Maintenance of fish diversity on disturbed coral reefs

Habitat perturbations play a major role in shaping community structure; however, the elements of disturbance-related habitat change that affect diversity are not always apparent. This study examined the effects of habitat disturbances on species richness of coral reef fish assemblages using annual surveys of habitat and 210 fish species from 10 reefs on the Great Barrier Reef (GBR). Over a period of 11 years, major disturbances, including localised outbreaks of crown-of-thorns sea star (Acanthaster planci), severe storms or coral bleaching, resulted in coral decline of 46–96% in all the 10 reefs. Despite declines in coral cover, structural complexity of the reef framework was retained on five and species richness of coral reef fishes maintained on nine of the disturbed reefs. Extensive loss of coral resulted in localised declines of highly specialised coral-dependent species, but this loss of diversity was more than compensated for by increases in the number of species that feed on the epilithic algal matrix (EAM). A unimodal relationship between areal coral cover and species richness indicated species richness was greatest at approximately 20% coral cover declining by 3–4 species (6–8% of average richness) at higher and lower coral cover. Results revealed that declines in coral cover on reefs may have limited short-term impact on the diversity of coral reef fishes, though there may be fundamental changes in the community structure of fishes.     

Principles for estimating fish productivity on coral reefs

Coral reefs provide major nutritional inputs to humans through fish production. Yet, our capacity to adequately assess fish productivity in these and other high-diversity aquatic systems is hampered by a lack of computationally accessible methods with realistic data requirements. Standing stock biomass is often assumed to reflect biomass productivity, yet theoretical and empirical evidence question this assumption. These methodological hurdles have stymied progress in managing this critical coral reef function, potentially jeopardising the future of many small-scale tropical fisheries struggling to respond to global changes. Here, we summarise the physiological and ecological processes that lead to the production of fish biomass. We outline principles and present a robust framework for quantifying fish productivity in high-diversity ecosystems that overcomes these shortcomings by integrating readily accessible individual-level data (e.g. from visual counts) with growth trajectories and predicted mortality rates. This framework provides fisheries-independent estimates of multispecies fish productivity without the need to specify often unknown detailed trophic relationships. We delineate five simple steps and provide a critical user-friendly interface (an easy-to-use R package) to make the calculation of fish productivity a readily accessible tool for coral reef scientists and managers.

     

Parasite infestation increases on coral reefs without cleaner fish

Coral Reefs - Mutualisms are pivotal in shaping ecological communities. Iconic images of cleaner fish entering the mouths of predatory fish clients to remove ectoparasites epitomize their mutual...     

Seascape-scale trophic links for fish on inshore coral reefs

It is increasingly accepted that coastal habitats such as inshore coral reefs do not function in isolation but rather as part of a larger habitat network. In the Caribbean, trophic subsidies from habitats adjacent to coral reefs support the diet of reef fishes, but it is not known whether similar trophic links occur on reefs in the Indo-Pacific. Here, we test whether reef fishes in inshore coral, mangrove, and seagrass habitats are supported by trophic links. We used carbon stable isotopes and mathematical mixing models to determine the minimum proportion of resources from mangrove or seagrass habitats in the diet of five fish species from coral reefs at varying distances (0–2,200 m) from these habitats in Moreton Bay, Queensland, eastern Australia. Of the fish species that are more abundant on reefs near to mangroves, Lutjanus russelli and Acanthopagrus australis showed no minimum use of diet sources from mangrove habitat. Siganus fuscescens utilized a minimum of 25–44 % mangrove sources and this contribution increased with the proximity of reefs to mangroves (R ² = 0.91). Seagrass or reef flat sources contributed a minimum of 14–78 % to the diet of Diagramma labiosum, a species found in higher abundance on reefs near seagrass beds, but variation in diet among reefs was unrelated to seascape structure. Seagrass or reef flat sources also contributed a minimum of 8–55 % to a fish species found only on reefs (Pseudolabrus guentheri), indicating that detrital subsidies from these habitats may subsidize fish diet on reefs. These results suggest that carbon sources from multiple habitats contribute to the functioning of inshore coral reef ecosystems and that trophic connectivity between reefs and mangroves may enhance production of a functionally important herbivore.     

Biogeographic variation in coral species diversity on coral reefs in three regions of Indonesia

Abstract . This paper assesses variation in coral species diversity within the Indonesian archipelago, and the influence of regional species pools, geomorphology and anthropogenic pollution on coral species diversity and occurrence. We obtained transects from 33 sites on 14 reefs in three regions of Indonesia: Ambon (Moluccas), South Sulawesi and the Java Sea. We determined the within‐site species richness by using species‐sampling curves. Cluster analysis and multi‐dimensional scaling showed that land‐based pollution was the primary determinant of coral species diversity and species occurrence on reefs. Relatively unaffected reference sites in eastern Indonesia were approximately 20% more diverse than Java Sea reference sites. Rare species formed a higher proportion of the coral fauna on eastern Indonesian sites, and eastern Indonesian apparent endemic species contributed approximately 25% of the total species pool sampled. Between‐site variation in species occurrence was lower on Java Sea reefs than on eastern Indonesian reefs. A larger species pool in eastern Indonesia than in the Java Sea probably accounted for most of the difference in within‐site species diversity between eastern Indonesian and Java Sea reference sites. High fishing intensity in the Java Sea, including destructive fishing practices, may have also contributed to reduced within‐site species diversity on Java Sea reference reefs. Despite the fact that the Java Sea was exposed during Pleistocene lowstands, and was recolonized by marine organisms only within the last 10 000 years, coral species diversity and assemblage composition on the Java Sea reefs was largely similar to open ocean reefs in eastern Indonesia.     

Recruitment and the structure of assemblages of fish on coral reefs

Much recent research has focused on the question of the high species richness and population densities of fish inhabiting coral reefs. Both density-dependent (post-recruitment) and density-independent (pre-recruitment) processes have been implicated in the regulation of these assemblages. These processes may operate at a variety of spatial and temporal scales. Here we review recent progress and point to some of the key questions for future research.     

High diversity,abundance and distinct fish assemblages on submerged coral reef pinnacles compared to shallow emergent reefs

Coral Reefs - Coral reefs exhibit consistent patterns in biodiversity across multiple spatial scales, from local to global clines in species richness, abundance and community structure. Knowledge...     

Demographic aspects of the soft coral Sinularia flexibilis leading to local dominance on coral reefs

We evaluated the role that demography may play in the formation of local aggregations of Sinularia flexibilis (Quoy & Gaimard, 1833), a soft coral that commonly dominates inshore coral reefs of the Great Barrier Reef (GBR), Australia. Two populations on inshore reefs of the Palm Islands were censused once a year for 3 years, starting 10 mo after the extensive bleaching mortality in early 1998. Larger colonies became more prevalent over time; mean colony size increasing by 35%, from 276 cm² in 1998 to 373 cm² in 2000. Growth rates were size dependent, with smaller colonies growing proportionally faster than larger colonies. Change in size relative to initial size indicated an expected mean annual growth of 128 cm² for a 50-cm² colony. Zero growth was predicted at 532±21cm², with colonies larger than this likely to undergo fission or shrink. Forty-three percent of colonies were undergoing fission at any time at both localities. Most new colonies were produced by fission (70%, n=285), with the remainder produced by the recruitment of sexually produced larvae (19%) or by colony translocation (11%). The sexual and asexual recruitment rates were 0.24 and 1.0 recruits m- ² year−¹, respectively. Opportunistic recruitment and rapid growth following disturbances are commonly assumed to be the mechanisms leading soft corals to dominate locally. In this study, these mechanisms operated more slowly than expected, with no net change in population size.     

Estimates of species diversity of free-living marine isopod crustaceans on coral reefs

 A core group of isopod crustacean genera appears to be present at many coral reef sites. Within these genera, however, species show high local endemicity. Based on the estimated percentage of endemism for the Atlantic (19% for individual sites, 90% for the tropical western Atlantic as a unit), the Indian Ocean (50%), the eastern-central Pacific (80%), and the western Pacific (40%), it is estimated that there are some 5,000 to 13,000 isopod species in the world’s coral reefs, and that some 2,000 to 6,000 of these are endemics. (At present, approximately 4,400 species of marine and 560 species of freshwater isopods have been described.) Based on the crudely estimated relative abundances of other peracaridan crustaceans in coral reefs (compared to isopods), the total diversity of reef amphipods, tanaidaceans, cumaceans, and mysidaceans is approximately 54,500 species. Accepted: 29 August 1997     

High rate of prey consumption in a small predatory fish on coral reefs

Coral Reefs - Small piscivores are regarded as important regulators of the composition of coral reef fish communities, but few studies have investigated their predatory ecology or impact on...     

Black reefs: iron-induced phase shifts on coral reefs

The Line Islands are calcium carbonate coral reef platforms located in iron-poor regions of the central Pacific. Natural terrestrial run-off of iron is non-existent and aerial deposition is extremely low. However, a number of ship groundings have occurred on these atolls. The reefs surrounding the shipwreck debris are characterized by high benthic cover of turf algae, macroalgae, cyanobacterial mats and corallimorphs, as well as particulate-laden, cloudy water. These sites also have very low coral and crustose coralline algal cover and are call black reefs because of the dark-colored benthic community and reduced clarity of the overlying water column. Here we use a combination of benthic surveys, chemistry, metagenomics and microcosms to investigate if and how shipwrecks initiate and maintain black reefs. Comparative surveys show that the live coral cover was reduced from 40 to 60% to <10% on black reefs on Millennium, Tabuaeran and Kingman. These three sites are relatively large (>0.75 km²). The phase shift occurs rapidly; the Kingman black reef formed within 3 years of the ship grounding. Iron concentrations in algae tissue from the Millennium black reef site were six times higher than in algae collected from reference sites. Metagenomic sequencing of the Millennium Atoll black reef-associated microbial community was enriched in iron-associated virulence genes and known pathogens. Microcosm experiments showed that corals were killed by black reef rubble through microbial activity. Together these results demonstrate that shipwrecks and their associated iron pose significant threats to coral reefs in iron-limited regions.     

Facilitation in Caribbean coral reefs: high densities of staghorn coral foster greater coral condition and reef fish composition

Recovery of the threatened staghorn coral (Acropora cervicornis) is posited to play a key role in Caribbean reef resilience. At four Caribbean locations (including one restored and three extant populations), we quantified characteristics of contemporary staghorn coral across increasing conspecific densities, and investigated a hypothesis of facilitation between staghorn coral and reef fishes. High staghorn densities in the Dry Tortugas exhibited significantly less partial mortality, higher branch growth, and supported greater fish abundances compared to lower densities within the same population. In contrast, partial mortality, branch growth, and fish community composition did not vary with staghorn density at the three other study locations where staghorn densities were lower overall. This suggests that density-dependent effects between the coral and fish community may only manifest at high staghorn densities. We then evaluated one facilitative mechanism for such density-dependence, whereby abundant fishes sheltering in dense staghorn aggregations deliver nutrients back to the coral, fueling faster coral growth, thereby creating more fish habitat. Indeed, dense staghorn aggregations within the Dry Tortugas exhibited significantly higher growth rates, tissue nitrogen, and zooxanthellae densities than sparse aggregations. Similarly, higher tissue nitrogen was induced in a macroalgae bioassay outplanted into the same dense and sparse aggregations, confirming greater bioavailability of nutrients at high staghorn densities. Our findings inform staghorn restoration efforts, suggesting that the most effective targets may be higher coral densities than previously thought. These coral-dense aggregations may reap the benefits of positive facilitation between the staghorn and fish community, favoring the growth and survivorship of this threatened species.     

Precision and accuracy of visual census technique for fish assemblages on coral patch reefs

Synopsis A visual census technique is described in which the results of three separate enumerations of fish at a site are combined to produce a best estimate of the fish fauna present. Its precision and accuracy are examined, and compared to those of censuses obtained by modifications of the technique. Visual censuses can display high repeatability, but they seldom (if ever) completely sample the fish present at a site. Accuracy varies with technique used. In our tests, the preferred method yielded 82% of species and 75% of individuals known to be present and potentially censurable at the time the observations were made. Visual censuses are of comparable accuracy to ichthyocide collections of unenclosed sites, but the two methods sample different components of the total fish fauna. It is important when using visual censuses to remember that their accuracy is not 100%.     

Large scale spatial and temporal variation in recruitment to fish populations on coral reefs

Summary Visual census was used to sample young of the year of fish species recruited to each of two habitats on seven lagoonal platform reefs of the Capricorn-Bunker Group, Great Barrier Reef. The reefs sampled span an area 70 km in extent. In 1983, 62 species from 13 families were detected as recruits on reef slope sites. The total number of cruits, and the number of each of 6 of 16 species tested, differed significantly among reefs, despite the fact that differences among sites within reefs did not exist, and that sampled slopes were chosen to be hydrographically, and physiographically as similar as possible. Lagoonal patch reefs were sampled in two years. In 1982, 76 species of 11 families occurred as recruits. In 1983, 86 species of 12 families were recorded. All of 22 species common enough to test showed some significant variation in abundance among reefs, years, or both. For 9 species, significant year x reef interactions occurred, demonstrating that relative recruitment success among reefs varied between years. Reasons for the substantial levels of variability are discussed, and implications for the organisation of reef fish communities are considered.