Comparison of nine different methods to assess fish communities in lentic flood‐plain habitats

This study compares the effectiveness and representativeness of electrofishing, snorkelling, seining, baited lift netting, multi‐mesh gillnetting, baited fish traps, fyke netting, angling and longline fishing, considering three typical lentic flood‐plain habitats at different times of day. Electrofishing was by far the most effective method yielding highest species richness, species trait representation and catch per unit of effort (CPUE), followed by seining. For single species like dace Leuciscus leuciscus, European ruffe Gymnocephalus cernua, common bream Abramis brama and silver bream Blicca bjoerkna, seining was more effective than electrofishing. With both methods, some species were more consistently caught during night, dusk or dawn than during daylight. All other methods tested cannot be generally recommended for fish community assessments in shallow backwaters due to their low representativeness of species inventory and generally low CPUE. Based on these results, electrofishing of 30 m transect replicates at different times of day for monitoring the fish community in shallow backwaters, can be recommended, enabling the maximum possible comparability to adjacent river habitats. Seining should be considered as an alternative if accessibility of habitats is restricted or electrofishing is prohibited. The 25 species detected in the backwaters also suggest that these habitats contribute a large proportion of fish diversity and should be included in standard assessments of river ecological status.     

Gear effectiveness and size selectivity for five cryptic madtom species (Noturus spp.)

More than half of the diminutive North American catfish species known as madtoms (Noturus spp.) are considered imperiled due to range reductions from habitat fragmentation and degradation. In response, government agencies regularly conduct sampling that targets these cryptic fishes. Unfortunately, the effectiveness and biases associated with common sampling gears has rarely been quantified for non‐game fish, including madtoms. To improve sampling protocols for diverse madtom species, we: (a) quantified the detection probabilities and effort necessary to detect five species of madtom with relative certainty using complimentary sampling gears, and (b) described biases associated with size‐selectivity of the same gears when used to sample madtoms. Five madtom species that are native to Mississippi streams and have two distinct coloration patterns were selected: uniform pattern, Brown Madtom (Noturus phaeus) and Freckled Madtom (Noturus nocturnus); patterned, Piebald Madtom (Noturus gladiator), Least Madtom (Noturus hildebrandi), and Brindled Madtom (Noturus miurus). These species were captured using four standardized sampling gears (i.e., backpack electrofisher, seine, dipnet, and Gee‐style minnow trap) from 13 isolated stream reaches in Mississippi during baseflow conditions between August and November 2016. Each gear was randomly assigned to up to five transects at each site resulting in a maximum of 20 unique samples per stream reach. We estimated the detection probability of each species with each gear using presence–absence capture data. Cumulative detection probabilities were calculated for each species and gear combination to describe the number of samples necessary to achieve a detection probability of 0.95. Additionally, we pooled all capture data across sampling sites to compare madtom size distributions among gears. Electrofishing was generally the most efficient gear for detecting madtoms, but slightly overestimated fish size distributions. By seining, managers can expect to capture all species evaluated with a modest amount of effort; however, are likely to catch relatively small individuals. Seining was the most effective gear for only Piebald Madtom, a species petitioned for listing under the Endangered Species Act. To be relatively certain that Piebald Madtom are absent from a stream reach would require sampling ten 100 m transects by seining. In contrast, the same certainty would require more than twenty 100 m electrofishing transects. Minnow traps and dipnetting generally performed poorly and failed to detect some species at occupied sites. Biologists can now consider using each of these complimentary gears assessed when evaluating species status and population structure for diverse madtom species, while explicitly acknowledging the biases associated with each gear.     

Optimising Bait for Pitfall Trapping of Amazonian Dung Beetles (Coleoptera: Scarabaeinae)

The accurate sampling of communities is vital to any investigation of ecological processes and biodiversity. Dung beetles have emerged as a widely used focal taxon in environmental studies and can be sampled quickly and inexpensively using baited pitfalls. Although there is now a wealth of available data on dung beetle communities from around the world, there is a lack of standardisation between sampling protocols for accurately sampling dung beetle communities. In particular, bait choice is often led by the idiosyncrasies of the researcher, logistic problems and the dung sources available, which leads to difficulties for inter-study comparisons. In general, human dung is the preferred choice, however, it is often in short supply, which can severely limit sampling effort. By contrast, pigs may produce up to 20 times the volume. We tested the ability of human and pig dung to attract a primary forest dung beetle assemblage, as well as three mixes of the two baits in different proportions. Analyses focussed on the comparability of sampling with pig or human-pig dung mixes with studies that have sampled using human dung. There were no significant differences between richness and abundance sampled by each bait. The assemblages sampled were remarkably consistent across baits, and ordination analyses showed that the assemblages sampled by mixed dung baits were not significantly different from that captured by pure human dung, with the assemblages sampled by 10% and 90% pig mixes structurally most similar to assemblages sampled by human dung. We suggest that a 10:90 human:pig ratio, or similar, is an ideal compromise between sampling efficiency, inter-study comparability and the availability of large quantities of bait for sampling Amazonian dung beetles. Assessing the comparability of assemblage samples collected using different baits represents an important step to facilitating large-scale meta-analyses of dung beetle assemblages collected using non-standard methodology.     

The effectiveness of two common sampling methods for assessing imperilled freshwater fishes

This study tested the hypothesis that the most common gear type used to sample fishes in wadeable systems, electrofishing, was more effective than another commonly used gear type, seining, for sampling fish species at risk. Five predictions were tested. At sites where species at risk were detected, (1) the probability of detecting the species at risk, (2) the probability of only one gear type detecting the species at risk and (3) the estimated catch per unit effort of the species at risk, was as high as, or higher, when using electrofishing than when using a seine. (4) The number of sample sites required to detect a species at risk within a watershed and (5) the number of subsections required to detect a species at risk within a site, were as low as, or lower, using electrofishing than the number required using a seine. Based on analyses of these measurements, electrofishing was a more effective gear type than seining for sampling fish species at risk, irrespective of the unit (presence or absence or catch per unit effort) or scale of measurement (watershed or site level). Dissolved oxygen, turbidity, specific conductivity and nitrate concentrations were measured at each site and did not account for the between gear differences. Selection of sampling gear can be a fundamental consideration for the assessment of fish species at risk, where, unlike common species, they may be particularly influenced by small population sizes, restricted geographic ranges and narrow habitat preferences. Resource managers must weigh differences in the risks of injury of fish species at risk against differences in the effectiveness of each gear type when deciding between gear types and the utility of the assessments they represent.     

A comparison of two methods for generating artificial multi-assemblage ecological datasets

Simulated ecological datasets have been widely used to assess the ability of ordination techniques to portray patterns in ecological assemblage data. Such datasets typically contain a single assemblage sampled over an environmental gradient or set of gradients. Little has been done on the generation of artificial datasets that contain a number of different species assemblages, to aid in the evaluation of multivariate techniques that test for differences between assemblages of species. This paper describes and compares two simulation methods that generate ecologically realistic artificial multi-assemblage datasets. Both methods provide multivariate data (e.g. species abundances) for replicate sites within discretely different assemblages. The first technique is a coenocline model based on species' responses to variation modeled by a five-parameter β-function, where variation in species abundances both within and between assemblages is governed by differences in the positions of sites and assemblages along environmental gradients. The second technique, the resampling method, involves bootstrap resampling of real assemblage datasets, with the addition of selected types of controlled differences between assemblages. Here we use it to generate turnover in species composition. We calibrate both simulation methods based on a field assemblage of bird species. The two different simulation methods portray different levels and types of between-assemblage variation. The resampling method allows greater control over some aspects of assemblage difference (e.g. independently varying differences in species richness and compositional turnover) than the coenocline method. Both can generate usable replicated simulated datasets for assessing the ability of multivariate tests to detect ecological variation among assemblages.     

Effects of sampling techniques on population assessment of invasive round goby Neogobius melanostomus

In this study, a comparison of point abundance sampling (PAS) electrofishing, angling with two different hook sizes and trap‐based fishing was performed in a non‐wadeable river to analyse their effects on catch per unit effort (CPUE) and population characteristics of invasive round goby Neogobius melanostomus. PAS electrofishing was identified as the most effective (mean ± s.e . CPUE = 57 ± 4 N. melanostomus min^?1) and least selective method in terms of size, feeding status and species composition. Angling had the second highest CPUE, but was more size selective and resulted in a higher proportion of males compared to electrofishing [overall sex ratio angling (female:male) = 1:0·92, electrofishing 1:0·65]. Owing to low CPUE (0·012 ± 0·004) and low frequency of occurrence, minnow traps were least suitable for N. melanostomus population assessment. The results of this study suggest that a higher degree of standardization and inter‐calibration is useful to achieve better comparability of population data of invasive N. melanostomus and other benthic fish species.     

Method-integrated fish assemblage structure at two spatial scales along a free-flowing stretch of the Austrian Danube

Fish assemblages in large rivers are highly diverse communities. However, a single method to describe these complex systems adequately is not available as of yet. In this paper we propose a combination of normalized catch-per-unit-effort data of three fish sampling methods, thus providing a more inclusive representation of the overall assemblage. We investigate fish species occurrence and abundance in a survey at three consecutive sites in the free-flowing main channel of the Austrian Danube east of Vienna. Using boat and wading electrofishing, as well as longline fishing we collected data in four mesohabitats at each site. We analysed richness, assemblage structure and abundance of single species and tested site and mesohabitat as differentiating forces on the fish assemblage structure, applying PERMANOVA and NMDS. The investigated sites are populated by a species-rich fish assemblage, of which we collected a total of 36 species. The composition of the assemblage was rather consistent except for the occasional occurrence of rare species. At the smaller mesohabitat scale, however, the assemblages differed considerably and hydromorphological conditions were apparently the major structuring force. Overall, the combination of methods considerably improves the assessment across sites and mesohabitats and contributes to our understanding of large river fish assemblages.     

Using new electrofishing technology to amp‐up fish sampling in estuarine habitats

A prototype, boat‐mounted electrofisher capable of operation in estuarine waters (where electrical conductivities often exceed 20 000 µS cm^?1) was assessed. Electrofishing was compared to fyke and mesh netting in four riverine estuaries and to seining in a lagoonal estuary (consisting of a series of brackish coastal lakes separated from the sea by a barrier system of sand dunes). Fish assemblage composition, length distributions and the probability of detecting ecological fish guilds (relating to estuary use, position in the water column and body size) were compared among gears. The assemblage composition of electrofishing samples differed from those of fyke nets in all riverine estuaries and from mesh netting in two. The assemblage composition of electrofishing samples differed from those of seining in structured seagrass habitats of the lagoonal estuary. When all species were pooled, the electrofisher sampled a broader range of lengths than either fyke or mesh netting in riverine estuaries or seining in lagoonal estuaries. The bias of electrofishing and netting towards particular species and size classes affected the probability of detecting some ecological guilds, highlighting the potential implications of gear choice on understanding estuarine ecological function. The detection of guilds varied with gear type and environmental conditions, including stratification, water depth and surface electrical conductivity. Assessments with the aim to characterize the structure of fish assemblages will benefit from the use of multiple gears. Electrofishing shows immense promise for discretely sampling highly structured habitats to test hypotheses about their use.     

Assemblage level variation in springtail lower lethal temperature: the role of invasive species on sub‐Antarctic Marion Island

Abstract. It is widely held both in the physiological literature, and more generally, that the average characteristics of species within an assemblage differ among sites. Such generalizations should be based on investigations of whole assemblages at sites, but this is rarely done. Here, such a study is undertaken for virtually the full assemblage of springtails found at sub‐Antarctic Marion Island, by investigating supercooling points (SCPs) of 12 of the 16 species that occur there. Assemblage level variation tends to be less than that documented for assemblages across northern hemisphere sites but similar to that found at some Antarctic locations. Across this set of species, the mean SCPs of the indigenous species (mean ± SE =?17.2 ± 0.4 °C) do not differ significantly from that of the invasive species (?16.3 ± 0.7 °C). Overall, the introduction of several species to the island does not appear to have led to functional homogenization (for this trait). By combining the assemblage‐level SCP data with information on the abundances of the species in each of four major habitats, it is also shown that severe but uncommon low temperature events could substantially alter species relative abundances. By resetting assemblage trajectories, such events could play an important role in the terrestrial system at the island.     

Seasonal variation in the composition of fish assemblages in small Amazonian forest streams: evidence for predictable changes

1. The effects of seasonal inundation on the biology of fishes on floodplains of large Amazonian rivers are well studied. However, the small seasonal changes in headwater streams are generally considered to have little effect on fish assemblages. 2. In this study, we analysed seasonal changes in the species composition and abundance of fish in small Amazonian forest streams. We sampled fish with hand and seine nets in headwater streams in a 10 000 ha terra‐firme forest reserve near Manaus, Brazil. Each stream was surveyed at the end of the 2005 dry season, at the beginning of the 2006 rainy season and at the beginning of the 2006 dry season, by means of a standardized sampling effort. 3. The numbers of individuals and species caught were higher in the dry season, but rarefaction analyses indicated that greater species numbers could have been due simply to the larger number of individuals caught. 4. Between the dry and rainy season, the direction of changes in species composition in multivariate space varied among sites, especially for quantitative (abundance) data. However, the observed variation among sites was the less than expected if the directions of change were random. 5. Fish assemblages in the second dry season were more similar to those in the previous dry season than expected if changes in species composition among seasons were random. This indicates that a general seasonal pattern in fish assemblages can be detected, despite the existence of some erratic site‐specific changes. 6. Most of the species that showed large seasonal variations in density occupy temporary ponds during the rainy season, when much of the valley is inundated and pond networks form adjacent to streams. Short‐duration lateral migrations to these ponds may play an important role in the seasonal fish‐assemblage dynamics in Amazonian headwater streams. 7. Our results contrast with previous studies on small Amazonian streams, which have found little seasonal change in fish assemblages, and highlight the importance of the abundance of common species as an indicator of general fish assemblage structure in biological monitoring programmes.     

Gauging megadiversity with optimized and standardized sampling protocols: A case for tropical forest spiders

Characterizing and monitoring biodiversity and assessing its drivers require accurate and comparable data on species assemblages, which, in turn, should rely on efficient and standardized field collection. Unfortunately, protocols that follow such criteria remain scarce and it is unclear whether they can be applied to megadiverse communities, whose study can be particularly challenging. Here, we develop and evaluate the first optimized and standardized sampling protocol for megadiverse communities, using tropical forest spiders as a model taxon. We designed the protocol COBRA‐TF (Conservation Oriented Biodiversity Rapid Assessment for Tropical Forests) using a large dataset of semiquantitative field data from different continents. This protocol combines samples of different collecting methods to obtain as many species as possible with minimum effort (optimized) and widest applicability and comparability (standardized). We ran sampling simulations to assess the efficiency of COBRA‐TF (optimized, non‐site‐specific) and its reliability for estimating taxonomic, phylogenetic, and functional diversity, and community structure by comparing it with (1) commonly used expert‐based ad hoc protocols (nonoptimized, site‐specific) and (2) optimal protocols (optimized, site‐specific). We then tested the performance and feasibility of COBRA‐TF in the field. COBRA‐TF yielded similar results as ad hoc protocols for species (observed and estimated) and family richness, phylogenetic and functional diversity, and species abundance distribution. Optimal protocols detected more species than COBRA‐TF. Data from the field test showed high sampling completeness and yielded low numbers of singletons and doubletons. Optimized and standardized protocols can be as effective in sampling and studying megadiverse communities as traditional sampling, while allowing data comparison. Although our target taxa are spiders, COBRA‐TF can be modified to apply to any highly diverse taxon and habitat as long as multiple collecting techniques exist and the unit effort per sample is comparable. Protocols such as COBRA‐TF facilitate studying megadiverse communities and therefore may become essential tools for monitoring community changes in space and time, assessing the effects of disturbances and selecting conservation areas.     

Spatial analyses of intertidal assemblages on sheltered rocky shores

Abstract Understanding processes in complex assemblages depends on good understanding of spatial and temporal patterns of structure at various spatial scales. There has been little quantitative information about spatial patterns and natural temporal changes in intertidal assemblages on sheltered rocky shores in temperate Australia. Natural changes and responses to anthropogenic disturbances in these habitats cannot be accurately measured and assessed without quantitative data on patterns of natural variability in space and through time. This paper describes some suitable quantitative methods for examining spatial and temporal patterns of diversity and abundances of highshore, midshore and lowshore intertidal assemblages and the important component species for a number of shores in a bay that has not been severely altered by human disturbance. Despite a diverse flora and fauna on these shores, the midshore and lowshore assemblages on sheltered shores were characterized by a few species which were also the most important in discriminating among assemblages on a shore and, for each assemblage, among different shores. The same set of species was also important for measuring small-scale patchiness within each assemblage (i.e. between replicate sites on a shore). Therefore, these data provide a rationale for selecting species that are useful for measuring differences and changes in abundance among places and times at different scales and, hence, can be used in the more complex sampling designs necessary to detect environmental impacts. There was considerable spatial variability in all assemblages and all species (or taxa) examined at scales of metres, tens of metres and kilometres. There were no clear seasonal trends for most measures, with as much or more variability at intervals of 3 months as from year to year. Most interactions between spatial and temporal measures were at the smallest scale, with different sites on the same shore generally showing different changes from time to time. The cause(s) of this apparently idiosyncratic variability1 were not examined, but some potential causes are discussed. These data are appropriate for testing hypotheses about the applicability of these findings to other relatively undisturbed sheltered shores, about effects of different anthropogenic disturbances on sheltered intertidal assemblages and to test hypotheses about differences in intertidal assemblages on sheltered versus wave-exposed shores.     

The role of sampling effort,taxonomical resolution and abundance weight in multivariate comparison of stream dwelling caddisfly assemblages collected from riffle and pool habitats

We examined how the sample representativeness of a single assemblage and the separation of two assemblages can be improved by finding appropriate combinations of sampling effort, taxonomical resolution and abundance weight for stream dwelling caddisflies as model organisms. We found that these parameters strongly influenced the outcome of multivariate analyses both individually and when considered jointly. This is the first study to show that sample representativeness of an assemblage can decrease with increasing sampling effort. We assume that the turnover rate of the assemblage and the rarity of the species are responsible for this phenomenon. We found that the separation of two assemblages can be improved by increasing sampling effort and applying abundance data. Further, we observed that the effect of taxonomical resolution on the separation of ecological assemblages was highly context-dependent. Decreased taxonomical resolution, i.e. changing from species to genus or to family, did not decrease, or even more increased the separation of assemblages. In sum, this study demonstrates the importance of the careful selection of sampling, laboratory and data processing related factors (i.e. sampling effort, taxonomical resolution, abundance weight) in the multivariate comparison of assemblages.     

Measuring and controlling data quality in biological assemblage surveys with special reference to stream benthic macroinvertebrates

1. Biological assemblage surveys primarily aim to characterise species composition and relative abundance at one or more spatial or temporal scales. Data interpretation and conclusions are dependent on how well samples characterise the assemblage of interest. 2. Conventional measures of data quality, e.g. standard deviations or coefficients of variation, were designed for single variable estimation, and they are either insufficient or invalid for assessing the quality of data describing entire assemblages. Similarity indices take species composition and relative abundance into account and may be used to effectively measure and control the quality of data used to characterise assemblage structure. 3. The average Jaccard coefficient (JC) calculated across multiple pairs of replicate samples, i.e. autosimilarity JC (AJC), is conceptually and numerically related to the average coefficient of variation in the densities of all species recorded, a measure of sampling precision, and to the proportion of total species richness sampled, a measure of sampling accuracy. 4. We explored how AJC can be used to assess the effect of different potential sources of error on the quality of assemblage survey data, including the sampling effort used both within regions and at individual sites, the individuals collecting samples, sub‐sampling procedures, and consistency of taxon identification. 5. We found that the autosimilarity‐based approach overcomes most weaknesses associated with conventional measures of data quality and can be used to effectively measure and control the quality of assemblage survey data.     

Patternizing of impoundment impact (1985–2002) on fish assemblages in a lowland river using the Kohonen algorithm

Impoundment impact on fish assemblage structure was investigated in the dammed middle course of the Warta River. A backwater site (AB) was located 2 km upstream of the Jeziorsko Reservoir, and a tailwater site (CD) 1.5 km downstream of the dam. Both sites were studied for 3 years in the pre‐impoundment period (1985–1987) and 15 years after damming (1988–2002). Quantitative electrofishing in spring and autumn assured obtaining yearly average biomass for each population. Most of the data analysis aimed to assess the dam impact on the fish assemblage structure but other accompanying impacts such as discharge manipulations, revetment, different forms of engineering, and water quality improvement in the tailwater and backwater reaches were also discussed. The Kohonen algorithm (self‐organizing map, SOM) was used for the analysis, and perfectly separated AB and CD samples into two clusters. Samples from the backwater (AB) proved that this reach of the Warta River had maintained its almost natural character and that fish assemblages had changed moderately, now occupying only five neighbouring hexagons out of a total of 16. In the tailwater (CD), however, because of considerable fluctuations in fish assemblages the SOM produced three subclusters, which engaged nine hexagons: (i) the pre‐impoundment period (1985–1987, two hexagons); (ii) 7 years after the definite closure of dam sluices (1988–1994, five hexagons); and (iii) the past 8 years of sampling (1995–2002, two hexagons), when stabilization in the assemblage was observed. The SOM also definitely proved profound changes in fish assemblage composition: most lithophilous species declined and many phytolithophilous and phytophilous species became dominants, particularly in the tailwater site where downstream migration of 0+ of successfully spawned species from the reservoir took place.     

Testing the enemy release hypothesis: trematode parasites in the non-indigenous Manila clam <Emphasis Type="Italic">Ruditapes philippinarum</Emphasis>

Knowledge of temporal variation in nearshore Laurentian Great Lakes fish assemblages is important for understanding species–habitat associations, how abiotic and biotic influences vary temporally, and when sampling should occur. Using spring and fall seining data from Lake Erie beaches, we compared day and night fish assemblages and tested for differences among sampling periods. Beaches were utilized by a diverse collection of Lake Erie basin fishes (one-third of known species). During all sampling periods, catches were dominated by cyprinid species (53–91%), and by invertivores and planktivorous fishes. Diel differences were detected in abundance, species richness and assemblage structure. Multivariate analyses (canonical analysis of principal coordinates) indicated that season had a larger influence on fish assemblage structure than diel period. Given observed temporal variation in assemblage structure, studies of Laurentian Great Lakes beach fishes should be restricted to a single time period (e.g. day-time spring sampling), or adopt sampling designs that permit diel period and season to be included as factors in analyses. Second, the large seasonal variation in assemblage composition combined with higher night species richness indicates that night sampling during both spring and fall would be the most efficient and comprehensive approach for beach fish inventory. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: J. Trexler     

Structural and functional losses in macroalgal assemblages in a southeastern Brazilian bay over more than a decade

Changes in macroalgae assemblages over more than a decade are described for Sepetiba Bay, Brazil. Variations in macroalgae abundances and functional diversity were compared with older data to test the hypothesis that their diversity decreases following anthropogenic stress that negatively impact environmental characteristics. Four field sampling excursions were undertaken at two different sites from December/2012 to May/2014. Destructive sampling per effort used six box cores (25 × 25 cm) distributed randomly along a shallow sublittoral rocky shore. Biomass was used to quantify macroalgae assemblages identified to the species level. Multivariate analyses demonstrated decreases in total biomass at both sites as well as changes in community physiognomies. The predominant corticated algae found were classified as Ecological Status Group IIA, characteristic of sites in the process of degradation and indicating that anthropogenic stress had negatively affected the macroalgae communities as evaluated by the Ecological Evaluation Index.     

Evaluating detectability of freshwater fish assemblages in tropical streams: Is hand-seining sufficient?

Unprecedented threats to natural ecosystems mean that accurate quantification of biodiversity is a priority, particularly in the tropics which are underrepresented in monitoring schemes. Data from a freshwater fish assemblage in Trinidad were used to evaluate the effectiveness of hand-seining as a survey method in tropical streams. We uncovered large differences in species detectability when hand-seining was used alone, in comparison with when hand-seining and electrofishing were used together. The addition of electrofishing increased the number of individuals caught threefold, and increased the biomass fivefold. Some species were never detected using hand-seining, resulting in significant underestimates of species richness; rarefaction curves suggest that even when hand-seining effort increases, species richness is still underestimated. Diversity indices (Shannon and Simpson index) reveal that diversity was also significantly lower for hand-seined samples. Furthermore, the results of multivariate analyses investigating assemblage structure also differed significantly depending on whether they were based on hand-seined data alone, or a combination of hand-seining and electrofishing. Despite the extra equipment and maintenance required, these findings underline the value of including electrofishing when sampling tropical freshwater streams.     

Fish foraging effects on benthic assemblages along a warm-temperate stream: differences among drift feeders, benthic predators and grazers

In streams, physical and biotic conditions change from the headwaters to the mouth, shaping longitudinal patterns in community structure. We examined how fish foraging effects on periphyton and benthic invertebrates changed along a longitudinal gradient of a warm-temperate stream in southwestern Japan. We established three study sites according to changes in the composition of fish feeding guilds (upper site characterized by drifting-invertebrate feeders, Oncorhynchus masou ; middle site by benthic invertebrate feeders, Rhinogobius spp.; lower site by the presence of periphyton grazers, Sicyopterus japonicus ), and performed two manipulative experiments to examine effects of different fish assemblages on periphyton and benthic invertebrate abundances. Results of an exclosure experiment suggested that fishes had no effect on the benthic assemblages at the upper and middle sites whereas fishes reduced the abundances of both periphyton and invertebrates on stone surfaces at the lower site, where both benthic invertebrate feeders and grazers inhabited. A subsequent enclosure experiment showed that the reduction of invertebrate densities at the lower site was caused by the grazers rather than benthic invertebrate feeders. These experimental results suggested that effects of fishes on benthic assemblages are intensified downstream, owing to the occurrence of the grazing fish. Furthermore, observational data based on field sampling suggested that such grazing effects were reflected in longitudinal patterns in periphyton and invertebrate abundances. Overall results emphasize an important role of the grazing fish ( S. japonicus ) in shaping longitudinal patterns in benthic assemblage structure.