Rising seal numbers in Thames estuary hide triple threat to populations

Scientists conducting annual count of grey and common seals warn of potential impact of dredging, a deadly virus and predation between species

Scores of seals loll on the riverbank of the Stour, snorting and bellyflopping as they sun themselves a couple of miles outside Ramsgate’s busy marina.

Far from exceptional, these are just a smattering of the hundreds of seals that the Zoological Society of London (ZSL) hopes to count this week in the Greater Thames estuary. The mammals are sighted as far up as Teddington Lock, and Canary Wharf is a hotspot for seeing them.

A total of 905 common and grey seals were found along the estuary last year, up from 708 during the first survey in 2013, counted from plane, boat and on foot. ZSL’s spotters take advantage of the seals’ moulting season in August, when they shuffle up sandbanks to shed their coat and grow a new one, making double-counting less likely.

But while experts have celebrated the rise of the seals along the Thames, there are fears this year of a triple threat to these marine predators.

Several miles south-east off the coast from Ramsgate is Goodwin Sands, a sort of “spaghetti junction” for seals to haul out after travelling from Europe. It is here that the port of Dover hopes to dredge for sand to expand its operations, a move opposed by many local people and environmentalists.

Joanna Barker, ZSL’s conservation scientist, said she was concerned at the potential impact, after 75% of last year’s grey seals were counted there. “I think, and many other scientists think, it could be quite an important meeting point for seals coming from mainland Europe because it’s one of most eastern sandbanks of its type.

“We are a bit worried about the impact on seals. We would like to see the dredging – if it does take place – it doesn’t take place in important times in seal biology. We wouldn’t want to see dredging in the pupping [breeding] times.”

A petition has attracted nearly 10,000 signatures and last month actor Mark Rylance backed a campaign against the dredging of the sandbanks, which have been proposed for designation as a marine conservation zone.

A total of 905 common and grey species were counted along the estuary last year from Deal in Kent to in Lowestoft in Suffolk.
A total of 905 common and grey species were counted along the estuary last year from Deal in Kent to in Lowestoft in Suffolk. Photograph: Gareth Fuller/PA

There are also concerns over a repeat of an outbreak of a deadly virus which wiped out thousands of seals in 1988 and 2002. Phocene distemper virus has been described by government scientists as one of the largest and most high-profile animal disease epidemics to have hit indigenous British wildlife.

Barker said the apparent 14-year cycle suggested it could emerge again in 2016, though ZSL’s fourth count so far has found no evidence yet in the estuary. One sign of human influence is the occasional “rusty seal” among the common seals here – iron oxide in the water has turned their fur orange.

Close to Ramsgate, these common seals are unfazed by daytrippers cruising by on boats, while those out at more remote sandbanks would take flight if humans approached. However, the common seals may have more to worry about from the much larger greys.

Weighing up to 300kg, there is an increasing body of evidence that grey seals are spurning their usual diet of 8kg of fish a day and instead preying on both common seals and harbour porpoises.

Video captured last year appeared to show a grey seal eating a harbour porpoise off the coast of Pembrokeshire, while one study laid the blame for a series of mystery porpoise deaths off the Dutch coast on greys. Researchers last year published photographs of a grey killing a common seal in 2013.

Scientists are concerned grey seals are preying on common seals.
Scientists are concerned grey seals are preying on common seals. Photograph: Gareth Fuller/PA

Greys also eat so much that they could be taking common seals’ dinner. “Grey seals are much bigger than harbour seals, they are really ferocious predators. They could outcompete the harbour seal due to their size at haul-out sites but also by eating a lot of their prey,” said Barker.

While the ZSL surveys have found common seal populations in the area to be relatively stable, the number of greys has increased dramatically, she said. The 2015 survey recorded 451 common seals and 454 grey seals in the estuary, versus around 500 of the former and 200 of the latter in 2013.

The results of the 2016 survey will be out in a few weeks, but in the meantime there is the practical work of counting them. Earlier in the morning, Barker flew around the estuary, tallying them on paper from the air, and taking photos to double check later with colleagues.

On the Stour, she tots up 72 common and two greys, before passing a grey chewing on a huge fish in the marina as her afternoon’s seal spotting draws to a close.


Adam Vaughan

The GuardianTramp

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