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

Local human impacts disrupt depth-dependent zonation of tropical reef fish communities

The influence of depth and associated gradients in light, nutrients and plankton on the ecological organization of tropical reef communities was first described over six decades ago but remains untested across broad geographies. During this time humans have become the dominant driver of planetary change, requiring that we revisit historic ecological paradigms to ensure they capture the dynamics of contemporary ecological systems. Analysing >5,500 in-water reef fish surveys between 0 and 30 m depth on reef slopes of 35 islands across the Pacific, we assess whether a depth gradient consistently predicts variation in reef fish biomass. We reveal predictable ecological organization at unpopulated locations, with increased biomass of planktivores and piscivores and decreased primary consumer biomass with increasing depth. Bathymetric steepness also had a striking influence on biomass patterns, primarily for planktivores, emphasizing potential links between local hydrodynamics and the upslope propagation of pelagic subsidies to the shallows. However, signals of resource-driven change in fish biomass with depth were altered or lost for populated islands, probably due to depleted fish biomass baselines. While principles of depth zonation broadly held, our findings expose limitations of the paradigm for predicting ecological dynamics where human impacts confound connections between ecological communities and their surrounding environment.

Immersion Time Determines Performance of Artificial Habitats in Commercial Harbours by Changing Biodiversity of Colonising Invertebrate Assemblages

In highly modified coastal environments, such as commercial harbours, the installation of artificial habitats has garnered support as a means of enhancing local biological recruitment and connectivity. The success of these measures depends largely on the patterns of species colonisation. Using post-installation monitoring data, we compared the composition of assemblages of invertebrates colonising artificial habitats that were immersed for different periods (~6 vs. ~18 months) in three commercial harbours along the French Mediterranean coast. The artificial habitats were colonised by taxonomically diverse invertebrate assemblages of ecological and economic importance, including molluscs, crustaceans, and echinoids. Composition differed significantly with the immersion time of the artificial habitats, with total abundance, species richness, and evenness being significantly higher after ~18 than after ~6 months of immersion, indicating that long periods are necessary to enrich these new habitats with economically and ecologically important species. These results can inform restoration protocols and emphasise the value of post-installation monitoring programs.

The El Niño Southern Oscillation drives multidirectional inter-reef larval connectivity in the Great Barrier Reef

The El Niño Southern Oscillation (ENSO) is the strongest source of interannual global climate variability, and extreme ENSO events are projected to increase in frequency under climate change. Interannual variability in the Coral Sea circulation has been associated with ENSO, although uncertainty remains regarding ENSO's influence on hydrodynamics and larval dispersal in the adjacent Great Barrier Reef (GBR). We investigated larval connectivity during ENSO events from 2010 to 2017 throughout the GBR, based on biophysical modelling of a widespread predatory reef fish, Lutjanus carponotatus. Our results indicate a well-connected system over the study period with high interannual variability in inter-reef connectivity associated with ENSO. Larval connectivity patterns were highly correlated to variations in the Southern Oscillation Index (SOI). During El Niño conditions and periods of weak SOI, larval dispersal patterns were predominantly poleward in the central and southern regions, reversing to a predominant equatorward flow during very strong SOI and extreme La Niña conditions. These ENSO-linked connectivity patterns were associated with positive connectivity anomalies among reefs. Our findings identify ENSO as an important source of variation in larval dispersal and connectivity patterns in the GBR, which can influence the stability of population dynamics and patterns of biodiversity in the region.

Young fishes persist despite coral loss on the Great Barrier Reef

Unprecedented global bleaching events have led to extensive loss of corals. This is expected to lead to extensive losses of obligate coral-dependent fishes. Here, we use a novel, spatially-matched census approach to examine the nature of fish-coral dependency across two mass coral bleaching events. Despite a >40% loss of coral cover, and the ecological extinction of functionally important habitat-providing Acropora corals, we show that populations of obligate coral-dependent fishes, including Pomacentrus moluccensis, persisted and - critically - recruitment was maintained. Fishes used a wide range of alternate reef habitats, including other coral genera and dead coral substrata. Labile habitat associations of 'obligate' coral-dependent fishes suggest that recruitment may be sustained on future reefs that lack Acropora, following devastating climatic disturbances. This persistence without Acropora corals offers grounds for cautious optimism; for coral-dwelling fishes, corals may be a preferred habitat, not an obligate requirement.

Contrasting patterns in habitat selection and recruitment of temperate reef fishes among natural and artificial reefs

Artificial reefs, a common management tool for stock enhancement of recreational fisheries and marine habitat restoration, have been deployed all over the world. However, little is known about the attractiveness of artificial compared to natural reefs to reef fishes. Here we investigated the habitat preferences of three reef fish species: Trachinops caudimaculatus, Vincentia conspersa and Trinorfoklia clarkei through the observation of recruitment patterns to three study habitats: Reef Ball reefs, custom-designed artificial reefs, and natural reefs in Port Phillip Bay, Victoria, Australia. Additionally, we examined habitat preferences of new recruits of T. caudimaculatus and V. conspersa using laboratory-based habitat choice experiments. In general, T. caudimaculatus recruitment was at least twice as high on natural reefs compared to both artificial reefs, whereas V. conspersa recruitment was almost three times greater on Reef Ball reefs compared to the other two habitats. T. clarkei recruited in equal numbers across all habitats. However, in the laboratory experiments T. caudimaculatus recruits selected the Reef Ball reef almost three times as often as the other two habitats, while V. conspersa exhibited no habitat preference. Little is known about the growth, condition, survival or reproduction of individuals that occupy artificial reefs. In areas where habitat is not limiting, the higher preference or equal attractiveness of some artificial habitats may negatively influence fish populations, if larvae are redirected to poorer quality artificial reef habitat, that lead to lower fitness advantages.

The short-lived neon damsel Pomacentrus coelestis: implications for population dynamics

Daily increments of Pomacentrus coelestis, an abundant and well-studied fish, were validated for the life of the fish and depending on the location, age-maxima were estimated to be 127-160 days on reefs separated by tens to hundreds of kilometres on the Great Barrier Reef. This contrasts with congeners and other damselfishes that live for 5 years or more. Otoliths of P. coelestis were thinner and had different patterns of banding when compared with relatively long-lived congeners. It is suggested that banding patterns in P. coelestis may be related to patterns of maturation and spawning. The consequences of a short life would have a great influence on the population dynamics of this widespread species. Further, the demographics and habitat preferences of this species suggest rapid colonization and establishment of breeding populations that would quickly change the relative abundance of sympatric fishes.

Regime shift indicators fail under noise levels commonly observed in ecological systems

Ecological regime shifts are rapid, potentially devastating changes in ecosystem state that last for extended periods of time. Previous theoretical work has generated numerous early-warning indicators of regime shifts, some of which have been empirically demonstrated in closed ecological systems. Here we evaluated a suite of indicators using a previously studied three-species model under conditions likely to be observed in field studies of open ecological systems. Simulations included large correlated fluctuations in extrinsic noise and a rapidly changing driving variable, while indicators were calculated using sparsely sampled time series. All indicators performed poorly under these conditions, particularly during the beginning of the regime shift. Overall, the best performing indicator was a rise in variance. Future research should focus on methods for setting benchmark values of early warning indicators and for identifying indicators that work for sparsely sampled data sets.

Effects of habitat modification on coastal fish assemblages

The purpose of this study was to assess the influence of anthropogenic modification of coastal habitats on fish assemblages in Taiwan, comparing the abundance, species richness and taxonomic composition of fishes on natural v. artificial habitats. While there was no significant variation in the abundance or richness of fishes between natural and artificial habitats, the species composition of fishes in artificial habitats was significantly different from that of natural habitats. Natural reefs were characterized by greater abundance of Stethojulis spp. (Labridae), Abudefduf spp. (Pomacentridae) and Thalassoma spp. (Labridae), whereas anthropogenic habitats were dominated by Parupeneus indicus (Mullidae), Pempheris oualensis (Pempheridae) and Parapriacanthus ransonneti (Pempheridae). In general, it appears that specialist reef-associated species are being replaced with fishes that are much more generalist in their habitat-use. The loss of natural coastal habitats may threaten some species that cannot live in anthropogenically altered habitats, though the overall abundance and diversity of coastal fishes was not significantly different between natural and artificial habitats in Taiwan.

Interhemispheric comparison of recruitment to intertidal communities: pattern persistence and scales of variation

Recruitment variation can be a major source of fluctuation in populations and communities, making it difficult to generalize results. Determining the scales of variation and whether spatial patterns in the supply of individuals are persistent over time can provide insight into spatial generality and the application of conservation and metacommunity models. We examined these issues using eight-year-long data sets of monthly recruitment of intertidal mussels (Mytilus spp., Perumytilus purpuratus, Semimytilus algosus, Brachidontes granulata) and barnacles (Balanus glandula, Chthamalus dalli, Jehlius cirratus, Notochthamalus scabrosus) at sites spanning > 900 km along the coasts of Oregon-northern California (OR-NCA, 45.47-39.43 degrees N) and central Chile (CC, 29.5-34.65 degrees S). We evaluated four general "null" hypotheses: that despite different phylogenies and great spatial separation of these taxa, their similar life history strategies and environmental settings lead to similar patterns of recruitment (1) between hemispheres, (2) in time, (3) in space, and (4) at larger and smaller spatial scales. Hypothesis 1 was rejected: along the OR-NCA coast, rates of recruitment were between two and three orders of magnitude higher, and patterns of seasonality were generally stronger and more coherent across space and time than along CC. Surprisingly, however, further analysis revealed regularities in both time and space for all species, supporting hypotheses 2 and 3. Temporal decorrelation scales were 1-3 months, and characteristic spatial scales of recruitment were approximately 250 km. Contrary to hypothesis 4, for the ecologically dominant species in both hemispheres, recruitment was remarkably persistent at larger mesoscales (kilometers) but was highly stochastic at smaller microscales (meters). Across species, increased recruitment variation at large scales was positively associated with increased persistence. Our results have several implications. Although the two regions span distinct latitudinal ranges, potential forcing processes behind these patterns include similar large-scale climates and topographically locked hydrographic features, such as upwelling. Further, spatial persistence of the recruitment patterns of most species at the mesoscale supports the view that marine protected areas can be powerful conservation and management tools. Finally, persistent and yet contrasting spatial patterns of recruitment among competing species suggest that recent metacommunity models might provide useful representations of the mechanisms involved in species coexistence.

Demographic theory of coral reef fish populations with stochastic recruitment: comparing sources of population regulation

The effects of three forms of density-dependent regulation were explored in model coral reef fish populations: top-down (predation), bottom-up (competition for food), and pelagic (non-reef-based mechanisms) control. We describe the demographic responses of both biomass and numbers of adult fish, predicting the mean and the variance of temporal fluctuations resulting from stochastic recruitment of juveniles. We find that top-down control acts by suppressing variability of numbers of fish, which in turn suppresses the variability of biomass. Bottom-up control has no effect on fluctuations of numbers of fish, though it strongly reduces fluctuations of biomass. Because fecundity of fish is directly linked to body mass, the regulation of biomass tightly regulates reproductive output independently of the number of individuals in the population. Finally, populations under pelagic control experience bounded fluctuations of biomass and numbers directly proportional to the bounded fluctuations of recruitment. The demographic signatures predicted from both bottom-up and pelagic control are consistent with current evidence supporting the recruitment limitation hypothesis in reef fish ecology. We propose tests to discriminate the dominant mode of density-dependent regulation using qualitative trends in time series demographic data across environmental clines.

Ecological structure of assemblages of coral reef fishes on isolated patch reefs

A 9-year study of the structure of assemblages of fish on 20 coral patch reefs, based on 20 non-manipulative censuses, revealed a total of 141 species from 34 families, although 40 species accounted for over 95% of sightings of fish. The average patch reef was 8.5 m² in surface area, and supported 125 fish of 20 species at a census. All reefs showed at least a two-fold variation among censuses in total numbers of fish present, and 12 showed ten-fold variations. There was also substantial variation in the composition and relative abundances of species present on each patch reef, such that censuses of a single patch reef were on average about 50% different from each other in percent similarity of species composition (Czekanowski's index). Species differed substantially in the degree to which their numbers varied from census to census, and in the degree to which their dispersion among patch reefs was modified from census to census. We characterize the 40 most common species with respect to these attributes. The variations in assemblage structure cannot be attributed to responses of fish to a changing physical structure of patch reefs, nor to the comings and goings of numerous rare species. Our results support and extend earlier reports on this study, which have stressed the lack of persistant structure for assemblages on these patch reefs. While reef fishes clearly have microhabitat preferences which are expressed at settlement, the variations in microhabitat offered by the patch reefs are insufficient to segregate many species of fish by patch reef. Instead, at the scale of single patch reefs, and, to a degree, at the larger scale of the 20 patch reefs, most of the 141 species of fish are distributed without regard to differences in habitat structure among reefs, and patterns of distribution change over time. Implications for general understanding of assemblage dynamics for fish over more extensive patches of reef habitat are considered.

Fine-scale temporal variation in recruitment of a temperate demersal fish: the importance of settlement versus post-settlement loss

In order to understand variability in recruitment to populations of benthic and demersal marine species, it is critical to distinguish between the contributions due to variations in larval settlement versus those caused by post-settlement mortality. In this study, fine-scale (1-2 days) temporal changes in recruit abundance were followed through an entire settlement season in a temperate demersal fish in order to determine 1) how dynamic the process of recruitment is on a daily scale, 2) whether settlement and post-settlement mortality are influenced by habitat structure and conspecific density, and 3) how the relationship between settlement and recruitment changes over time. "Settlement" is considered to be the arrival of new individuals from the pelagic habitat, and "recruitment" is defined as the number of individuals surviving arbitrary periods of time after settlement. Replicate standardized habitat units were placed in 2 spatial configurations (clumped and randomly dispersed) and monitored visually for cunner (Tautogolabrus adspersus) settlement and recruitment every 1-2 days throughout the settlement season. The process of recruitment in T. adspersus was highly variable at a fine temporal scale. Changes in the numbers of recruits present on habitat units were due to both settlement of new individuals and mortality of animals previously recruited. The relative importance of these two processes appeared to change from day to day. The magnitude of the change in recruit number did not differ between the clumped and random habitats. However, post-settlement loss was significantly greater on randomly dispersed than clumped habitats. During several sampling dates, the extent of the change in recruit abundance was correlated with the density of resident conspecifics; however, on other dates no such relationship appeared to exist. Despite the presence of significant relationships between the change in recruit number and density, there was no evidence of either density-dependent mortality or settlement. Initially, there was a strong relationship between settlement and recruitment; however, this relationship weakened over time. Within 2 months after the cessation of settlement, post-settlement loss was greater than 99%, and no correlation remained between recruitment and the initial pattern of settlement. The results of this study demonstrate that the spatial arrangement of the habitat affects the rate and intensity of post-settlement loss. Counter to much current thinking, this study suggests that in order to understand the population ecology of reef fishes, knowledge of what habitats new recruits use and how mortality varies with structural aspects of the habitats is essential.