The largest sea inlet anywhere in the UK or Ireland is proving the perfect nursery for the species, with 181 pups born in its waters this autumn – the most since numbers were first counted in 1992.
There were also 282 adults recorded in the lough at the peak of the breeding season in the autumn.
It is a remarkable turnaround for the species, given that back in the early 1990s, when the National Trust started proactively recording the population, there were only around six or seven grey seal pups born each year.
Hugh Thurgate, the National Trust’s lead ranger on Strangford Lough, said several factors have contributed to the growth.
He said the lough is benefiting from significant population increases in Scottish waters, with seals being forced to seek out new places to breed.
He explained that the shelter of Strangford is a strong draw for seals on the look-out for alternative spots to “haul out” on shore during the breeding and moulting seasons.
He said: “From a security point of view and favourable habitat, it’s perfect for them.
“The seals are looking for a safe and protected place to raise their young – somewhere that provides protection from the elements and is relatively free from disturbance – and Strangford Lough offers them that.
“Pups are quite vulnerable. Seals give birth on the beach and if you get stormy weather and tidal surges, high spring tides, the pups aren’t capable of thermo-regulating properly when they are young and you get high pup fatalities because of weather.
“So if you’ve got calmer conditions it’s going to give your pups a better chance of survival.”
The mid lough islands of Strangford are a favourite spot for the seals to haul out, with large groups, known as rookeries, now a regular sight for boat users who know where to look.
Mr Thurgate, who has worked on Strangford for two decades, has seen the population grow from a low base.
He said: “In the early 90s, you would have been lucky to get six or seven pups.
“We started counting seals in 1992. So in 93, 94, 95 you were getting six or seven, maybe 11 – so very low numbers of grey seal pups.
“Then slowly but surely they increased and then by the mid-noughties they were into the 40s.
“And then it jumped quite significantly and by 2015 we got 100 pups for the first time. And then in 2017 they went past 150 – there were 172 that year.
“So they went from just over 100 to 172 pretty rapidly, and this year was an all-time high at 181.”
But it is not all good news for Strangford seals. While the greys are thriving, another species – the common/harbour seal – is struggling.
It used to be the more prevalent species in the lough, with an estimated population of around 850 in the mid-1980s.
But the common seal numbers were more than halved by a devastating distemper virus that swept through Europe around that time and the species has never really re-established itself in Strangford.
This year the total population was below 100, with an average of 25-30 pups born annually.
Mr Thurgate counts himself as very fortunate to have the job of monitoring the Strangford seals.
He said: “If you like the outdoor life it’s a fantastic job and I know I’m very lucky.
“If you look at my job description there’s lots of people who would like my job, but I’ll hang on to it for the time being.”
The grey seal baby boom is recorded in the National Trust’s review of 2019 for Northern Ireland – a year that saw several other types of wildlife thrive in Strangford Lough and the Ards Peninsula.
Populations of terns, brent geese and otters also showed an increase.
On dry land it was another good year for the resurgent red squirrels, whose population is now up to around 120 as a result of efforts by conservationists to keep the dominate greys out of the peninsula.
Warm spells of weather in the early half of the year saw migrant species of butterflies set course for Northern Ireland, with significant numbers of Painted Lady butterflies arriving en masse for the first time in a decade.
This was mirrored by a drop in sightings of native species, such as the common white, which are struggling as a consequence of intensive land use, habitat fragmentation and climate change.