Mass mortality of sea urchins in strandings on Sydney beaches

ABSTRACT Echinoderms are highly sensitive to decreased salinity and are vulnerable to mass stranding caused by flood-driven freshwater runoff and high wave conditions from storms. The coast of New South Wales is prone to east coast low (ECL) weather systems that produce conditions that combine storm surge with intense rainfall, which reduces the salinity of sea surface waters. We documented mass mortality and strandings of locally abundant sea urchins, Centrostephanus rodgersii, Heliocidaris erythrogramma and Holopneustes purpurascens, along with other biota in 2016, 2020 and 2022. These events were likely caused by storms. The 2022 storm resulted in freshening of sea urchin habitat (∼22 PSU). Sea urchins play key ecological roles in benthic communities, with C. rodgersii being the major species in the region. Sea urchins also support a commercial fishery. Understanding the impacts of storm-driven mortality may assist in predicting ecological and economic consequences, particularly as storms are becoming more severe due to climate change.