Recent storms appear to have badly hit a grey seal colony on the Farne Islands off the Northumberland coast.
Many newborn seal pups have died in the storms
Naturalist Simon King, who presents the BBC's Autumnwatch, said bad weather and high tides had "greatly affected" the colony - one of the largest in the UK.
He said a survey was being carried out, but that it appeared a significant number of newborn seals had perished.
Storms have also forced thousands of Arctic seabirds, called little auks, to the islands over the past week.
Grey seals have made their home on the Farnes for the last 800 years.
Mr King said: "The grey seals have been severely affected by the storms we have experienced over the past few days.
"Early signs are pretty grave. There is no doubt the colony has been severely affected by the great surge of waves that washed over the islands a few days ago.
"Each year you would expect about 1,200 pups to be born on the islands. Naturally you would expect about 50% of them would die. But this year it looks as though the death toll has been very dramatic indeed.
"We have yet to get a complete picture simply because the weather has been so bad."
Meanwhile more than 50,000 little auks, which are more used to the Arctic Circle, have been spotted off the Farnes.
The Royal Society for the Protection of Birds (RSPB) and National Trust put the influx down to recent stormy weather.
The small black and white bird is a relative of the puffin, but is only around the size of a starling and is normally found in Greenland.