Reef Clusters Research Articles

Coral reef potential connectivity in the southwest Indian Ocean

The tropical southwest Indian Ocean is a coral biodiversity hotspot, with remote reefs physically connected by larval dispersal through eddies and a complex set of equatorial and boundary currents. Based on multidecadal, 2 km resolution hydrodynamic and larval dispersal models that incorporate temporal variability in dispersal, we find that powerful zonal currents, current bifurcations, and geographic isolation act as leaky dispersal barriers, partitioning the southwest Indian Ocean into clusters of reefs that tend to consistently retain larvae, and therefore gene flow, over many generations. Whilst exceptionally remote, the Chagos Archipelago can broadcast (and receive) considerable numbers of larvae to (and from) reefs across the wider southwest Indian Ocean, most significantly exchanging larvae with the Inner Islands of Seychelles, but also the Mozambique Channel region. Considering multi-generational dispersal indicates that most coral populations in the southwest Indian Ocean are physically connected within a few hundred steps of dispersal. These results suggest that regional biogeography and population structure can be largely attributed to geologically recent patterns of larval dispersal, although some notable discrepancies indicate that palaeogeography and environmental suitability also play an important role. The model output and connectivity matrices are available in full and will provide useful physical context to regional biogeography and connectivity studies, as well as supporting marine spatial planning efforts.

Connectivity modelling identifies sources and sinks of coral recruitment within reef clusters

Connectivity aids the recovery of populations following disturbances, such as coral bleaching and tropical cyclones. Coral larval connectivity is a function of physical connectivity and larval behaviour. In this study, we used OceanParcels, a particle tracking simulator, with 2D and 3D velocity outputs from a high resolution hydrodynamic-biogeochemical marine model (RECOM) to simulate the dispersal and settlement of larvae from broadcast spawning Acropora corals in the Moore Reef cluster, northern Great Barrier Reef, following the annual spawning events in 2015, 2016 and 2017. 3D velocity simulations showed 19.40–68.80% more links and sinks than those of 2D simulations. Although the patterns of connectivity among sites vary over days and years, coral larvae consistently dispersed from east to west in the cluster domain, with some sites consistently acting as sources or sinks for local larval recruitment. Results can inform coral reef intervention plans for climate change, such as the design of marine protected areas and the deployment of proposed interventions within reef clusters. For example, the wider benefits of interventions (e.g., deployment of heat adapted corals) may be optimised when deployed at locations that are a source of larvae to others within comparable habitats across the reef cluster.

Submerged reef and inter-reef morphology in the Western South Atlantic, Abrolhos Shelf (Brazil)

Submarine geomorphology is a main environmental driver defining the seascape and associated benthic habitat. In this sense, eustatic sea-level fluctuations plays a major role in shaping the morphology of the seafloor. During the last deglaciation, changes in rates of sea-level rise led to moments of stillstand and rapid rise acceleration, forming and drowning distinct shelf geomorphological features. Here, we present and discuss the morphology and associated habitats related to submerged reefs and inter-reef areas along the southern Abrolhos inner shelf (western South Atlantic, Brazil). Our aim is to discuss the morphological evidence of reef drowning and related sea-level/climate events, and the formation of distinct habitats during the post-Last Glacial Maximum transgression. Multibeam echosounder data and sub-aquatic video footage are the basic datasets. Automatic seabed classification was carried out by combining morphometric analysis, acoustic backscatter and ground-truthing. Results show the presence of two clusters of submerged reefs (14-23 m and 24-35 m deep), associated with longitudinal and transverse bedforms, banks, and shelf-incised valleys. Rhodolith beds comprise the main inter-reef benthic habitat. Distinct reef clusters defined by reef top depths indicate that reef drowning occurred in two different moments. The shallower reefs are likely related to the rapid sea-level rise associated with MWP 1C, while deeper reefs are interpreted as remanent carbonate build-ups. Habitats are strongly influenced by the marine geodiversity forming a mosaic of reef, inter-reef and bedforms classes that seem to be associated with an antecedent paleo landscape marked by shelf-incised valleys and paleochannels. Submarine geomorphology is an environmental component that drives geodiversity and is driven by distinct seafloor processes acting in different time and spatial scales, as well as being able to quantify biogenic phenomena, for example, surrogacy and provide records of sea-level changes.

Global analysis of reef ecosystem services reveals synergies, trade-offs and bundles

Millions of people around the world depend on the ecosystem services produced by rocky and coral reef ecosystems, including nutrition, aesthetic value, and coastal protection. Rocky and coral reefs also contribute to critical global and regional processes through the cycling of nitrogen, phosphorus, and carbon. The increased stress experienced by reefs in the Anthropocene threatens their ability to provide vital ecosystem services. This study investigates bundles of ecosystem services, ecosystem services that occur together, to identify trade-offs and synergies among services produced by coral reefs. To do this, we bring together estimates of seven ecosystem services: productivity, nitrogen cycling, phosphorus cycling, inorganic carbon cycling, aesthetic value, nutritional value, and coastal protection. We use correlations analysis to understand trade-offs and synergies between these seven ecosystem services and cluster analysis to identify clusters of reefs with distinct suites of ecosystem services, or ecosystem service bundles. Our analysis reveals (1) synergies and trade-offs among the seven ecosystem services, and (2) three distinct clusters of reefs, which differ on the basis of their overall and relative delivery of ecosystem services. Differences in service production among the clusters appear to be linked to differences in key ecological traits, including total reef fish biomass and species richness. Similar applications of ecosystem service bundles analysis in other marine and coastal systems could result in improved understanding of the spatial distributions and relationships between marine ecosystem services, which is a key input to marine policy.

Biophysical models resolution affects coral connectivity estimates

Estimating connectivity between coral reefs is essential to inform reef conservation and restoration. Given the vastness of coral reef ecosystems, connectivity can only be simulated with biophysical models whose spatial resolution is often coarser than the reef scale. Here, we assess the impact of biophysical models resolution on connectivity estimates by comparing the outputs of five different setups of the same model with resolutions ranging from 250 m to 4 km. We show that increasing the model resolution around reefs yields more complex and less directional dispersal patterns. With a fine-resolution model, connectivity graphs have more connections but of weaker strength. The resulting community structure therefore shows larger clusters of well-connected reefs. Virtual larvae also tend to stay longer close to their source reef with a fine-resolution model, leading to an increased local retention and self-recruitment for species with a short pre-competency period. Overall, only about half of the reefs with the largest connectivity indicator values are similar for the finest and coarsest resolution models. Our results suggest that reef management recommendations should only be made at scales coarser than the model resolution. Reef-scale recommendations can hence only be made with models not exceeding about 500 m resolution.

Partitioning of marine transition zone reefs among temperate, sub-tropical and tropical fishes is related more to depth and habitat than temperature

Changes in fish communities as oceans warm and cool and competition for space between established and novel species can be evaluated in biogeographic transition zones such as the west coast of Australia. At ~30° S in this region, a cool anomaly occurred in the 2000s, between marine heatwaves. Over 2 yr of that anomaly, surveyed reef fishes were 57% temperate, 18% sub-tropical and 25% tropical. The most numerous fishes included a wrasse, herring, bullseye, drummer and damselfish. Based on similarities in the composition of fishes, 7 significant clusters of reefs were identified along a gradation from deep, exposed reefs to shallow, protected lagoonal reefs. Endemic sub-tropical and temperate wrasses and damselfishes typified all reefs. Some of these were ubiquitous over exposed and lagoonal reefs and others prevalent in only one reef type, demonstrating habitat preferences and partitioning among closely related species. This was reflected in the differing order of importance of fishes that typified different reefs. Linear modelling indicated that abiotic (depth, distance from shore) and biotic factors (e.g. algae) explained most of the variation in the fish communities among reefs. Additional variation, particularly within lagoonal reefs, was related to relief, turf and corals, rather than water temperature. Occurrence and reproductive activity of a group of tropical/sub-tropical wrasses and damselfish in some lagoonal reefs with abundant tropical habitats (e.g. corals) suggested that they supported novel communities during cool anomalies. Better predictions of future change and interactions between existing and novel species with environmental cycles requires knowledge of species-specific habitat relationships and biology.

Species Composition and Distribution of Tropical Marine Macroalgae In The Pari Island Reef Cluster, Jakarta

The objective of this study was to analyze the species composition and distribution of marine macroalgae at Pari Island reef cluster-Seribu Islands reef system Jakarta based on specimen information available in Reference Collection of Research Center for Oceanography (RCO-LIPI). The Macroalgae specimens were collected irregularly by researchers during marine expedition projects from 1973 until 2014. Taxonomic identification of specimens was conducted in 2014, validation of species names was updated in May 2020 through www.macroalgaebase.org. The floristic composition value of macroalgae was analyzed using the C/P, R/P, and (R+C)/P ratio. Bray-Curtis similarity index analyzes were performed to visualize differences in macroalgae species from the different islands. We recorded a total of 41 species from 25 genus and 18 families. Rhodophyta represented the most significant number of taxa with 20 species (49%), followed by Chlorophyta with 11 species (27%) and Phaeophyta with ten species (24%). In this study, we found that Sargassum polycystum, Gracilaria salicornia, Amphiroa fragilisima, and Halimeda opuntia are dominant species in the study area. The highest macroalgae diversity found in Kongsi Island with 33 species, while the lower diversity found in Pari Island with five species. Bray-Curtis Similarity index showed that Kongsi Island and Burung Island have the highest similarity value with 29,26%, while Tikus Island and Pari Island have the lowest similarity value with 11,76%. The macroalgae distribution in the Pari island reef cluster influenced by the difference of substrate profile and environmental pressure derived from human activities. Proper management of macroalgae resources is necessary to preserve the sustainability of the macroalgae ecosystem. Keywords: macroalgae, composition, distribution, Pari Island

An Assessment of Computational Fluid Dynamics as a Tool to Aid the Design of the HCMR-Artificial-ReefsTM Diving Oasis in the Underwater Biotechnological Park of Crete

Since recreational diving activities have increased in recent decades, resulting in additional environmental pressure on the coastal zone, the deployment of artificial reefs as a conservation strategy to divert mass ecotourism from fragile natural reefs has been proposed and realized in many areas of the world. Twelve units of a patented naturoid artificial reef technology developed by the Hellenic Centre for Marine Research (HCMR) were deployed in 2015 in the Underwater Biotechnological Park of Crete (UBPC) in order to create an experimental diving oasis and investigate the potential of achieving this aim for the over-exploited coastal ecosystems of this part of the Eastern Mediterranean. Assessment of the degree of establishment of artificial reefs and their ability to mimic natural ecosystems is often monitored through biological surveys and sampling. The measurement of the chemical, physical, and hydrodynamic characteristics of the water mass surrounding artificial reefs is also essential to fully understand their comparison to natural reefs. In particular, the flow field around reefs has been shown to be one of the most important physical factors in determining suitable conditions for the establishment of a number of key species on reef habitats. However, the combination of biological establishment monitoring and realistic flow-field simulation using computational fluid dynamics as a tool to aid in the design improvement of already existing reef installations has not been fully investigated in previous work. They are often reported separately as either ecological or engineering studies. Therefore, this study examined a full-scale numerical simulation of the field flow around individual already installed naturoid reef shapes, and part of their present arrangement on the sea bottom of the UPBC combined with the field-testing of the functionality of the installed artificial reefs concerning fish species aggregation. The results show that the simulated flow characteristics around the HCMR diving oasis artificial reefs were in good general agreement with the results of former studies, both for flows around a single deployed unit and for flows around a cluster of more than one unit. The results also gave good indications of the performance of individual reef units concerning key desirable characteristics such as downstream shadowing and sediment/nutrient upwelling and resuspension. In particular, they confirmed extended low flow levels (less than 0.3 m/s) and in some cases double vortexes on the downstream side of reef units where observed colonization and habitation of some key fish species had taken place. They also showed how the present distribution of units could be optimized to perform better as an integrated reef cluster. The use of computational fluid dynamics, with field survey data, is therefore suggested as a useful design improvement tool for installed reef structures and their deployment arrangement for recreational diving oases that can aid the sustainable development of the coastal zone.

Spatio‐temporal variability of benthic macroalgae in a coral reef system highly influenced by fluvial discharge: Veracruz, Gulf of Mexico

AbstractThe Veracruz Reef System, in the southern Gulf of Mexico, is a suitable area for the study of the temporal and spatial variability of macroalgae abundance, at reef settings influenced by the fluvial discharge of the Jamapa River, and by human activities in the city and port of Veracruz. With this purpose, the bottom cover of each morpho‐functional group of benthic macroalgae (frondoses, turf, and crustose corallines), and hermatypic corals, was determined at ten selected coral reefs, on a seasonal basis (rainy and dry seasons), for the 2009–2015 period. The average cover of benthic macroalgae was high (53.1%), with turf as the dominant morpho‐functional group (31.9%), as in several reef ecosystems in the tropical Atlantic, followed by crustose corallines (15.2%), and frondoses (6.1%). Although turf macroalgae is dominant, due to their high temporal and spatial variability, the Veracruz Reef System could not be considered to be in a stable state, but just in an intermediate unstable equilibrium state, which is highly influenced by a high sediment load. As expected, nearshore reefs presented higher macroalgae covers, and unexpectedly, the outer‐shelf reefs presented the highest cover of frondoses. Despite fluvial discharge influence, no differences in cover were found between the rainy and dry seasons. There was a negative and significantly correlation between the cover of frondoses and turf, which suggests that the driver/s of the abundance of these macroalgae, act in opposite ways for each group. Three clusters of reefs, defined by community structure and conservation degree, were determined: nearshore or degraded, offshore or moderately conserved, and conserved; and the entire Veracruz Reef System is considered to be in a moderately state of conservation.

Larval connectivity patterns of the North Indo-West Pacific coral reefs

Coral reefs of the North Indo-West Pacific provide important ecosystem services to the region but are subjected to multiple local and global threats. Strengthening management measures necessitate understanding the variability of larval connectivity and bridging global connectivity models to local scales. An individual-based Lagrangian biophysical model was used to simulate connectivity between coral reefs for three organisms with different early life history characteristics: a coral (Acropora millepora), a sea urchin (Tripneustes gratilla), and a reef fish (Epinephelus sp). Connectivity metrics and reef clusters were computed from the settlement probability matrices. Fitted power law functions derived from the dispersal kernels provided relative probabilities of connection given only the distance between reefs, and demonstrated that 95% of the larvae across organisms settled within a third of their maximum settlement distances. The magnitude of the connectivity metric values of reef cells were sensitive to differences both in the type of organism and temporal variability. Seasonal variability of connections was more dominant than interannual variability. However, despite these differences, the moderate to high correlation of metrics between organisms and seasonal matrices suggest that the spatial patterns are relatively similar between reefs. A cluster analysis based on the Bray-Curtis Dissimilarity of sink and source connections synthesized the inherent variability of these multiple large connectivity matrices. Through this, similarities in regional connectivity patterns were determined at various cluster sizes depending on the scale of interest. The validity of the model is supported by 1) the simulated dispersal kernels being within the range of reported parentage analysis estimates; and, 2) the clusters that emerged reflect the dispersal barriers implied by previously published population genetics studies. The tools presented here (dispersal kernels, temporal variability maps and reef clustering) can be used to include regional patterns of connectivity into the spatial management of coral reefs.

Large-scale, multidirectional larval connectivity among coral reef fish populations in the Great Barrier Reef Marine Park.

Larval dispersal is the key process by which populations of most marine fishes and invertebrates are connected and replenished. Advances in larval tagging and genetics have enhanced our capacity to track larval dispersal, assess scales of population connectivity, and quantify larval exchange among no-take marine reserves and fished areas. Recent studies have found that reserves can be a significant source of recruits for populations up to 40km away, but the scale and direction of larval connectivity across larger seascapes remain unknown. Here, we apply genetic parentage analysis to investigate larval dispersal patterns for two exploited coral reef groupers (Plectropomus maculatus and Plectropomus leopardus) within and among three clusters of reefs separated by 60-220km within the Great Barrier Reef Marine Park, Australia. A total of 69 juvenile P.maculatus and 17 juvenile P.leopardus (representing 6% and 9% of the total juveniles sampled, respectively) were genetically assigned to parent individuals on reefs within the study area. We identified both short-distance larval dispersal within regions (200m to 50km) and long-distance, multidirectional dispersal of up to ~250km among regions. Dispersal strength declined significantly with distance, with best-fit dispersal kernels estimating median dispersal distances of ~110km for P.maculatus and ~190km for P.leopardus. Larval exchange among reefs demonstrates that established reserves form a highly connected network and contribute larvae for the replenishment of fished reefs at multiple spatial scales. Our findings highlight the potential for long-distance dispersal in an important group of reef fishes, and provide further evidence that effectively protected reserves can yield recruitment and sustainability benefits for exploited fish populations.

Cold-water coral reefs in the Hola glacial trough off Vesterålen, North Norway

Coral reefs, including cold-water coral reefs in deep water, grow most favourably in areas of firm seabed and strong ocean currents. Data collected by the MAREANO programme (www.mareano.no) from the Hola glacial trough off Vesteralen, North Norway, reveal a spectacular seafloor morphology. Features found include large clusters of coral reefs and sand-wave fields, and moraines from the last glacial period (Boe et al. 2009; Buhl-Mortensen et al. 2010). More than 300 coral reefs occur in water depths of 200–270 m, where measured bottom current speeds exceed 0.3 m s−1 and water temperature is about 7–8°C. The area offshore of Vesteralen, North Norway (Fig. 1a), comprises alternating shallow banks and deeper troughs formed during successive glaciations (Boe et al. 2009). The banks Vesteralsgrunnen and Eggagrunnen, NE and SW of the Hola trough, respectively, have water depths of 70–90 m, whereas the Hola trough is 200–270 m deep (Fig. 1b). A gentle moraine ridge (10–35 m high) crosses the central part of the Hola trough between the two banks. Water depth on this ridge increases from c. 170 m in the SW to c. 200 m in the NE. Several smaller moraine ridges are also …

Environmental triggers for primary outbreaks of crown-of-thorns starfish on the Great Barrier Reef, Australia

In this paper, we postulate a unique environmental triggering sequence for primary outbreaks of crown-of-thorns starfish (COTS, Acanthaster planci) on the central Great Barrier Reef (GBR, Australia). Notably, we extend the previous terrestrial runoff hypothesis, viz. nutrient-enriched terrestrial runoff→elevated phytoplankton ‘bloom’ concentrations→enhanced COTS larval survival, to include the additional importance of strong larvae retention around reefs or within reef groups (clusters) that share enhanced phytoplankton concentrations. For the central GBR, this scenario is shown to occur when El Niño-Southern Oscillation (ENSO) linked hydrodynamic conditions cause the ‘regional’ larval connectivity network to fragment into smaller ‘local’ reef clusters due to low ocean current velocities. As inter-annual variations in hydrodynamic circulation patterns are not amenable to direct management intervention, the ability to reduce the future frequency of COTS outbreaks on the central GBR is shown to be contingent on reducing terrestrial bioavailable nutrient loads ~20–40%.

Connectivity networks reveal the risks of crown‐of‐thorns starfish outbreaks on the Great Barrier Reef

Summary Many ecosystems suffer systemwide outbreaks of damaging species propagating from primary outbreak sites. Connectivity patterns can identify parts of the ecosystem that help turn local outbreaks into a systemwide contagion through a series of transmission events. Here, we show that patterns of larval connectivity among reefs can help explain periodic crown‐of‐thorns starfish (COTS) epidemics across the Great Barrier Reef (GBR). We simulated potential dispersal of COTS larvae to obtain a connectivity network of coral reefs across the entire GBR. Network analysis revealed areas of high local connectivity where any outbreaks could be amplified locally, as well as those areas with potential to cause large‐scale epidemics with ecosystem‐wide impacts. We find that the regions where COTS epidemics are known to originate are predictable from their high local and systemwide connectivity. Extensive larval exchanges among reef clusters in these regions can start a chain reaction of COTS population build‐up. The same regions also have high potential to reach and affect other parts of the GBR, thereby maximizing the likelihood that any outbreaks would eventually propagate throughout the ecosystem. Hydrodynamic properties and geography of the GBR make it vulnerable to COTS epidemics. Using network analysis to identify regions with high‐risk high‐impact sources could help control these devastating events in future. Synthesis and applications. The observed centre of origin for COTS epidemics (the Cooktown–Cairns region) can be predicted from its elevated short‐ and long‐range levels of larval connectivity. Connectivity analysis of per‐reef risks provides spatially explicit targets to guide surveillance and control measures that might help curtail COTS epidemics through prioritization of highly connected reefs. The analytical approach developed here for COTS connectivity can also be applied to identify well‐connected patches and regions in other interconnected ecological systems.

Habitat complexity and predator size mediate interactions between intraguild blue crab predators and mud crab prey in oyster reefs

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 488:209-219 (2013) - DOI: https://doi.org/10.3354/meps10386 Habitat complexity and predator size mediate interactions between intraguild blue crab predators and mud crab prey in oyster reefs Jennifer M. Hill1,2,*, Marc J. Weissburg1 1Georgia Institute of Technology, 310 Ferst Drive, Atlanta, Georgia 30332, USA 2Dauphin Island Sea Lab, 101 Bienville Boulevard, Dauphin Island, Alabama 36528, USA *Email: jhill@disl.org ABSTRACT: The prevalence of inherently unstable intraguild (IG) predation in natural systems suggests that many biotic and abiotic factors, including habitat complexity and population size structure, affect interactions between IG predators and prey. We investigated the role of shared oyster resource use (Crassostrea virginica), predator size, and habitat complexity (i.e. sand, shell hash, and simulated oyster clusters) in mediating the interactions of IG predators, the blue crabs Callinectes sapidus, and IG prey, the mud crabs Panopeus herbstii, in oyster reef communities. In mesocosms, mud crabs consumed the shared oyster resource at higher rates than did blue crabs. Constructed oyster reef clusters in mesocosms reduced blue crab predation on mud crabs, suggesting increasing habitat complexity provides mud crabs with a more effective refuge from IG predators. Survival of mud crabs in less complex habitats depended on predator size; small blue crabs were not able to consume mud crabs in any habitat type. The refuge effects of oyster reefs were confirmed in the field using mud crabs tethered in sand, shell hash, and oyster clusters. These experiments demonstrate the importance of habitat complexity and body size in mediating intraguild interactions between blue crabs and mud crabs on oyster reefs; they also elucidate the impacts of these predators in oyster reef communities. KEY WORDS: Intraguild predation · Body size · Tethering · Callinectes sapidus · Panopeus herbstii · Oyster reef Full text in pdf format PreviousNextCite this article as: Hill JM, Weissburg MJ (2013) Habitat complexity and predator size mediate interactions between intraguild blue crab predators and mud crab prey in oyster reefs. Mar Ecol Prog Ser 488:209-219. https://doi.org/10.3354/meps10386 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 488. Online publication date: August 15, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Growth and pelagic larval duration of presettlement and newly settled neon damselfish, Pomacentrus coelestis, at multiple spatial scales

Variation in planktonic larval duration (PLD) and growth of Pomacentrus coelestis was investigated on the southern Great Barrier Reef at multiple spatial scales. A tetracycline experiment, using presettlement fish, demonstrated that increments were formed daily. Variation in PLD was low between reef clusters (0%), reefs within clusters (0.4–8.5%) and sites within reefs (13.1%), but high among individuals within sites (86.5–91.5%); PLD ranged from 15 to 27 days. It was predicted that PLD would vary at greater spatial scales, but differences were low and a review of all studies on P. coelestis in tropical waters had a similar range of PLDs to our study. Contrary to a hypothesis that fish with slower growth would have longer PLDs, there was no significant relationship between mean presettlement increment width of otoliths and PLD for fish from the two reef clusters examined. There were, however, differences in presettlement growth rates between reef clusters (over 100 km apart) over the last 5–6 days of planktonic life. Warmer waters at the Swain Reefs (0.1–1°C) may have contributed to these differences in growth. Stochastic transport of larvae, habitat choice by presettlement fish in a reef mosaic, and variable conditions in the plankton may contribute to variation in PLD, presettlement growth and size-at-settlement in P. coelestis. We propose that prolonged periods of settlement choice may obscure simple relationships between PLD and size-at-settlement.

Dispersal in the spiny damselfish, Acanthochromis polyacanthus, a coral reef fish species without a larval pelagic stage

The spiny damselfish, Acanthochromis polyacanthus, is widely distributed throughout the Indo-Australian archipelago. However, this species lacks a larval dispersal stage and shows genetic differentiation between populations from closely spaced reefs. To investigate the dispersal strategy of this unique species, we used microsatellite markers to determine genetic relatedness at five dispersal scales: within broods of juveniles, between adults within a collection site (approximately 30 m(2)), between sites on single reefs, between nearby reefs in a reef cluster, and between reef clusters. We sampled broods of juveniles and adults from seven reefs in the Capricorn-Bunker and Swain groups of the Great Barrier Reef. We found that extra-pair mating is rare and juveniles remain with their parents until fledged. Adults from single sites are less related than broods but more related than expected by chance. However, there is no evidence of inbreeding suggesting the existence of assortative mating and/or adult migration. Genetic differences were found between all of the reefs tested except between Heron and Sykes reefs, which are separated only by a 2-km area of shallow water (less than 10 m). There was a strong correlation between genetic distance, geographical distance and water depth. Apparently, under present-day conditions spiny damselfish populations are connected only between sites of shallow water, through dispersal of adults over short distances. Assuming that dispersal behaviour has not changed, the broad distribution of A. polyacanthus as a species is likely based on historical colonization patterns when reefs were connected by shallow water at times of lower sea levels.

Elemental signatures of Acanthochromis polyacanthus otoliths from the Great Barrier Reef have significant temporal, spatial, and between-brood variation

We evaluated the spatial and temporal scales over which otolith signatures varied in a reef fish on the Great Barrier Reef (GBR) using the non-dispersing damselfish Acanthochromis polyacanthus. We found a robust multi-element separation in otolith signatures from reef clusters in the northern and southern GBR. Variance components indicated that this spatial scale accounted for the majority of the variation in two elemental ratios (Ba/Ca and Sr/Ca) over the 2 years of the study. There was also significant variation in elemental signatures between otoliths collected over two consecutive years, as well as within a season. Individual reefs within clusters were less distinguishable based on otolith chemistry and were probably observed by differences within reefs (among sites and broods within sites). These results indicate that it may be difficult to determine the reef of origin for individual fish using otolith chemistry, while determining natal region seems a realistic goal.

Contrasting effects of marine protected areas on the abundance of two exploited reef fishes at the sub-tropical Houtman Abrolhos Islands, Western Australia

A unique assemblage of tropical and temperate marine organisms characterizes the Houtman Abrolhos Islands, four clusters of islands and reefs off the coast of mid-Western Australia. Four reef observation areas or marine protected areas (MPAs) were established in 1994 to examine their value in protecting vulnerable reef fish species, including the sub-tropical wrasse, Choerodon rubescens, and the coral trout, Plectropomus leopardus. In 1993 and 1994 (prior to protection), population densities and body sizes were monitored at two island groups (Easter and Wallabi). At each location, the MPAs and the equivalent ‘control’ areas to remain open to fishing were monitored by underwater visual censuses. These closed and open areas were subsequently monitored four more times between 1995 and 2002 to assess long-term trends in abundance and population structure. Populations of the wrasse, C. rubescens, did not appear to respond to protection, exhibiting irregular fluctuations in both closed and open areas throughout the study. In contrast, although there were no significant increases for P. leopardus for the first three years of closure, after eight years of protection there were significantly larger numbers of P. leopardus in the closed areas. There was a three-fold increase at the Easter Group and a seven-fold increase at the Wallabi Group, relative to open fishing areas, attributed primarily to reduced fishing mortality. Significant closed areas (17% of the P. leopardus habitat) provided substantial stock-wide impacts. The data also indicate that the MPAs can be effective where some forms of fishing (such as lobster fishing) are permitted which are compatible with the objectives of the MPA. While MPAs are clearly an effective tool for increasing the local abundance of some reef fishes, the spatial and temporal scales required for their success may vary among species.

Elemental signatures of Pomacentrus coelestis otoliths at multiple spatial scales on the Great Barrier Reef, Australia

Elemental signatures in both the natal and post-settlement regions of otoliths from newly settled Pomacentrus coelestis were measured from multiple sites, reefs, years and reef clusters at different latitudes on the Great Barrier Reef, Australia. Significant differences in latitude and year were found for Sr/Ca and Ba/Ca in the post-settlement portion of the otoliths, and discriminant function analyses identified clear separations in otolith signatures from different reef clusters and years. The spatial and annual variation found in the post-settlement signatures was similar to that determined for the brooding damselfish Acanthochromis polyacanthus, and suggests that these 2 species record environmental variation in a similar manner. This similarity also extended to reefs, although some reefs showed persistent trends for individual elemental ratios. However, the natal portion of the otoliths proved less useful in discriminating groups, and appeared to be decoupled from the post-settlement portion of the otoliths. Such a decoupling could be the result of differences in the food source during the natal period, protein content of the 2 otolith portions which could potentially influence affinity for trace elements, or ontogenetic changes in the depositional process itself. Our findings indicate that robust elemental signatures are likely to be found among clusters of reefs rather than individual reefs, which has important ramifications for future studies attempting to discriminate reef fish populations.