Thames breeding success for harbour seals revealed

Scores of harbour seal pups were born in the Thames Estuary in 2018, the first survey of its kind has revealed.

A team at the Zoological Society of London (ZSL) tallied up 138 harbour seals born in the river by analysing hundreds of photos taken during last year’s pupping season.

The experts said the results, which indicate the river is providing seals with a reliable food source, show how far restoration of the Thames has come since it was declared “biologically dead” in the 1950s.

ZSL has been conducting seal population estimates each year since 2013, with the most recent results from 2017 showing 1,104 harbour seals and 2,406 grey seals across the estuary.

The survey has found numbers of seals in the Thames are on the rise, but it was not known whether the increase was due to resident seals having pups or adults arriving from other areas where colony numbers are falling.

This prompted ZSL to undertake a breeding survey for the first time in 2018, analysing the number of pups born in the river from photos taken from a light aircraft.

This is an easier way of counting seals than in real life when they are constantly on the move, the experts said.

The Thames is home to both harbour seals and grey seals, but only harbour seals breed there.

Conservation biologist Thea Cox said: “We were thrilled to count 138 pups born in a single season.

“The seals would not be able to pup here at all without a reliable food source, so this demonstrates that the Thames ecosystem is thriving and shows just how far we have come since the river was declared biologically dead in the 1950s.”

Project manager Anna Cucknell, who leads ZSL’s Thames conservation, explained: “The restored Mother Thames – as we call her – is an essential nursery habitat and home to many animals, including more than 100 species of fish, including two species of shark, short-snouted seahorses and the critically endangered European eel.

“Incredibly, harbour seal pups can swim within hours of birth, which means they are well adapted to grow up in tidal estuaries like the Thames.

“By the time the tide comes in, they can swim away on it.

“Grey seals, on the other hand, take longer to be comfortable in the water, so breed elsewhere and come to the Thames later to feed.”

The aim is for the population survey and the breeding survey to complement each other and give researchers at ZSL a better understanding of the seals in the Thames and the reason behind their changing numbers, the experts said.