Habitat Use by Fishes in Coral Reefs,Seagrass Beds and Mangrove Habitats in the Philippines

Understanding the interconnectivity of organisms among different habitats is a key requirement for generating effective management plans in coastal ecosystems, particularly when determining component habitat structures in marine protected areas. To elucidate the patterns of habitat use by fishes among coral, seagrass, and mangrove habitats, and between natural and transplanted mangroves, visual censuses were conducted semiannually at two sites in the Philippines during September and March 2010–2012. In total, 265 species and 15,930 individuals were recorded. Species richness and abundance of fishes were significantly higher in coral reefs (234 species, 12,306 individuals) than in seagrass (38 species, 1,198 individuals) and mangrove (47 species, 2,426 individuals) habitats. Similarity tests revealed a highly significant difference among the three habitats. Fishes exhibited two different strategies for habitat use, inhabiting either a single (85.6% of recorded species) or several habitats (14.4%). Some fish that utilized multiple habitats, such as Lutjanus monostigma and Parupeneus barberinus, showed possible ontogenetic habitat shifts from mangroves and/or seagrass habitats to coral reefs. Moreover, over 20% of commercial fish species used multiple habitats, highlighting the importance of including different habitat types within marine protected areas to achieve efficient and effective resource management. Neither species richness nor abundance of fishes significantly differed between natural and transplanted mangroves. In addition, 14 fish species were recorded in a 20-year-old transplanted mangrove area, and over 90% of these species used multiple habitats, further demonstrating the key role of transplanted mangroves as a reef fish habitat in this region.     

Spatial distribution of the upside-down jellyfish Cassiopea sp. within fringing coral reef environments of the Northern Red Sea: implications for its life cycle

The zooxanthellate mangrove jellyfish Cassiopea sp. represents a prominent invasive species and a potential bioindicator for nutrient monitoring in coral reefs. However, information about its spatial distribution in combination with abundance, habitat specificity and life cycle elements is barely available. This study, therefore, presents the results of field surveys conducted within four different benthic habitat types (coral reef, seagrass meadow, reef-sand transition and sand flat) in the Northern Red Sea. Cassiopea sp. exhibited a highly patchy distribution within the entire study area with mean abundance of 1.6 ± 0.3 animals m⁻² and benthic coverage of 3.2%. Within coral reef habitats, maximum abundance of up to 31 animals m⁻² and benthic coverage of up to 20% were detected. Additionally, this study revealed that 65% of all observed Cassiopea specimens were associated with the commensalistic crustacean mysid Idiomysis tsurnamali. Cassiopea abundance and size as well as association patterns with mysids differed between most of the surveyed habitats. In summary, the findings of the present study (1) characterize Cassiopea as one of the key organisms in investigated benthic habitats, (2) indicate active habitat selection by the jellyfish and (3) may hint to an unexplored life cycle of Cassiopea with central role of seagrass meadows providing cues for larval settlement and metamorphosis in the absence of mangroves.     

Variation in fish density,assemblage composition and relative rates of predation among mangrove,seagrass and coral reef habitats

We tested the hypothesis for several Caribbean reef fish species that there is no difference in nursery function among mangrove, seagrass and shallow reef habitat as measured by: (a) patterns of juvenile and adult density, (b) assemblage composition, and (c) relative predation rates. Results indicated that although some mangrove and seagrass sites showed characteristics of nursery habitats, this pattern was weak. While almost half of our mangrove and seagrass sites appeared to hold higher proportions of juvenile fish (all species pooled) than did reef sites, this pattern was significant in only two cases. In addition, only four of the six most abundant and commercially important species (Haemulon flavolineatum, Haemulon sciurus, Lutjanus apodus, Lutjanus mahogoni, Scarus iserti, and Sparisoma aurofrenatum) showed patterns of higher proportions of juvenile fish in mangrove and/or seagrass habitat(s) relative to coral reefs, and were limited to four of nine sites. Faunal similarity between reef and either mangrove or seagrass habitats was low, suggesting little, if any exchange between them. Finally, although relative risk of predation was lower in mangrove/seagrass than in reef habitats, variance in rates was substantial suggesting that not all mangrove/seagrass habitats function equivalently. Specifically, relative risk varied between morning and afternoon, and between sites of similar habitat, yet varied little, in some cases, between habitats (mangrove/seagrass vs. coral reefs). Consequently, our results caution against generalizations that all mangrove and seagrass habitats have nursery function.     

Importance of seagrass vegetation for habitat partitioning between closely related species,mobile macrofauna <Emphasis Type="Italic">Neomysis</Emphasis> (Misidacea)

Seagrass meadows provide both habitats and a range of food sources for macrofaunal communities. These functions facilitate the coexistence of less mobile invertebrates (in comparison with mysids, such as amphipods) that are associated with seagrass leaves, and may also enhance the coexistence of highly mobile invertebrates such as mysid. We investigated the function of seagrass in supporting the coexistence of two mysid species, Neomysis awatschensis and N. mirabilis. These taxa are dominant in seagrass ecosystems of temperate coastal areas. We compared patterns of habitat use between the two species at mesoscales (among seagrass patches) and microscales (among seagrass leaves) by performing field surveys and laboratory experiments. The field survey results showed positive correlations in the abundance of the two mysid species, indicating that both species select similar habitats at the mesoscale level. In the laboratory experiments, the pattern of microhabitat selection (fundamental habitat) was similar for both species, even at increased densities and with the presence of an immobile habitat-competitor (the gastropod Barleeia angustata) on the leaves. However, this pattern changed significantly when a food source (epiphytic microalgae) was present on the leaves. This result indicates that (i) inter- and intraspecific interference competition does not affect microhabitat selection in these two mysids and (ii) both Neomysis species use similar habitats at the feeding stage. Although these two closely related mysids species may have similar requirements for microhabitat and food, the evidence that they did not act as competitors is attributable to unrestricted microhabitat and food (e.g., epiphytic algae) in the presence of seagrass vegetation.     

Habitat use patterns of fishes across the mangrove-seagrass-coral reef seascape at Ishigaki Island,southern Japan

To clarify seascape-scale habitat use patterns of fishes in the Ryukyu Islands (southern Japan), visual censuses were conducted in the mangrove estuary, sand area, seagrass bed, coral rubble area, branching coral area on the reef flat, and tabular coral area on the outer reef slope at Ishigaki Island in August and November 2004, and May, August and November 2005. During the study period a total of 319 species were observed. Species richness and abundance were highest in the branching and tabular coral areas, followed in order by the seagrass bed and mangrove estuary, and coral rubble and sand areas, in each month. Cluster analysis resulted in a clear grouping of assemblage structures by habitat type rather than by census month. SIMPER analysis showed that fish assemblages in the tabular coral area were mainly characterized by Acanthurus nigrofuscus, Pomacentrus lepidogenys, P. philippinus and P. vaiuli, the branching coral area by Chromis viridis and Pomacentrus moluccensis, the coral rubble area by Amblyeleotris steinitzi and Ctenogobiops pomastictus, the seagrass bed by Cheilio inermis, Lethrinus atkinsoni and Stethojulis strigiventer, the sand area by Valenciennea longipinnis, and the mangrove estuary by Gerres oyena, Lutjanus fulvus and Yongeichthys criniger. Moreover, fishes exhibited two habitat use strategies, inhabiting either a single or several specific habitats throughout their benthic life history stages, or having a possible ontogenetic habitat shift from the mangrove estuary or seagrass bed to coral-dominated habitats (e.g., Lethrinus atkinsoni, Lethrinus obsoletus, Lutjanus fulviflamma, Lutjanus fulvus, Lutjanus gibbus, Lutjanus monostigma and Parupeneus barberinus), suggesting that the mangrove estuary and seagrass bed have a nursery function.     

A baseline for prioritizing the conservation of the threatened seagrass Cymodocea nodosa in the oceanic archipelago of Madeira

Seagrasses are experiencing fragmentation and regression globally; thus, protection and recovery of meadows are a preservation priority. However, conservation actions must consider inherent regional conditions, since certain coastal areas are not suitable for the settlement of extensive meadows. Likewise, small oceanic archipelagos are not always able to fulfil the habitat requirements of seagrass habitats but can harbour small patches that in turn provide unique research opportunities. In this study, we focused on the seagrass Cymodocea nodosa in the archipelago of Madeira (NE Atlantic Ocean). Here we compile historical and contemporary records of this species along with characterization of associated communities (fish and invertebrates). A bionomic map with potentially suitable areas for the establishment and settlement of this species is also included. Lastly, we highlight coastal management and restoration actions and future research directions to preserve this species in Madeira Island.     

What lies beneath: Why knowledge of belowground biomass dynamics is crucial to effective seagrass management

Conservation of seagrasses meadows is important, because these habitats are ecologically important and under threat. Monitoring and modelling are essential tools for assessing seagrass condition and potential threats, however there are many seagrass indicators to choose from, and differentiating between natural variability and declining conditions poses a serious challenge. Tropical seagrass meadows in the Indo-Pacific, in contrast to most temperate meadows, are characterized by a multi-species composition and a year-round growth. Differences in characteristics between species growing within one meadow could induce uncertainty in the assessment of the dynamics of these meadows if variation in productivity and related biomass turnover timescales are not taken into consideration. We present data on biomass distribution, production and turnover timescales of above- and belowground tissues for three key tropical seagrass species (Thalassia hemprichii, Cymodocea rotundata and Halodule uninervis) in two mixed-species meadows in the Spermonde Archipelago, Indonesia. Seagrass leaf turnover time scales were comparable for the three studied seagrass species and varied between 25 and 30 days. Variation in leaf and rhizome turnover timescales were small (or insignificant) between the two meadows. In contrast, rhizome turnover time scales were around ten times longer than leaf turnover timescales, and large differences in rhizome turnover time scales (200–500 days) were observed between the species. The late-successional species T. hemprichii had much slower rhizome turnover compared to the two early successional species. Furthermore, since rhizome biomass has a much longer turnover time compared to leaf biomass, changes in rhizome biomass reflect effects on seagrass meadows on a much longer timescale compared to changes in leaf biomass for these tropical meadows. We conclude that belowground biomass dynamics are an important proxy to assess long-term effects of environmental stressors on seagrass ecosystems and should be included in tropical seagrass management programmes.     

The functional importance of <Emphasis Type="Italic">Acropora austera</Emphasis> as nursery areas for juvenile reef fish on South African coral reefs

Many coral reef fish species use mangrove and seagrass beds as nursery areas. However, in certain regions, the absence or scarcity of such habitats suggests that juvenile coral reef fish may be seeking refuge elsewhere. The underlying biogenic substratum of most coral reefs is structurally complex and provides many types of refuge. However, on young or subtropical coral reefs, species may be more reliant on the living coral layer as nursery areas. Such is the case on the high-latitude coral reefs of South Africa where the coral communities consist of a thin veneer of coral overlaying late Pleistocene bedrock. Thus, the morphology of coral species may be a major determinant in the availability of refuge space. Acropora austera is a branching species that forms large patches with high structural complexity. Associated with these patches is a diverse community of fish species, particularly juveniles. Over the past decade, several large (>100 m²) A. austera patches at Sodwana Bay have been diminishing for unknown reasons and there is little evidence of their replacement or regrowth. Seven patches of A. austera (AP) and non-A. austera (NAP) were selected and monitored for 12 months using visual surveys to investigate the importance of AP as refugia and nursery areas. There were significant differences in fish communities between AP and NAP habitats. In total, 110 species were recorded within the patches compared to 101 species outside the patches. Labrids and pomacentrids were the dominant species in the AP habitats, while juvenile scarids, acanthurids, chaetodons and serranids were also abundant. The diversity and abundance of fish species increased significantly with AP size. As the most structurally complex coral species on the reefs, the loss of APs may have significant implications for the recruitment and survival of certain fish species.     

Movement of carbon among estuarine habitats and its assimilation by invertebrates

We measured the extent of movement of carbon and its assimilation by invertebrates among estuarine habitats by analysing carbon stable isotopes of invertebrates collected along transects crossing the boundary of two habitats. The habitats were dominated by autotrophs with distinct isotope values: (1) mudflats containing benthic microalgae (mean −22.6, SE 0.6‰) and (2) seagrass and its associated epiphytic algae (similar values, pooled mean −9.8, 0.5‰). Three species of invertebrates were analysed: a palaemonid shrimp, Macrobrachium intermedium, and two polychaete worms, Nephtys australiensis and Australonereis ehlersi. All species had a similar narrow range of isotope values (−9 to −14‰), and showed no statistically significant relationship between position along transect and isotope values. Animals were relying on carbon from seagrass meadows whether they were in seagrass or on mudflats hundreds of metres away. Particulate organic matter collected from superficial sediments along the transects had similar values to animals (mean −11.1, SE 1.3‰) and also showed no significant relationship with position. The isotope values of these relatively immobile invertebrates and the particulate detritus suggest that carbon moves from subtidal seagrass meadows to mudflats as particulate matter and is assimilated by invertebrates. This assimilation might be direct in the case of the detritivorous worm, A. ehlersi, but must be via invertebrate prey in the case of the carnivorous worm, N. australiensis and the scavenging shrimp, M. intermedium. The extent of movement of carbon among habitats, especially towards shallower habitats, is surprising since in theory, carbon is more likely to move offshore in situations such as the current study where habitats are in relatively open, unprotected waters.     

Comparative Evidence that Salt Marshes and Mangroves May Protect Seagrass Meadows from Land-derived Nitrogen Loads

Seagrass meadows within estuaries are highly sensitive to increased supplies of nitrogen (N). The urbanization of coastal watersheds increases the delivery of N to estuaries, threatening seagrass habitats; both seagrass production per unit area and the area of seagrass meadows diminish as land-derived N loads increase. The damaging effects of land-derived N loads may be lessened where there are fringes of coastal wetlands interposed between land and seagrass meadows. Data compiled from the literature showed that production per unit area by seagrasses increased and losses of seagrass habitat were lower in estuaries with relatively larger areas of fringing wetlands. Denitrification and the burial of land-derived N within fringe wetlands may be sufficient to protect N-sensitive seagrass habitats from the detrimental effects of land-derived N. The protection furnished by fringing wetlands may be overwhelmed by increases in anthropogenic N loads in excess of 20–100 kg N ha⁻¹ y⁻¹. The relationships of land-derived N loadings, fringing coastal wetlands, and seagrass meadows demonstrate that different units of the landscape mosaic found in coastal zones do not exist as separate units, but instead are coupled and uncoupled by biogeochemical transformations and transport among environments. Received 12 December 2000; accepted 15 August 2001.     

First experimental evidence of corals feeding on seagrass matter

We present the first experimental evidence of a coral (Oulastrea crispata) ingesting and assimilating seagrass material. Tropical seagrass meadows export a substantial portion of their productivity and can provide an important source of nutrients to neighbouring systems such as coral reefs; however, little is known about the mechanisms of this link. To investigate whether seagrass nutrient uptake via coral heterotrophy is possible, we conducted a feeding experiment with seagrass particulate and dissolved organic matter. Using gut extractions and stable isotope analyses, we determined that O. crispata ingested ¹⁵N-enriched seagrass particles and assimilated the nitrogen into its tissue at a rate of 0.75 μg N cm^?2 h^?1. Corals took up nitrogen from dissolved matter at a comparable rate of 0.98 μg N cm^?2 h^?1. While other ecological connections between seagrass meadows and reef ecosystems are well known, our results suggest a previously unstudied direct nutritional link between seagrasses and corals.     

Comparison between community structures of fishes in <Emphasis Type="Italic">Enhalus acoroides</Emphasis>- and <Emphasis Type="Italic">Thalassia hemprichii</Emphasis>-dominated seagrass beds on fringing coral reefs in the Ryukyu Islands,Japan

To clarify differences in community structures and habitat utilization patterns of fishes in Enhalus acoroides- and Thalassia hemprichii-dominated seagrass beds on fringing coral reefs, visual censuses were conducted at Iriomote and Ishigaki islands, southern Japan. The numbers of fish species and individuals were significantly higher in the E. acoroides bed than in the T. hemprichii bed, although the 15 most dominant fishes in each seagrass bed were similar. Cluster and ordination analyses based on the number of individuals of each fish species also demonstrated that fish community structures were similar in the two seagrass beds. Species and individual numbers of coral reef fishes which utilized the seagrass beds numbered less than about 15% of whole coral reef fish numbers, although they comprised about half of the seagrass bed fishes. Of the 15 most dominant species, 5 occurred only in the two seagrass beds, including seagrass feeders. Ten other species were reef species, their habitat utilization patterns not differing greatly between the two seagrass beds. Some reef species, such as Lethrinus atkinsoni and L. obsoletus, showed ontogenetic habitat shifts with growth, from the seagrass beds to the coral areas. These results indicate that community structures and habitat utilization patterns of fishes were similar between E. acoroides- and T. hemprichii-dominated seagrass beds, whereas many coral reef fishes hardly utilized the seagrass beds.     

Spatial organization of the clouded Apollo population (<Emphasis Type="Italic">Parnassius mnemosyne</Emphasis>) in Onega Lake Basin

The distribution and interaction of local populations of the clouded Apollo Parnassius mnemosyne were studied on Bolshoi Klimenetskii Island (Lake Onega, Russia). Within an area of 10 km², the butterflies occurred in 27 discrete patches—small meadows located within and on the periphery of forests. The mark-recapture results confirmed that the patches were connected by a flow of individuals. Based on exponential distribution, the local and migratory movements of the butterflies were separated, and a model describing the movement frequencies in relation to distance was proposed. The generalization of the habitats using the PCA Q-technique and isolation indices as measures of similarity between the objects in initial matrices revealed four subpopulations occupying separate plots of the area with a network of suitable habitats. According to the available data on the local movements of butterflies, an area of settlement was determined for each subpopulation, characterized by two zones: the central one (0.3 km in diameter), within which the movements of butterflies are most intensive, and the peripheral one (0.3 to 0.5 km) which is rarely visited by butterflies. Long-distance migrations between different occupied areas were recorded for 8% of marked butterflies; they included all the subpopulations. The male to female ratio among the migrants was 6: 1; the maximum distance covered by a butterfly was 2.68 km. The common opinion that the clouded Apollo is an extremely sedentary species with low migratory potentials is refuted.     

Trophic Transfers from Seagrass Meadows Subsidize Diverse Marine and Terrestrial Consumers

In many coastal locations, seagrass meadows are part of a greater seascape that includes both marine and terrestrial elements, each linked to the other via the foraging patterns of consumers (both predators and herbivores), and the passive drift of seagrass propagules, leaves, roots and rhizomes, and seagrass-associated macroalgal detritus. With seagrasses declining in many regions, the linkages between seagrass meadows and other habitats are being altered and diminished. Thus, it is timely to summarize what is known about the prevalence and magnitude of cross-habitat exchanges of seagrass-derived energy and materials, and to increase awareness of the importance of seagrasses to adjacent and even distant habitats. To do so we examined the literature on the extent and importance of exchanges of biomass between seagrass meadows and other habitats, both in the form of exported seagrass biomass as well as transfers of animal biomass via migration. Data were most abundant for Caribbean coral reefs and Australian beaches, and organisms for which there were quantitative estimates included Caribbean fishes and North American migratory waterfowl. Overall, data from the studies we reviewed clearly showed that seagrass ecosystems provide a large subsidy to both near and distant locations through the export of particulate organic matter and living plant and animal biomass. The consequences of continuing seagrass decline thus extend far beyond the areas where seagrasses grow.     

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.     

Positive Feedbacks in Seagrass Ecosystems: Implications for Success in Conservation and Restoration

Abstract Seagrasses are threatened by human activity in many locations around the world. Their decline is often characterized by sudden ecosystem collapse from a vegetated to a bare state. In the 1930s, such a dramatic event happened in the Dutch Wadden Sea. Before the shift, large seagrass beds (Zostera marina) were present in this area. After the construction of a large dam and an incidence of the “wasting disease” in the early 1930s, these meadows became virtually extinct and never recovered despite restoration attempts. We investigated whether this shift could be explained as a critical transition between alternative stable states, and whether the lack of recovery could be due to the high resilience of the new turbid state. We analyzed the depth distribution of the historical meadows, a long-term dataset of key factors determining turbidity and a minimal model based on these data. Results demonstrate that recovery was impossible because turbidity related to suspended sediment was too high, probably because turbidity was no longer reduced by seagrass itself. Model simulations on the positive feedback suggest indeed the robust occurrence of alternative stable states and a high resilience of the current turbid state. As positive feedbacks are common in seagrasses, our findings may explain both the worldwide observed collapses and the low success rate of restoration attempts of seagrass habitats. Therefore, appreciation of ecosystem resilience may be crucial in seagrass ecosystem management.     

Small-scale removal of seagrass (Zostera marina L.): effects on the infaunal community

Eelgrass meadows are a common feature in shallow waters along the Norwegian coast, where they provide a habitat for a diverse infaunal community. Recreational boat anchoring and moorings physically scour seagrass and may affect the ecosystem functioning and resilience of the system to natural and anthropogenic disturbances. A small-scale eelgrass (Zostera marina) removal experiment was conducted to study the effects on macro- and meiofauna. Entire plants, including the rhizomes, were removed from 4?m² patches in three eelgrass meadows in the inner Oslofjord in October 2010. Core samples were taken after a recovery period of 10 months, from the removed patches as well as from the surrounding meadow. Macrofauna (>500?μm) and meiofauna (63–500?μm) in the sediment were investigated for possible effects of the eelgrass removal. Macrofauna and meiofauna composition were site specific and therefore location was identified as the main determinant for the infaunal community. The eelgrass did not regrow within the recovery period and bare sediment patches with only single eelgrass shoots were present during the sampling. Our analyses support an influence of the removal on individual species, but not the complete community. In particular one species, the gastropod Peringia ulvae, was encountered in higher numbers in samples from the removed patches than in control samples. From a management perspective, such minor removal of eelgrass, on the scale of square metres, appears to have no long-lasting detrimental effect to the infaunal community in sheltered meadows with muddy sediments.     

Habitat-dependent growth in a Caribbean sea urchin <Emphasis Type="Italic">Tripneustes ventricosus</Emphasis>: the importance of food type

The sea urchin Tripneustes ventricosus is a common, yet relatively poorly known, grazer of seagrass beds and coral reefs throughout the Caribbean. We compared the size and abundance of urchins between adjacent seagrass and coral reef habitats (where macroalgae are the dominant primary producers). We also conducted a laboratory experiment comparing the growth rate of juvenile urchins fed a diet of either macroalgae or seagrass. Reef urchins had significantly larger test diameter than those in the seagrass on some sampling dates. This size difference may be at least partially explained by diet, because laboratory-reared urchins fed macroalgae grew significantly faster than those fed seagrass. The seagrass population, however, was stable over time, whereas the reef population exhibited strong fluctuations in abundance. Overall, our study indicates that both the seagrass and coral reef habitats are capable of supporting healthy, reproductive populations of T. ventricosus. Each, however, appears to offer a distinct advantage: faster growth on the reef and greater population stability in the seagrass.     

Are artificial reefs surrogates of natural habitats for corals and fish in Dubai,United Arab Emirates?

Artificial reefs are often promoted as mitigating human impacts in coastal ecosystems and enhancing fisheries; however, evidence supporting their benefits is equivocal. Such structures must be compared with natural reefs in order to assess their performance, but past comparisons typically examined artificial structures that were too small, or were immature, relative to the natural reefs. We compared coral and fish communities on two large (>400,000 m³) and mature (>25 year) artificial reefs with six natural coral patches. Coral cover was higher on artificial reefs (50%) than in natural habitats (31%), but natural coral patches contained higher species richness (29 vs. 20) and coral diversity (H′ = 2.3 vs. 1.8). Multivariate analyses indicated strong differences between coral communities in natural and artificial habitats. Fish communities were sampled seasonally for 1 year. Multivariate fish communities differed significantly among habitat types in the summer and fall, but converged in the winter and spring. Univariate analysis indicated that species richness and abundance were stable throughout the year on natural coral patches but increased significantly in the summer on artificial reefs compared with the winter and spring, explaining the multivariate changes in community structure. The increased summer abundance on artificial reefs was mainly due to adult immigration. Piscivores were much more abundant in the fall than in the winter or spring on artificial reefs, but had low and stable abundance throughout the year in natural habitats. It is likely that the decreased winter and spring abundance of fish on the artificial reefs resulted from both predation and emigration. These results indicate that large artificial reefs can support diverse and abundant coral and fish communities. However, these communities differ structurally and functionally from those in natural habitats, and they should not be considered as replacements for natural coral and fish communities.     

Thresholds in seascape connectivity: the spatial arrangement of nursery habitats structure fish communities on nearby reefs

Ecosystems are linked by the movement of organisms across habitat boundaries and the arrangement of habitat patches can affect species abundance and composition. In tropical seascapes many coral reef fishes settle in adjacent habitats and undergo ontogenetic habitat shifts to coral reefs as they grow. Few studies have attempted to measure at what distances from nursery habitats these fish migrations (connectivity) cease to exist and how the abundance, biomass and proportion of nursery species change on coral reefs along distance gradients away from nursery areas. The present study examines seascape spatial arrangement, including distances between habitats, and its consequences on connectivity within a tropical seascape in Mozambique using a seascape ecology approach. Fish and habitat surveys were undertaken in 2016/2017 and a thematic habitat map was created in ArcGIS, where cover and distances between habitat patches were calculated. Distance to mangroves and seagrasses were significant predictors for abundance and biomass of most nursery species. The proportions of nursery species were highest in the south of the archipelago, where mangroves were present and decreased with distance to nurseries (mangroves and seagrasses). Some nursery species were absent on reef sites farthest from nursery habitats, at 80 km from mangroves and at 12 km from seagrass habitats. The proportion of nursery/non-nursery snapper and parrotfish species, as well as abundance and biomass of seagrass nursery species abruptly declined at 8 km from seagrass habitats, indicating a threshold distance at which migrations may cease. Additionally, reefs isolated by large stretches of sand and deep water had very low abundances of several nursery species despite being within moderate distances from nursery habitats. This highlights the importance of considering the matrix (sand and deep water) as barriers for fish migration.