Fish Assemblage Composition Research Articles

Sulawesi stream fish communities depend on connectivity and habitat diversity.

Streams provide an array of habitat niches that may act as environmental filters for fish communities. The tropical island of Sulawesi in Indonesia is located in the Wallacea, a region isolated by marine barriers from the Asian and Australian faunas. Primary freshwater fishes are naturally absent in the Wallacea, including Sulawesi's numerous coastal streams. Diadromous species are in contrast species-rich in the area. The knowledge available on stream fishes in the Wallacea is largely restricted to taxonomic work and studies targeting single species groups, whereas baseline data on fish ecology remain extremely scarce. Such data and a deeper understanding of stream fish ecology are, however, urgently required for purposes such as informed management. We assumed that the stream fish assemblages are dominated by recruitment from the sea and are structured by macro- and microhabitat diversity. To test this hypothesis, we quantified the occurrence of individual fishes by point abundance electrofishing at 33 streams across Sulawesi. The 4632 fishes obtained represent 58 species out of 24 families. The native fishes recorded are mainly amphidromous (34 species), euryhaline (five species), and catadromous (five species). Gobiiformes make up the vast majority of records, dominated by Oxudercidae (22 species) and Eleotridae (five species). Only two of the species recorded are endemic to Sulawesi, including a single species strictly confined to freshwaters. Ten species, making up 6% of the fishes caught, are not native to Sulawesi. The outlying mean index (OMI) and BIOENV analyses suggest that effects on the scale of macro- and microhabitat shape fish assemblage composition, ranging from pH, conductivity, and temperature to current velocity, substrate, canopy cover, and elevation. Habitat niche use of species along the first two OMI axes is complementary and fine-scaled, covering a wide range of the available habitat space. Juvenile and adult conspecifics share similar habitat niches in most of the cases. Niche breadths overlap, but niche specialization is significant in most of the species. Non-native fishes link into the assemblages at the margins of habitat space, with substantial niche overlaps to native species. The present findings show that the native fish communities in coastal streams of Sulawesi are largely composed of species depending on access to the sea, highlighting the importance of connectivity down to the estuaries and sea. The ichthyofauna shows a rich diversity in habitat use, and the availability of alternative habitats along the altitudinal gradient provides plausible filters for species establishment. Non-native fishes are locally abundant, pose substantial potential for changing communities, but are still stocked intentionally. We stress the need for incorporating the need for connectivity and maintained habitat quality into management decisions, and a critical evaluation of stocking activities.

Habitat trumps biogeography in structuring coral reef fishes

AbstractAs one of the world’s most diverse ecosystems, coral reefs have been the focus of numerous biogeographic analyses. With strong biodiversity gradients across the Indo-Pacific, coral reefs have shed light on the effects of evolutionary history, isolation, and human exploitation on local assemblages. However, there are also strong environmentally driven local gradients in faunal assemblages. We ask, does reef fish community composition and trait space vary to a greater extent across small scales (i.e. along habitat gradients) or across large scales (i.e. across geographic regions separated by up to 12,000 km)? Using a standardized survey method that explicitly includes habitats (i.e. the slope, crest, and flat), we surveyed a highly diverse family of reef fishes (Labridae) in nine regions across the Indo-Pacific, from the Cocos (Keeling) Islands to French Polynesia. We demonstrate that small-scale habitat gradients represent a greater axis of variation, in both the taxonomic and trait composition of fish assemblages, than large-scale biogeographic gradients. Indeed, fish assemblages just 10 m apart, along a habitat gradient, appear to differ more than assemblages in the same habitats separated by over 12,000 km along the world’s largest biodiversity gradient. Essentially, fish assemblages cluster by habitat regardless of their biogeographic region, with habitat associations trumping biogeographic affiliations. This emphasizes the primacy of local environmental factors, such as hydrodynamics, in shaping the ecology of reef fishes. It also raises serious concerns over the use of combined datasets, where data from different habitats are used, without explicit recognition, in global-scale analyses.

Trait Composition and Assemblage Structure Analyses of Lacustrine Fishes: Synthesizing a Proposal for Better Fishing Practices

Fish provide ecosystem services and contribute to human well-being through fishing. In Chile, subsistence fishing provides food to local communities, whereas recreational fishing contributes to economic income via tourism. In the Lanalhue coastal lake basin, unregulated subsistence fishing and formal recreational fishing primarily target large fish species to ensure satisfactory catches and food provision. However, the development of unplanned fishing activities and lack of scientific knowledge can lead to socioecological conflicts and tension between users. To address this issue, this study focuses on improving fishing practices through the analysis of fish assemblages of the Lanalhue lake basin. The life cycle traits of each fish species were analyzed, as well as their abundance and biomass in each habitat. Twelve fish species were identified, and their distribution and abundance explain the differences in fish assemblage composition and structure among habitats. To promote better fishing practices, we propose four target species and specific management actions. These include defining fishing seasons, delineating fishing sites, and establishing fishing quotas. This approach, considering both community structure and functional aspects of fish assemblages, can serve as a valuable guide for developing sustainable fishing practices in the Lanalhue coastal lake basin.

Relationships between environmental variables and fish functional groups in impounded reaches of the Upper Mississippi and Yangtze Rivers

Large rivers throughout the world have been transformed by anthropogenic stressors that are known to influence the structure and composition of fish assemblages. Management of large rivers requires balancing socio-economic and political considerations with biodiversity conservation efforts. By exchanging best management practices between rivers, management efforts can be improved. However, data limitations have largely prevented comparative analyses among fish assemblages in large rivers, potentially limiting the effectiveness of shared management strategies. To improve understanding of the similarities and differences between the Upper Mississippi and Yangtze Rivers, we (1) compared environmental variables and functional traits of fish assemblages between the two systems, (2) identified traits responsible for distinguishing functional groups from one another, and (3) investigated relationships between similar functional groups of fishes and environmental variables to establish expectations for how fish assemblages in large rivers might respond to anthropogenic stressors. Regional species pools in the Upper Mississippi and Upper Yangtze Rivers were characterized by a similar composition of functional traits; the majority of species were omnivorous, had affinities for gravel or sand substrates, and produced sinking eggs. Few species were pelagic, planktivorous, or herbivorous. Functional groups in both rivers were primarily distinguished according to species' trophic habits and substrate preferences, with secondary contributions from species’ water column positions and life history characteristics. Pelagic planktivores and small-bodied guarders with an affinity for structural habitat complexity tended to increase downriver in both systems, in direct association with total phosphorus concentrations, agricultural land use, and temperature. In contrast, proportions of lithophilic species with affinities for gravel or cobble substrates were highest in segments located furthest upriver. By highlighting the sensitivity of different groups of fishes to anthropogenic stressors, we provide insights into the functional ecology of fishes inhabiting the Mississippi and Yangtze Rivers.

The role of the environment and connectivity on multiple facets of local fish diversity in tropical headwater streams

Abstract The identification of multiple facets of biodiversity (e.g., phylogenetic and functional) can help to understand how evolutionary and ecological mechanisms shape the assembly of biological communities. Here, we employed multiple regressions and variance partitions to investigate the effects of environmental conditions, connectivity and space on aspects of the functional and phylogenetic facets of fish fauna in tropical headwater streams of the Xingu River basin, in Eastern Amazonia. We found that the functional and phylogenetic diversities of fish communities are related to local environmental conditions and stream connectivity in the drainage network, with marked differences occurring between the rainy and dry periods. Furthermore, we discovered that functional uniqueness was influenced mainly by environmental factors, whereas phylogenetic uniqueness had a more complex relationship with the influence of local environmental factors and connectivity. Our findings indicate that the fish assemblages of small headwater streams of the Xingu River basin share a close phylogenetic history. This is probably because of the climatic and environmental stability experienced by Amazonian ichthyofauna over evolutionary time. Our findings emphasise the role of niche‐based processes in shaping the unique functional composition of fish assemblages in headwater streams, and highlight the importance of environmental heterogeneity in maintaining the ecological integrity of these freshwater ecosystems.

Estructura comunitaria de peces, asteroideos y equinoideos en el Farallón de San Ignacio, arrecife situado en la “brecha faunística de Sinaloa”, México

Even though there is extensive research on the rocky reefs in the Gulf of California, some areas still have scarce information, such as the Farallón de San Ignacio, which is well known for its high biodiversity and its location in the biogeographic transition zone known as the "Sinaloa gap." Due to the limited information available for this site, the present study aimed to describe the composition and structure of asteroid and echinoid fish assemblages at the Farallón de San Ignacio. For this purpose, visual surveys were conducted in November 2016 using belt transects of 25 m long by 4 m wide for fish and 25 m long by 2 m wide for invertebrates, to estimate species abundance, richness, diversity, and evenness indices, and to compare them with other rocky reefs located at the same latitude on the western coast of the Gulf of California that were visited the same year. A total of 34 species were recorded, including two asteroids and four echinoids. The most abundant species were the fish Azurina atrilobata, the starfish Phataria unifacialis, and the sea urchin Diadema mexicanum. Despite the Farallón’s isolation, the complexity of its community structure is similar to that of other islands in the Gulf of California, although there were differences in species composition due to the high abundance of herbivorous species at this site.

Spatio-Temporal Dynamics of Larval Fish Assemblage in the Nakdong River Estuary, South Korea

Estuaries are crucial fish nursery habitats owing to their high productivity and the presence of various microhabitats for the early development of aquatic organisms. This study investigated the temporal and spatial patterns of the species composition and abundance of larval fish assemblages in the Nakdong River estuary, South Korea, through bi-monthly sampling in the inner and outer estuaries. Fifty-five larval taxa were collected, and the larval fish assemblages were dominated by a few species. Engraulis japonicus (28.8%) was the most abundant, followed by Gobiidae sp.1 (22.6%), Clupea pallasii (13.9%), and Omobranchus sp. (6.1%). The species richness, abundance, and diversity tended to be higher during the warm season in the inner estuary. Multivariate analyses revealed that the structures of larval fish assemblages were significantly influenced by the season and site. Spatial and seasonal changes in larval fish assemblages resulted from the different occurrence patterns of common fish larvae in relation to water temperature and salinity. Among the predominant fish larvae, E. japonicus was captured more frequently in more saline outer estuaries during the warm season, whereas Gobiidae sp.1 and C. pallasii were more abundant in less saline inner estuaries during the warm and cold seasons, respectively. The results of this study improve our understanding of larval biodiversity in estuarine habitats in relation to environmental modification and contribute to the evaluation of nursery functions in the Nakdong River estuary.

A Comparison of Methods of Visual Census and Cryptobenthic Fish Collecting, an Integrative Approach to the Qualitative and Quantitative Composition of the Mediterranean Temperate Reef Fish Assemblages

The present research quantitatively compared the fish composition among two methods for non-cryptic benthic fish species and one method for cryptobenthic fish species for the first time for the Mediterranean temperate reef fish assemblage. A visual census of fishes was performed within a cylinder of 4 m radius and within a cylinder of 2 m radius, while the cryptobenthic fishes were collected using a square of 1 m2 with anesthetic. The data and material were collected at fifty sampling points. The visual census methods together recorded 31 species, and the square with anesthetic method recorded 18 species. The quantitative comparison of methods of visual census and cryptobenthic fish collecting showed significantly different species richness, total fish abundance, and fish assemblage structure among methods. The applied methods were highly complementary. The cylinder of 2 m radius is well suited for epibenthic fishes and the cylinder of 4 m radius is reliable for hyperbenthic and benthopelagic fishes. Therefore, each of the methods well covered one of three components of ichthyobenthos (hyperbenthic, epibenthic, and cryptobenthic fishes), and all three methods together provided a far more complete assessment of fish species composition than any individual census method for the Mediterranean littoral benthic fishes.

Mesopelagic fish assemblages in the Mauritanian Upwelling System off Northwest Africa with oxygen as a major driving force

The aims of this study were to determine the community composition and environmental drivers of mesopelagic fishes in the Mauritanian Upwelling System. Mesopelagic fish communities were sampled at 17 stations in August of 2016 off the coast of Senegal and Mauritania using a non-closing rectangular midwater trawl (RMT). This subregion is the most productive area of the Canary Current and has a pronounced and permanent oxygen minimum zone (OMZ). The sampling area was mainly composed of low-oxygenated South Atlantic Central Water, except for 1 station, where North Atlantic Central Water prevailed. Three distinct mesopelagic fish communities were identified. One community occurred in the vicinity of a coldwater eddy and was dominated by the myctophid Benthosema glaciale. In contrast, the gonostomatid genus Cyclothone prevailed in the other 2 communities and made up 62 and 51% of the total fish abundance. Our analyses revealed that the environmental factors that primarily influenced the composition of these mesopelagic fish assemblages were the oxygen concentrations at 10, 100, and 600 m depth. The present study shows that the mixing of water masses at frontal zones leads to communities representative of both water masses. In the case of the Mauritanian upwelling region, a high portion of the mesopelagic fish species were well adapted to OMZ conditions.

Water quality, habitat, and fish assemblage relationships in middle-order agriculture and forest streams of the Mississippi Alluvial Plain

BackgroundAgriculture has greatly influenced water quality, habitats, and fish assemblages in streams of the Mississippi Alluvial Plain (MAP) ecoregion. However, MAP streams have historically been understudied compared to streams in other agricultural regions of the USA. In this study, water quality, habitat, and fish assemblage composition were assessed seasonally (spring, summer, and fall) in eight representative MAP streams located across three U.S. states. The study design included four streams containing highly agricultural watersheds (herein termed “agriculture” streams) and four streams containing mostly forested watersheds (herein termed “forest” streams), which were intended to represent reference conditions for MAP streams.ResultsIn general, forest streams contained significantly better instream and riparian habitats than agriculture streams (P = 0.010–0.040) whereas agriculture streams contained significantly greater levels of primary nutrients (P < 0.001–0.010). Differences between agriculture and forest streams with respect to other physical and chemical variables were intermittent and season dependent. Fish assemblages in agriculture and forest streams were structured primarily along an environmental gradient reflecting instream habitat conditions, water nutrient concentrations, and benthic chlorophyll-a production. Structurally, fish assemblages in both stream types contained many regionally common species, though some species appeared to exhibit affinities for a particular stream type. Functionally, fish assemblages in agriculture streams contained more tolerant species, more omnivores, and fewer insectivores compared to forest stream assemblages, which were nearly all insectivores. Overall, one-third of the fish specimens collected in forest streams classified as intolerant species.ConclusionsOur results suggested that stream water quality, habitat, and fish assemblages differed between agriculture and forest streams in the MAP, with fish assemblages exhibiting both structural and functional differences. Results were consistent with a larger body of literature from smaller, headwater streams whereby land-use changes (e.g., row-crop agriculture) impacted the physical, chemical, and biological characteristics of stream ecosystems. Results further highlight the importance of land use management and its effects on habitat diversity in stream ecosystems, and that protecting the few remaining undisturbed or less-disturbed streams should be a priority.

Local habitat composition and complexity outweigh seascape effects on fish distributions across a tropical seascape

ContextThe distribution of animals is influenced by a complex interplay of landscape, environmental, habitat, and anthropogenic factors. While the effects of each of these forces on fish assemblages have been studied in isolation, the implications of their combined influence within a seascape remain equivocal.ObjectivesWe assessed the importance of local habitat composition, seascape configuration, and environmental conditions for determining the abundance, diversity, and functional composition of fish assemblages across a tropical seascape.MethodsWe quantified fish abundance in coral, macroalgal, mangrove, and sand habitats throughout the Dampier Archipelago, Western Australia. A full-subsets modelling approach was used that incorporated data from benthic habitat maps, a hydrodynamic model, in situ measures of habitat composition, and remotely sensed environmental data to evaluate the relative influence of biophysical drivers on fish assemblages.ResultsMeasures of habitat complexity were the strongest predictors of fish abundance, diversity, and assemblage composition in coral and macroalgal habitats, with seascape effects playing a secondary role for some functional groups. Proximity to potential nursery habitats appeared to have minimal influence on coral reef fish assemblages. Consequently, coral, macroalgal, and mangrove habitats contained distinct fish assemblages that contributed to the overall diversity of fish within the seascape.ConclusionsOur findings underscore the importance of structural complexity for supporting diverse and abundant fish populations and suggest that the value of structural connectivity between habitats depends on local environmental context. Our results support management approaches that prioritise the preservation of habitat complexity, and that incorporate the full range of habitats comprising tropical seascapes.

Composición histórica de los ensamblajes de peces en el sistema lagunar de alvarado, golfo de México

The composition of fish assemblages in the Alvarado Lagoon System was updated with information collected between 1966 and 2008, divided into nine collection periods. A temporal analysis of the variation in species richness, species richness indices, rarity, and composition of ecological and trophic guilds was also carried out. A total of 113 species were recognized, 66 of which were present in most of the collection periods, forming a group with a constant presence in the lagoon system, in which marine and freshwater species predominated. This group showed the highest levels of similarity between collection periods; this was confirmed by the richness and rarity indices. Functionally, the composition of the ecological and trophic guilds did not show significant differences. The composition of the trophic guilds showed a prevalence of invertebrate consumers, macrocrustaceans, and small fish, supporting the fact that migratory marine species were the most numerous, with minor contributions from the other guilds. The contribution of marine or freshwater visitor species was not important in the characteristics of the assemblages. These results suggest that the environmental conditions of the Alvarado Lagoon System have remained without severe changes during the study period, thus allowing both the constancy of the fish community and that of the ecological and trophic guilds.

Putting eDNA to the test: A field comparison of eDNA metabarcoding to established protocols for assessing biodiversity in Missouri's Ozark Highland streams

AbstractAquatic biodiversity monitoring to inform conservation and management efforts in‐stream systems has increasingly begun to incorporate environmental DNA (eDNA)‐based sampling methods. We conducted a comparison of eDNA metabarcoding to a traditional protocol of combined seining and electrofishing methods to assess fish biodiversity of wadeable stream sites in six separate drainages in the Ozark Highlands of Missouri (USA). The study further focused on the headwaters of the Meramec River, which included 11 sites and seasonal sampling (summer and winter). We compared estimates of diversity across sampling methods, assessed the influences of water flow (depth, velocity, and discharge) and season, and tested the effects of sampling method and site locality on fish assemblage composition. eDNA sampling detected approximately double the number of species compared to traditional methods, providing higher diversity estimates while maintaining the relative ranking of sites. eDNA detection probabilities were positively associated with stream depth and velocity and were generally higher in summer than in winter but not for all species. Estimated species richness was positively associated with discharge for both methods but the relationship was stronger with eDNA sampling. Assemblage differences between tributary and mainstem sites were attributable to a small number of species that were found predominantly in one stream size category or the other, indicating that eDNA was sensitive to within‐drainage assemblage structure relationships. We highlight improved species detection, a more comprehensive understanding of assemblage structural dynamics, and the potential ability to integrate data across sampling methods as important benefits that encourage the use of eDNA metabarcoding as a primary collection method in future stream biodiversity assessment and monitoring programs.

Variáveis ambientais como preditoras da assembleia de peixes em sistemas aquáticos semiáridos

Abstract Aim To understand the distribution of fish species and their use of habitat, a range of environmental variables were evaluated as predictors of fish assemblage composition and richness in tropical semiarid aquatic systems. Methods We surveyed the species composition of fish assemblages in semiarid aquatic systems and established their degree of association with the structure of the aquatic habitat. Sites consisted of stream reaches with surface water flow, isolated temporary pools, and man-made reservoirs. Fish sampling was conducted on four occasions during the wet (April and June 2006) and dry seasons (September and December 2006). The correlation between richness and abundance with the habitat structure was evaluated using stepwise multiple regression. Patterns of variation in fish assemblage composition across sites were evaluated using Nonmetric Multidimensional Scaling, and the Indicator Species Analysis was used to determine which species were significant indicators of sites. To establish multivariate correlations and test the hypothesis of local fish composition being associated with the environmental variables we used a Canonical Correspondence Analysis. Results Morphometric variables, stream reach width, stream length and elevation explained 75.6% of the variation in fish richness. Macrophyte cover and overhanging vegetation added to the predictive power of the model equation, where the final model explained 86.9% of the variation in fish richness. Canonical Correspondence Analysis showed a significant relationship between fish composition data and site morphology (altitude, bank slope and littoral depth). Among the water quality, habitat composition and substrate variables, temperature, sand, and gravel showed higher correlation with the CCA axes. Conclusions These results indicated that fish communities assume different structures and compositions across different habitat types following the environmental heterogeneity in dryland aquatic systems.

Comparison of Abundance and Structure of Larval Fish Assemblages between Autumn and Spring in the Waters Surrounding Taiwan Bank, Western North Pacific

The fluctuations in both time and space of larval fish assemblages in relation to hydrographic characteristics in the waters surrounding Taiwan Bank were studied in October 2021 (autumn) and March 2022 (spring). Throughout the study period, we identified a total of 149 taxa of fish larvae, encompassing 96 genera and 71 families. Engraulis japonicus, Diaphus slender type, unidentified Gobiidae, Apogon sp., unidentified Clupeidae, and Benthosema pterotum were the six dominant taxa and together constituted 47.39% of the total catch. There were no notable temporal variations observed in the abundance of fish larvae, but the species number of fish larvae was greater in spring than in autumn. Significant variations in species composition were observed between the different cruises, and the cluster analysis unveiled a distinct temporal structure of the assemblages of fish larvae. The dynamics of the prevailing currents induced by seasonal monsoons contribute significantly to the transportation of fish larvae. The distribution of fish larvae showed a good association with hydrographic characteristics, where seawater temperature and salinity emerged as the primary explanatory factors influencing the composition of larval fish assemblages in the waters surrounding Taiwan Bank. While the increased influx of nutrients from upwelling ensures abundant food availability, the hydrographic conditions may not be suitable for every fish larva.

Similar functional composition of fish assemblages despite contrasting levels of habitat degradation on shallow Caribbean coral reefs.

Functional trait-based approaches provide an opportunity to assess how changes in habitat affect the structure of associated communities. Global analyses have found a similarity in the composition of reef fish functional traits despite differences in species richness, environmental regimes, and habitat components. These large-scale patterns raised the question of whether this same stability can be observed at smaller spatial scales. Here, we compared the fish trait composition and their functional diversity in two Caribbean shallow coral reefs with contrasting levels of habitat degradation: Limones (>30% cover), constituted mainly by colonies of Acropora palmata and Bonanza, a reef with extensive areas of dead Acropora structures, dominated by algae. To characterize the functional structure of fishes on each reef, we calculated the community-weighted mean trait values (CWM), functional richness, functional evenness, functional dispersion, and functional originality. Despite the differences in habitat quality, reefs exhibited a similar proportion and common structure on fish functional traits. Functional richness and functional evenness differed significantly, but functional dispersion and functional originality did not show differences between reefs. The greater niche complexity driven by the high availability of microhabitats provided by A. palmata may explain the higher functional richness in Limones, whereas the reef degradation in Bonanza may contribute to a higher functional evenness because of a similar distribution of abundance per fish trait combinations. Our results suggest that widespread degradation on Caribbean reefs has limited the type, variety, and range of traits, which could lead to a functional homogenization of fish communities even at local scales.

Temperate rocky reef fish community patterns in a coastal marine protected area (MPA) from northern Chile, utilizing remote underwater video cameras (RUVs)

Shallow water seascapes are complex systems, important for the life history and connectivity of many marine fish species. Remote Underwater Video cameras (RUVs) are an increasingly used, non-lethal technique able to record the occurrence of reef fish species with little disturbance. In this study, RUVs were installed in a Marine Protected Area (Chañaral Island, northern Chile, 29°2'S, 71°34'W) at three different depth strata (5, 10, 15 m) in three different habitats (sand and boulder, bedrock, and kelp forest) during October 2018, July 2021 and October 2021. The RUV system recorded a total of 20 species belonging to 16 families over 73.5 h of videotaping. The most recurrent fishes were damselfish Chromis crusma, Chilean sandperch Pinguipes chilensis, and marblefish Aplodactylus punctatus. There was a significant difference in fish assemblage composition among depth strata and seasons, but not among habitats. In July 2021 (austral winter) higher densities of small cryptobenthic fish (Helcogrammoides spp.) and medium-sized carnivorous Chirodactylus variegatus were detected, whereas P. chilensis was associated to the deeper stratum, and the herbivorous blenny Scartichthys viridis was related to the shallower depths, which is likely due to differences in the habitat structure and composition. Winter 2021 showed a higher species richness than in spring 2018, while the evenness (Pielou’s J’) was significantly higher in spring 2018 than winter/spring 2021. There was no other strong or consistent pattern, which suggests that most fish species use the entire depth gradient throughout the year. Finally, the results suggest that the use of RUVs for the estimation of biodiversity of rocky reef fishes is a cost-effective way to improve our understanding of nearshore community shifts in relation to environmental forcing.

Habitat and nutrients, but not artificial lighting at night, drive fish assemblage composition in urban streams

Habitat and nutrients, but not artificial lighting at night, drive fish assemblage composition in urban streams

Effects of territorial damselfish on corallivorous fish assemblage composition and coral predation in the Mauritian lagoon

Predation is a significant stressor for many coral species. Understanding how predation interacts with factors such as climate change is key to coral conservation. Territorial damselfish of the genus Stegastes form territories on branching corals (genus Acropora) on which they cultivate algae, and while defending these territories, provide corals with indirect protection from predation. However, it is not known how the protection afforded by Stegastes spp. varies with corallivorous fish assemblage composition, nor whether corallivore assemblages themselves may be affected by the presence of Stegastes spp. and their associated Acropora spp. habitats. This study examined relationships among predation protection by Stegastes spp., branching Acropora coral cover, and the number and species richness of corallivorous fish present within a given area, namely the Mauritian lagoon in the western Indian Ocean. Predation on bleaching-resilient massive Porites lutea corals within and outside Stegastes territories was surveyed at sites around Mauritius island. Corallivorous fish assemblages, branching coral cover, and Stegastes spp. density were also surveyed at each site visit. Results show that high predation was correlated with the presence of high numbers of corallivores, but predation was lower within Stegastes spp. territories irrespective of all observed corallivore densities. Greater numbers of Stegastes spp. were correlated with increasing density of obligate corallivores. Non-Acroporid coral cover was positively correlated with species richness of obligate corallivores, while branching Acropora coral cover was negatively correlated with overall corallivore density. This study shows for the first time that predation on bleaching-resilient massive corals is lower within Stegastes spp. territories regardless of the number of corallivores present, adding to the growing body of knowledge on the complex relationships between Stegastes spp. and their environment. The findings also indicate possible effects of the presence of Stegastes spp. and their branching Acropora habitats on corallivorous fish density in areas with low coral diversity, which may warrant further study.

Effects of repeated sediment bypassing on stream ecosystems in a Japanese mountainous river

Sediment bypassing connects the upstream and downstream regions of dam impoundments through tunnels and is expected to partly restore natural processes (flood pulse and sediment regime) in dam‐altered river ecosystems. However, few reports examined the effects of restoration on stream biota. We examined the effectiveness of repeated sediment bypassing (12 times) over a period of 5 years in a Japanese dammed river using a before‐after‐reference‐impact design. We compared the abiotic parameters (water quality and substrate environment), biotic assemblages (algae, macroinvertebrates, and fish), and similarities between the dam downstream and undammed reference reaches. The reference sites exhibited high temporal variability in abiotic environments and species composition over the 5‐year period. Most abiotic and biotic parameters downstream of the dam improved and approached those of the changing reference states. However, these indices returned to pretrial conditions after the last trial. Biotic similarities between the downstream and reference reaches gradually increased throughout successive sediment‐bypassing trials for macroinvertebrates and algae; however, these similarities diminished after the last trial. Conversely, the abundance‐based composition of fish assemblages in the restored reach consistently diverged from that of the reference conditions throughout the study period. Our results show that the success and failure of restoration by sediment bypassing may be affected by the amount of bedload in the flushing flows and other regional processes such as migratory connectivity. Sediment bypassing can partially re‐establish natural processes and is a useful tool for improving sediment‐associated environments and biota.