Granite Island will be closed at night for two weeks, starting Monday, August 3, as two specialist pest controllers scour the park for foxes in a bid to protect the dwindling little penguin population.
At the end of June, up to nine little penguins - almost one-third of the population - had been found dead on the island as a result of a fox attack.
Two foxes were seen on the island's video camera footage used to monitor the colony, and a number of baits were laid to eliminate them.
While one bait has been taken, no fox bodies have been found, so the specialists will use firearms and night vision to ensure the threat has been eliminated.
The two experts undertaking the task have operated a lot of the pest control done by Natural Resources Management (NRM) and National Parks and Wildlife Service SA.
The operation is being run by the Department of Environment and Water (DEW), and conservation and wildlife director Lisien Loan said it would close the island to the public from 6pm to 6am Monday to Thursday for two weeks.
She said it comes after a member of the public reported a sighting of a fox on the island "in the last few days".
"We haven't seen any foxes on the cameras (recently), but they can be pretty wily," she said.
"We've said, 'Let's get these specialists in to check and make sure.
"There haven't been any more penguins disturbed or found dead."
Ms Loan said it was a number of measures being taken to protect penguins on the island from foxes.
A few weeks ago, a high-pitched sonic deterrent was put on the Causeway and activated at night to discourage them from entering the island.
DEW is also looking at logistics, including accessibility, of changing the current gate on the Causeway.
The current gate has gaps around and under it, and DEW is looking at changing it to a bigger, spring-loaded gate to be closed at night.
"We're working with DPTI (Department of Planning, Transport and Infrastructure) and the (City of Victor Harbor) council to change the gate on the Causeway," Ms Loan said.
Local National Parks and Wildlife Service SA staff are working with Flinders University to create better educational signage to reduce human impact penguins.
Ms Loan said the deaths of the penguins was "heart-wrenching" and felt for those who work to protect the colony, particularly the local tour guides who had dedicated countless hours to their cause.
"It's distressing that close to one-third of the population is gone," she said.
"I feel for the guides quite a bit - they get close and get to know the individual (penguins).
"Those people put so much time and effort into preserving the colony."
She said foxes were not the only factor in the decline of the population; seals, human impact, and climate change have also contributed.
"As oceans warm, it affects the penguins' food supply," she said.
"Where conservation activities occur and where (little penguins) live with humans, they are more likely to survive."