A plague of black rats is destroying the native wildlife population on Penguin Island, threatening to ruin one of WA's best nature havens and ecotourism attractions.
Thousands of the rats have inundated the small island 700m off Rockingham to feed on hundreds of eggs and chicks of its famous little penguin colony and other birds.
Penguin Island was, as ever, an idyllic escape from the heat and bustle of the mainland yesterday but the scene belied the huge environmental threat.
As families, tourists and holidaying children swam in the blue shallows and watched the penguins, birds and seals, a major offensive began to try to rid the island of its unwanted pests.
Penguin Island manager David Charles said the rats were having a devastating impact on wildlife.
"The bridled terns have had no chicks survive at all this year," he said. "From our nest boxes, we're talking about only 20 little penguin chicks that have survived.
"In previous years, it's been in the 40s and 50s."
The rat infestation is at a terrible time for WA's biggest breeding colony of little penguins, which already faces a battle for survival.
Mr Charles said the penguin population had fallen to 1200 breeding pairs from about 2000 just two years ago. Rising ocean temperatures are blamed for penguin deaths because the warmer waters drive away whitebait, their favourite food.
"We're not quite sure what we can do apart from eliminate as many of the factors that are affecting them as possible," Mr Charles said.
A dozen wildlife officers and volunteers are laying more than 270 poison bait traps over the 12.5ha island this week.
Black rats were first spotted on the island a year ago.
Mr Charles said a few rats were thought to have managed to cross the sandbar to the island at low tide or stowed away on a visiting boat.
In just one year, the number of rats has exploded to plague proportions and thousands now roam the island.
With a gestation period of just 21 days, female rats can have eight or nine litters of about six babies a year.
Mr Charles said they had designed the bait traps so only rats - and not skinks and other native wildlife - could get inside and take the poison.
Some secondary poisoning of birds that ate the dead rats was expected.
But Mr Charles said the mass rat poisoning was necessary to save the rest of Penguin Island's diverse wildlife.