The species, which was eradicated from the island in 2013 to protect native red squirrels, are now being spotted in increasing numbers, prompting conservationists to fear for the future of the largest red squirrel population in Wales.
Since February 2024, more than 40 grey squirrels have been caught in the south of the island with others currently remaining at large.
The public have also submitted videos of animals from as far north as Amlwch and Llanfwrog near Holyhead indicating the species is becoming widespread.
“Despite concerted efforts to detect and quickly remove animals, sadly we are now frequently discovering grey squirrels breeding once again on Anglesey,” said Dr Craig Shuttleworth, a research scientist at Bangor University.
The grey squirrel originates from North America and it competes with the much smaller native red squirrels for food and space.
Grey squirrels carry the Squirrel pox virus which doesn’t harm them but is deadly to red squirrels and can rapidly kill large numbers.
Volunteers and conservation teams are working hard to catch grey squirrels on Anglesey as quickly as possible.
They are urging the general public to be vigilant and report any sightings via the Red Squirrels Trust Wales website.
“We would like to thank local communities and wildlife enthusiasts who have already reported grey squirrel sightings from woodlands and urban parks” said Robert Macaulay, of Red Squirrels Trust Wales.
“Together, we must do everything possible to prevent the loss of red squirrels from Anglesey.”