Diet specificity is likely to be the key predictor of a predator's vulnerability to changing habitat and prey conditions. Understanding the degree to which predatory coral reef fishes adjust or maintain prey choice, in response to declines in coral cover and changes in prey availability, is critical for predicting how they may respond to reef habitat degradation. Here, we use stable isotope analyses to characterize the trophic structure of predator–prey interactions on coral reefs of the Keppel Island Group on the southern Great Barrier Reef, Australia. These reefs, previously typified by exceptionally high coral cover, have recently lost much of their coral cover due to coral bleaching and frequent inundation by sediment‐laden, freshwater flood plumes associated with increased rainfall patterns. Long‐term monitoring of these reefs demonstrates that, as coral cover declined, there has been a decrease in prey biomass, and a shift in dominant prey species from pelagic plankton‐feeding damselfishes to territorial benthic algal‐feeding damselfishes, resulting in differences in the principal carbon pathways in the food web. Using isotopes, we tested whether this changing prey availability could be detected in the diet of a mesopredator (coral grouper, Plectropomus maculatus). The δ13C signature in grouper tissue in the Keppel Islands shifted from a more pelagic to a more benthic signal, demonstrating a change in carbon sources aligning with the change in prey availability due to habitat degradation. Grouper with a more benthic carbon signature were also feeding at a lower trophic level, indicating a shortening in food chains. Further, we found a decline in the coral grouper population accompanying a decrease in total available prey biomass. Thus, while the ability to adapt diets could ameliorate the short‐term impacts of habitat degradation on mesopredators, long‐term effects may negatively impact mesopredator populations and alter the trophic structure of coral reef food webs.

Regime shifts between alternative stable ecosystem states are becoming commonplace due to the combined effects of local stressors and global climate change. Alternative states are characterised as substantially different in form and function to pre-disturbance states, disrupting the delivery of ecosystem services and functions. On coral reefs, regime shifts are typically characterised by a change Accepted ArticleThis article is protected by copyright. All rights reserved.in the benthic composition from coral-to macroalgal-dominance. Such fundamental shifts in the benthos are anticipated to impact associated fish communities that are reliant on the reef for food andshelter, yet there is limited understanding of how regime shifts propagate through the fish community over time, relative to initial or recovery conditions. This study addresses this knowledge gap using long-term data of coral reef regime shifts and recovery on Seychelles reefs following the 1998 mass bleaching event. It shows how trophic structure of the reef fish community becomes increasingly dissimilar between alternative reef ecosystem states (regime-shifted vs recovering) with time since disturbance. Regime-shifted reefs developed a concave trophic structure, with increased biomass in base trophic levels as herbivorous species benefitted from increased algal resources. Mid trophic level species, including specialists such as corallivores, declined with loss of coral habitat, while biomass was retained in upper trophic levels by large-bodied, generalist invertivores. Recovering reefs also experienced an initial decline in mid trophic level biomass, but moved towards a bottom-heavy pyramid shape, with a wide range of feeding groups (e.g. planktivores, corallivores, omnivores) represented at mid trophic levels. Given the importance of coral reef fishes in maintaining the ecological function of coral reef ecosystems and their associated fisheries, understanding the effects of regime shifts on these communities is essential to inform decisions that enhance ecological resilience and economic sustainability.

Climate-induced increases in micronutrient availability for coral reef fisheriesHighlights d Coral reef fishes are important sources of essential dietary nutrients d Nutrients available to fisheries increased after mass coral bleaching d Iron and zinc were higher in reef fishes caught on macroalgal habitats d Coral reefs can remain key sources of nutritious food despite climate impacts

Predator populations are in decline globally. Exploitation, as well as habitat degradation and associated changes in prey availability are key drivers of this process of trophic downgrading. In the short term, longevity and dietary adaptability of large‐bodied consumers can mask potential sublethal effects of a changing prey base, producing a delayed effect that may be difficult to detect. In coral reef ecosystems, regime shifts from coral‐ to algae‐dominated states caused by coral bleaching significantly alter the assemblage of small‐bodied reef fish associated with a reef. The effects of this changing prey community on reef‐associated mesopredators remains poorly understood. This study found that the total diversity, abundance and biomass of piscivorous mesopredators was lower on regime‐shifted reefs than recovering reefs, 16 years after the 1998 mass coral bleaching event. We used stable isotope analyses to test for habitat‐driven changes in the trophic niche occupied by a key piscivorous fishery target species on reefs that had regime‐shifted or recovered following climatic disturbance. Using morphometric indices, histology, and lipid analyses, we also investigated whether there were sublethal costs for fish on regime‐shifted reefs. Stable isotopes demonstrated that fish from regime‐shifted reefs fed further down the food chain, compared to recovering reefs. Lower densities of hepatocyte vacuoles in fish from regime‐shifted reefs, and reduced lipid concentrations in spawning females from these reefs, indicated a reduction in energy stores, constituting a sublethal and potential delayed effect on populations. Reduced energy reserves in mesopredators could lead to energy allocation trade‐offs, and decreased growth rates, fecundity and survivorship, resulting in potential population declines in the longer term. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13012/suppinfo is available for this article.

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Ecosystems are becoming vastly modified through disturbance. In coral reef 23 ecosystems, the differential susceptibility of coral taxa to climate-driven bleaching is predicted 24 to shift coral assemblages towards reefs with an increased relative abundance of taxa with high 25 thermal tolerance. Many thermally tolerant coral species are characterised by low structural complexity, with reduced habitat niche space for the small-bodied coral reef fishes on which piscivorous mesopredators feed. This study used a patch reef array to investigate the potential impacts of climate-driven shifts in coral assemblages on the trophodynamics of reef mesopredators and their prey communities. The 'tolerant' reef treatment consisted only of coral taxa of low susceptibility to bleaching, while 'vulnerable' reefs included species of moderate to high thermal vulnerability. 'Vulnerable' reefs had higher structural complexity, and the fish assemblages that established on these reefs over 18 months had higher species diversity, abundance and biomass than those on 'tolerant' reefs. Fish assemblages on 'tolerant' reefs were also more strongly influenced by the introduction of a mesopredator (Cephalopholis boenak).Mesopredators on 'tolerant' reefs had lower lipid content in their muscle tissue by the end of the six-week experiment. Such sublethal energetic costs can compromise growth, fecundity and survivorship, resulting in unexpected population declines in long-lived mesopredators.This study provides valuable insight into the altered trophodynamics of future coral reef ecosystems, highlighting the potential increased vulnerability of reef fish assemblages to predation as reef structure declines, and the cost of changing prey availability on mesopredator condition. 42

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