Mass changes within taxa added 10.3 g to mean recorded fish mass at six Moorea fore-reef sites between 2006–2010 and 2021–2025

We tested whether fish counts, total estimated mass and mean estimated mass showed the same change at six Moorea fore-reef sites. We compared 2006–2010 with 2021–2025, using two fixed 250 m² transects per site and the same main observers. We split the change in mean mass into exact terms for species shares, mass within shared taxa, and taxa recorded in only one period. Mass changes within shared taxa added 10.3 g per fish. This term was positive at all six sites. Its nominal 95% interval was 6.9 to 13.9 g when we sampled three shores and shared blocks of two years. Changes in shares added 0.8 g. Taxa recorded only late added 8.9 g, while taxa recorded only early removed 11.8 g. Total mean mass change was +8.2 g, with an interval of −7.5 to +28.2 g. The valid-size count fell by 38.7%, while total estimated biomass change was uncertain. The within-taxon term stayed positive with other windows, either main observer, missing-size imputation and a late window that ended before the 2022 protocol change. An independent CSV calculation reproduced all primary site values. This is accounting of visual survey records. It does not measure growth of the same fish or identify an environmental cause. The package preserves unchanged source data, provenance, code, all results and figures.

coral reef · fish body size · Moorea · species composition · biomass · long-term survey · reproducible analysis