Effects of floods on fish assemblages in an intermittent prairie stream

1. Floods are major disturbances to stream ecosystems that can kill or displace organisms and modify habitats. Many studies have reported changes in fish assemblages after a single flood, but few studies have evaluated the importance of timing and intensity of floods on long‐term fish assemblage dynamics. 2. We used a 10‐year dataset to evaluate the effects of floods on fishes in Kings Creek, an intermittent prairie stream in north‐eastern, Kansas, U.S.A. Samples were collected seasonally at two perennial headwater sites (1995–2005) and one perennial downstream flowing site (1997–2005) allowing us to evaluate the effects of floods at different locations within a watershed. In addition, four surveys during 2003 and 2004 sampled 3–5 km of stream between the long‐term study sites to evaluate the use of intermittent reaches of this stream. 3. Because of higher discharge and bed scouring at the downstream site, we predicted that the fish assemblage would have lowered species richness and abundance following floods. In contrast, we expected increased species richness and abundance at headwater sites because floods increase stream connectivity and create the potential for colonisation from downstream reaches. 4. Akaike Information Criteria (AIC) was used to select among candidate regression models that predicted species richness and abundance based on Julian date, time since floods, season and physical habitat at each site. At the downstream site, AIC weightings suggested Julian date was the best predictor of fish assemblage structure, but no model explained >16% of the variation in species richness or community structure. Variation explained by Julian date was primarily attributed to a long‐term pattern of declining abundance of common species. At the headwater sites, there was not a single candidate model selected to predict total species abundance and assemblage structure. AIC weightings suggested variation in assemblage structure was associated with either Julian date or local habitat characteristics. 5. Fishes rapidly colonised isolated or dry habitats following floods. This was evidenced by the occurrence of fishes in intermittent reaches and the positive association between maximum daily discharge and colonisation events at both headwater sites. 6. Our study suggests floods allow dispersal into intermittent habitats with little or no downstream displacement of fishes. Movement of fishes among habitats during flooding highlights the importance of maintaining connectivity of stream networks of low to medium order prairie streams.     

Effects of deforestation on fish community structure in Ecuadorian Amazon streams

SUMMARY 1. There is little information on the impacts of deforestation on the fish fauna in neotropical streams, and on parameters influencing species diversity and community structure of fish. We analysed these aspects in 12 stream sites in the Ecuadorian Amazon. The stream sites represented a large gradient in canopy cover and were located in an area of fragmented forest. While some streams had been deforested, they had not suffered gross degradation of the habitat.
2. The species richness of stream fish was not related to deforestation. Local fish diversity (Fisher's Alpha) was positively related to the surface area of stream pools (m²). Beta diversity was higher among forested than deforested sites, indicating greater heterogeneity in species composition among forested than deforested sites. The percentage of rare species was positively correlated with canopy cover.
3. Total fish density increased with deforestation, and the fish community changed from dominance by omnivorous and insectivorous Characiformes at forested sites to dominance of periphyton-feeding loricariids at deforested sites.
4. Multidimensional statistical analysis of fish community structure showed that six environmental variables (the area of stream bottom covered by leaves, relative pool area, particulate organic matter, mean depth, conductivity and suspended solids) were related to the ordination axes. The presence of leaves, which was strongly correlated to canopy cover, was the variable most closely related to fish community structure, while relative pool area was the second strongest variable. Thus, fish community structure was strongly affected by deforestation.     

Indirect food web interactions increase growth of an algivorous stream fish

1. We tested the hypothesis that indirect food web interactions between some common, invertivorous fishes and their prey would positively affect growth of an algivorous fish species. Specifically, we predicted that orangethroat darter (Etheostoma spectabile) would increase periphyton biomass via a top‐down pathway, indirectly enhancing growth of the algivorous central stoneroller minnow (Campostoma anomalum). Moreover, we predicted that sand shiner (Notropis stramineus) would increase periphyton biomass via a bottom‐up pathway and indirectly enhance growth of the stoneroller minnow. 2. In an 83‐day experiment in large, outdoor, stream mesocosms, we stocked two fish species per mesocosm (stoneroller and either darter or shiner), estimated the effects of the invertivorous and grazing fishes on periphyton biomass and estimated growth of the algivorous fish. 3. The darter consumed grazing invertebrates, indirectly increasing periphyton biomass. The shiner consumed terrestrial insects as predicted, but it did not affect periphyton biomass. 4. In support of our hypothesis, the darter indirectly enhanced stoneroller growth. As predicted, stonerollers consumed the increased periphyton in streams with darters, resulting in greater growth, condition and gut fullness compared to streams without darters. No indirect interaction was observed between stonerollers and shiners. 5. Our study suggests that some invertivorous fish species can positively affect growth of algivorous fishes through indirect food web interactions. Thus, in stream communities, it is possible that the loss of a single, invertivorous fish taxon could have negative consequences on algivorous fish populations via the removal of positive indirect food web interactions.     

Influence of habitat heterogeneity on the distribution of larval Pacific lamprey (Lampetra tridentata) at two spatial scales

1. Spatial patterns in channel morphology and substratum composition at small (1–10 metres) and large scales (1–10 kilometres) were analysed to determine the influence of habitat heterogeneity on the distribution and abundance of larval lamprey. 2. We used a nested sampling design and multiple logistic regression to evaluate spatial heterogeneity in the abundance of larval Pacific lamprey, Lampetra tridentata, and habitat in 30 sites (each composed of twelve 1‐m² quadrat samples) distributed throughout a 55‐km section of the Middle Fork John Day River, OR, U.SA. Statistical models predicting the relative abundance of larvae both among sites (large scale) and among samples (small scale) were ranked using Akaike's Information Criterion (AIC) to identify the ‘best approximating’ models from a set of a priori candidate models determined from the literature on larval lamprey habitat associations. 3. Stream habitat variables predicted patterns in larval abundance but played different roles at different spatial scales. The abundance of larvae at large scales was positively associated with water depth and open riparian canopy, whereas patchiness in larval occurrence at small scales was associated with low water velocity, channel‐unit morphology (pool habitats), and the availability of habitat suitable for burrowing. 4. Habitat variables explained variation in larval abundance at large and small scales, but locational factors, such as longitudinal position (river km) and sample location within the channel unit, explained additional variation in the logistic regression model. The results emphasise the need for spatially explicit analysis, both in examining fish habitat relationships and in developing conservation plans for declining fish populations.     

Habitat structure along channel-unit sequences for juvenile salmon: a subunit-based analysis of in-stream landscapes

1. Habitat structure and habitat use by juvenile masu salmon, Oncorhynchus masou Brevoort, in small streams in northern Hokkaido, Japan, were examined by considering 'subunits' (patches within channel units) as structural elements of stream reaches.
2. Whole wetted channel surfaces of three study reaches were divided into 0.5 × 0.5 m quadrats, which were grouped into eight subunit types according to water depth and velocity, and substratum conditions by a cluster analysis. The subunit distribution showed a regular mosaic pattern corresponding to the channel-unit sequence in each of the three reaches.
3. Juvenile masu salmon exhibited a strong preference for a subunit type characterized by greater depth and moderate current velocity (deep–moderate subunit; mean depth = 0.29 m; mean velocity = 0.19 m s⁻¹). This subunit type usually occurred downstream of stretches with fast current. The preference of masu salmon for the deep–moderate subunit could be because of its usual spatial position in relation to other subunit types as well as to the characteristics of the subunit itself.
4. The results suggest that the value of a habitat is determined not only by the characteristics of the habitat itself, but also by those of adjacent habitats. Therefore, habitat use by stream fish should be studied in the context of the whole 'in-stream landscapes'.     

Role of habitat in determining macroinvertebrate production in an intermittent-stream system

1. The effect of channel drying on macroinvertebrate production was studied at the habitat and reach scale in a catchment drained by intermittent streams in Maine, U.S.A. The catchment includes two first‐order streams and their second‐order confluence. Six reaches were selected for study based on differences in channel slope and habitat cover (bedrock, riffle/run, debris dam and pool). Stream water in each reach was acidic and oligotrophic. 2. The study reaches had different degrees of channel drying. In the first‐order reaches, surface flow ceased earlier in the season and for longer periods than second‐order reaches. Regardless of reach, pool and debris dam habitats retained water longer than riffle/runs and bedrock. Unlike other habitats, debris dams retained moisture for relatively long periods following cessation of surface flow. 3. Reach‐specific macroinvertebrate production ranged from approximately 1.7 to 2.9 g AFDM m⁻² year⁻¹ which are among the lowest values ever reported. Habitat‐specific production ranged from approximately 0.5 to 5.0 g AFDM m⁻² year⁻¹ (bedrock and debris dams, respectively). 4. At the reach scale, quantities of stored benthic organic matter (range approximately 200–600 g AFDM⁻²) decreased in a downstream direction. 5. A combination of differences in the timing and duration of channel drying, habitat structure and detritus standing stocks appeared to influence levels of invertebrate production among the study reaches. 6. Our interpretation of a canonical correspondence analysis indicates that drying is more important than habitat in affecting macroinvertebrate production in this intermittent stream system.     

Population size,habitat fragmentation,and the nature of adaptive variation in a stream fish

Whether and how habitat fragmentation and population size jointly affect adaptive genetic variation and adaptive population differentiation are largely unexplored. Owing to pronounced genetic drift, small, fragmented populations are thought to exhibit reduced adaptive genetic variation relative to large populations. Yet fragmentation is known to increase variability within and among habitats as population size decreases. Such variability might instead favour the maintenance of adaptive polymorphisms and/or generate more variability in adaptive differentiation at smaller population size. We investigated these alternative hypotheses by analysing coding-gene, single-nucleotide polymorphisms associated with different biological functions in fragmented brook trout populations of variable sizes. Putative adaptive differentiation was greater between small and large populations or among small populations than among large populations. These trends were stronger for genetic population size measures than demographic ones and were present despite pronounced drift in small populations. Our results suggest that fragmentation affects natural selection and that the changes elicited in the adaptive genetic composition and differentiation of fragmented populations vary with population size. By generating more variable evolutionary responses, the alteration of selective pressures during habitat fragmentation may affect future population persistence independently of, and perhaps long before, the effects of demographic and genetic stochasticity are manifest.     

Conflicting effects of woody debris on stream fish populations: implications for management

1. Coarse woody debris (CWD) in stream channels causes changes in flow, sedimentation and ratios of pool to riffle areas. There is a consensus among fishery managers and scientists that CWD is beneficial to stream fish communities because of its enhancement of habitat diversity, invertebrate production and cover. Our hypothesis was that CWD accumulation or introduction would not increase in‐stream habitat capacity for all species and their ontogenic stages at reach and stream scales. 2. The study used a system of gravel‐bed streams with naturally dynamic CWD accumulations and a fish community consisting of Salmo trutta, Cotttus gobio, Phoxinus phoxinus, Lampetra cf planeri, Nemacheilus barbatulus and Anguilla anguilla. Cotttus gobio and L. cf planeri are protected by an EU Directive and S. trutta is exploited for angling. Riffles, pools and CWD matrices, considered as the basic habitat/spatial units of channel structure, were sampled separately and abundance of each fish species quantified seasonally at each spatial scale. 3. Multiple‐pass electric fishing techniques were used. Capture efficiencies were calculated for species, habitat and season. Areal densities (number m^?2) were compared for habitat types and season using nonparametric anova . Canonical analysis and stepwise multiple regression were used to show the most influential physical variables on fish density. Densities were also compared by unit volume (numbers m^?3) for pools and CWD matrices to investigate direct three‐dimensional use for cover. Reach‐scale densities for each fish species in relation to habitat composition were made using Spearman rank correlation of habitat‐scale densities with proportionate areas of the different habitat units in the reach. 4. Habitat‐scale densities of bullheads and age 0+ trout were negatively correlated with depth and CWD areas for some seasons. Densities of lampreys, older trout, eels and minnows were positively correlated with depth in some seasons. Water depth had the most consistent influence on fish abundance at the habitat unit scale. Three‐dimensional comparisons of pools and CWD matrices indicated that only trout older than 1+ may use CWD habitats as cover. 5. Reach‐scale densities of 0+ trout and bullheads were significantly correlated with proportion of riffle area and negatively with CWD and combined CWD‐pool habitat area in the reach. Densities of older trout, large eels and lampreys were positively correlated with CWD area and combined CWD‐pool area in some seasons. Inundation of riffles caused by impoundment upstream of CWD accumulations reduced spawning habitat for trout, bullheads, brook lampreys, minnows and stone loach. A trade‐off was an increase in refugia for older trout, minnows and eels. 6. Coarse woody debris accumulation in streams is not beneficial to all species or ontogenic stages in a mixed species population and could severely limit essential habitat areas for some species. Thus, physical manipulation of channels should be implemented only after a thorough study of the habitat relationships of all species present, especially where protected species coexist with target species. The relative importance of in‐stream morphological changes depends on the spatial and temporal scale of the species life histories.     

Temporal variation in foraging group structure of a size-structured stream fish community

Temporal variation in foraging group structure of a fish assemblage was examined in a flood-prone stream in southern Hokkaido, Japan. Foraging behaviour was observed underwater for four species which inhabit the water column: ayu, Plecoglossus altivelis, white-spotted charr, Salvelinus leucomaenis, masu salmon, Oncorhynchus masou, and Japanese dace, Tribolodon hakonensis, with each species being categorized into five size classes (species-size group; SSG). Based on foraging behaviour, each SSG of the fish assemblage was classified into one of four foraging groups: algae grazers, drift foragers, benthos-drift foragers, and omnivores, defined as SSG exhibiting similar foraging behaviour. All size classes of ayu, and of charr and salmon were categorized as algae grazers and drift foragers, respectively, throughout the study period. In contrast, size classes of dace were categorized as drift foragers, benthos-drift foragers, or omnivores with the same size classes often assigned to different foraging groups from month to month. Digestive tract contents of the fishes in the four foraging groups reflected their observed foraging behaviour, and foraging groups were therefore regarded as representing trophic groups. Abundance and membership of each foraging group varied in accordance with changes in abundance of SSG due to their growth, immigration, emigration, and/or mortality. Moreover, due to numerical dominance within the assemblage, plasticity in foraging behaviour of small- and medium-sized dace also played a key role in determining variability in the foraging group structure. Relative frequencies of two types of foraging behaviour, algae nipping and benthos foraging, of the small-sized dace were significantly correlated with the level of each resource, whereas no significant relationship was detected between foraging frequencies of the medium-sized dace and either resource. Fluctuations in foraging group structure within this assemblage occurred through niche shifts of some component members and by changes in SSG composition.     

Local habitat restoration in streams: Constraints on the effectiveness of restoration for stream biota

Summary   The restoration of physical habitat has emerged as a key activity for managers charged with reversing the damage done by humans to streams and rivers, and there has been a great expenditure of time, money and other resources on habitat restoration projects. Most restoration projects appear to assume that the creation of habitat is the key to restoring the biota ('the field of dreams hypothesis'). However, in many streams where new habitat is clearly required if populations and communities are to be restored, there may be numerous other factors that cause the expected link between habitat and biotic restoration to break down. We discuss five issues that are likely to have a direct bearing on the success, or perceived success of local habitat restoration projects in streams: (i) barriers to colonization, (ii) temporal shifts in habitat use, (iii) introduced species, (iv) long-term and large-scale processes, and (v) inappropriate scales of restoration. The purpose of the study was primarily to alert ecologists and managers involved in stream habitat restoration to the potential impacts of these issues on restoration success. Furthermore, the study highlights the opportunities provided by habitat restoration for learning how the factors we discuss affect populations, communities and ecosystems.     

Climate change hastens the turnover of stream fish assemblages

Stream fish are expected to be significantly influenced by climate change, as they are ectothermic animals whose dispersal is limited within hydrographic networks. Nonetheless, they are also controlled by other physical factors that may prevent them moving to new thermally suitable sites. Using presence–absence records in 655 sites widespread throughout nine French river units, we predicted the potential future distribution of 30 common stream fish species facing temperature warming and change in precipitation regime. We also assessed the potential impacts on fish assemblages' structure and diversity. Only cold-water species, whose diversity is very low in French streams, were predicted to experience a strong reduction in the number of suitable sites. In contrast, most cool-water and warm-water fish species were projected to colonize many newly suitable sites. Considering that cold headwater streams are the most numerous on the Earth's surface, our results suggested that headwater species would undergo a deleterious effect of climate change, whereas downstream species would expand their range by migrating to sites located in intermediate streams or upstream. As a result, local species richness was forecasted to increase greatly and high turnover rates indicated future fundamental changes in assemblages' structure. Changes in assemblage composition were also positively related to the intensity of warming. Overall, these results (1) stressed the importance of accounting for both climatic and topographic factors when assessing the future distribution of riverine fish species and (2) may be viewed as a first estimation of climate change impacts on European freshwater fish assemblages.     

Effect of habitat structure on the feeding efficiency of young stages of razor fish (Pelecus cultratus (L.)): an experimental approach

The effects of a submerged macrophyte (Potamogeton pectinatus), turbidity (produced by 50 mg l^–1 kaolin), and their combination on the feeding rate of young razor fish (Pelecus cultratus (L.)) were studied in the laboratory. Young razor fish showed preference for Copepoda in most of the simulated habitats. Razor fish fed more efficiently in the absence of any environmental structure, but the more structured habitats, 3 g wet wt l^–1 of simulated submerged macrophyte and turbidity combined with vegetation, had a negative impact on food consumption rates.     

Variability in the distribution and abundance of stream salmonids, and the associated use of habitat models

The effective management of salmonid fisheries requires that the factors influencing variation in the abundance of stream populations are understood. The use of habitat models to explain the spatial component of population variance offers potential for management, but has not previously been set in the context of long term variation in population abundance because of the lack of suitable data sets. This paper examines contributions of spatial and temporal factors lo fish density variance using a 10-year data set from five tributaries of the River Conwy, North Wales. Recently developed habitat models were applied to the data to test their ability to explain nominal spatial variance. Spatial variance accounted for between 21 and 62% of the overall variance of salmonid abundance, and habitat models explained up to 95% of the spatial variance component. Synchrony in population variation amongst sites within and between tributaries is described, and some of the factors that may influence this are discussed.     

Stream fish communities and their associations to habitat variables in a rain forest reserve in southeastern Brazil

This paper describes the spatial variability of fish communities and identifies patterns of association between fish communities and habitat variables, including anthropogenic factors. We sampled streams inside and in the surroundings of a rain forest reserve in the southeast of Brazil in the rainy season. We could distinguish three main groups of streams: upland streams (draining the upland, flat portions of the mountain ridge of Serra de Paranapiacaba), adventitious streams of clearwater mountain torrents (small streams draining confined valleys in the slopes of this sierra), and large streams of clearwater mountain torrents (relatively unconstrained large streams close to the foots of the mountains). Despite the high variability of fish communities associated with these streams we identified some patterns using exploratory statistical analyses. These patterns were corroborated by additional field observations and information from the scientific literature. The main differences in fish community composition and diversity among the three groups of streams are probably related to large-scale factors such as elevation and position of the stream in the watershed. However, differences within these three groups seem to be mostly due to site-specific factors. Differences of instream characteristics are likely to be caused by natural variability of the ecosystems but also, in some cases, by human disturbances like pollution from human settlements, agriculture and mining.     

Influences of habitat complexity on the diversity and abundance of epiphytic invertebrates on plants

SUMMARY 1. The compound influence of habitat complexity and patch size on stream invertebrate assemblages associated with submerged macrophytes was investigated through field sampling of two natural macrophyte species with contrasting leaf morphologies (complex, Ranunculus yezoensis; simple, Sparganium emersum) and an experiment with two artificial plants with different levels of morphological complexity. 2. The artificial plant experiment was designed to separate the effects of habitat area (patch size) and habitat complexity, thus enabling a more rigorous assessment of complexity per se than in previous studies where only a single patch size was used. Simple and complex artificial plants were established with five different patch sizes corresponding to the range found in natural plants. 3. Invertebrates occurred on both complex and simple forms of natural and artificial plants at similar abundances with dipterans and ephemeropterans being predominant. Taxon richness was higher on structurally complex Ranunculus than on simple Sparganium and was similarly higher on the complex artificial plant than on the simple one, over the entire range of habitat patch sizes. Thus, architectural complexity affected the taxon richness of epiphytic invertebrates, independently of habitat scale. 4. On the natural plants there was no difference in the abundance (both number of individuals and biomass) of invertebrates between simple and complex forms, while on artificial plants more invertebrates occurred on complex than on simple forms. The amount of particulate organic matter, >225 μm (POM) and chlorophyll a showed mixed patterns on natural and artificial plants, suggesting that the availability of these resources is not an overriding proximate factor controlling invertebrate abundance on plants. The difficulty of extrapolating from experimental results involving use of artificial plants is discussed, especially when considering the relationship between habitat structure and the occurrence of epiphytic invertebrates on natural plants.     

Multi-scale effects of resource patchiness on foraging behaviour and habitat use by longnose dace, Rhinichthys cataractae

1. We examined the response of a predatory benthic fish, the longnose dace ( Rhinichthys cataractae ), to patchiness in the distribution of benthic macroinvertebrates on cobbles at three hierarchical spatial scales during summer and autumn 1996, and spring 1997 in a southern Appalachian stream. 2. At the primary scale (four to five individual cobbles separated by <1 m), the intensity of foraging was not correlated with the biomass of benthic macroinvertebrates/cobble, regardless of season. 3. At the secondary scale (i.e. foraging patches <5 m in diameter) we found that benthic macroinvertebrates were patchily distributed in summer, but not in autumn or spring. Concomitantly, in summer, longnose dace foraged on cobbles with a significantly higher biomass of benthic macronvertebrates than nearby, randomly selected cobbles with similar physical conditions (i.e. longnose dace tended to avoid low-prey foraging patches). In contrast, when benthic macroinvertebrates were distributed homogeneously (spring and autumn), dace did not select patches with a significantly higher biomass of benthic macroinvertebrates than that available on randomly selected cobbles. 4. At the tertiary scale (i.e. stream reaches 11–19 m long), the biomass of benthic macroinvertebrates (per cobble per reach) was patchily distributed (i.e. differed significantly among reaches) in all seasons. Among reaches with physical characteristics preferred by longnose dace, (i.e. erosional reaches dominated by cobble/boulder substratum and high current velocity), we detected a significant, positive correlation between the biomass of benthic macroinvertebrates/cobble and longnose dace density in all seasons. 5. Our results demonstrated that both spatial and temporal patchiness in resource availability influenced significantly the use of both foraging patches and stream reaches by longnose dace.     

Assemblage structure of stream fishes in the Western Ghats (India)

Macro and microhabitat analyses were applied to characterise the fish assemblage structure in 10 streams of the Western Ghat mountains of Peninsular India. Macrohabitat features, such as channel gradient, stream depth, stream width, riparian cover, instream cover, habitat types and substrates, were used. Microhabitat requirements of the abundant cyprinids (35 species) were also analysed. Macrohabitat assessment indicated that a high habitat diversity was associated with a high species diversity, and that habitat volume was a major determining factor for species diversity and abundance. In all streams, cyprinids were the dominant group in the assemblage and almost all cyprinids were confined to pools with a varied habitat diversity. Riffle dwelling species included ancient forms such as Glyptothorax madraspatnum, G. trewasae and Homaloptera santhamparaiensis. Suitable microhabitats for dominant cyprinid species were pools and riffle edges. Big-sized barbs and mahseers such as Hypselobarbus dobsoni, H. curmuca, H. dubius, Labeo calbasu, Puntius sarana, Tor khudree and Tor khudree malabaricus were confined to deep pools with a large area. Smaller Puntius species like P. fasciatus, P. melanampyx, P. narayani, P. sophore, P. ticto and P. vittatus lived in shallow backwater pools and pools with low flow. Species like P. arulius tambiraparniei, P. amphibius, P. bimaculatus and P. filamentosus were found towards shallow pools with moderate flow. Surface-dwelling species such as Danio aequipinnatus, Rasbora daniconius, Salmostoma spp. and Barilius spp. preferred deepwater habitats with high flow. Habitat-based multivariate analysis revealed four guilds: surface dwellers, column dwellers, generalized bottom dwellers and specialized bottom dwellers.     

Tidal and diurnal variations in larval fish abundance in an estuarine inlet in Ariake Bay,Japan: implication for selective tidal stream transport

We have conducted a preliminary study of tidal and diurnal variations in the distribution of dominant larval and juvenile fishes in the Chikugo River inlet (Ariake Bay, Kyushu, Japan) to determine whether selective tidal stream transport (STST) occurs. Larval and juvenile fish were collected from the mesohaline zone of the Chikugo River inlet during spring 2002. Temperature, salinity, depth, and current velocity were measured. Larval and juvenile abundance were compared among four tidal conditions, flooding tide, high tide, ebbing tide, and low tide, and between day and night. A total of 12 families, 15 species, and 5,577 individuals were collected. Temperature did not vary significantly with tidal conditions whereas salinity, depth, and current velocity varied significantly. Salinity also was correlated significantly and positively with depth. The abundance of most of the fishes was correlated positively and significantly with salinity and depth. Lateolabrax japonicus, Trachidermus fasciatus, Acanthogobius hasta, and other gobiid larvae (Gobiidae spp.) were significantly more abundant during high tide; in contrast, Coilia nasus and Neosalanx reganius were most abundant during low tide. The abundance of most of the fishes was higher during high tides at night than during the day, indicating the existence of STST, which may be strategically associated with ascending progress to upstream nursery areas.     

Composition and trophic structure of a fish community of a clear water Atlantic rainforest stream in southeastern Brazil

As part of a larger project that aimed to determine the factors that regulate fish productivity in a clear water river of the Atlantic rainforest, São Paulo State, Brazil, composition and the trophic structure of the fish community of the middle course of the river were studied from January 1995 to May 1996. Of the 17 species collected, Mimagoniates microlepis, Schizolecis guentheri, Phalloceros caudimaculatus and Kronichthys heylandi accounted for 83.2% of the total fish abundance. Most of the species were insectivorous (35.7%), followed by detritivores (21.4%), benthivores (14.2%), omnivores (14.2%), herbivores (7.1%), and piscivores (7.1%). Results of food overlap analysis as calculated by the Index of Morisita showed that 17.5% of the species pairs overlapped. Habitat segregation, however was observed among most of the species, suggesting some degree of food partitioning. Despite the characteristics of this escarpment river, which produces spates year round, no seasonal variation in diet was observed, suggesting that although food abundance may fluctuate throughout the year, most items are in constant supply. The importance of allochthonous food was considered and it appears that, though only a few species feed on this source (including detritivores), they make up 87.2% of the total fish abundance. The low abundance of most of the fishes, the high number of endemic species and the strong dependence of the species on a few food resources suggest that these systems are sensitive to anthropogenic impacts and require future studies.