Is increased calcarinid (foraminifera) abundance indicating a larger role for macro-algae in Indonesian Plio-Pleistocene coral reefs?

An important question in coral reef ecology is whether algal abundance in coral reef eco-systems is a natural phenomenon, or has increased as a result of coral reef degradation ultimately resulting in coral–algal regime shifts. Regime shifts, from coral to macro-algae dominated, alter the three-dimensional habitat structure in coral reef ecosystems. Surprisingly, few studies have looked at the effects for species that inhabit the reefs without being the architects of the three-dimensional structure. In this study the effects of a change in habitat characteristics on the community structure of large benthic foraminifera (LBF) is compared between an area with high (Kepulauan Seribu) and lower (Spermonde Archipelago) anthropogenic influence. The results indicate a general relationship between habitat and LBF assemblage structure. The largest difference was observed in shallow habitats. Habitats dominated by algae are inhabited by a specific group of LBF, the Calcarinidae, and domination of this group increases with higher algal prevalence. The fossil record of this group indicates that they evolved following a major change in settings of the central Indo-West-Pacific coral reefs from land detached platforms to fringing reefs, about 5 million years ago. Understanding the biotic response to this transition in reef morphology and the associated increase in terrestrially derived nutrients forms an excellent challenge to gain insights in present-day threats to coral reef ecosystems.     

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.     

Thermal stress induces persistently altered coral reef fish assemblages

Ecological communities are reorganizing in response to warming temperatures. For continuous ocean habitats this reorganization is characterized by large‐scale species redistribution, but for tropical discontinuous habitats such as coral reefs, spatial isolation coupled with strong habitat dependence of fish species imply that turnover and local extinctions are more significant mechanisms. In these systems, transient marine heatwaves are causing coral bleaching and profoundly altering habitat structure, yet despite severe bleaching events becoming more frequent and projections indicating annual severe bleaching by the 2050s at most reefs, long‐term effects on the diversity and structure of fish assemblages remain unclear. Using a 23‐year time series spanning a thermal stress event, we describe and model structural changes and recovery trajectories of fish communities after mass bleaching. Communities changed fundamentally, with the new emergent communities dominated by herbivores and persisting for >15 years, a period exceeding realized and projected intervals between thermal stress events on coral reefs. Reefs which shifted to macroalgal states had the lowest species richness and highest compositional dissimilarity, whereas reefs where live coral recovered exceeded prebleaching fish richness, but remained dissimilar to prebleaching compositions. Given realized and projected frequencies of bleaching events, our results show that fish communities historically associated with coral reefs will not re‐establish, requiring substantial adaptation by managers and resource users.     

Seaweed fails to prevent ocean acidification impact on foraminifera along a shallow‐water CO2 gradient

Ocean acidification causes biodiversity loss, alters ecosystems, and may impact food security, as shells of small organisms dissolve easily in corrosive waters. There is a suggestion that photosynthetic organisms could mitigate ocean acidification on a local scale, through seagrass protection or seaweed cultivation, as net ecosystem organic production raises the saturation state of calcium carbonate making seawater less corrosive. Here, we used a natural gradient in calcium carbonate saturation, caused by shallow‐water CO₂ seeps in the Mediterranean Sea, to assess whether seaweed that is resistant to acidification (Padina pavonica) could prevent adverse effects of acidification on epiphytic foraminifera. We found a reduction in the number of species of foraminifera as calcium carbonate saturation state fell and that the assemblage shifted from one dominated by calcareous species at reference sites (pH ~8.19) to one dominated by agglutinated foraminifera at elevated levels of CO₂ (pH ~7.71). It is expected that ocean acidification will result in changes in foraminiferal assemblage composition and agglutinated forms may become more prevalent. Although Padina did not prevent adverse effects of ocean acidification, high biomass stands of seagrass or seaweed farms might be more successful in protecting epiphytic foraminifera.     

Relationships between fishes and habitat in rainforest streams in Sabah, Malaysia

Distinct fish assemblages were found at the mesohabitat scale in 14 streams in eastern Sabah, Malaysia. Sites were designated a priori as pool, run or riffle on the basis of physical habitat structure and properties. Principal components analysis of physical habitat data confirmed the validity of the a priori designation with a major axis of three correlated variables: water velocity, depth and substratum type. Canonical discriminant analysis on fish abundance and biomass data confirmed the existence of a specialized assemblage of fishes from riffle areas of all streams. Overall, pool and run assemblages were highly variable, dependent on stream size, but also variable between streams of the same size. Multiple regression of species richness, diversity, abundance and biomass data on principal components revealed significant but low correlations with measured habitat variables. Riffle habitats showed lower species richness and diversity but high abundance. The fish assemblage in riffles was dominated by balitorid species, specialized for fast-water conditions. Pool assemblages had the highest species diversity and were dominated by cyprinid species of a number of morphological and ecological guilds. Run assemblages were intermediate in assemblage characteristics between riffle and pool assemblages. Between-stream variation in assemblage composition was less than within-stream variation. Of 38 species collected, seven could be designated as riffle specialists, 18 as pool specialists and 13 as ubiquitous, although most of the latter showed size-specific habitat use with larger size classes found in slower, deeper water.     

Holocene trends in distribution and diversity of benthic foraminifera assemblages in atoll lagoons,Belize, Central America

Seven benthic foraminiferal assemblages were identified in vibracores through Holocene lagoons of three Belize atoll lagoons (Glovers Reef, Lighthouse Reef, Turneffe Islands). These include (1) the low-diversity Cribroelphidium assemblage (2) the Cribroelphidium-Elphidium assemblage (3) the Quinqueloculina-Triloculina-Peneroplis assemblage (4) the high-diversity miliolid assemblage (5) the Archaias-miliolid assemblage (6) the low-diversity miliolid assemblage, and (7) the mixed assemblage. Altogether, 109 species and 56 genera were identified. The highest diversities are observed in the largest lagoon (Turneffe Islands), whereas one of the smaller lagoons (Glovers Reef) exhibits the lowest diversities during the Holocene. No significant changes in diversity over time occur, however, a slight trend to higher diversity may be observed through the Holocene, suggesting that the foraminiferal faunas in the atolls are in a diversification stage. Faunal diversity in atoll lagoons appears to be controlled largely by habitat size, habitat heterogeneity, and water circulation. Habitat age and water depth only play minor roles. Substrate texture, water depth, and turbidity influence the predominant modes of life of benthic foraminifera encountered in the lagoons (epifaunal versus infaunal versus symbiont-bearing). Time-averaging effects were not observed, even though lagoonal sedimentation rates fluctuate in individual cores and the three lagoons, and despite the fact that sediments are modified through bioturbation by callianassid shrimps. This finding underlines the potential of benthic foraminifera for paleoecological studies in the fossil record of reefs and carbonate platforms.     

Fish assemblages on sunken vessels and natural reefs in southeast Florida,USA

Derelict ships are commonly deployed as artificial reefs in the United States, mainly for recreational fishers and divers. Despite their popularity, few studies have rigorously examined fish assemblages on these structures and compared them to natural reefs. Six vessel-reefs off the coast of southeast Florida were censused quarterly (two ships per month) to characterize their associated fish assemblages. SCUBA divers used a non-destructive point-count method to visually assess the fish assemblages over 13- and 12-month intervals (March 2000 to March 2001 and March 2002 to February 2003). During the same intervals, fish assemblages at neighboring natural reefs were also censused. A total of 114,252 fishes of 177 species was counted on natural and vessel-reefs combined. Mean fish abundance and biomass were significantly greater on vessel-reefs in comparison to surrounding natural reef areas. Haemulidae was the most abundant family on vessel-reefs, where it represented 46% of total fish abundance. The most abundant family on natural reefs was Labridae, where it accounted for 24% of total fish abundance. Mean species richness was significantly greater on vessel-reefs than neighboring natural reefs and also differed among vessel-reefs. Both mean fish abundance and mean species richness were not significantly different between natural reefs neighboring vessel-reefs and natural reefs with no artificial structures nearby. This suggests the vessel-reefs are not, in general, attracting fish away from neighboring natural reefs in our area. Additionally, economically important fish species seem to prefer vessel-reefs, as there was a greater abundance of these species on vessel-reefs than surrounding natural reef areas. Fish assemblage structure on natural versus artificial reefs exhibited a low similarity (25.8%). Although no one species was responsible for more than 6% of the total dissimilarity, fish assemblage trophic structure differed strikingly between the two reef types. Planktivores dominated on vessel-reefs, accounting for 54% of the total abundance. Conversely, planktivores only made up 27% of total abundance on natural reefs. The results of this study indicate vessel-reef fish assemblages are unique and that these fishes may be utilizing food resources and habitat characteristics not accessible from or found at natural reefs in our area. Production may also be occurring at vessel-reefs as the attraction of fish species from nearby natural reefs seems to be minimal. Electronic supplementary material Supplementary material is available for this article at and accessible for authorised users     

Fish Assemblages on Estuarine Artificial Reefs: Natural Rocky-Reef Mimics or Discrete Assemblages?

If the primary goal of artificial reef construction is the creation of additional reef habitat that is comparable to adjacent natural rocky-reef, then performance should be evaluated using simultaneous comparisons with adjacent natural habitats. Using baited remote underwater video (BRUV) fish assemblages on purpose-built estuarine artificial reefs and adjacent natural rocky-reef and sand-flat were assessed 18 months post-deployment in three south-east Australian estuaries. Fish abundance, species richness and diversity were found to be greater on the artificial reefs than on either naturally occurring reef or sand-flat in all estuaries. Comparisons within each estuary identified significant differences in the species composition between the artificial and natural rocky-reefs. The artificial reef assemblage was dominated by sparid species including Acanthopagrus australis and Rhabdosargus sarba. The preference for a range of habitats by theses sparid species is evident by their detection on sand-flat, natural rocky reef and artificial reef habitats. The fish assemblage identified on the artificial reefs remained distinct from the adjacent rocky-reef, comprising a range of species drawn from naturally occurring rocky-reef and sand-flat. In addition, some mid-water schooling species including Trachurus novaezelandiae and Pseudocaranx georgianus were only identified on the artificial reef community; presumably as result of the reef''s isolated location in open-water. We concluded that estuarine artificial reef assemblages are likely to differ significantly from adjacent rocky-reef, potentially as a result of physical factors such as reef isolation, coupled with species specific behavioural traits such as the ability of some species to traverse large sand flats in order to locate reef structure, and feeding preferences. Artificial reefs should not be viewed as direct surrogates for natural reef. The assemblages are likely to remain distinct from naturally occurring habitat comprised of species that reside on a range of adjacent natural habitats.     

The relative influence of local to regional drivers of variation in reef fishes

In this study, fishes and habitat attributes were quantified, four times over 1 year, on three reefs within four regions encompassing a c. 6° latitudinal gradient across south-western Australia. The variability observed was partitioned at these spatio-temporal scales in relation to reef fish variables and the influence of environmental drivers quantified at local scales, i.e. at the scale of reefs (the number of small and large topographic elements, the cover of kelp, fucalean and red algae, depth and wave exposure) and at the scale of regions (mean and maximum nutrient concentrations and mean seawater temperature) with regard to the total abundance, species density, species diversity and the multivariate structure of reef fishes. Variation in reef fish species density and diversity was significant at the regional scale, whereas variation in the total abundance and assemblage structure of fishes was also significant at local scales. Spatial variation was greater than temporal variation in all cases. A systematic and gradual species turnover in assemblage structure was observed between adjacent regions across the latitudinal gradient. The cover of red algae within larger patches of brown macroalgae (a biological attribute of the reef) and the number of large topographic elements (a structural attribute of the reef) were correlated with variation observed at local scales, while seawater temperature correlated with variation at the scale of regions. In conclusion, conservation efforts on reef fishes need to incorporate processes operating at regional scales with processes that shape local reef fish communities at local scales.     

Climate-mediated mechanical changes to post-disturbance coral assemblages

Increasingly severe storms and weaker carbonate materials associated with more acidic oceans will increase the vulnerability of reef corals to mechanical damage. Mechanistic predictions based on measurements of colony mechanical vulnerability and future climate scenarios demonstrate dramatic shifts in assemblage structure following hydrodynamic disturbances, including switches in species' dominance on the reef and thus potential for post-disturbance recovery. Larger colonies are more resistant to factors such as disease and competition for space, and complex morphologies support more associated reef species. Future reefs are thus expected to have lower colony abundances and be dominated by small and morphologically simple, yet mechanically robust species, which will in turn support lower levels of whole-reef biodiversity than do present-day reefs.     

Locomotion in labrid fishes: implications for habitat use and cross-shelf biogeography on the Great Barrier Reef

Coral reefs exhibit marked zonation patterns within single reefs and across continental shelves. For sessile organisms these zones are often related to wave exposure. We examined the extent to which wave exposure may shape the distribution patterns of fishes. We documented the distribution of 98 species of wrasses and parrotfishes at 33 sites across the Great Barrier Reef. The greatest difference between labrid assemblages was at the habitat level, with exposed reef flats and crests on mid- and outer reefs possessing a distinct faunal assemblage. These exposed sites were dominated by individuals with high pectoral fin aspect ratios, i.e. fishes believed to be capable of lift-based swimming which often achieve high speeds. Overall, there was a strong correlation between estimated swimming performance, as indicated by fin aspect ratio, and degree of water movement. We propose that swimming performance in fishes limits access to high-energy locations and may be a significant factor influencing habitat use and regional biogeography of reef fishes.     

Habitat variables determining the occurrence of large benthic foraminifera in the Berau area (East Kalimantan, Indonesia)

Knowledge of the assemblage composition of large benthic foraminifera (LBF) in relation to environmental conditions is needed to interpret fossil records. In this study the assemblage composition of LBF is described for a carbonate shelf with a barrier reef system and some reefs outside the barrier. In a total of 140 samples, 34–35 species of LBF were found. Four clusters, roughly corresponding to substrate type were identified. Several small groups of samples were recognized that were collected locally inside, on, or outside the barrier. Microscale environmental gradients within the substrate or in the benthic boundary layer resulted in spatial differentiation of the microhabitat of each species.     

Hierarchical and scale‐dependent effects of fishing pressure and environment on the structure and size distribution of parrotfish communities

Parrotfishes are considered to have a major influence on coral reef ecosystems through grazing the benthic biota and are also primary fishery targets in the Indo‐Pacific. Consequently, the impact of human exploitation on parrotfish communities is of prime interest. As anthropogenic and environmental factors interact across spatial scales, sampling programs designed to disentangle these are required by both ecologists and resource managers. We present a multi‐scale examination of patterns in parrotfish assemblage structure, size distribution and diversity across eight oceanic islands of Micronesia. Results indicate that correlates of assemblage structure are scale‐dependent; biogeographic distributions of species and island geomorphology hierarchically influenced community patterns across islands whereas biophysical features and anthropogenic pressure influenced community assemblage structure at the within‐island scale. Species richness and phylogenetic diversity increased with greater broad‐scale habitat diversity associated with different island geomorphologies. However, within‐island patterns of abundance and biomass varied in response to biophysical factors and levels of human influence unique to particular islands. While the effect of fishing activities on community composition and phylogenetic diversity was obscured across island types, fishing pressure was the primary correlate of mean parrotfish length at all spatial scales. Despite widespread fishery‐induced pressure on Pacific coral reefs, the structuring of parrotfish communities at broad spatial scales remains a story largely dependent on habitat. Thus, we propose better incorporation of scale‐dependent habitat effects in future assessments of overexploitation on reef fish assemblages. However, strong community‐level responses within islands necessitate an improved understanding of the phylogenetic and functional consequences of altering community structure.     

Spatial Patterns in the Distribution,Diversity and Abundance of Benthic Foraminifera around Moorea (Society Archipelago,French Polynesia)

Coral reefs are now subject to global threats and influences from numerous anthropogenic sources. Foraminifera, a group of unicellular shelled organisms, are excellent indicators of water quality and reef health. Thus we studied a set of samples taken in 1992 to provide a foraminiferal baseline for future studies of environmental change. Our study provides the first island-wide analysis of shallow benthic foraminifera from around Moorea (Society Archipelago). We analyzed the composition, species richness, patterns of distribution and abundance of unstained foraminiferal assemblages from bays, fringing reefs, nearshore and back- and fore-reef environments. A total of 380 taxa of foraminifera were recorded, a number that almost doubles previous species counts. Spatial patterns of foraminiferal assemblages are characterized by numerical abundances of individual taxa, cluster groups and gradients of species richness, as documented by cluster, Fisher α, ternary plot and Principal Component Analyses (PCA). The inner bay inlets are dominated by stress-tolerant, mostly thin-shelled taxa of Bolivina, Bolivinella, Nonionoides, Elongobula, and Ammonia preferring low-oxygen and/or nutrient-rich habitats influenced by coastal factors such as fresh-water runoff and overhanging mangroves. The larger symbiont-bearing foraminifera (Borelis, Amphistegina, Heterostegina, Peneroplis) generally live in the oligotrophic, well-lit back- and fore-reef environments. Amphisteginids and peneroplids were among the few taxa found in the bay environments, probably due to their preferences for phytal substrates and tolerance to moderate levels of eutrophication. The fringing reef environments along the outer bay are characterized by Borelis schlumbergeri, Heterostegina depressa, Textularia spp. and various miliolids which represent a hotspot of diversity within the complex reef-lagoon system of Moorea. The high foraminiferal Fisher α and species richness diversity in outer bay fringing reefs is consistent with the disturbance-mosaic (microhabitat heterogeneity) hypothesis.Calculations of the FORAM Index (FI), a single metric index to assess reef vitality, indicate that all fore- and most back-reef environments support active carbonate accretion and provide habitat suitability for carbonate producers dependent on algal symbiosis. Lowest suitability values were recorded within the innermost bays, an area where natural and increasing anthropogenic influences continue to impact the reefs. The presence of habitat specific assemblages and numerical abundance values of individual taxa show that benthic foraminifera are excellent recorders of environmental perturbations and good indicators useful in modern and ancient ecological and environmental studies.     

Coral and fish distribution patterns in Mafia Island Marine Park, Tanzania: fish–habitat interactions

We examined coral reef communities at 11 sites within Mafia Island Marine Park using a point count method for substrate and visually censused belt transects for fish populations. Multivariate ordinations showed that the benthic habitat differed among reefs. The patterns were mainly attributed to variations in depth, hydrodynamics and benthic composition. In total, the substratum was dominated by dead coral (49%) and algae (25%), with a live coral cover of only 14%. Three hundred and ninety-four fish species belonging to 56 families were recorded. According to MDS-ordinations and RELATE procedures, fish assemblage composition varied among sites in concordance with the habitats provided. Sites with highest proportion of dead coral exhibited highest degree of dispersion in the multivariate ordinations of fish assemblages. Stepwise multiple regression was used to determine the proportion of variance among sites which could be explained by depth, exposure, rugosity, substrate diversity, branching substrate, live coral cover, dead coral cover and different types of algae. The results showed that habitat variables explained up to 92% of the variation in species numbers and in total, and taxon-specific, abundance. Live coral cover was the foremost predictor of both numerical and species abundance, as well as of corallivores, invertivores, planktivores and of the families Pomacentridae, Chaetodontidae and Pomacanthidae. Our results suggest that habitat characteristics play a dominant role in determining fish assemblage composition on coral reefs.     

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.     

Foraminiferal distribution and diversity,Madang Reef and Lagoon,Papua New Guinea

In the Madang Lagoon, on the northern coast of Papua New Guinea (PNG), distinct groups of foraminifera, defined by numerical Q-mode cluster analysis of foraminiferal species occurrences, occupy four major environments and sedimentary regimes, generally aligned parallel to the coast: (1) the harbor and bay inlets, which have large fresh-water runoff and organic detrital inputs; (2) the fringing reefs along the west side of the lagoon which are influenced by coastal factors such as overhanging mangroves or fresh-water runoff; (3) the central lagoon floor which is over 50 m deep and covered with fine sand and patch reefs rising from it; and (4) the reef barrier with adjacent live coral-covered fore-reef slope and generally sandy back-reef slope. The four clusters are also mirrored in both species richness and Fisher alpha diversity analysis. Cluster 4 includes 79 species of large, thick-shelled miliolids, robust agglutinated species, calcarinids, and amphisteginids (Fisher α ≥20) that occur on the coral-rich barrier reef and back-reef. Cluster 3 has 50 species (Fisher α=8–20) and occupies the central lagoon floor. Cluster 2 has 25 or fewer species (Fisher α=2–6) and occurs on the shallow fringing reefs. Cluster 1 is the least diverse (≤7 species, Fisher α ≤2) and occurs in the harbors and bays in the mouths of larger rivers and streams. The larger, endosymbiont-bearing foraminifera (alveolinellids, soritids, amphisteginids, nummulitids, and calcarinids) generally live on the back- and fore-reef slopes and in the lagoon, avoid the organic-rich coastal and harbor habitats, and preferentially dwell in well-lit environments to the bottom of the lagoon. The river mouths and bays are unusual for reef systems because of their high organic content, which creates low-oxygen and nutrient-rich conditions. Here the foraminiferal fauna is dominated by only a few and, for the most part, particularly thin-shelled and highly fragile species. Each faunal group contains a number of numerically abundant indicator species that do not occur in other faunal clusters. This implies low horizontal transport rates within the reef and lagoon complex and signifies that faunal mixing among the cluster groups is limited. Foraminiferal death assemblages may thus be autochthonous and retain information regarding the original community structure. They may also preserve environmental information useful in paleoecological studies and they are good ecological indicators of reef and lagoon habitats.     

The invasive green alga Avrainvillea sp. transforms native epifauna and algal communities on a tropical hard substrate reef

Some introduced species compete directly with native species for resources and their spread can alter communities, while others do not proliferate and remain benign. This study compares community structure and diversity in adjacent areas dominated by the introduced alga Avrainvillea sp. or native algal species on a hard substrate reef. The biomass and species composition of 15 paired plots (30 in total, plot type based on dominance of Avrainvillea sp. or native species) were quantified. Plots dominated by Avrainvillea sp. had a significantly different assemblage of species characterized by lower algal diversity, mostly Dictyota spp. and Laurencia sp., and a higher abundance and diversity of invertebrates, such as small arthropods, polychaetes, and brittlestars. These results suggest that as Avrainvillea sp. becomes more abundant on hard substrate reefs, it will engineer a different community composed of algal epiphytes and an invertebrate assemblage more typically associated with algae in soft sediments.     

Fish distribution and ontogenetic habitat preferences in non-estuarine lagoons and adjacent reefs

We surveyed fish distribution in three lagoons and adjacent forereefs in the British Virgin Islands recording about 28,000 fish from 40 families and 118 species. Canonical correspondence indicated that rock, sand, fleshy algae, gorgonians, mangroves and live hard coral were the most important habitat types influencing fish assemblage composition. About 47% of fishes occurring at more than 10 stations displayed evidence of ontogenetic partitioning between reefs and lagoons but post-settlement ontogenetic life history strategies were quite varied depending on the species. For example Chaetodon striatus juveniles occurred exclusively in lagoons and all sexually mature adults were found on reefs. Some differences were less pronounced as seen in Halichoeres bivittatus where individuals of all sizes occurred on reefs and lagoons, but when analysed it was found that reefs had larger individuals than lagoons. Some species, such as Acanthurus bahianus, were primarily reef species whose juveniles also used lagoon habitats while others, such as Gerres cinereus, were generally lagoon species whose adults occasionally moved onto reefs. Even with all this variation in life-history strategies, all the species that exhibited bay-reef partitioning used the lagoons as juveniles then moved onto reefs as adults and not vice versa, supporting the hypothesis that bays are important nursery areas for reef-dwelling fishes. These results show that a detailed review of the natural life-history strategies and habitat requirements are required before making further generalisations about the role of near-shore habitat types as nurseries for reef fishes. This is especially important given the rapid changes in tropical near-shore habitats around the world.