The Thames’s healthy seal population is proof that the river is teeming with life, according to conservationists.
Despite a decline in numbers over the past two years, researchers from the Zoological Society of London (ZSL) found that the river was home to around 2,866 gray seals and 797 common seals after the most recent puppy season.
The seal population has been counted every year since 2013 – except for 2020 – due to the coronavirus pandemic.
Marine biologists use the mammals as a barometer of river health, with stable numbers indicating good water quality and reliable fish stocks.
Despite being a protected species, they are exposed to a range of threats including disease, marine litter, getting caught in abandoned gear – and being caught up in shipping.
Seals are also prone to disturbance when they have their pups, especially from curious members of the public, free-range dogs, and others who use the water, such as canoeists and kayakers.
Meanwhile, in 2002, the population was hit hard by an outbreak of distemper virus, which causes a range of symptoms including rash, conjunctivitis and, often fatal, pneumonia.
To monitor their population, the researchers compared the seals from photos taken by a light aircraft at various fishing sites in the Thames estuary over a period of three days.
These numbers are then counted before adjusting the final number to account for the fact that there will always be a line of seals at sea.
Despite this drop in numbers since 2019, the researchers said this wasn’t definitive evidence that the two seal populations were facing trouble.
“It’s the long-term picture that is most telling, and that’s why it’s important to conduct these surveys regularly,” conservation biologist Thea Cox told PA news agency.
“Changes in numbers may be due to a number of factors, such as the variability in the proportion of seals at sea during the survey, but we want to keep a close eye on that.
“As top predators, (seals) are a great indicator of environmental health, so tell us how the Thames is doing.”
The seals in bloom show how far the Thames’s recovery has come since it was declared “biologically dead” in the 1950s.
“People think the Thames is dead because it is brown, but the Thames is full of life – the water quality has improved so much,” added Ms. Cox.
“Seals have always been here, but the dates didn’t necessarily exist so we wanted to understand how they use the Thames, where they go, and their numbers.
“At the national level, some seal populations are increasing and some are decreasing, and we don’t necessarily know the exact cause.
“It could have to do with increased puppy play, it could have to do with animals moving along the coast across the country – a question we would like to answer.”
The competition for space between gray seals and harbor seals for space has resulted in attacks by gray seals on seals, Ms. Cox said.
Near one of the most popular seal fishing spots near Ramsgate, Kent, there is a lot of plastic litter visible along the coast, but it’s not clear how great the threat to the animals is.
“There is definitely some litter and there have been studies to show how far we are seeing this in marine mammals,” Ms. Cox said.
“But knowing the impact of this and the impact on the population level – we don’t know that either, but there are a lot of studies looking at it to find out what the threat might be.
“Entanglement (in fishing nets) is a threat to the seal population, it is less of a problem here in the Thames, but it is definitely a threat to the seal population in general.
“We are still trying to develop the evidence of what these effects on animals really are.”
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