After the 2019 coral loss, Moorea fore-reef parrotfish biomass increased 1.1–1.4 times, not 4–8 times as after the 2007–2010 loss; the local coral link did not change detectably (6 sites, 2006–2025)

Question: after a second large coral loss on the Moorea fore reef (French Polynesia), did herbivorous fishes increase again, as they did after the 2007–2010 loss? Data: Moorea Coral Reef LTER annual fish counts (6 sites x 3 habitats x 4 transects, 2006–2025) and benthic cover (2005–2025). Event 1 was a crown-of-thorns outbreak and a cyclone. Event 2 was the 2019 heatwave. In both events, mean coral cover at 10 m fell to approximately 10%. I compared six post-event surveys with two pre-event surveys. On transects that the same two divers counted in all years, parrotfish biomass increased 7.5 times (95% CI 3.8–16.0) in event 1 and 1.13 times (0.91–1.47) in event 2. Parrotfish density increased 5.8 times and 1.03 times. All six sites had a smaller response in event 2 (exact sign-flip p = 0.031). Two other observer-aware estimators and 12 window choices gave the same result (event 1: 3.1–8.1 times; event 2: 1.1–1.6 times). After event 2, absolute parrotfish biomass was also lower, although it started higher. Fore-reef macroalgae cover was 8.9% after event 1 and 18.9% after event 2, at similar coral cover. Corallivores decreased by about two thirds in both events. But the event-1 increase was not only on the fore reef. Lagoon parrotfish also increased 2.3–3.4 times, while lagoon coral also decreased. In event 2 the lagoon did not change. Relative to the backreef, the fore-reef response was not significantly different between events. A panel model with site-habitat and year effects found 12.5% less parrotfish biomass for each 10 points of local coral cover. Its change after 2016 was +0.068 per 10 points (CI -0.045 to +0.199). Thus the local link did not change detectably, but the CI is wide. An observer audit found that one transect had new divers at the start of the event-1 window and at the end of the event-2 window. These divers recorded 3.6–4.7 times more fore-reef parrotfish. Without adjustment, this artefact makes the two biomass responses look equal. Limitations: six sites, observational data, two events of different type, primary estimator chosen after the audit. The package has raw data, code and all results.

coral reef · herbivory · parrotfish · Moorea · MCR LTER · coral bleaching · crown-of-thorns · macroalgae · resilience · observer effect · time series · replication