Conservation teams are scouring the Thames Estuary by land, sea and air to track the fortunes of the area's seals.
Scientists are using aerial surveys of the Essex and Kent coastlines, as well as boat and land-based assessments, to count seal numbers.
The Zoological Society of London's (ZSL) fourth annual seal survey will record numbers of harbour and grey seals, revealing any changes to harbour seal populations, and how they are doing in relation to the larger and more dominant greys.
Grey seals have increased in number in the past 15 years, particularly along the east coast of England, and compete with the harbour seals for food and territory.
Last year the survey counted 451 harbour seals and 454 grey seals in the Thames Estuary, and this year's count will provide an update on figures and inform conservation and management of the marine mammals in the region.
The experts will also look out for emerging health issues as they conduct the survey, which coincides with the annual August moult when harbour seals shuffle on to sandbanks to shed their coat and grow a new layer of fur for the winter.
Experts say the Thames, with its proximity to mainland Europe, could serve as an entry point for disease outbreaks which affect seals.
The last outbreak of a virus known as phocine distemper, which has killed thousands of seals in previous outbreaks in Europe, happened 14 years ago, in 2002, and scientists fear it could return in 2016.
ZSL's European conservation projects manager, Joanna Barker, said: "This is the fourth year in a row that we've conducted a comprehensive survey of seal populations in the Thames.
"But from a conservation point of view it's one of the most important surveys yet.
"It's a really interesting time to study seals in the UK.
"Firstly, grey seal numbers have rapidly increased over the last 15 years, especially on the east coast of England, which is good news for the species.
"We believe this has also led to a greater amount of competition between grey and harbour seals, however, both for food and for sites where they can 'haul-out' or leave the water.
"In addition, new behaviour of grey seals predating upon harbour seals has recently been observed in other European seal colonies.
"As the large intertidal sandbanks in the Thames are preferred harbour seal habitat, we are interested to see what impact increased competition could have for the species."
And with the possibility of disease, the combination of various threats could amount to "a perfect storm" for the harbour seal population of the Thames, making the 2016 survey potentially the most important one yet, she said.