Biologists are optimistic their 2019 seal population census in the Thames Estuary will see a growth in the population.

A team from the Zoological Society of London (ZSL) make an annual trip to the area to count the seals. The latest figures estimate there are 2,406 grey seals and 1,104 harbour seals across the estuary, a respective 19% and 14% increase since 2016. The team will release the next figures and this year's trends in September.

In a river that was declared a biologically dead open sewer in the 1950s, an increase in marine life in the estuary is a sign that the river's health is improving. Today, the Thames is a vital nursery for seals, say marine scientists.

The Thames is the second-longest river in the UK and runs from Gloucestershire in the southwest of England all the way to the North Sea in the east. The estuary, at the exit point of the river to the sea, is a sanctuary for animals from the often hostile waters of the North Sea.

"If you imagine the North Sea and it can be fairly wild and woolly out there, and actually it provides this protective environment that has a lot of species like plankton which allows there to be a lot of fish, we have over 120 fish species that live here, many of them have their young in the estuary and that allows us to have the top predators, like the seals and the sharks as well in the outer estuary," says conservationist Anna Cucknell.

The estuary is also an ideal location for young pups to learn how to swim.

"Harbour seal pups are able to swim within hours of being born, which makes sense that in the Thames Estuary, where you have these seven-metre tides, they can swim on the tide exactly as it comes up after they're born," she explains.

Grey seals, who take about six weeks to learn how to swim, also benefit from the estuary's asylum

"They're born with a sort of very white, furry coat, you might have seen, and they need to develop their adult coat before going out to sea. So, what they do is they tend to hide beyond beaches, they're born, and the adults wean them over a long period of time before they slowly are more introduced to the sea and then to fish, once they have adult coats," says Cucknell.

Despite the thriving seals on the estuary, Cucknell advises against making premature assumptions about the ecosystem of the river as a whole, although signs are promising. When the river was declared biologically dead in the 1950s it meant that the oxygen content had completely disappeared in some parts of the river. With no oxygen, these parts could not support life.

The European eel, for example once flourished in the Thames but has seen a dramatic decrease in population since the 1980s. Now, the animal is classified as "critically endangered" by the International Union for Conservation of Nature (IUCN).

Other creatures, however, including the fish population, seahorses, sharks and seals are thriving in the Thames.