They are one of the worlds rarest seal species, officially endangered by a combination of water pollution, fishing nets and seaside tourism. But on the Farne Islands, off the Northumberland coast, the population of grey seal pups has increased by 57% in five years, with 2,737 born last year.

Rangers from the National Trust, which owns the islands, were predicting another bumper year as they began their pup count this week, taking aerial shots by drone and spraying newborns with special paint to keep tabs on their movements.

They use four colours in rotation to work out how many are born and how many die over the six-week pupping season. Natural selection means up to 50% of baby seals do not make it past their first year, with 30% dying in their first month, says Tom Hendry, who for the past four years has been one of the small group of rangers who are the only humans allowed to live on the islands.

He pointed to a forlorn bundle of white fur lying in the grass near the boat landing on Brownsman, one of 20 Farne islands visible at low tide. The pup was born at the weekend, but had not survived the bitter northerly winds and icy rain. It can be quite sad when you see the mum tapping the baby with their flipper to try to rouse them, but so many die that you do become a bit desensitised to it, he said.

Seal pups are not waterproof. It takes three weeks until their birth fluff is replaced with water-resistant fur and they have drunk enough fatty milk to develop a thick layer of insulating blubber.

Counting the pups can be tricky, says Hendry. Despite their image as the friendly teddy bears of the sea, seals can be quite aggressive, especially the males, known as bulls, who can be 2.7 metres long and weigh up to 233kg. Seals can move quite fast, particularly the bulls. Its all about not getting hemmed in between them and the sea, he said.

The bulls fight over territory and cows (females), competing to get a whole harem pregnant, says Hendry. They even have a macho name: beachmasters. The rangers try to spray the pups when the beachmasters are off hunting, or at least looking the other way.

They work as a team of five, two armed with a spray bottle of the sort a gardener might use to spritz roses, two creating a distraction, and another keeping watch. I do like seals, but by the end of the count I do tend to have developed a love/hate relationship with them, says Hendry, watching as a mother tries to get her newborn to understand the concept of suckling 20 metres further down the beach.

At least two rangers live on Brownsman for nine months of the year, sharing a rundown cottage with no running water. Usually they do five-day stints and then go back to the mainland for a shower, but on Wednesday Hendry was 10 days in and very ready for a wash that did not just involve baby wipes.

In the early 19th century, the cottage was the childhood home of Grace Darling, who became a national hero after helping to rescue survivors from the shipwrecked Forfarshire in 1838. Little seems to have changed since her day, the floors still primitive stone slabs and the grubby sash windows still single-glazed, rattling in the wind. The rangers have a TV to pass the time and play board games; this years favourite has been a card game called Unstable Unicorns, where players compete to build a unicorn army while betraying their friends.

The rangers do not know for sure why the grey seal population has grown so quickly in recent years, with an estimated 300,000 now living worldwide. The main theory is that they have plenty of food and a lack of predators, Hendry said. In the North Sea they have not had an enemy since orcas stopped patrolling the waters in the 1970s and are omnivorous, eating pretty much anything that swims or scampers on the seabed.

While celebrating the grey seal baby boom, the National Trust has begun to worry that these large mammals may come to over-dominate the fragile ecosystem of the Farne Islands, which is also a breeding ground for puffins, Arctic terns, shags and many other birds. An electric fence has been put up around one area of Brownsman to protect the flora and fauna, and to stop the lumbering seals flopping over the puffin burrows, destroying their summer homes.

But for now, they are enjoying the renaissance. Normally, National Trust stories are doom and gloom, about what climate change is doing to the natural world, said the trusts press officer, Jessica Harrison-Thomas, cooing over a newborn pup trying to work out how to use his flippers. This is lovely.

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Seal of approval: Farne Islands population boom gathers pace - The Guardian