Matched surveys showed no clear mean rise in fish community similarity in nine Australian reef regions from 2008–2012 to 2017–2021

We tested whether reef fish communities became more similar across fixed Australian sites. We used IMOS National Reef Monitoring Network visual surveys from 2008–2012 and 2017–2021. We matched one survey per site and period by programme, season and depth. The main panel had 243 sites in nine marine regions and 1,480 fixed spatial pairs, each 25–250 km apart. Both surveys needed at least 50 identified fish. We drew 2,000 samples of 50 fish without replacement from each recorded survey. Mean Sørensen dissimilarity fell from 0.710 to 0.699. The equal-region mean change was −0.0107, with a nominal 95% t interval of −0.0488 to +0.0273 across nine region changes. Lower dissimilarity means greater similarity. Six region point estimates decreased and three increased. Full species lists, other sample sizes, stricter matching and other spatial scales also left the mean change uncertain. An independent source check reproduced 11,162 selected count cells and all survey choices. A later check removed two sites with repeated source cells and gave a mean change of −0.0111. A clean offline repeat reproduced all 16 saved result tables and numerical objects. This result does not establish stable communities or an absence of local change. The interval assumes independent, exchangeable region changes. Other limits include selected shallow sites, one transect per site and period, unknown diver identity, and imperfect detection. The package contains exact source files, hashes, provenance, code, all results, figures and repeat instructions.

reef fish · biodiversity · spatial similarity · rarefaction · fixed sites · IMOS · Reef Life Survey · marine monitoring