Spatial mismatch in fish and coral loss following 2016 mass coral bleaching.

Record-breaking temperatures between 2015 and 2016 led to unprecedented pan-tropical bleaching of scleractinian corals. On the Great Barrier Reef (GBR), the effects were most pronounced in the remote, northern region, where over 90% of reefs exhibited bleaching. Mass bleaching that results in widespread coral mortality represents a major disturbance event for reef organisms, including reef fishes. Using 133 replicate 1 m quadrats, we quantified short-term changes in coral communities and spatially associated reef fish assemblages, at Lizard Island, Australia, in response to the 2016 mass bleaching event. Quadrats were spatially matched, permitting repeated sampling of fish and corals in the same areas: before, during and 6 months after mass bleaching. As expected, we documented a significant decrease in live coral cover. Subsequent decreases in fish abundance were primarily driven by coral-associated damselfishes. However, these losses, were relatively minor (37% decrease), especially compared to the magnitude of Acropora loss (>95% relative decrease). Furthermore, at a local, 1 m scale, we documented a strong spatial mismatch between fish and coral loss. Post-bleaching fish losses were not highest in quadrats that experienced the greatest loss of live coral. Nor were fish losses associated with a proliferation of cyanobacteria. Several sites did, however, exhibit increases in fish abundance suggesting substantial spatial movements. These results challenge common assumptions and emphasize the need for caution when ascribing causality to observed patterns of fish loss at larger spatial scales. Our results highlight the potential for short-term resilience to climate change, in fishes, through local migration and habitat plasticity.