South Australia’s 2025–26 harmful algal bloom (HAB), dominated by Karenia spp., has had devastating and highly visible impacts on marine ecosystems, including widespread fish and invertebrate beach washups. To quantify effects on nearshore fish communities, the South Australian Department for Environment and Water conducted baited remote underwater video system (BRUVS) surveys across Gulf St Vincent and Spencer Gulf at sites exposed to varying bloom intensities and durations as part of the HAB impact assessment on nearshore marine biodiversity project. These data were compared with baseline surveys spanning the last decade at some sites, enabling robust assessment of temporal change.
We analysed shifts in species richness, abundance, and community composition, with a focus on site-specific responses and variation in functional and trophic group structure. Preliminary results indicate substantial declines in overall abundance and diversity at sites exposed to prolonged, high-intensity Karenia blooms, alongside marked changes in trophic structure, including reductions in higher-order predators and increases in opportunistic and scavenging taxa. In contrast, sites with lower exposure exhibited more moderate changes.
Our findings demonstrate that HAB intensity and duration are likely key drivers of ecological impact. This study highlights the value of long-term BRUVS datasets for detecting ecosystem impacts and provides critical insights into the resilience and recovery trajectories of temperate reef fish communities under increasing climate and HAB pressure.