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During 2015-2016, record temperatures triggered a pan-tropical episode of coral bleaching, the third global-scale event since mass bleaching was first documented in the 1980s. Here we examine how and why the severity of recurrent major bleaching events has varied at multiple scales, using aerial and underwater surveys of Australian reefs combined with satellite-derived sea surface temperatures. The distinctive geographic footprints of recurrent bleaching on the Great Barrier Reef in 1998, 2002 and 2016 were determined by the spatial pattern of sea temperatures in each year. Water quality and fishing pressure had minimal effect on the unprecedented bleaching in 2016, suggesting that local protection of reefs affords little or no resistance to extreme heat. Similarly, past exposure to bleaching in 1998 and 2002 did not lessen the severity of bleaching in 2016. Consequently, immediate global action to curb future warming is essential to secure a future for coral reefs.

© 2016 by the author(s). Around the world, the decreasing health of coral reef ecosystems has highlighted the need to better understand the processes of reef degradation. The development of more sensitive tools, which complement traditional methods of monitoring coral reefs, may reveal earlier signs of degradation and provide an opportunity for pre-emptive responses. We identify new, sensitive metrics of ecosystem processes and benthic composition that allow us to quantify subtle, yet destabilizing, changes in the ecosystem state of an inshore coral reef on the Great Barrier Reef. Following severe climatic disturbances over the period 2011-2012, the herbivorous reef fish community of the reef did not change in terms of biomass or functional groups present. However, fish-based ecosystem processes showed marked changes, with grazing by herbivorous fishes declining by over 90%. On the benthos, algal turf lengths in the epilithic algal matrix increased more than 50% while benthic sediment loads increased 37-fold. The profound changes in processes, despite no visible change in ecosystem state, i.e., no shift to macroalgal dominance, suggest that although the reef has not undergone a visible regime-shift, the ecosystem is highly unstable, and may sit on an ecological knife-edge. Sensitive, process-based metrics of ecosystem state, such as grazing or browsing rates thus appear to be effective in detecting subtle signs of degradation and may be critical in identifying ecosystems at risk for the future.

© 2015 The Authors. Many species live in stable pairs, usually to breed and raise offspring together, but this cannot be assumed. Establishing whether pairing is based on mating, or an alternative cooperative advantage, can be difficult, especially where species show no obvious sexual dimorphism and where the act of reproduction itself is difficult to observe. In the tropical marine fishes known as rabbitfish (Siganidae), half of extant species live in socially monogamous, territorial pairs. It has been assumed that partnerships are for mating, but the reproductive mode of pairing rabbitfish is currently unconfirmed. Using passive acoustic telemetry to track movements of fishes belonging to one such species (Siganus doliatus), we provide the first evidence that paired adult fish undertake highly synchronized migrations with multiple conspecifics on a monthly cycle. All tagged individuals migrated along the same route in three consecutive months and were absent from home territories for 2–3 days just after the new moon. The timing and directionality of migrations suggest that S. doliatus may form spawning aggregations, offering the potential for exposure to multiple reproductive partners. The finding raises fundamental questions about the basis of pairing, mate choice and partnership longevity in this family.

© 2015 The Author(s) Published by the Royal Society. All rights reserved. Patterns of ecological specialization offer invaluable information about ecosystems. Yet, specialization is rarely quantified across several ecological niche axes and variables beyond the link between morphological and dietary specialization have received little attention. Here, we provide a quantitative evaluation of ecological specialization in a coral reef fish assemblage (f. Acanthuridae) along one fundamental and two realized niche axes. Specifically, we examined ecological specialization in 10 surgeonfish species with regards to morphology and two realized niche axes associated with diet and foraging microhabitat utilization using a recently developed multidimensional framework. We then investigated the potential relationships between morphological and behavioural specialization. These relationships differed markedly from the traditional ecomorphological paradigm. While morphological specialization showed no relationship with dietary specialization, it exhibited a strong relationship with foraging microhabitat specialization. However, this relationship was inverted: species with specialized morphologies were microhabitat generalists, whereas generalized morphotypes were microhabitat specialists. Interestingly, this mirrors relationships found in plant–pollinator communities and may also be applicable to other ecosystems, highlighting the potential importance of including niche axes beyond dietary specialization into ecomorphological frameworks. On coral reefs, it appears that morphotypes commonly perceived as most generalized may, in fact, be specialized in exploiting flat and easily accessible microhabitats.

Summary: The application of network theory within the field of ecology has predominantly been based around the examination of species interactions (e.g. food webs). Extension of networks to spatial ecology is currently limited, yet the approach has significant potential as a framework for analysing habitat connectivity and ecosystem stability. Examination of network structures characterizing the spatial dynamics of organisms has the potential to highlight the relative robustness or vulnerability of the ecosystem processes they support. On coral reefs, grazing by large herbivorous fishes is critical in maintaining the balance between coral and algae and is widely recognized as a key process in reef ecosystem functioning. However, the spatial dynamics exhibited by roving herbivores, and the implications of these dynamics for the overall stability and resilience of herbivory on reefs are not well understood. Here, we present a novel extension of network theory to the analysis of movements of marine herbivorous fishes within a reef system. Using an array of remote acoustic receivers, over a period of 12 months, we tracked the movements of fishes belonging to the three dominant species of roving herbivore within a particular coral reef community. Analysing these movements in terms of network graph theory, we found that individual fishes clustered their activities within small sections of the available linear reef habitat, and movements within and outside of these home ranges were made along predictable routes. All three species were heavily reliant on particular, well-connected parts of the reef (nodes), mirroring an 'ultra-small-world' network. This underlying dynamic creates vulnerability in the system, making targeted removal by fishing or the degradation of individual pockets of reef habitat more likely to result in a complete collapse in the overall process of herbivory at that particular area of reef. Our results highlight an inherent vulnerability in the movement

Niche theory predicts that coexisting species minimise competition by evolving morphological or behavioural specialisations that allow them to spread out along resource axes such as space, diet and temporal activity. These specialisations define how a species interacts with its environment and, by extension, determine its functional role. Here, we examine the feeding niche of three species of coral reef-dwelling rabbitfishes (Siganidae, Siganus). By comparing aspects of their feeding behaviour (bite location, bite rate, foraging distance) with that of representative species from two other abundant herbivorous fish families, the parrotfishes (Labridae, Scarus) and surgeonfishes (Acanthuridae, Acanthurus), we examine whether rabbitfishes have a feeding niche distinct from other members of the herbivore guild. Measurements of the penetration of the fishes' snouts and bodies into reef concavities when feeding revealed that rabbitfish fed to a greater degree from reef crevices and interstices than other herbivores. There was just a 40 % overlap in the penetration-depth niche between rabbitfish and surgeonfish and a 45 % overlap between rabbitfish and parrotfish, compared with the almost complete niche overlap (95 %) recorded for parrotfish and surgeonfish along this spatial niche axis. Aspects of the morphology of rabbitfish which may contribute to this niche segregation include a comparatively longer, narrower snout and narrower head. Our results suggest that sympatric coexistence of rabbitfish and other reef herbivores is facilitated by segregation along a spatial (and potentially dietary) axis. This segregation results in a unique functional role for rabbitfishes among roving herbivores that of "crevice-browser": a group that specifically feeds on crevice-dwelling algal or benthic organisms. This functional trait may have implications for reef ecosystem processes in terms of controlling the successional development of crevice-based algal communities, reducing their po

Despite high diversity and abundance of nominally herbivorous fishes on coral reefs, recent studies indicate that only a small subset of taxa are capable of removing dominant macroalgae once these become established. This limited functional redundancy highlights the potential vulnerability of coral reefs to disturbance and stresses the need to assess the functional role of individual species of herbivores. However, our knowledge of species-specific patterns in macroalgal consumption is limited geographically, and there is a need to determine the extent to which patterns observed in specific reefs can be generalised at larger spatial scales. In this study, video cameras were used to quantify rates of macroalgae consumption by fishes in two coral reefs located at a similar latitude in opposite sides of Australia: the Keppel Islands in the Great Barrier Reef (eastern coast) and Ningaloo Reef (western coast). The community of nominally herbivorous fish was also characterised in both systems to determine whether potential differences in the species observed feeding on macroalgae were related to spatial dissimilarities in herbivore community composition. The total number of species observed biting on the dominant brown alga Sargassum myriocystum differed dramatically among the two systems, with 23 species feeding in Ningaloo, compared with just 8 in the Keppel Islands. Strong differences were also found in the species composition and total biomass of nominally herbivorous fish, which was an order of magnitude higher in Ningaloo. However, despite such marked differences in the diversity, biomass, and community composition of resident herbivorous fishes, Sargassum consumption was dominated by only four species in both systems, with Naso unicornis and Kyphosus vaigiensis consistently emerging as dominant feeders of macroalgae.

Remote monitoring technologies are increasingly being implemented in the marine environment to better understand the movement patterns of taxa. Coral reefs are no exception. However, there is a paucity of information relating to the performance of acoustic receivers on coral reefs. Our results suggest that the detection performance of acoustic receivers may be significantly impacted by the unique nature of the reef environment. This study assessed the performance of passive acoustic receivers on a typical inner-shelf fringing reef, Orpheus Island, on the Great Barrier Reef, Australia. The detection range and diel performance variability of acoustic receivers was assessed using two parallel lines of 5 VR2W receivers spanning 125 m, deployed on the reef base and reef crest. Two 9-mm acoustic transmitters were moored at opposite ends of each receiver line. The working detection range for receivers was found to be approximately 90 m for the transmitter moored on the reef base and just 60 m for the transmitter moored on the reef crest. However, the detection range on the reef crest increased to 90 m when just the reef crest receivers were considered, highlighting importance of optimal receiver deployment. No diel patterns in receiver performance or detection capacities were detected, suggesting that no corrections are required when interpreting nocturnal versus diurnal activity patterns. We suggest that studies aiming for complete coverage of a site within a reef environment will require receivers in close (<100 m) proximity, and that the placement depth of receivers must be a major consideration, with shallow receivers exhibiting a greater detection range than those on the reef slope. Our results highlight the challenges imposed by coral reefs for acoustic telemetry and the importance of receiver placement for studies conducted within these habitats. © 2012 Springer-Verlag.

Expert opinion was canvassed to identify crucial knowledge gaps in current understanding of climate change impacts on coral reef fishes. Scientists that had published three or more papers on the effects of climate and environmental factors on reef fishes were invited to submit five questions that, if addressed, would improve our understanding of climate change effects on coral reef fishes. Thirty-three scientists provided 155 questions, and 32 scientists scored these questions in terms of: (i) identifying a knowledge gap, (ii) achievability, (iii) applicability to a broad spectrum of species and reef habitats, and (iv) priority. Forty-two per cent of the questions related to habitat associations and community dynamics of fish, reflecting the established effects and immediate concern relating to climate-induced coral loss and habitat degradation. However, there were also questions on fish demographics, physiology, behaviour and management, all of which could be potentially affected by climate change. Irrespective of their individual expertise and background, scientists scored questions from different topics similarly, suggesting limited bias and recognition of a need for greater interdisciplinary and collaborative research. Presented here are the 53 highest-scoring unique questions. These questions should act as a guide for future research, providing a basis for better assessment and management of climate change impacts on coral reefs and associated fish communities. © 2010, Published by The Company of Biologists Ltd.

Many coral reefs worldwide have undergone phase shifts to alternate, degraded assemblages because of the combined effects of overfishing, declining water quality, and the direct and indirect impacts of climate change [1–9]. Here, we experimentally manipulated the density of large herbivorous fishes to test their influence on the resilience of coral assemblages in the aftermath of regional-scale bleaching in 1998, the largest coral mortality event recorded to date. The experiment was undertaken on the Great Barrier Reef, within a no-fishing reserve where coral abundances and diversity had been sharply reduced by bleaching [10]. In control areas, where fishes were abundant, algal abundance remained low, whereas coral cover almost doubled (to 20%) over a 3 year period, primarily because of recruitment of species that had been locally extirpated by bleaching. In contrast, exclusion of large herbivorous fishes caused a dramatic explosion of macroalgae, which suppressed the fecundity, recruitment, and survival of corals. Consequently, management of fish stocks is a key component in preventing phase shifts andmanaging reef resilience. Importantly, local stewardship of fishing effort is a tractable goal for conservation of reefs, and this local action can also provide some insurance against larger-scale disturbances such as mass bleaching, which are impractical to manage directly.

Many coral reefs worldwide have undergone phase shifts to alternate, degraded assemblages because of the combined effects of overfishing, declining water quality, and the direct and indirect impacts of climate change [1–9]. Here, we experimentally manipulated the density of large herbivorous fishes to test their influence on the resilience of coral assemblages in the aftermath of regional-scale bleaching in 1998, the largest coral mortality event recorded to date. The experiment was undertaken on the Great Barrier Reef, within a no-fishing reserve where coral abundances and diversity had been sharply reduced by bleaching [10]. In control areas, where fishes were abundant, algal abundance remained low, whereas coral cover almost doubled (to 20%) over a 3 year period, primarily because of recruitment of species that had been locally extirpated by bleaching. In contrast, exclusion of large herbivorous fishes caused a dramatic explosion of macroalgae, which suppressed the fecundity, recruitment, and survival of corals. Consequently, management of fish stocks is a key component in preventing phase shifts andmanaging reef resilience. Importantly, local stewardship of fishing effort is a tractable goal for conservation of reefs, and this local action can also provide some insurance against larger-scale disturbances such as mass bleaching, which are impractical to manage directly.