Coral–algal phase shifts alter fish communities and reduce fisheries production

Anthropogenic stress has been shown to reduce coral coverage in ecosystems all over the world. A phase shift towards an algae‐dominated system may accompany coral loss. In this case, the composition of the reef‐associated fish assemblage will change and human communities relying on reef fisheries for income and food security may be negatively impacted. We present a case study based on the Raja Ampat Archipelago in Eastern Indonesia. Using a dynamic food web model, we simulate the loss of coral reefs with accompanied transition towards an algae‐dominated state and quantify the likely change in fish populations and fisheries productivity. One set of simulations represents extreme scenarios, including 100% loss of coral. In this experiment, ecosystem changes are driven by coral loss itself and a degree of habitat dependency by reef fish is assumed. An alternative simulation is presented without assumed habitat dependency, where changes to the ecosystem are driven by historical observations of reef fish communities when coral is lost. The coral–algal phase shift results in reduced biodiversity and ecosystem maturity. Relative increases in the biomass of small‐bodied fish species mean higher productivity on reefs overall, but much reduced landings of traditionally targeted species.     

荣成俚岛人工鱼礁区游泳动物群落特征及其与主要环境因子的关系

根据2009年5月份至2010年2月份于山东荣成俚岛人工鱼礁区和对照区4个季度月的游泳动物及同步的环境调查数据,采用Margalef种类丰富度指数,Shannon-Wiener多样性指数,Pielou均匀度指数分析了鱼礁区游泳动物群落组成和物种多样性的时空分布特征,运用梯度分析法对调查区域游泳动物群落格局与环境因子进行排序分析,结合蒙特卡罗检验确定影响鱼礁区游泳动物群落结构的控制因子。调查期间共捕获游泳动物18种,隶属于8目15科,优势种为许氏平鲉(Sebastes schlegeli)、大泷六线鱼(Hexagrammos otakii)和斑头六线鱼(Hexagrammos agrammus),其中许氏平鲉在渔获量组成中占绝对优势(41%)。鱼礁区游泳动物的渔获种类数和渔获量高于各自对照区,但差异不显著(P>0.05), 季度月平均CPUE最高值出现在近岸礁区,达到830.24 g ·网^-1·d^-1。群落组成特征值上,春秋季的参数值最大,鱼礁区与对照区间并无显著差异(P>0.05)。聚类分析表明,春季、夏季和冬季均是季度月内不同区域的样方相似性较高,而秋季的远岸礁区及其对照区样方与春季各区域样方的相似性高于秋季近岸礁区及其对照区。RDA分析表明,水温对游泳动物群落组成变化的解释量为37.1%,是主要的解释因子(P=0.002)。水温和水深等环境因子主导了俚岛人工鱼礁区游泳动物群落结构的时空变化。     

A standard unit for monitoring recruitment of fishes to coral reef rubble

We developed a benthic standard monitoring unit for recruitment of fishes (SMURF) to sample fishes that settle in coral rubble and tested it on three fringing reefs on the west coast of Barbados, West Indies. These rubble SMURFs are inexpensive to construct and permit newly settled fishes, including cryptic, secretive and small species, to be quickly and fairly completely (> 83%) removed by divers without returning the unit to the surface. Over a 3-month period involving 9 collections, 32 units on 3 reefs yielded 948 newly settled fishes belonging to 28 taxa (3.3 ± 0.2 S.E. recruits per sample, n = 287) in addition to numerous crustaceans. Most specimens were Sparisoma spp. (Scaridae) (41.8%) and Stegastes partitus (Pomacentridae) (13.6%), with moderate numbers of Scarus spp. (Scaridae) (8.6%), Lythrypnus spp. (Gobiidae) (7.4%), and Pseudogramma gregoryi (Serranidae) (7.2%). Sampling SMURFs at 1-day rather than 11-day intervals yielded approximately twice as many fish, although some taxa were not affected by sampling frequency, indicating taxon-specific differences in post-settlement loss rates. Netting, intended to reduce predation on settlers, did not affect settlement estimates unless algae grew on it, suggesting that the SMURF design minimizes fish predation even in the absence of the netting. Over time, however, the netting resulted in substantial algal growth that was associated with an increase in abundance of Sparisoma spp. and a decrease in the abundance of S. partitus. This suggests that settler estimates are more affected by microhabitat changes from algal growth than by predation. During low settlement periods, SMURFs performed better than light-traps in assessing the daily input of new settlers of two abundant taxa. Eight SMURFs per reef were sufficient to demonstrate differences in settlement rates and temporal correlations in settlement within and among reefs. This study underscores the potential of benthic SMURFs to measure settlement of coral reef fishes at a range of spatio-temporal scales with moderate research cost and effort, and to facilitate the study of the settlement stage of several taxa.     

Current Patterns and the Distribution of Benthic Habitats in a Coastal Lagoon of Mauritius

The oceanography around Mauritius (in the Western Indian Ocean) remains largely unstudied, hence there is an acute scarcity of marine environmental data for management purposes. Rigorous water depth and current measurements were made on a system of grids inside Le Morne lagoon (in the south western part of Mauritius) in March–April 2000 to generate semi-quantitative models of general flow pattern in the form of contour maps using SURFER 6 computer programme. A simultaneous survey on composition of bottom cover was conducted to examine possible relationships with current speed. A separate investigation recorded surface and bottom currents prevailing amongst various habitat types to demonstrate the nature of the resulting damping effect on surface current speed. Significant correlations generated from data analysis were discussed as a basis for real biophysical relationships. Some of the limitations in the current analysis and some of the seemingly contradictory results are acknowledged and addressed in the light of the general assumption that the structure of the lagoon is conditioned by current speeds. Much stronger current speeds just outside the reef (e.g. >0.5 m s⁻¹) than inside the lagoon (e.g. <0.32 m s⁻¹) indicated a substantial slow-down of water current by the reef barrier. Inshore bottom currents were weaker than at the surface and current speed correlated well with water depth. Bottom and surface current directions were generally similar, i.e. going northward during flood tide and southward during ebb. The lagoon would be classified as ‘restricted’, exhibiting well-defined tidal circulation, which is modified by wind forcing. Dense fields of branching Acropora corals slowed down surface current speed by as much as 87%, but the relationship between current speed and bottom cover appears to be variable, depending on the specific location within the lagoon under consideration. The contour plots of the flow pattern model generated reasonably high qualitative modelling of spatial current speed pattern in the lagoon, with stronger currents generally along the reef areas, at the reef passes and in the deeper zones. However, these plots did not match closely those displaying distribution of bottom cover, thus confirming results obtained from pair-wise correlation tests, namely the lack of a significant relationship between current speed and bottom cover. Most of the correlations would appear to represent biological relationships, with different types of communities enabling or excluding other types. Thus, the biophysical structure of the lagoon would be driven a priori by the distribution and abundance of corals rather than current speeds, which contradicts the above hypothesis. Recurrent natural hazards subject the benthic communities to a state of ‘perpetual knock-down and recovery’. Recovery, however, can be seriously impaired by the chronic ongoing degradation of the coastal marine environment of Mauritius. An urgent review of its coastal zone management and protection strategy would be desirable for the island.     

Sexual reproduction of Acropora reef corals at Moorea, French Polynesia

Little information is available on reproductive processes among corals in isolated central Pacific reef regions, including French Polynesia. This study examined the timing and mode of sexual reproduction for Acropora reef corals at Moorea. Spawning was observed and/or inferred in 110 Acropora colonies, representing 12 species, following full moon periods in September through November 2002. Gamete release was observed and inferred in four species of Acropora between 9 and 13 nights after the full moon (nAFM) in September 2002. Twelve Acropora spp. spawned gametes between 5 and 10 nAFM in October 2002, with six species spawning 7 nAFM and four species spawning 9 nAFM. In November 2002, spawning of egg and sperm bundles was observed and inferred in 27 colonies of Acropora austera, 6 nAFM. These are the first detailed records of spawning by Acropora corals in French Polynesia.     

Grazing pressure of herbivorous coral reef fishes on low coral-cover reefs

The impact of grazing by herbivorous fishes (Acanthuridae, Scaridae, and Pomacentridae) on low coral-cover reefs was assessed by measuring rates of benthic algal production and consumption on inshore and offshore reefs in the upper Florida Keys. Algal production rates, determined in situ with caged and uncaged experimental plates, were low (mean 1.05 g C m⁻² day⁻¹) and similar among reef types. Algal consumption rates were estimated using two different models, a detailed model incorporating fish bite rates and algal yield-per-bite for one species extrapolated to a guild-wide value, and a general regression relating fish biomass to algal consumption. Algal consumption differed among reef types: a majority of algal production was consumed on offshore reefs (55–100%), whereas consumption on inshore patch reefs was 31–51%. Spatial variation in algal consumption was driven by differences in herbivorous fish species composition, density, and size-structure among reef types. Algal consumption rates also varied temporally due to seasonal declines in bite rates and intermittent presence of large-bodied, vagile, schooling species. Spatial coherence of benthic community structure and temporal stability of algal turf over 3 years suggests that grazing intensity is currently sufficient to limit further spread of macroalgal cover on these low coral-cover reefs, but not to exclude it from the system.     

Hormonally mediated maternal effects shape offspring survival potential in stressful environments

In most egg-laying vertebrates, maternal responses to stressful conditions are translated into the release of glucocorticoid hormones such as cortisol, which are then transmitted to their developing embryos. Although such maternally transmitted hormonal resources have been shown to influence or even interfere with the optimal developmental trajectories of offspring in many taxa, their influence on the dynamics of wild fish populations remains largely unexplored. Here, we examined the extent to which simulated hormonally mediated maternal effects influence the development and early survival of the coral reef damselfish, Pomacentrus amboinensis. Concentrations of cortisol in the eggs were manipulated within naturally occurring limits by immersion. We found that the proportion of embryos that delayed hatching when exposed to high levels of cortisol was considerably lower than in the other two treatments (low cortisol dose and control). High cortisol levels in P. amboinensis eggs resulted in increased egg mortality and greater asymmetry in hatchlings. For embryos that successfully hatched, individuals from the elevated cortisol treatments (especially low dose) survived longer after hatching. Although individuals that originated from eggs with elevated cortisol levels survived longer after hatching, they may not gain an overall survival advantage. Our results suggest that subtle increases in the allocation of maternally derived hormones, such as cortisol, to offspring are a direct way for stressed mothers to endow their young with an immediate survival advantage. We propose that this immediate benefit outweighs the developmental costs which may be expressed as reduced fitness at later life stages. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Searching for heat in a marine biodiversity hotspot

Coral reefs exhibit highly congruent patterns of biodiversity, with a prominent hotspot in the Indo-Australian Archipelago (IAA). Unlike many terrestrial systems, the IAA hotspot exhibits extensive latitudinal and longitudinal biodiversity gradients. Conflicting hypotheses have highlighted the importance of the area as a centre of origin, overlap or accumulation, with the location of endemics being used as the primary criterion for testing these hypotheses, by identifying the presumed geographical origins of species. We evaluate the utility of marine endemics for resolving these hypotheses, and examine recent molecular phylogenetic evidence for coral reef species that has revealed the antiquity of the endemics and the other species that make up this hotspot. These analyses emphasize the importance of the IAA in the survival rather than the origins of species.     

Genetic structure of juvenile cohorts of bicolor damselfish ( Stegastes partitus ) along the Mesoamerican barrier reef: chaos through time

Dispersal in marine systems is a critical component of the ecology, evolution, and conservation of such systems; however, estimating dispersal is logistically difficult, especially in coral reef fish. Juvenile bicolor damselfish (Stegastes partitus) were sampled at 13 sites along the Mesoamerican Barrier Reef System (MBRS), the barrier reefs on the east coast of Central America extending from the Yucatan, Mexico to Honduras, to evaluate genetic structure among recently settled cohorts. Using genotype data at eight microsatellite loci genetic structure was estimated at large and small spatial scales using exact tests for allele frequency differences and hierarchical analysis of molecular variance (AMOVA). Isolation-by-distance models of divergence were assessed at both spatial scales. Results showed genetic homogeneity of recently settled S. partitus at large geographic scales with subtle, but significant, genetic structure at smaller geographic scales. Genetic temporal stability was tested for using archived juvenile S. partitus collected earlier in the same year (nine sites), and in the previous year (six sites). The temporal analyses indicated that allele frequency differences among sites were not generally conserved over time, nor were pairwise genetic distances correlated through time, indicative of temporal instability. These results indicate that S. partitus larvae undergo high levels of dispersal along the MBRS, and that the structure detected at smaller spatial scales is likely driven by stochastic effects on dispersal coupled with microgeographic effects. Temporal variation in juvenile cohort genetic signature may be a fundamental characteristic of connectivity patterns in coral reef fishes, with various species and populations differing only in the magnitude of that instability. Such a scenario provides a basis for the reconciliation of conflicting views regarding levels of genetic structuring in S. partitus and possibly other coral reef fish species.