Mice Check Out, Rats Move in on Great Barrier Reef Island

Rats have quickly established at an island recently declared pest-free after extensive rodent eradication efforts. 

A Great Barrier Reef island declared pest-free just months ago has succumbed to an infestation of black rats. Rangers are said to be devastated that a full-scale eradication plan may need to be enacted so soon after recent efforts.

North West Island, off the central Queensland coast, is a pristine coral cay that is the largest breeding island for wedge-tailed shearwaters and black noddies on Australia’s east coast.

Surveys conducted between 2013 and 2016 indicated a decline in wedge-tailed shearwaters of approximately 40% as mice had been preying on seabirds, including their eggs and hatchlings. Mice were also a threat to turtle hatchlings on the island.

In 2020 utilising Reef Trust indirect offset funding through the Reef Joint Field Management Program, rangers used a helicopter and a bait dispenser to drop rodent bait across the island. In August 2022 the common house mouse was officially eradicated from the island after the extensive effort that took two years to fully execute.

It was with deep disappointment that assistant principal ranger Damon Shearer confirmed that black rats had turned up on the island sometime between August and December 2022.

Following a possible rat sighting that was reported to the Queensland Parks and Wildlife Service on December 2, rangers subsequently installed 21 ink track tunnels, two trail cameras, and 60 baited rodent stations. When they were inspected a week later on December 9, ten of the 21 tunnels showed rodent tracks, and the trail cameras also detected rats (pictured above).

So how did the rats end up there? Griffith University ecologist Guy Castley said that although rats can swim considerable distances, the most likely scenario was that the rats arrived by boat, perhaps on one of the barges or privately owned boats that had visited the island.

He added that rangers have grave concerns for the native seabirds on the island as ”rats potentially pose more of a risk because they’re bigger than mice [and] they breed faster than mice.”

A full control program is underway to understand the size of the current rat population. Extra rangers have been tasked with monitoring, adding more bait stations, tracking, and actively trapping the rodents.

Mr Shearer emphasised the exhaustive effort that had gone into declaring the island pest-free. The arrival of black rats posed a potentially greater problem than the prior house mouse infestation.

“It’s a lot of expense – a lot of time went into the planning of the eradication,” he said. “Having to repeat a full-scale eradication, including helicopter aerial baiting, would be a huge blow to rangers.“

Government agencies are urging visitors to rigorously check clothing, footwear, and gear before arriving on the island, stressing that the public’s help will be vital in minimising the impact of rodents on the island.