Rottnest Island reefs under threat as marine species dwindle

The marine life at one of Australia’s biggest tourism drawcards is under threat, with some species numbers declining by up to 90 per cent in recent years.

New research into the health of the reefs at Rottnest Island in Western Australia has revealed a “catastrophic” decline in biodiversity.

Marine molluscs and echinoderms including sea snails, clams, starfish and sea urchins are the most under threat.

Curtin University School of Molecular and Life Sciences Professor Fred Wells, who led the research, said the west end of the island was the worst hit.

“Despite being sanctuary zones with the highest level of protection from human activities, we found that Radar Reef and Cape Vlamingh suffered a catastrophic decline in biodiversity between 2007 and 2021, likely due to exposure to the warm Leeuwin Current,” he said.

The Leeuwin Current brings water down the coast towards Perth.Credit:Bureau of Meteorology

“By contrast, the metropolitan coastline, which is not under the influence of the Leeuwin Current, was found to have well-preserved biodiversity and species richness.

“Overall, at the west end the rocky reefs are badly depleted with a decline of 90 percent or more in biodiversity and density of molluscs – the most abundant species type on the reef.”

Wells said there had been a number of marine heatwaves along the WA coast which caused abnormally high ocean temperatures, carried down on the current.

He said even tropical species, more acclimatised to warmer waters, had been affected.

“[There is a] mixture of tropical, temperate and WA endemic species. With increased sea temperatures we expected to see the proportion of tropical species increase, but this did not happen at the west end of Rottnest where all three groups declined substantially,” he said.

“These findings demonstrate that even with the high degree of protection from direct human activities, these areas are not immune to the effects of global climate change.”

The “quokka selfie” has helped turn the island into one of Australia’s biggest drawcards, including for world-famous stars like Roger Federer.Credit:Getty Images

The island is promoted as having “some of the most biodiverse marine gardens you’ll find anywhere,” with around 400 species of fish and 20 species of coral that call the island home.

Wells said addressing climate change needed to be an urgent priority to ensure the island, and other biodiverse hotspots remained into the future.

“This is not a local issue, it is a global climate change issue,” he said. “Hopefully, the reef system can bounce back if conditions improve.”

Marine heatwaves have also caused problems in other areas of the state.

All of WA’s coral reefs have bleached at least once since 2010. In 2016, during an El Nino year when there are historically warmer waters, there was a reduction in coral cover and bleaching at northern locations like Christmas Island, Ashmore and Scott Reef as well as inshore at the Kimberley.

Further south Ningaloo and Pilbara reefs, seagrass and kelp forests copped a beating in 2011, 2021 and 2022 – as the Leeuwin Current intensified and brought warm waters south.

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