It says it received a credible sighting from a contractor of a stoat or weasel on the nature reserve the week before last.
However, there has not been another sighting since and there has been no signs of destruction.
Area manager Rob Stone says it is still operating tracking tunnels and traps and is preparing a plan to set up larger traps around the island.
He says it will be a long job, but the department will keep going until it feels the threat has been eliminated.
