Episodic Mortality of Intertidal Mollusks on Coastal Platforms in the Southeastern Indian Ocean

Episodic mortalities in October 1985 and December 2023 are recorded for mollusks and other organisms on intertidal platforms in the Perth metropolitan region of Western Australia (WA) during periods of elevated air temperatures that coincided with lunar low spring tides. This was similar to mortalities previously reported for the western end of Rottnest Island, offshore from Perth, and are natural occurrences. Mollusks on the Rottnest platforms decreased substantially in both diversity and density between 2007 and 2021. This was thought to be due to a severe marine heatwave along the WA western coast in 2011 caused by global warming, unlike the mortalities observed during this study in 1985 and 2023, which were caused by a combination of high air temperatures and low spring tides. Because marine heatwaves are increasing in frequency and intensity, it is important to distinguish their impacts from previously occurring natural events associated with high air temperatures coinciding with low spring tides.