IT is one of Scotland’s most mysterious islands, a wildlife haven that has become known as “seal city”.

But today, more than a century since its last human residents left, experts are struggling to establish exactly how many of the marine mammals are living on Mingulay in the Outer Hebrides.

Now drones are being trialled to obtain a more accurate count of the island’s grey seals.

Greys are the larger of the two Scottish species, growing to almost 7ft long and weighing more than 660lbs.

Approximately 38 per cent of the world’s grey seals breed in the UK and 88% of these are at colonies in Scotland, with the main concentrations in the Outer Hebrides and Orkney.

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Aerial surveys to estimate pup births were carried out in Scotland in 2016 using a digital camera system, but the numbers were based on wide averages.

It is hoped drones will give a more accurate count of important local colonies, such as Mingulay, which is owned by the National Trust for Scotland (NTS).

Experts calculated previously that there were 23,800 grey seal pups in the Outer Hebrides, with estimates ranging from 20,700 to 27,550.

Dr Richard Luxmore, the NTS’s senior nature conservation adviser, said drones may help to provide a better picture of the true population.

“It is almost impossible to count seals individually in large congregations,” he said. “It is difficult to separate them looking horizontally at them using binoculars. They are just a black mass.

“We have done some preliminary work on Mingulay using drones and we are looking at using them to get a better idea of the population there.”

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Mingulay is the second largest of the Bishop’s Isles, which also comprise Berneray and Pabbay. After 2,000 years or more of continuous habitation, the island was abandoned by its last few Gaelic-speaking residents in 1912.

Grey seal numbers, meanwhile, have grown substantially.

The animals congregate above the high water mark on remote beaches and islands between September and December to pup.

Harbour seals, however, tend to pup on intertidal sandbanks or rocks in June and July.

The greys are also famous for their haunting howl. Harbour seals are silent in comparison. At this time of year they are also wary of predators, such as killer whales.

Man-made pollution, fishing nets and shooting at salmon rivers or at fish farms are also threats.

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The Special Committee on Seals’ last report assessed the UK grey seal population at about 150,000 – based on estimates ranging between 131,000 and 171,600 – with harbour seals about 45,100.

“Overall, the UK population has increased since the late 2000s and is close to the 1990s level,” says the report.

“However, there are significant differences in the population dynamics between regions, with general declines in counts of harbour seals in several regions around Scotland. But the declines are not universal, with some populations either stable or increasing.”

Grey seals are long-lived animals. Males may survive for more than 20 years, while females often live for more than 30.

They are generalist feeders, foraging mainly on the sea bed at depths of up to 330ft, although they are probably capable of feeding at all the depths found across the UK continental shelf.

They take a wide variety of prey species including sandeels, cod, whiting, haddock, ling and flatfish.

Grey seals forage in the open sea and return regularly to land where they moult and breed.