.
In May, UK Seal Alliance launched its Give Seals Space, external campaign, calling on the government to reconsider previous recommendations and provide the two species “with the legal protection they deserve”.
The campaign warns that they are highly sensitive to disturbance from the public, animals and drones.
It states these can have long-lasting consequences to the animals’ health and that it can lead to pup abandonment, wasted energy from staying alert and injuries from panic fleeing.
The Department for Environment, Food and Rural Affairs (Defra) says: “We have a suite of legislative protections in place which make it an offence to take, injure, capture or kill a seal.”
