Rowley Shoals Research Articles

Spatially varying selection between habitats drives physiological shifts and local adaptation in a broadcast spawning coral on a remote atoll in Western Australia.

At the Rowley Shoals in Western Australia, the prominent reef flat becomes exposed on low tide and the stagnant water in the shallow atoll lagoons heats up, creating a natural laboratory for characterizing the mechanisms of coral resilience to climate change. To explore these mechanisms in the reef coral Acropora tenuis, we collected samples from lagoon and reef slope habitats and combined whole-genome sequencing, ITS2 metabarcoding, experimental heat stress, and transcriptomics. Despite high gene flow across the atoll, we identified clear shifts in allele frequencies between habitats at relatively small linked genomic islands. Common garden heat stress assays showed corals from the lagoon to be more resistant to bleaching, and RNA sequencing revealed marked differences in baseline levels of gene expression between habitats. Our results provide new insight into the complex mechanisms of coral resilience to climate change and highlight the potential for spatially varying selection across complex coral reef seascapes to drive pronounced ecological divergence in climate-related traits.

A tale of two reef systems: Local conditions, disturbances, coral life histories, and the climate catastrophe.

Coral reefs have evolved over millennia to survive disturbances. Yet, in just a few decades chronic local pressures and the climate catastrophe have accelerated so quickly that most coral reefs are now threatened. Rising ocean temperatures and recurrent bleaching pose the biggest threat, affecting even remote and well-managed reefs on global scales. We illustrate how coral bleaching is altering reefs by contrasting the dynamics of adjacent reef systems over more than two decades. Both reef systems sit near the edge of northwest Australia's continental shelf, have escaped chronic local pressures and are regularly affected by tropical storms and cyclones. The Scott reef system has experienced multiple bleaching events, including mass bleaching in 1998 and 2016, from which it is unlikely to fully recover. The Rowley Shoals has maintained a high cover and diversity of corals and has not yet been impacted by mass bleaching. We show how the dynamics of both reef systems were driven by a combination of local environment, exposure to disturbances and coral life history traits, and consider future shifts in community structure with ongoing climate change. We then demonstrate how applying knowledge of community dynamics at local scales can aid management strategies to slow the degradation of coral reefs until carbon emissions and other human impacts are properly managed.

129I/127I and Δ14C records in a modern coral from Rowley Shoals off northwestern Australia reflect the 20th-century human nuclear activities and ocean/atmosphere circulations

Radionuclides produced by 20th-century human nuclear activities from 1945 (e.g., atmospheric nuclear explosions and nuclear-fuel reprocessing) made significant impacts on earth's surface environments. Long-lived shallow-water corals living in tropical/subtropical seas incorporate the anthropogenically-produced radionuclides, including 129I and 14C, into their skeletons, and provide time series records of the impacts of nuclear activities. Here, we present 129I/127I and Δ14C time series records of an annually-banded modern coral skeleton from Rowley Shoals, off the northwestern coast of Australia, in the far eastern Indian Ocean. The 129I/127I and Δ14C records, covering the period 1930s−1990s, exhibit distinct increases caused by the nuclear activities, and their increasing profiles are clearly different from each other. The first distinct 129I/127I increase occurs from 1955 to 1959, followed by a decrease in 1960–1963. The increase is probably due to US atmospheric nuclear explosions in Bikini and Eniwetok Atolls in 1954, 1956 and 1958. The 129I produced in those nuclear tests would be transported by the North Equatorial Current, a portion of which passes through the Indonesian Throughflow and then reaches Rowley Shoals. This initial increase from 1955 is, however, absent in the Δ14C record, which shows a distinct increase from 1959 and its peak around the mid-1970s, followed by a gradual decrease. This absence and the 4-year-delayed Δ14C increase are likely due to dilution of explosion-produced 14C with natural carbon (by seawater mixing and air-sea gas exchange) being much more intense than that of explosion-produced 129I with natural iodine (by the same processes), suggesting that the 129I/127I ratio is a more conservative anthropogenic tracer in surface ocean waters, as compared to Δ14C. The second 129I/127I increase is contemporaneous with a rapid Δ14C increase during 1964–1967, followed by a rapid 129I/127I decrease in 1968–1969; the increases can be ascribed to very large atmospheric nuclear explosions conducted in the former Soviet Union in 1961–1962. The third 129I/127I increase appears between 1969/1970 and 1992, which can be attributed to airborne 129I released from nuclear-fuel reprocessing facilities in Europe, the former Soviet Union and the US. The coral 129I/127I and Δ14C time series records, combined with previous studies, enhance our understanding of the behavior of anthropogenic 129I and 14C in the global ocean and atmosphere.

Coral monitoring in northwest Australia with environmental DNA metabarcoding using a curated reference database for optimized detection

AbstractThe need for efficient and more accurate ways of monitoring threatened ecosystems is becoming increasingly urgent as climate change intensifies. Coral reefs are an example of an ecosystem in crisis, with widespread declines in coral cover and diversity documented over recent decades. Novel molecular approaches such as biomonitoring using environmental DNA (eDNA) from seawater samples show great potential to complement future coral reef monitoring programs, especially when used in combination with conventional methods. However, eDNA metabarcoding studies often rely on public databases (e.g., GenBank) for assigning taxonomy, which generally limits the number of sequences that can be taxonomically identified. The extent to which building reference tissue sequences improves taxonomic resolution has yet to be fully examined. Here, we combined traditional coral reef monitoring data with eDNA assessments derived from seawater collected at the highly diverse Rowley Shoals in Western Australia. Using two ITS2 assays developed to target basal metazoan DNA and a reference database spiked with 70 local coral specimens, we identified 37 genera and 40 species from 56 1 L seawater samples. We identified considerable overlap of taxa with visual survey data and showed that assignment of amplicon sequence variants was significantly improved when “spiking” the taxonomic classifier with curated sequences of locally collected species. Our findings showcase the potential of eDNA metabarcoding for monitoring the biodiversity of reef corals and highlight the importance of custom reference sequence databases for improving taxonomic resolution in metabarcoding studies.

Isolated reefs support stable fish communities with high abundances of regionally fished species.

Anthropogenic impacts at isolated and inaccessible reefs are often minimal, offering rare opportunities to observe fish assemblages in a relatively undisturbed state. The remote Rowley Shoals are regarded as one of the healthiest reef systems in the Indian Ocean with demonstrated resilience to natural disturbance, no permanent human population nearby, low visitation rates, and large protected areas where fishing prohibitions are enforced. We used baited remote underwater video systems (BRUVS) to quantify fish assemblages and the relative abundance of regionally fished species within the lagoon, on the slope and in the mesophotic habitat at the Rowley Shoals at three times spanning 14 years and compared abundances of regionally fished species and the length distributions of predatory species to other isolated reefs in the northeast Indian Ocean. Fish assemblage composition and the relative abundance of regionally fished species were remarkably stable through time. We recorded high abundances of regionally fished species relative to other isolated reefs, including globally threatened humphead Maori wrasse (Cheilinus undulatus) and bumphead parrotfish (Bolbometopon muricatum). Length distributions of fish differed among habitats at the Rowley Shoals, suggesting differences in ontogenetic shifts among species. The Cocos (Keeling) Islands typically had larger‐bodied predatory species than at the Rowley Shoals. Differences in geomorphology, lagoonal habitats, and fishing history likely contribute to the differences among remote reefs. Rowley Shoals is a rare example of a reef system demonstrating ecological stability in reef fish assemblages during a time of unprecedented degradation of coral reefs.

Latitude, depth and environmental variables influence deepwater fish assemblages off Western Australia

Continental shelf and upper slope fishes are important global resources which have significant economic and biodiversity values. Understanding the drivers of their abundance and distribution are fundamental to guide sustainable fisheries management arrangements. Deepwater demersal fish assemblages were sampled between depths of 91–570 m at six locations along the Western Australian coastline spanning 17° of latitude (i.e. 17–34°S at the Rowley Shoals, Montebello Islands, Ningaloo Reef, Abrolhos Islands, Perth Canyon and South West Capes) using Baited Remote Underwater Stereo-Video systems (stereo-BRUVs, n = 417 deployments). A total of 5965 individual fishes were identified belonging to 252 species and 92 families. Assemblage composition data were modelled against in-situ habitat data, temperature, salinity, current speed, and current direction. Multivariate statistical analyses revealed that fish assemblage composition was predominantly influenced by latitude (9.5% of variation), depth (7.3%) and salinity (7.2%). Three distinct assemblages were defined in response to latitude; a tropical, a sub-tropical and a southern temperate bioregion. Species richness declined with increasing latitude, and average biomass was highest in the southern bioregion. A mid-depth peak in biomass and lengths indicated the dominance of meso-predator species in the 300–399 m depth range. The information provided by this study on the factors influencing the abundance and distribution of fish assemblages over an exceptionally broad depth and latitudinal range along a continuous coastline will be useful for predicting the effects of future climate shifts on this and similar teleost species occupying tropical and temperate oceans elsewhere.

Unprecedented longevity of unharvested shallow-water snappers in the Indian Ocean

Identification of latitudinal trends in growth and maximum ages provides important insights into the vulnerability of coral reef fishes to human exploitation. Here, we sampled three species of unharvested tropical snappers from 4 locations along the tropical Western Australian coast and from the Chagos Archipelago in the central Indian Ocean. Interpretation of sectioned sagittal otoliths identified 2 species as the longest-lived tropical reef-associated fishes recorded to date, with a combined eleven specimens aged > 60 yrs, a single Lutjanus bohar aged 79 yrs and a single Macolor macularis aged 81 yrs (both from Rowley Shoals, Western Australia). These maximum ages are two decades greater than previous estimates of maximum age for reef fishes. Lifetimes of such long durations confirm the low rates of natural mortality for these species, their associated low production potential, and the need for effective systems of governance to enable sustainable harvests across the distributions of these species.

Contrasting patterns of genetic connectivity in brooding and spawning corals across a remote atoll system in northwest Australia

An understanding of larval dispersal and connectivity in corals provides valuable insight into the processes of population maintenance and replenishment and is vital for effective management. Here, we used a genotyping by sequencing approach to explore patterns of genetic connectivity in two species of coral with different reproductive modes (brooding and broadcast spawning) at the Rowley Shoals in northwest Australia. Our data revealed strikingly different patterns of genetic structure in the two species. High levels of genetic subdivision in the brooding coral I. brueggemanni was consistent with extensive self-recruitment and restricted connectivity. In contrast, the broadcast spawning coral A. digitifera formed a single panmictic population with extensive gene flow. Our results highlight the wide variation in metapopulation connectivity that exists among corals and support the well-established link between reproductive mode and population genetic structure in the marine environment.

A Cenozoic Great Barrier Reef on Australia's North West shelf

Abstract The Great Barrier Reef of Australia is the largest modern coral reef system on Earth. However, a similar-sized barrier reef (~2000 km long) existed on Australia's north-western margin in the middle Miocene and to some extent, still exists today. Seismic profiles reveal that this reef system was first initiated in the late Paleogene (~34–28 Ma) and reached its zenith as a prograding barrier reef during the middle Miocene (16–15 Ma). This giant reef system backstepped in the late Miocene (~10 Ma), forming a series of isolated atolls and pinnacle reefs. These remaining reefs were gradually extinguished, leaving only a few surviving isolated atolls today (Rowley Shoals, Scott Reef, Seringapatam Reef and Ashmore Reef). The distribution of these extant reefs therefore outlines the former presence of the mid-Miocene Great Barrier Reef of the Australian North West Shelf. This middle Miocene shallow-water rimmed-reefal shelf grew contemporaneously on a deeper-water heterozoan ramp, producing an unusual mixed heterozoan/photozoan carbonate system for most of its length. The initiation of reef growth appears related to Australia's northward drift into lower latitudes, combined with warm mid-Miocene climatic optimum conditions. However, the sudden and simultaneous initiation of reef growth in the mid-Miocene across the North West Shelf was probably also related to changes in paleoceanography (increased aragonite saturation and/or nutrient-depletion) perhaps influenced by changing climatic conditions (increased aridity). From the late Miocene (~10 Ma) to Holocene, high rates of subsidence across the North West Shelf probably set the scene for ultimate demise of this giant barrier reef system and led to widespread backstepping with the development of a series of isolated atolls and pinnacle reefs. It is likely that other paleoceanographic and climatic events (like nutrient supply, carbonate saturation, eustasy etc) combined with continued high subsidence rates acted to gradually extinguish the reef system in a series of steps over a period of 10 million years. The onset (~1 Ma) of a strong Leeuwin Current (with the delivery of warm and nutrient depleted water from the north) and its effect on the Quaternary reefs of the Western Australian coast is a good example of a paleoceanographic process that would be difficult to recognize in an ancient setting. It is likely that similar paleoceanographic events have controlled the rich and complex history of reef growth and drowning since the mid-Miocene on the North West Shelf.

Protection from illegal fishing and shark recovery restructures mesopredatory fish communities on a coral reef.

The recovery of communities of predatory fishes within a no‐take marine reserve after the eradication of illegal fishing provides an opportunity to examine the role of sharks and other large‐bodied mesopredatory fishes in structuring reef fish communities. We used baited remote underwater video stations to investigate whether an increase in sharks was associated with a change in structure of the mesopredatory fish community at Ashmore Reef, Western Australia. We found an almost fourfold increase in shark abundance in reef habitat from 0.64 hr−1 ± 0.15 SE in 2004, when Ashmore Reef was being fished illegally, to 2.45 hr−1 ± 0.37 in 2016, after eight years of full‐time enforcement of the reserve. Shark recovery in reef habitat was accompanied by a two and a half‐fold decline in the abundance of small mesopredatory fishes (≤50 cm TL) (14.00 hr−1 ± 3.79 to 5.6 hr−1 ± 1.20) and a concomitant increase in large mesopredatory fishes (≥100 cm TL) from 1.82 hr−1 ± 0.48 to 4.27 hr−1 ± 0.93. In contrast, near‐reef habitats showed an increase in abundance of large mesopredatory fishes between years (2.00 hr−1 ± 0.65 to 4.56 hr−1 ± 1.11), although only smaller increases in sharks (0.67 hr−1 ± 0.25 to 1.22 hr−1 ± 0.34) and smaller mesopredatory fishes. Although the abundance of most mesopredatory groups increased with recovery from fishing, we suggest that the large decline of small mesopredatory fish in reef habitat was mostly due to higher predation pressure following the increase in sharks and large mesopredatory fishes. At the regional scale, the structure of fished communities at Ashmore Reef in 2004 resembled those of present day Scott Reefs, where fishing still continues today. In 2016, Ashmore fish communities resembled those of the Rowley Shoals, which have been protected from fishing for decades.

Geography and island geomorphology shape fish assemblage structure on isolated coral reef systems.

We quantify the relative importance of multi‐scale drivers of reef fish assemblage structure on isolated coral reefs at the intersection of the Indian and Indo‐Pacific biogeographical provinces. Large (>30 cm), functionally‐important and commonly targeted species of fish, were surveyed on the outer reef crest/front at 38 coral reef sites spread across three oceanic coral reef systems (i.e. Christmas Island, Cocos (Keeling) Islands and the Rowley Shoals), in the tropical Indian Ocean (c. 1.126 x 106 km2). The effects of coral cover, exposure, fishing pressure, lagoon size and geographical context, on observed patterns of fish assemblage structure were modelled using Multivariate Regression Trees. Reef fish assemblages were clearly separated in space with geographical location explaining ~53 % of the observed variation. Lagoon size, within each isolated reef system was an equally effective proxy for explaining fish assemblage structure. Among local‐scale variables, ‘distance from port’, a proxy for the influence of fishing, explained 5.2% of total variation and separated the four most isolated reefs from Cocos (Keeling) Island, from reefs with closer boating access. Other factors were not significant. Major divisions in assemblage structure were driven by sister taxa that displayed little geographical overlap between reef systems and low abundances of several species on Christmas Island corresponding to small lagoon habitats. Exclusion of geographical context from the analysis resulted in local processes explaining 47.3% of the variation, highlighting the importance of controlling for spatial correlation to understand the drivers of fish assemblage structure. Our results suggest reef fish assemblage structure on remote coral reef systems in the tropical eastern Indian Ocean reflects a biogeographical legacy of isolation between Indian and Pacific fish faunas and geomorphological variation within the region, more than local fishing pressure or reef condition. Our findings re‐emphasise the importance that historical processes play in structuring contemporary biotic communities.

Evidence for rapid recovery of shark populations within a coral reef marine protected area

There is limited evidence on the rate at which the shark populations of coral reefs can rebound from over-exploitation, the baselines that might signify when recovery has occurred and the role of no-take Marine Protected Areas (MPA) in aiding this process. We surveyed shark assemblages at Ashmore Reef in Western Australia using baited remote underwater video stations in 2004 prior to enforcement of MPA status and then again in 2016 after eight years of strict enforcement. We found an increase in the relative mean abundance of Carcharhinus amblyrhynchos from 0.16 ± 0.06 individuals h−1 in 2004 to 0.74 ± 0.11 individuals h−1 in 2016, a change that was also accompanied by a shift in the assemblage of sharks to greater proportions of apex species (from 7.1% to 11.9%) and reef sharks (from 28.6% to 57.6%), and a decrease in the proportional abundance of lower trophic level species (from 64.3% to 30.5%). Abundances and trophic assemblage of sharks at Ashmore Reef in 2004 resembled those of the Scott Reefs, where targeted fishing for sharks still occurs, whereas in 2016, abundances and trophic structures had recovered to resemble those of the Rowley Shoals, a reef system that has been a strictly enforced MPA for over 25 years. The shift in abundance and community structure coincident with strict enforcement of the MPA at Ashmore Reef has occurred at a rate greater than predicted by demographic models, implying the action of compensatory processes in recovery. Our study shows that shark communities can recover rapidly after exploitation in a well-managed no-take MPA.

Predator declines and morphological changes in prey: evidence from coral reefs depleted of sharks

We thank the Western Australian Departments of Fisheries, Parks and Wildlife (DPaW), the Commonwealth Department of Sustainability, Environment, Water, Population and Communities for arranging permits to conduct research at the Scott Reefs and the Rowley Shoals. We acknowledge the crew on board the RV 'Solander,' Kim Brooks, Paul Tinkler and numerous volunteers for aid in the field. Thanks also to Paddy Ryan who provided images for the figures. Special thanks to Sarah Hirth, Gammon Koval, Rachel Skubel and Marcelo Costa Lopez for help with some figures. This research was funded in part by the Australian Institute of Marine Science, an International Postgraduate Research Scholarship to S.C.B., the Save our Seas Foundation and the University of Western Australia, Perth, under UWA Ethics Approvals: RA3/100/1279, RA3/100/1172. We thank Ashley Frisch and several reviewers who provided valuable comments that improved our manuscript.

Innovation and technology in marine science: AIMS' North West Shoals to Shore Research Program

The North West Shoals to Shore Research Program began in July 2017 and encompasses four themes, each of which addresses major gaps in scientific knowledge relevant to the environmental management of the offshore petroleum industry in north-west Australia. The themes are: 1. Marine Noise Monitoring and Impacts: investigate selected potential impacts of exposure to a seismic survey on demersal fishes and infauna across different spatial and temporal scales. 2. Benthic Habitats and Demersal Biodiversity: understand physical and biological characteristics of the Ancient Coastline Key Ecological Feature (125 m) and the ecological processes that maintain benthic communities from the ancient to contemporary coastlines, focusing on the distribution and genetic connectivity of pearl oysters. 3. Protected and Iconic Species Movement, Distribution and Threats: determine biologically important areas for pygmy blue whales and hawksbill and green turtles. This will lead to better quantification and mitigation of the risks to megafauna posed by vessel movements, industrial infrastructure and activities. 4. Spatial Dynamics of Isolated Coral Reef Atolls: developing a bio-physical model for benthic and fish communities at the Rowley Shoals and an adaptive monitoring program that quantifies and predicts their spatial dynamics. In this talk, we will discuss planning and progress of the program achieved to date, including innovative and technological approaches used to address scientific uncertainty currently faced by industry.

Diet and condition of mesopredators on coral reefs in relation to shark abundance.

Reef sharks may influence the foraging behaviour of mesopredatory teleosts on coral reefs via both risk effects and competitive exclusion. We used a “natural experiment” to test the hypothesis that the loss of sharks on coral reefs can influence the diet and body condition of mesopredatory fishes by comparing two remote, atoll-like reef systems, the Rowley Shoals and the Scott Reefs, in northwestern Australia. The Rowley Shoals are a marine reserve where sharks are abundant, whereas at the Scott Reefs numbers of sharks have been reduced by centuries of targeted fishing. On reefs where sharks were rare, the gut contents of five species of mesopredatory teleosts largely contained fish while on reefs with abundant sharks, the same mesopredatory species consumed a larger proportion of benthic invertebrates. These measures of diet were correlated with changes in body condition, such that the condition of mesopredatory teleosts was significantly poorer on reefs with higher shark abundance. Condition was defined as body weight, height and width for a given length and also estimated via several indices of condition. Due to the nature of natural experiments, alternative explanations cannot be discounted. However, the results were consistent with the hypothesis that loss of sharks may influence the diet and condition of mesopredators and by association, their fecundity and trophic role. Regardless of the mechanism (risk effects, competitive release, or other), our findings suggest that overfishing of sharks has the potential to trigger trophic cascades on coral reefs and that further declines in shark populations globally should be prevented to protect ecosystem health.

Species diversity, abundance, biomass, size and trophic structure of fish on coral reefs in relation to shark abundance

We thank the University of Western Australia (UWA), Perth, which partly funded this research via an International Postgraduate Research Scholarship and the Australian Institute of Marine Science. We also thank the Department of Fisheries, the Department of Parks and Wildlife (DPaW) and the Department of Sustainability, Environment, Water, Population and Communities for arranging permits to conduct research at the Scott Reefs and the Rowley Shoals. Thank you also to the crew on board the RV 'Solander'. This research was permitted under UWA Ethics Approvals: RA3/100/1279, RA3/100/1172.

The Mermaid Atoll Shipwreck-A mysterious early 19th-century loss MYRA STANBURY with 16 Contributors 307 pp., including 9 appendices, 222 figs mostly colour, 46 tables Australian National Centre of Excellence for Maritime Archaeology and the Australasian I

The Mermaid Atoll Shipwreck-A mysterious early 19th-century loss MYRA STANBURY with 16 Contributors 307 pp., including 9 appendices, 222 figs mostly colour, 46 tables Australian National Centre of Excellence for Maritime Archaeology and the Australasian I

Phylogeography of the reef fish Cephalopholis argus(Epinephelidae) indicates Pleistocene isolation across the indo-pacific barrier with contemporary overlap in the coral triangle

BackgroundThe Coral Triangle (CT), bounded by the Philippines, the Malay Peninsula, and New Guinea, is the epicenter of marine biodiversity. Hypotheses that explain the source of this rich biodiversity include 1) the center of origin, 2) the center of accumulation, and 3) the region of overlap. Here we contribute to the debate with a phylogeographic survey of a widely distributed reef fish, the Peacock Grouper (Cephalopholis argus; Epinephelidae) at 21 locations (N = 550) using DNA sequence data from mtDNA cytochrome b and two nuclear introns (gonadotropin-releasing hormone and S7 ribosomal protein).ResultsPopulation structure was significant (ΦST = 0.297, P < 0.001; FST = 0.078, P < 0.001; FST = 0.099, P < 0.001 for the three loci, respectively) among five regions: French Polynesia, the central-west Pacific (Line Islands to northeastern Australia), Indo-Pacific boundary (Bali and Rowley Shoals), eastern Indian Ocean (Cocos/Keeling and Christmas Island), and western Indian Ocean (Diego Garcia, Oman, and Seychelles). A strong signal of isolation by distance was detected in both mtDNA (r = 0.749, P = 0.001) and the combined nuclear loci (r = 0.715, P < 0.001). We detected evidence of population expansion with migration toward the CT. Two clusters of haplotypes were detected in the mtDNA data (d = 0.008), corresponding to the Pacific and Indian Oceans, with a low level of introgression observed outside a mixing zone at the Pacific-Indian boundary.ConclusionsWe conclude that the Indo-Pacific Barrier, operating during low sea level associated with glaciation, defines the primary phylogeographic pattern in this species. These data support a scenario of isolation on the scale of 105 year glacial cycles, followed by population expansion toward the CT, and overlap of divergent lineages at the Pacific-Indian boundary. This pattern of isolation, divergence, and subsequent overlap likely contributes to species richness at the adjacent CT and is consistent with the region of overlap hypothesis.

Quantifying movement patterns for shark conservation at remote coral atolls in the Indian Ocean

Grey reef sharks (Carcharhinus amblyrhynchos) are apex predators found on many Indo-Pacific coral reefs, but little is known about their movement patterns and habitat requirements. We used acoustic telemetry to determine movements and habitat use of these sharks at the isolated Rowley Shoals atolls, 250 km off the coast of north-western Australia. We equipped 12 male and 14 female sharks ranging from 0.79 to 1.69 m in total length with transmitters that were detected by an array of 11 strategically placed receivers on two atoll reefs. Over 26,000 detections were recorded over the 325 days of receiver deployment. No sharks were observed to move between reefs. Receivers on the outer slopes of reefs provided nearly all (99%) of the detections. We found no differences in general attendance parameters due to size, sex or reef, except for maximum period of detection where larger sharks were detected over a longer period than smaller sharks. Male and female sharks were often detected at separate receivers at the outer slope habitat of one reef, suggesting sexual segregation, but this pattern did not occur at the second reef where males and females were detected at similar frequencies. We identified two patterns of daily behaviour: (1) sharks were present at the reef both day and night or (2) sharks spent more time in attendance during day than at night. Fast Fourier transforms identified 24-h cycles of attendance at the reef and a secondary peak of attendance at 12 h for most sharks, although no individuals shared the same attendance patterns. Our study provides baseline data that can be used to optimise the minimum area and habitat requirements for conservation of these apex predators.

Quaternary development of resilient reefs on the subsiding kimberley continental margin, Northwest Australia

The Kimberley region in remote northwest Australia has poorly known reef systems of two types; coastal fringing reefs and atoll-like shelf-edge reefs. As a major geomorphic feature (from 12ºS to 18ºS) situated along a subsiding continental margin, the shelf edge reefs are in a tropical realm with warm temperatures, relatively low salinity, clear low nutrient waters lacking sediment input, and Indo-West Pacific corals of moderate diversity. Seismic architecture of the Rowley Shoals reveals that differential pre-Holocene subsidence and relative elevation of the pre-Holocene substrate have controlled lagoon sediment infill and reef morphology, forming an evolutionary series reflecting differential accommodation in three otherwise similar reef systems. The Holocene core described for North Scott Reef confirms previous seismic interpretations, and provides a rare ocean-facing reef record. It demonstrates that the Indo-Pacific reef growth phase (RG111) developed during moderate rates of sea level rise of 10 mm/year from 11 to about 7-6.5 ka BP until sea level stabilization, filling the available 27 m of pre-Holocene accommodation. Despite the medium to high hydrodynamic energy imposed by the 4m tides, swell waves and cyclones the reef-building communities represent relatively low-wave energy settings due to their southeast facing and protection afforded by the proximity of the South Reef platform. This study demonstrates the resilience of reefs on the subsiding margin whilst linking Holocene reef morphology to the relative amount of pre-Holocene subsidence.