Decadal consistency and change in urban reef fish assemblages after a marine heatwave

Reef fish assemblages are dynamic and heterogeneous across both temporal and spatial scales. Understanding these patterns is essential for effective management strategies that aim to protect marine communities. We investigated whether previously described cross and longshore patterns in reef fish assemblages persisted after a decade of high ocean temperature variability driven by the El Niño-Southern oscillation and a significant marine heatwave in 2011. Two nearshore marine parks near Perth, Western Australia, located 70 km apart, were sampled in 2012 and 2023 using Baited Remote Underwater stereo-Video systems. At each location, distinct inshore, midshore, and offshore reef lines were targeted across a depth gradient of 3-23 m. Inshore-to-offshore assemblage patterns described in 2012 remained temporally consistent in 2023, with each reef line representing distinct compositions at both locations. The mean relative number of species, the mean relative abundance of fish per deployment, and the mean relative abundances of most individual feeding guilds, were also similar between years. Although assemblage and feeding guild compositions differed at a few locations between years, these changes were likely driven by the localised variation in the distribution of some focal species across reef lines. Planktivores were the only feeding guild to significantly increase in abundance over time. As cumulative anthropogenic and climate pressures increase, it is likely assemblage modification will occur more often. To protect the full range of fish assemblages, it would be appropriate for future spatial management to include fully protected sanctuary zones across inshore, midshore, and offshore reef lines at both locations.