Effect of waterfalls and the flood pulse on the structure of fish assemblages of the middle Xingu River in the eastern Amazon basin

Efeito das cachoeiras e do pulso de inundação na estrutura das assembleias de peixes do Médio Rio Xingu, Amazônia oriental

Abstract

The structure of fish assemblages in Neotropical rivers is influenced by a series of environmental, spatial and/or temporal factors, given that different species will occupy the habitats that present the most favourable conditions to their survival. The present study aims to identify the principal factors responsible for the structuring of the fish assemblages found in the middle Xingu River, examining the influence of environmental, spatial, and temporal factors, in addition to the presence of natural barriers (waterfalls). For this, data were collected every three months between July 2012 and April 2013, using gillnets of different sizes and meshes. In addition to biotic data, 17 environmental variables were measured. A total of 8,485 fish specimens were collected during the study, representing 188 species. Total dissolved solids, conductivity, total suspended matter, and dissolved oxygen concentrations were the variables that had the greatest influence on the characteristics of the fish fauna of the middle Xingu. Only the barriers and hydrological periods played a significant deterministic role, resulting in both longitudinal and lateral gradients. This emphasizes the role of the connectivity of the different habitats found within the study area in the structuring of its fish assemblages.

Keywords:
natural barriers; connectivity; hydrological periods; community ecology; impacts of hydroelectric dams

Resumo

A estrutura da ictiofauna em rios neotropicais é constantemente influenciada por fatores ambientais, espaciais e/ou temporais, uma vez que as espécies tendem a ocupar ambientes com condições favoráveis à sua sobrevivência. Dessa forma, esta pesquisa tem como objetivo responder qual o principal fator responsável pela estruturação das assembleias de peixes no Médio Rio Xingu, testando a influência dos fatores ambientais, espaciais e temporais, além da presença de barreiras naturais (cachoeiras). Os dados foram coletados, trimestralmente, entre os meses de julho de 2012 e abril de 2013, utilizando redes de emalhe de tamanhos de malha variados. Foram mensuradas 17 variáveis ambientais. Foram coletados 8.485 indivíduos distribuídos em 188 espécies. Observou-se que sólidos dissolvidos totais, condutividade, material em suspensão total e oxigênio dissolvido foram as variáveis que mais influenciaram a ictiofauna do médio Rio Xingu. Observou-se que apenas as barreiras naturais e os períodos hidrológicos foram determinantes, ocorrendo tanto variação longitudinal quanto lateral, ficando claro que a conectividade entre os diferentes trechos do médio rio Xingu é de suma importância na estruturação das assembleias de peixes.

Palavras-chave:
barreiras físicas naturais; conectividade; períodos hidrológicos; ecologia de comunidades; impactos de hidrelétricas

1 Introduction

In natural riverine communities, the distribution of species, resources, and biological processes fluctuate in response to a range of processes that occur on different scales (Humphries et al., 2014). At the larger (regional) scale, climate, hydrology, and geomorphology are among the principal factors contributing to assemblage structure, while biotic and abiotic factors, such as inter-specific interactions and fluctuations in limnological variables, tend to function on more local scales (Hoeinghaus et al., 2007; Suarez and Petrere-Junior, 2007; Scarabotti et al., 2011). Variations in all these factors along the course of a river determine the distribution patterns of fish species, which tend to occupy the habitats that present the most favourable biotic and abiotic conditions for their survival and the maintenance of viable populations, as established in Hutchinson’s (1957) theory of the ecological niche, and Southwood’s (1977) habitat template. This variation in the composition of the fauna may be modified by different factors, such as the spatial configuration of environment and changes in local abiotic factors (Nekola and White, 1999), resources availability, among others.

The existence of barriers to dispersal, whether natural, such as rapids or waterfalls, or man-made, like dams, hampers species movements (Agostinho et al., 2008; Torrente-Vilara et al., 2011), separating the assemblages in each side of the barrier. In the absence of ostensible barriers, dissimilarities in the composition of assemblages would be expected to be related to the distance between them, considering the distinct dispersal capacities of the different component species (Hubbell, 2001; Morlon et al., 2008). Another factor that may also have a role in fish population structure is the response of each species to alterations in local abiotic factors, according to their environmental requirements, where each species will be present in an environment which presents a set of abiotic variables favourable to its existence (Hutchinson, 1957). Given all these aspects, the composition of aquatic assemblages would be expected to vary longitudinally along rivers, with more distant assemblages being less similar to one another than those located at shorter distances.

In addition to spatial variations, Neotropical floodplains areas are characterised by an annual change in water levels, which alternates between rainy and dry seasons, modifying the availability of habitats, and producing major fluctuations in the abundance and diversity of fish species (Goulding, 1980; Rodríguez and Lewis Junior, 1994). These fluctuations are characterised by an increase in connectivity in High Water period, with more similar assemblages due to higher dispersion, and greater isolation in Low Water period, with more dissimilar assemblages (Junk, 1980; Thomaz et al., 2007; Scarabotti et al., 2011). Changes in the hydrological cycle may alter local abiotic factors, such as limnological variables. During the High Water period, the river water carries a higher sediment load as a consequence of the pluvial runoff and the inundation of the floodplain, and the body of water becomes wider and deeper (e.g. Marques et al., 2003). This means that the temporal variation in Neotropical aquatic assemblages may be at least partly related to modifications in abiotic factors, and not only to changes in the connectivity of habitats.

Based on these considerations, the present study aimed to identify the principal determinant of the structure of fish assemblages in the middle Xingu River, Amazon Basin. Three predictions were tested: (i) the composition of assemblages located at shorter distances from one another will be more similar than that of more distant ones, given their enhanced potential for dispersal; (ii) given the distinct environmental requirements of species, the composition of assemblages among sites will be affected by modifications in local abiotic variables; (iii) as the hydrological cycle affects the availability of habitats, assemblages found at Low and High Water will have distinct compositions; (iv) as the presence of waterfalls and rapids may affect the connectivity of a river, distinct assemblages will be expected up- and down-stream of these features.

2 Material and Methods

2.1 Study area

The Xingu River is a major right-bank tributary of the Amazon River, which originates in the Brazilian state of Mato Grosso, in the Serra do Roncador region, and discharges into the Amazon just downstream from the town of Porto de Moz, in Pará state. The river is 2,045 km long and flows predominantly in a south-north direction. Its principal tributary is the Iriri River, which originates approximately 100 km to the southwest of the town of Altamira, and other important tributaries of Xingu river are the Bacajá and Bacajaí rivers, on the Volta Grande do Xingu, downstream from Altamira (Eletronorte, 2001; Salomão et al., 2007; Eletrobras, 2009; Castilhos and Buckup, 2011).

The climate of this region is Am in the Köppen-Geiger classification, that is, tropical hot and humid (Peel et al., 2007). During the study period, monthly rainfall varied from 10.8 mm to 478.3 mm (INMET, 2014) and between 1971 and 2013, flow ranged from 1142.53 m³/s to 19518.23 m³/s on average, creating four distinct hydrological periods: Receding Water (June-August), Low Water (September-November), Flooding (December and February), and High Water (March-May). Because of this variation in river level, reaching on average 4.8 m in High Water period (Goulding et al., 2003), different environments become available during the year, including floodplains and flooded forests. In addition, some streams and lakes that connect with the river in High Water become isolated during the Low Water season.

The region is covered by typical lowland Amazon rainforest, with some enclaves of open vegetation. The waters of the Xingu are clear, with a transparency of 1 m to 5 m, light green in shallower parts, and dark green in the deeper environments (Castilhos and Buckup, 2011). The bottom is sandy or rocky, and rapids and waterfalls can be found in many areas, representing barriers to the dispersal of fish populations. The main waterfall is located in the region known as Volta Grande do Xingu at coordinates 03°23'24.9” S and 051°43'55.9” W; known as Jericoá, it presents a fairly sharp waterfall, where only large migratory fish can pass through. In addition, rapids in Bacajá River (a tributary of Xingu River) can be barriers too.

The present study focused on the middle Xingu, between the mouth of the Iriri River (20 km upstream of Altamira city) and the town of Senador José Porfírio (Figure 1). Data were collected tri-monthly between July 2012 and April 2013. In total, 36 sites were sampled, 9 in each hydrological period. Each sampling site was approximately 40 km, in fluvial distance, from each other.

Figure 1
Sites visited during the present study for the collection of data on the local fish fauna of the middle Xingu River between July, 2012, and April, 2013. The points are represented by black circles, and the black bars show waterfalls.

2.2 Collection of biological samples and environmental data

Fish specimens were collected using a sequential set of gillnets with meshes of different sizes, with each set being referred to as a “battery”. Each battery was composed of seven, 20 m-long rectangular nets of 2 m in height made of monofilament nylon, with diagonal stretch meshes of 2, 4, 7, 10, 12, 15, and 18 cm. Each net had an area of 40 m², with a total area of 280 m² or 0.00028 km² per battery.

The flood period is characterised by the availability of new habitats, such as swamps and floodplain lakes. Due to the presence of these environments, sampling effort increased during this period, including one battery per swamp or floodplain lake sampled. Thus, the data were standardised using a Capture Per Unit Effort (CPUE), where the abundance of each species during a given month was divided by the area of the batteries set at the site in that month, providing a metric in the form of a number of individuals per km² of net per hour (ind./km²/h). In other words, the CPUE was used as an index of species relative abundance, defined as the number of individuals captured per km² of gillnet per hour.

Three batteries were set at each site, with a distance of at least 5 km between each battery, in order to avoid problems of spatial autocorrelation. All the nets remained in the water for 15 hours, between 5 pm and 8 am of the following morning. The set of three batteries at each site was considered a single sample. Total sampling effort for each period of the hydrological cycle was 88.2 km² at High Water, 52.92 km² during the Receding Water, 52.92 km² at Low Water, and 48.51 km² during the Flooding period. The difference in sampling effort was due to amount of habitat, such as flooded forests, which are available only at High Water.

Once collected, the specimens were identified to the lowest possible taxonomic level (to species in most cases), fixed in 10% formaldehyde for 48 hours, and conserved in 70% ethanol. All specimens were deposited in the ichthyological collection at the Laboratório de Ictiologia de Altamira (LIA) of Universidade Federal do Pará (UFPA), as well as in the Museu Paraense Emílio Goeldi (MPEG) in Belém (Pará, Brazil).

In addition to the biological data, a number of environmental variables were obtained from the Norte Energia database, derived from samples collected by the International Ecology Institute (IIEGA). These data were collected near the sites of fish sampling. A total of 17 variables were analysed: alkalinity (acronym: Alk, unit: mg-CaCO3/L), total carbon (C, mg/g sed), chlorophyll a (cloa, µg/L), conductivity (cond, mS/cm), Biochemical Demand for Oxygen (BDO, mg/L), suspended organic matter (SOM, mg/L), suspended inorganic matter (SIM, mg/L), total suspended matter (TSM, mg/L), total nitrogen (N, mg/L), dissolved oxygen (DO, mg/L), pH, redox potential (redox, mV), depth (depth, m), total dissolved solids (DisSol, mg/L), temperature (temp, °C), transparency (transp, m), and turbidity (turb, UNT).

2.3 Data analysis

A Pearson Correlation Analysis was used to examine multicollinearity between variables, excluding those with correlation above a threshold of 0.8. A Principal Components Analysis (PCA) was used to determine which environmental variables were important in the differentiation of sites (Jongman, 1995). The axes were selected using the Broken Stick criterion. The environmental variables selected through this method were used for subsequent analyses. Prior to these analyses, the environmental variables were standardised by subtracting each value from the mean and then dividing it by the standard deviation in order to remove the effects of the different scales of measurement.

The pairwise distance between sites was measured following the course of the river, using 1:100,000 scale shape files of the local hydrography. To evaluate longitudinal variation in fish assemblage composition, the CPUE data (ind/km²/h) from each site were ordinated distances (Clarke and Warwick, 2001). After NMDS, data were tested using a Permutational Analysis of Variance (PERMANOVA) with sums of squares type III (partial), permutation of residuals under a reduced model and 999 permutations. The PERMANOVA was based on the null hypothesis that the composition of the fish assemblages did not vary significantly among hydrological periods and spatially. Lastly, an Indicator Species Analysis (IndVal) was run to investigate which species were responsible for the differences among sites and/or hydrological periods (Clarke and Warwick, 2001).

We used Mantel analysis to evaluate the correlation of four matrices with fish assemblage composition (environmental variables, hydrological periods, presence of waterfalls/rapids, and fluvial distance between points), based on Pearson’s correlation coefficient. We also tested the correlation among these four matrices with Mantel. When it was significant, we used partial Mantel to control the effect of each explanatory matrix on fish assemblages. Partial Mantel determines the partial correlation of two distance matrices, while controlling the effect of a third matrix (Legendre and Legendre, 2012), which allows us to see the individual effect of each matrix on the response matrix.

The matrix for the analysis of the hydrological periods was based on the pairwise comparison of sites by sample period. A score of zero was applied to pairs of samples from the same period (e.g., Flooding-Flooding), 1 for adjacent periods (e.g., Flooding-High Water), and 2 for alternate periods (e.g., Flooding-Receding Water). The matrix for the presence of waterfalls or rapids was also based on pairs of sites, which were scored zero for the absence of barriers and 1 when a barrier existed between them.

All statistical analyses were run in the R program (R Development Core Team, 2011) using the Vegan (Oksanen et al., 2011) and Ecodist packages (Goslee and Urban, 2007). All tests considered a 5% significance level.

3 Results

3.1 Environmental variables

The High Water period was characterised by the highest alkalinity, BDO, depth, and redox potential. The highest temperatures and dissolved oxygen concentrations were recorded at Low Water. The highest values for all other variables were recorded during the transitional periods, that is, the Flooding and Receding Water cycles (see ^{Appendix 1}).

The variables total dissolved solids and turbidity were excluded of the analysis because presented large correlation with conductivity. The same occurred with suspended inorganic matter that was correlated to total suspended matter. The first PCA axis explained 28.35% of the variation, and the second, 19.92%, with a total of 48.28% for the first two ordination axes (Table 1). The most important variables of the first axis (loading > 0.7) were conductivity and total suspended matter, both negatively associated with the first Principal Component. Dissolved oxygen was the variable that contributed most to the formation of axis 2 (Table 1), with a negative association. The plot (Figure 2) shows a clear grouping of the Receding Water and Low Water periods, characterised by increased dissolved oxygen and reduced conductivity and total suspended matter. The Flooding and High Water periods presented the opposite values, with increased values of conductivity and total suspended matter and reduced dissolved oxygen. The points showed no spatial pattern, meaning that environmental variables did not group per sampling site.

Table 1
Results of the Principal Components Analysis (PCA) for the nine sample points surveyed on the middle Xingu River between April, 2012 and April, 2013.

Variable	Axis 1	Axis 2
Alk	–0.111	0.451
C	–0.612	0.207
Cloa	0.12	–0.594
Cond	–0.774	–0.152
BDO	–0.486	–0.57
SOM	–0.518	–0.364
TSM	–0.804	–0.229
N	0.57	0.001
DO	0.348	–0.817
pH	0.661	–0.218
Depth	0.088	0.296
Redox	0.355	0.67
Temp	0.644	–0.631
Transp	0.622	0.083
Eigenvalue	3.97	2.79
% explanation	28.35	19.92
% accumulated explanation	28.35	48.28
Broken Stick	3.25	2.25

• The loading values for each environmental variable are shown for each axis, with those in bold type being characterised by a strong (>0.7) correlation between axes. The acronyms are defined in ^{Appendix 1}.

Figure 2
Plot of the PCA for the hydrological periods and sample sites surveyed on the middle Xingu River between July, 2012, and April, 2013.

3.2 Fish assemblages

A total of 8,485 fish specimens were collected during the present study, representing 188 species belonging to 33 families in nine orders (See ^{Appendix 2}). The most abundant order was Characiformes (5,765 specimens; 1,354.44 ind/km²/h of net), followed by Siluriformes (2,803; 678.65 ind/km²/h), and Perciformes (444; 104.91 ind/km²/h). The most abundant family was Hemiodontidae (1,424; 333.48 ind/km²/h), followed by Curimatidae (1,095; 261.97 ind/km²/h), and Characidae (1,144; 254.97 ind/km²/h). The most common species was Hemiodus unimaculatus (Bloch, 1794) (780; 185.87 ind/km²/h), then Ageneiosus ucayalensis Castelnau, 1855 (396; 70.56 ind/km²/h), and Tocantinsia piresi (Miranda Ribeiro, 1920) (387; 56.69 ind/km²/h).

The PERMANOVA indicated significant temporal variation in the characteristics of the fish fauna (pseudo-F = 3.45; d.f. = 3; p = 0.001), despite a certain degree of overlap between the Flooding and High Water period, as shown in the NMDS plot (Figure 3). Significant spatial differentiation was also observed (pseudo-F = 2.32; d.f. = 8; p = 0.001). After the PERMANOVA, it was possible to realize the formation of three groups, the first encompassing sites 8 and 9 (group 1), the second, sites 1 through 5 (group 2), and the third by sites 6 and 7, forming group 3 (Figure 3). Groups 1 and 2 were separated by the Bacajá rapids and groups 2 and 3 were separated by the Jericoá falls.

Figure 3
Results of the Non-Metric Multidimensional Scaling (NMDS) analysis of the data collected on the fish fauna of the middle Xingu River by sampling site and hydrological period between July, 2012, and April, 2013. The ellipses correspond to the three groups formed by the influence of Jericoá falls and Bacajá rapids.

Different species contributed to the formation of the spatial groups and to the differentiation among hydrological periods (Table 2). The IndVal test selected 31 species with occurrence linked to the sampling sites, while 24 species were related to hydrological periods.

Table 2
Results of Indicator Species Analysis per hydrologic period (a) and per sampling site (b) of the fish assemblages on the middle Xingu River.

(a) Periods
Period	Species	IndVal	p
Receding Water	Cyphocharax gouldingi	0.812	0.01
Receding Water	Triportheus rotundatus	0.73	0.015
Receding Water	Poptella brevispina	0.571	0.02
Flooding	Cyphocharax festivus	0.733	0.02
Flooding	Oxydoras niger	0.629	0.05
High Water	Triportheus albus	0.836	0.005
Receding Water + Low Water	Baryancistrus aff. niveatus	0.9	0.005
Receding Water + High Water	Myloplus arnoldi	0.749	0.01
Low Water + High Water	Limatulichthys griseus	0.758	0.01
Low Water + High Water	Brycon falcatus	0.684	0.035
Low Water + High Water	Brycon aff. pesu “adiposa preta”	0.667	0.02
Flooding + High Water	Ageneiosus ucayalensis	0.923	0.005
Flooding + High Water	Auchenipterichthys longimanus	0.892	0.005
Flooding + High Water	Auchenipterus nuchalis	0.861	0.005
Flooding + High Water	Myloplus rubripinnis	0.848	0.005
Flooding + High Water	Tocantinsia piresii	0.782	0.005
Flooding + High Water	Hypoptopoma inexpectatum	0.763	0.02
Flooding + High Water	Curimatella immaculata	0.745	0.015
Flooding + High Water	Chilodus punctatus	0.699	0.02
Receding Water + Low Water + High Water	Hemiodus sp. “xingu”	0.804	0.035
Receding Water + Low Water + High Water	Pimelodella cristata	0.72	0.015
Low Water + Flooding + High Water	Hydrolycus armatus	0.917	0.005
Low Water + Flooding + High Water	Boulengerella cuvieri	0.856	0.005
Low Water + Flooding + High Water	Geophagus altifrons	0.787	0.04

(b) Sites
Sites	Species	IndVal	p
IC07	Hemiodontichthys acipenserinus	0.707	0.045
IC09	Acestrorhynchus falcatus	1	0.005
IC09	Moenkhausia intermedia	0.932	0.005
IC09	Pterygoplichthys xinguensis	0.877	0.005
IC09	Pristobrycon striolatus	0.866	0.01
IC09	Ctenobrycon spilurus	0.824	0.02
IC09	Poptella compressa	0.812	0.015
IC01 + IC03	Agoniates halecinus	0.872	0.01
IC01 + IC03	Bivibranchia velox	0.799	0.01
IC06 + IC07	Ilisha amazonica	0.935	0.005
IC06 + IC07	Pseudoloricaria laeviuscula	0.935	0.005
IC06 + IC07	Hassar orestis	0.791	0.01
IC06 + IC07	Pseudotylosurus microps	0.791	0.005
IC07 + IC09	Hypoptopoma inexpectatum	0.83	0.03
IC08 + IC09	Cynopotamus xinguano	0.782	0.005
IC01 + IC02 + IC09	Hypostomus hemicochliodon	0.772	0.025
IC02 + IC03 + IC04	Geophagus argyrostictus	0.828	0.015
IC01 + IC02 + IC03 + IC04	Hemiodus vorderwinckleri	0.91	0.005
IC02 + IC03 + IC07 + IC09	Leporinus friderici	0.848	0.015
IC01 + IC02 + IC03 + IC04 + IC05	Leporinus maculatus	0.858	0.02
IC01 + IC02 + IC03 + IC04 + IC06	Chilodus punctatus	0.753	0.045
IC01 + IC02 + IC07 + IC08 + IC09	Hassar gabiru	0.874	0.005
IC01 + IC02 + IC03 + IC04 + IC05 + IC06	Caenotropus labyrinthicus	0.885	0.05
IC01 + IC02 + IC03 + IC04 + IC05 + IC06	Serrasalmus manueli	0.883	0.035
IC01 + IC02 + IC03 + IC04 + IC05 + IC09	Acnodon normani	0.764	0.035
IC01 + IC02 + IC03 + IC04 + IC06 + IC09	Pimelodus blochii	0.927	0.01
IC01 + IC02 + IC03 + IC04 + IC08 + IC09	Loricaria birindellii	0.854	0.02
IC01 + IC03 + IC04 + IC07 + IC08 + IC09	Acestrorhynchus microlepis	0.879	0.02
IC01 + IC02 + IC03 + IC04 + IC05 + IC06 + IC09	Pachyurus junkii	0.92	0.045
IC01 + IC02 + IC03 + IC04 + IC05 + IC08 + IC09	Squaliforma aff. emarginata	0.91	0.04
IC01 + IC02 + IC03 + IC04 + IC05 + IC08 + IC09	Hemiodus sp. “xingu”	0.845	0.035

3.3 Factors that affect the distribution of fish species

The Mantel analysis between the four explanatory matrices indicated a very weak correlation between the environmental variables and the hydrological periods, so this correlation was not considered in this study, although it was statistically significant (Table 3). On the other hand, there was a strong correlation between the presence of barriers and the fluvial distance. The effect of each matrix was then analysed separately to see their effects on fish assemblages.

Table 3
Results of the Mantel analysis between the four explanatory matrices ̶ environmental variables, hydrological periods, the presence of barriers (waterfalls or rapids), and fluvial distance between points ̶ and their effects on the composition of the fish assemblages on the middle Xingu River.

	R	p
Environment x Period	0.07	0.01
Environment x Waterfall	–0.05	0.78
Environment x Fluvial distance	–0.10	0.91
Period x Waterfall	–0.05	1
Period x Fluvial distance	–0.06	1
Waterfall x Fluvial distance	0.43	<0.01
Fish assemblages x Environment	–0.25	0.99
Fish assemblages x Period	0.23	<0.01
Fish assemblages x Waterfall*	0.39	<0.01
Fish assemblages x Fluvial distance*	0.01	0.41

• The values in bold type are significant (P < 0.05). Marked (*) analysis were performed with partial Mantel.

The Mantel analysis indicated that the explanatory variables were responsible for 62% of the variation in the data (Table 3). Only the hydrological period and the presence of barriers (waterfalls or rapids) affected the distribution of the fish fauna, with the latter (barriers) being the most important. There was no influence of distance between sample points neither of environmental variables on fish assemblages.

4 Discussion

The hydrological cycle (temporal effect) and the presence of waterfalls (structural effect) are the main determinants of the fish species distribution in the middle Xingu River, confirming predictions iii and iv. Habitat connectivity among hydrologic periods is the main factor regulating the dispersal of individuals to new areas and to access new resources. In the case of lotic ecosystems, connectivity is observed longitudinally in relation to the course of the river, and laterally in relation to the influence of the hydrological cycle (Kondolf et al., 2006), with the formation of a vast flood plain.

Rivers and streams are dynamic and complex systems with a unidirectional flow of matter and energy. These processes modify gradually environmental conditions and the distribution of resources exploited by fishes, these variations being explained by the River Wave Theory (Humphries et al., 2014). This results in variations in the structure of fish assemblages along a longitudinal gradient, although the continuity of this gradient, the autochthonous production or allochthonous inputs. These factors may be interrupted abruptly and modified by the presence of physical barriers, such as waterfalls and rapids, resulting in distinct assemblages on either side of the barrier (Agostinho et al., 2008; Torrente-Vilara et al., 2011). Our study confirms this, since we observed the formation of groups between the waterfalls.

The characteristics of Neotropical fish assemblages also vary considerably in relation to the fluctuations caused by the seasonal flood pulse (Goulding, 1980; Junk et al., 1989; Scarabotti et al., 2011; Silva et al., 2013; Humphries et al., 2014). This process results in the inundation of the floodplain swamps, expanding the availability of resources (food and refuges, for example) and increasing the connectivity among habitats, resulting in a random redistribution of the fish fauna and reducing spatial variability (Thomaz et al., 2007). As the water drains back into the main channel, nutrients are washed out, while fish density and biotic interactions increase, some environments being isolated (Goulding, 1980; Junk et al., 1989). In the present study, the composition of the assemblages was affected by hydrologic periods, varying significantly among seasons, as recorded in a number of previous studies in the Neotropical region (Scarabotti et al., 2011; Silva et al., 2013). However, the effects of the Jericoá falls and Bacajá rapids were more pronounced than those of the flood pulse, and represent a major factor in the structuring of the fish assemblages of the middle Xingu. A similar pattern has been recorded in a number of previous studies of the effects of natural barriers on the abundance and distribution of fish species (Ingênito and Buckup, 2007; Torrente-Vilara et al., 2011).

Given the importance of physical barriers such as waterfalls and the habitat connectivity caused by the annual flood pulse, the construction of hydroelectric dams may have a significant impact on the composition of fish assemblages. In the specific case of the Belo Monte project on the Xingu River, which is being constructed in the middle of the study area, there is a predicted reduction in river discharge on the stretch that includes the Jericoá falls (Eletronorte, 2001; Norte Energia, 2010). This would result in the permanent loss of connectivity between the fish assemblages located up- and down-stream of these falls, as well as a marked change in the types of habitat available for the different species, which would affect species composition, as well as reproductive patterns and the recruitment processes of the majority of taxa (Agostinho et al., 2004). The impacts of the construction of hydroelectric dams are well documented (Junk and Mello, 1990; Agostinho et al., 2008; Mims and Olden, 2013; Sakaris, 2013; Freedman et al., 2014) and are related primarily to processes such as the loss and homogenization of habitats, and the replacement of species. This emphasizes the need for the systematic collection of data on the characteristics of local fish assemblages prior to the flooding of reservoirs, in order to provide a sound database for the development of effective management strategies. However, little is known about the effect of the construction of reduced flow hydroelectric dams and this knowledge is nil when it comes to Amazon. Thus, this study is important because it allows the understanding of the structure of fish populations in the Middle Xingu River, forming bases for possible conservation measures.

The results of the present study indicated that the presence of waterfalls and the fluctuations of the flood pulse were the primary factors determining the distribution of fish species within the study area, creating both longitudinal and lateral gradients. This supports two of the operational hypotheses tested in the study, but rejects those on the possible effects of local environmental variables or the distance between sites. The difference in the composition of the assemblies due to hydrological periods and physical barriers are clearly the most important determinants of the structure of ichthyofauna in the study area, and is also one of the characteristics that may be most impacted by the construction of the Belo Monte hydroelectric dam. This re-emphasizes the need for the consideration of the region’s unique characteristics in the planning of future management strategies.

Appendix 1 Values recorded for the different environmental variables at the nine sampling sites on the middle Xingu River during each of the four principal hydrological periods (ebb, low water, flood, and high water) between July, 2012, and April, 2013. Alk = Alkalinity (mg-CaCO3/L), C = Total carbon (mg/g sed), Cloa = Chlorophyll a (µg/L), Cond = Condutivity (mS/cm), BDO = Biochemical Demand for Oxygen (mg/L), SIM = Suspended Inorganic Matter (mg/L), SOM = Suspended Organic Matter (mg/L), TSM = Total Suspended Matter (mg/L), N = Total nitrogen (mg/L), DO = Dissolved oxygen (mg/L), pH, Depth = depth (m), Redox = Redox potential (mV), SolDiss = Total Dissolved Solids (mg/L), Temp = Temperature (°C), Transp = Transparency (m), and Turb = Turbidity (UNT).

Period	Alk	C	Cloa	Cond	BDO	SIM	SOM	TSM	N	DO	pH	Depth	Redox	SolDiss	Temp	Transp	Turb
Receding Water	6.59± 3.16	0.27± 0.16	12.92± 5.82	0.02± 0.001	0.67± 0.28	3.82± 2.34	2.31± 0.55	6.13± 2.71	2.76± 5.74	7.12± 0.26	7.98± 0.30	3.16± 5.21	117.23± 14.09	0.01± 0.01	29.85± 0.68	1.06± 0.32	4.60± 2.44
Low water	7.36± 5.73	6.96± 5.96	4.87± 3.18	0.02± 0.02	1.05± 0.60	4.57± 6.21	2.58± 1.60	7.14± 7.70	0.87± 0.50	7.54± 0.22	7.47± 0.60	2.02± 4.28	77.94± 28.04	0.01± 0.01	30.85± 0.61	0.81± 0.71	5.02± 5.19
Flooding	7.05± 6.28	7.71± 6.53	7.56± 4.55	0.03± 0.02	0.77± 0.40	2.93± 6.01	2.99± 2.15	5.92± 7.98	0.44± 0.11	7.37± 0.77	7.89± 0.79	2.12± 3.60	116.42± 28.67	0.01± 0.01	31.24± 0.36	0.92± 0.86	6.43± 5.44
High water	5.00± 2.02	25.69± 35.27	10.98± 8.87	0.07± 0.03	1.75± 0.24	10.96± 6.84	5.18± 5.47	16.13± 6.13	0.31± 0.20	7.05± 0.84	13.52± 21.11	3.03± 3.41	68.47± 33.35	0.04± 0.02	28.60± 1.28	0.49± 0.21	22.17± 13.36

Appendix 2 Taxonomic list of the fish species collected during the present study and their respective CPUE (ind./km2/h) for each hydrological period between July/2012 and April/2013 (Laboratório de Ictiologia de Altamira - LIA and Museu Paraense Emílio Goeldi - MPEG).

Taxon / Authority	Voucher	High Water (ind./km2/h)	Flooding (ind./km2/h)	Low Water (ind./km2/h)	Receding Water (ind./km2/h)	Total (ind./km2/h)
BELONIFORMES
Belonidae
Pseudotylosurus microps (Günther, 1866)	LIA 404	3.741	2.721	0.000	0.227	6.689
CHARACIFORMES
Acestrorhynchidae
Acestrorhynchus falcatus (Bloch, 1794)	MPEG 29909	6.198	0.454	1.361	4.308	12.320
Acestrorhynchus microlepis (Jardine, 1841)	MPEG 28903	22.751	15.420	8.503	7.370	54.044
Anostomidae
Anostomoides passionis Santos & Zuanon, 2006	LIA 417	0.227	0.227	0.000	0.000	0.454
Hypomasticus julii (Santos, Jégu & Lima, 1996)	MPEG 29314	2.494	0.227	0.000	2.041	4.762
Laemolyta fernandezi Myers, 1950	MPEG 29078	0.454	0.227	0.000	0.000	0.680
Laemolyta proxima (Garman, 1890)	LIA 161	6.311	5.442	0.680	0.000	12.434
Leporinus aff. fasciatus	LIA 134	15.004	2.041	3.061	8.277	28.328
Leporinus brunneus Myers, 1950	LIA 418	0.454	0.000	0.000	0.227	0.680
Leporinus desmotes Fowler, 1914	LIA 313	1.663	0.000	1.701	0.113	3.477
Leporinus friderici (Bloch, 1794)	MPEG 28073	10.204	2.721	2.834	3.515	19.274
Leporinus maculatus Müller & Troschel, 1844	LIA 370	12.207	3.628	2.154	4.082	22.071
Leporinus sp. 1	MPEG 28837	0.000	0.000	0.000	0.227	0.227
Leporinus sp. 2	MPEG 28938	0.756	0.000	0.227	0.000	0.983
Leporinus tigrinus Borodin, 1929	MPEG 28996	0.907	0.000	0.000	0.907	1.814
Petulanos intermedius (Winterbottom, 1980)	MPEG 29626	0.529	0.227	0.000	0.000	0.756
Pseudanos trimaculatus (Kner, 1858)	MPEG 29440	0.869	0.227	0.000	0.340	1.436
Sartor respectus Myers & Carvalho, 1959	MPEG 28924	0.227	0.000	0.227	0.000	0.454
Schizodon vittatus (Valenciennes, 1850)	MPEG 29057	1.134	0.680	0.113	0.340	2.268
Synaptolaemus latofasciatus (Steindachner, 1910)		0.151	0.000	0.000	0.000	0.151
Characidae
Acestrocephalus stigmatus Menezes, 2006	MPEG 28849	0.302	0.000	0.113	0.000	0.416
Agoniates halecinus Müller & Troschel, 1845	LIA 409	4.460	2.721	1.701	3.741	12.623
Astyanax gr. bimaculatus	LIA 419	0.756	0.000	0.000	0.680	1.436
Brycon sp. 1	LIA 203	5.518	1.361	1.927	1.134	9.939
Brycon sp. 2	LIA 420	2.230	0.000	1.020	0.000	3.250
Brycon falcatus Müller & Troschel, 1844	MPEG 28992	2.381	0.227	1.361	0.227	4.195
Bryconops alburnoides Kner, 1858	MPEG 28979	2.608	0.227	0.794	1.587	5.215
Bryconops caudomaculatus (Günther, 1864)	MPEG 29003	0.227	0.227	0.000	0.000	0.453
Bryconops giacopinii (Fernández-Yépez, 1950)	MPEG 29402	0.000	0.567	0.000	0.000	0.566
Chalceus epakros Zanata & Toledo-Piza, 2004	LIA 157	1.134	0.000	0.680	0.000	1.814
Charax gibbosus (Linnaeus, 1758)	LIA 342	0.907	0.000	0.907	0.000	1.814
Ctenobrycon spilurus (Valenciennes, 1850)	MPEG 29817	2.305	0.000	0.340	1.587	4.232
Cynopotamus xinguano Menezes, 2007	MPEG 29587	3.364	0.227	1.020	2.154	6.764
Jupiaba polylepis (Günther, 1864)	MPEG 29017	1.020	0.907	0.113	0.000	2.040
Moenkhausia heikoi Géry & Zarske, 2004	MPEG 28982	6.274	4.762	0.227	0.000	11.262
Moenkhausia intermedia Eigenmann, 1908	MPEG 28844	36.168	16.440	9.524	11.451	73.582
Moenkhausia lepidura (Kner, 1858)	MPEG 28867	0.567	0.227	0.000	0.340	1.133
Moenkhausia xinguensis (Steindachner, 1882)	MPEG 28083	8.428	3.175	0.454	1.134	13.189
Poptella brevispina Reis, 1989	LIA 410	5.329	0.000	0.227	5.669	11.224
Poptella compressa (Günther, 1864)	MPEG 29019	9.259	2.381	4.875	0.000	16.515
Roeboexodon guyanensis (Puyo, 1948)	MPEG 29407	0.567	0.227	0.000	0.340	1.133
Roeboides sp.	MPEG 28899	2.608	0.227	2.041	0.340	5.215
Tetragonopterus argenteus (Puyo, 1948)	MPEG 29871	1.474	0.680	0.000	0.794	2.947
Tetragonopterus chalceus Spix & Agassiz, 1829	MPEG 28943	3.288	1.134	1.247	0.794	6.462
Triportheus albus Cope, 1872	MPEG 29112	19.048	5.556	0.000	1.361	25.963
Triportheus auritus (Valenciennes, 1850)	MPEG 29631	4.649	0.680	0.000	4.535	9.863
Triportheus rotundatus (Jardine, 1841)	MPEG 29352	10.280	1.587	0.000	9.864	21.730
Chilodontidae
Caenotropus labyrinthicus (Kner, 1858)	MPEG 28838	27.211	4.308	6.009	15.646	53.174
Chilodus punctatus Müller & Troschel, 1844	MPEG 29059	15.684	10.091	2.041	1.814	29.629
Ctenoluciidae
Boulengerella cuvieri (Spix & Agassiz, 1829)	MPEG 28070	13.568	6.689	4.082	0.227	24.565
Boulengerella maculata (Valenciennes, 1850)	LIA 272	1.587	1.134	0.454	0.000	3.174
Curimatidae
Curimata inornata Vari, 1989	MPEG 28876	37.906	18.367	4.308	15.193	75.774
Curimata vittata (Kner, 1858)	LIA 421	0.907	0.907	0.000	0.000	1.814
Curimatella dorsalis (Eigenmann & Eigenmann, 1889)	MPEG 29838	1.512	0.000	0.907	0.000	2.418
Curimatella immaculata (Fernández-Yépez, 1948)	MPEG 29526	10.506	7.596	0.000	0.000	18.102
Cyphocharax festivus Vari, 1992	LIA 366	39.985	43.084	0.000	0.000	83.068
Cyphocharax gouldingi Vari, 1992	MPEG 28893	16.289	2.268	4.308	37.528	60.393
Cyphocharax leucostictus (Eigenmann & Eigenmann, 1889)	MPEG 29535	7.181	8.957	1.020	0.000	17.157
Cyphocharax stilbolepis Vari, 1992	LIA 367	2.230	0.454	0.000	0.000	2.683
Psectrogaster falcata (Eigenmann & Eigenmann, 1889)	LIA 308	0.340	0.227	0.000	0.000	0.566
Cynodontidae
Cynodon gibbus (Agassiz, 1829)	MPEG 28908	2.683	0.454	0.340	1.134	4.610
Hydrolycus armatus (Jardine, 1841)	LIA 401	10.242	2.948	3.855	0.227	17.271
Hydrolycus tatauaia Toledo-Piza, Menezes & Santos, 1999	MPEG 29086	3.477	0.907	1.134	0.680	6.198
Rhaphiodon vulpinus Spix & Agassiz, 1829	MPEG 29085	3.704	0.227	1.134	1.474	6.538
Erythrinidae
Hoplias aimara (Valenciennes, 1847)	MPEG 29904	0.680	0.567	0.000	0.227	1.473
Hoplias malabaricus (Bloch, 1794)	MPEG 28067	1.701	0.680	0.794	0.113	3.287
Hemiodontidae
Argonectes robertsi Langeani, 1999	MPEG 28961	21.958	10.431	2.608	1.814	36.810
Bivibranchia fowleri (Steindachner, 1908)	MPEG 28883	5.102	1.814	4.195	0.000	11.111
Bivibranchia velox (Eigenmann & Myers, 1927)	MPEG 29105	6.463	2.154	2.381	3.401	14.399
Hemiodus cf. semitaeniatus	LIA 422	6.122	6.122	0.000	0.000	12.244
Hemiodus sp. 1	MPEG 29072	22.373	0.680	7.937	9.864	40.854
Hemiodus unimaculatus (Bloch, 1794)	MPEG 28887	99.471	39.569	19.955	26.871	185.865
Hemiodus vorderwinckleri (Géry, 1964)	LIA 371	16.667	7.256	2.268	6.009	32.199
Prochilodontidae
Prochilodus nigricans Spix & Agassiz, 1829	LIA 298	3.401	1.361	1.020	0.454	6.235
Semaprochilodus brama (Valenciennes, 1850)	MPEG 28968	6.236	2.721	2.948	1.814	13.718
Serrasalmidae
Acnodon normani Gosline, 1951	LIA 181	4.611	1.927	2.608	0.113	9.259
Metynnis cf. luna	LIA 423	4.157	2.494	0.454	1.134	8.238
Myleus setiger Müller & Troschel, 1844	LIA 413	3.099	0.907	1.701	0.454	6.160
Myloplus arnoldi (Ahl, 1936)	MPEG 28966	6.387	0.454	0.567	5.669	13.076
Myloplus rhomboidalis (Cuvier, 1818)	LIA397	2.948	1.134	0.907	0.907	5.895
Myloplus rubripinnis (Müller & Troschel, 1844)	LIA 374	11.300	3.741	2.494	0.000	17.535
Myloplus schomburgkii (Jardine, 1841)	LIA163	2.494	0.227	1.361	0.794	4.875
Pristobrycon eigenmanni (Norman, 1929)	LIA 297	0.227	0.000	0.227	0.000	0.453
Pristobrycon striolatus (Steindachner, 1908)	LIA 411	3.401	1.587	1.247	0.567	6.802
Pygocentrus nattereri Kner, 1858	LIA 300	1.474	0.000	0.680	0.794	2.947
Serrasalmus altispinis Merckx, Jégu & Santos, 2000	LIA 424	2.834	0.000	2.834	0.000	5.668
Serrasalmus gouldingi Fink & Machado-Allison, 1992	MPEG 28860	1.587	0.227	0.000	1.361	3.174
Serrasalmus manueli (Fernández-Yépez & Ramírez, 1967)	LIA 393	22.978	2.494	13.152	6.689	45.313
Serrasalmus rhombeus (Linnaeus, 1766)	LIA 351	17.385	1.020	4.195	10.317	32.917
Tometes sp.	LIA 59	2.948	0.907	0.567	0.794	5.215
CLUPEIFORMES
Engraulidae
Anchoviella sp.	MPEG 28064	0.680	0.680	0.000	0.000	1.360
Lycengraulis batesii (Günther, 1868)	LIA 360	0.907	0.454	0.000	0.454	1.814
Pristigasteridae
Ilisha amazonica (Miranda & Ribeiro, 1920)	MPEG 28870	12.812	5.215	0.907	0.907	19.841
Pellona castelnaeana Valenciennes, 1847	LIA 425	0.794	0.000	0.227	0.227	1.247
GYMNOTIFORMES
Electrophoridae
Electrophorus electricus (Linnaeus, 1766)	LIA 426	0.113	0.000	0.000	0.113	0.226
Gymnotidae
Gymnotus carapo Linnaeus, 1758	LIA 427	0.529	0.000	0.000	0.000	0.529
Hypopomidae
Steatogenys elegans (Steindachner, 1880)	MPEG 29292	0.227	0.000	0.000	0.227	0.453
Rhamphichthyidae
Rhamphichthys drepanium Triques, 1999	LIA 428	0.416	0.227	0.113	0.000	0.755
Sternopygidae
Archolaemus janeae Vari, de Santana & Wosiacki, 2012	MPEG 28896	4.308	0.000	1.701	2.494	8.503
Eigenmannia aff. trilineata	MPEG 29595	0.076	0.000	0.000	0.000	0.075
MYLIOBATIFORMES
Potamotrygonidae
Paratrygon aiereba (Müller & Henle, 1841)	LIA 314	0.227	0.227	0.000	0.000	0.453
OSTEOGLOSSIFORMES
Osteoglossidae
Osteoglossum bicirrhosum (Cuvier, 1829)	LIA 276	1.361	0.000	1.361	0.000	2.721
PERCIFORMES
Cichlidae
Aequidens michaeli Kullander, 1995	MPEG 28846	0.227	0.227	0.000	0.000	0.453
Caquetaia spectabilis (Steindachner, 1875)	MPEG 28840	0.454	0.227	0.000	0.113	0.793
Cichla melaniae Kullander & Ferreira, 2006	LIA 64	2.268	0.000	0.113	1.587	3.968
Cichla monoculus Agassiz, 1831	LIA 63	0.227	0.000	0.227	0.000	0.453
Crenicichla gr. saxatilis	LIA 429	0.113	0.000	0.000	0.113	0.226
Crenicichla lugubris Heckel, 1840	MPEG 28959	0.227	0.000	0.113	0.340	0.680
Crenicichla sp.	LIA 81	1.134	0.454	0.000	0.680	2.267
Geophagus altifrons Heckel, 1840	MPEG 28081	12.094	4.422	7.143	0.794	24.452
Geophagus argyrostictus Kullander, 1991	MPEG 28962	7.521	0.794	4.649	1.247	14.210
Retroculus xinguensis Gosse, 1971	MPEG 29203	1.020	0.227	0.567	0.567	2.380
Satanoperca sp.	MPEG 29334	0.340	0.000	0.113	0.113	0.566
Teleocichla sp.	LIA 5	0.227	0.000	0.227	0.000	0.453
Sciaenidae
Pachyurus junki Soares & Casatti, 2000	MPEG 28085	20.446	4.422	9.751	4.649	39.266
Plagioscion squamosissimus (Heckel, 1840)	LIA 362	8.957	1.814	3.175	0.794	14.739
PLEURONECTIFORMES
Achiridae
Hypoclinemus mentalis (Günther, 1862)	MPEG 29117	0.000	0.000	0.113	0.000	0.113
SILURIFORMES
Auchenipteridae
Ageneiosus inermis (Linnaeus, 1766)		4.308	2.608	0.454	0.227	7.596
Ageneiosus ucayalensis Castelnau, 1855	MPEG 29114	43.915	23.696	2.381	0.567	70.559
Auchenipterichthys longimanus (Günther, 1864)	MPEG 28834	19.992	5.329	0.567	1.020	26.908
Auchenipterus nuchalis (Spix & Agassiz, 1829)	LIA 383	63.492	22.789	1.247	0.794	88.321
Centromochlus heckelii (De Filippi, 1853)	MPEG 28063	12.245	9.977	0.000	2.268	24.489
Centromochlus schultzi Rössel, 1962	MPEG 29752	0.454	0.567	0.000	0.113	1.133
Tatia intermedia (Steindachner, 1877)	MPEG 28925	0.529	0.454	0.000	0.000	0.982
Tocantinsia piresi (Miranda Ribeiro, 1920)	LIA 363	34.014	22.676	0.000	0.000	56.689
Trachelyopterus ceratophysus (Kner, 1858)	LIA 339	0.454	0.000	0.000	0.000	0.453
Callichthyidae
Megalechis picta (Müller & Troschel, 1849)	MPEG 29903	0.113	0.000	0.000	0.113	0.226
Cetopsidae
Cetopsis coecutiens (Lichtenstein, 1819)	MPEG 28061	3.628	0.000	0.227	3.401	7.256
Doradidae
Doras higuchii Sabaj Pérez & Birindelli, 2008	MPEG 29368	8.919	1.474	5.215	2.494	18.102
Hassar gabiru Birindelli, Fayal & Wosiacki, 2011	MPEG 28965	17.952	5.669	7.596	2.268	33.484
Hassar orestis (Steindachner, 1875)	MPEG 28871	21.542	6.803	0.000	14.286	42.630
Leptodoras hasemani (Steindachner, 1915)	MPEG 29120	2.494	1.701	2.041	0.000	6.235
Leptodoras praelongus (Myers & Weitzman, 1956)	MPEG 29740	0.113	0.000	0.000	0.113	0.226
Megalodoras uranoscopus (Eigenmann & Eigenmann, 1888)	MPEG 28076	1.058	0.340	0.000	0.567	1.965
Nemadoras elongatus (Boulenger, 1898)	LIA 430	0.227	0.227	0.000	0.000	0.453
Ossancora asterophysa Birindelli & Sabaj Pérez, 2011	LIA 275	1.134	0.680	0.000	0.454	2.267
Oxydoras níger (Valenciennes, 1821)	LIA 369	1.587	1.701	0.000	0.113	3.401
Platydoras armatulus (Valenciennes, 1840)	MPEG 28062	2.343	0.907	0.454	0.567	4.270
Platydoras sp.	LIA 139	4.119	2.494	0.000	0.454	7.067
Rhinodoras boehlkei Glodek, Whitmire & Orcés, 1976	MPEG 28857	0.454	0.454	0.000	0.000	0.907
Heptapteridae
Imparfinis aff. hasemani	LIA 431	0.113	0.000	0.000	0.113	0.226
Pimelodella cristata (Müller & Troschel, 1849)	MPEG 28892	4.535	0.000	2.154	2.154	8.843
Pimelodella sp. 1	MPEG 29481	0.567	0.000	0.000	0.000	0.567
Pimelodella sp. 2	MPEG 28969	0.113	0.000	0.000	0.113	0.226
Loricariidae
Ancistrus ranunculus Muller, Rapp Py-Daniel & Zuanon, 1994	LIA 131	0.227	0.227	0.000	0.000	0.453
Ancistrus sp. 1	LIA 169	0.454	0.454	0.000	0.000	0.907
Ancistrus sp. 2	LIA 77	0.680	0.340	0.227	0.000	1.247
Baryancistrus aff. niveatus	LIA 170	6.236	0.227	2.948	3.288	12.698
Baryancistrus chrysolomus Rapp Py-Daniel, Zuanon & Ribeiro de Oliveira, 2011	LIA 387	1.587	0.227	1.134	0.227	3.174
Baryancistrus xanthellus Rapp Py-Daniel, Zuanon & Ribeiro de Oliveira, 2011	LIA 171	1.474	0.340	1.020	0.113	2.947
Hemiodontichthys acipenserinus (Kner, 1853)	LIA 432	1.134	0.000	0.000	0.907	2.040
Hopliancistrus sp.	LIA 433	0.227	0.227	0.000	0.000	0.454
Hypancistrus sp.	LIA 21	0.227	0.000	0.227	0.000	0.453
Hypoptopoma inexpectatum (Holmberg, 1893)	LIA 321	27.022	21.995	1.020	0.113	50.151
Hypostomus aff. plecostomus	LIA 434	0.227	0.000	0.000	0.000	0.226
Hypostomus hemicochliodon Armbruster, 2003	LIA 359	2.041	0.454	0.680	0.340	3.514
Limatulichthys griseus (Eigenmann, 1909)	LIA 380	5.556	0.454	2.268	0.000	8.276
Loricaria birindellii Thomas & Sabaj Pérez, 2010	LIA 85	8.163	2.268	2.041	4.989	17.460
Loricaria cataphracta Linnaeus, 1758	LIA 365	7.143	0.000	2.721	5.215	15.079
Panaque armbrusteri Lujan, Hidalgo & Stewart, 2010	LIA 137	0.794	0.227	0.340	0.227	1.587
Parancistrus nudiventris Rapp Py-Daniel & Zuanon, 2005	LIA 177	0.794	0.000	0.794	0.000	1.587
Peckoltia cf. cavatica	LIA 435	0.454	0.227	0.227	0.000	0.907
Peckoltia feldbergae de Oliveira, Rapp Py-Daniel, Zuanon & Rocha, 2012	LIA 107	0.680	0.000	0.000	1.020	1.700
Peckoltia sabaji Armbruster, 2003	LIA 358	0.454	0.454	0.000	0.000	0.907
Peckoltia vittata (Steindachner, 1881)	LIA 291	4.611	1.134	1.927	1.927	9.599
Pseudacanthicus sp.	LIA 178	0.794	0.000	0.000	0.794	1.587
Pseudancistrus sp.	LIA 309	0.794	0.000	0.794	0.000	1.587
Pseudoloricaria laeviuscula (Valenciennes, 1840)	LIA 415	6.916	3.401	0.680	2.041	13.038
Pterygoplichthys xinguensis (Weber, 1991)	LIA 299	2.116	1.701	0.113	0.227	4.157
Rineloricaria sp.	LIA 248	0.113	0.000	0.000	0.000	0.113
Scobinancistrus aureatus Burgess, 1994	LIA 111	0.340	0.000	0.227	0.113	0.680
Scobinancistrus pariolispos Isbrücker & Nijssen, 1989	LIA 141	0.567	0.113	0.000	0.567	1.247
Squaliforma aff. emarginata	LIA 294	17.763	4.989	7.370	3.401	33.522
Spectracanthicus punctatissimus (Steindachner, 1881)	LIA 118	1.361	0.227	0.454	0.680	2.721
Spectracanthicus sp.	LIA 136	2.381	0.227	1.474	1.134	5.215
Pimelodidae
Brachyplatystoma filamentosum (Lichtenstein, 1819)		0.227	0.000	0.000	0.227	0.453
Hemisorubim platyrhynchos (Valenciennes, 1840)	MPEG 29084	0.000	0.000	0.113	0.000	0.113
Megalonema sp.	MPEG 29868	0.907	0.454	0.454	0.000	1.814
Phractocephalus hemioliopterus (Bloch & Schneider, 1801)	LIA 389	2.646	0.680	0.000	1.701	5.026
Pimelodus blochii Valenciennes, 1840	LIA 348	22.071	10.884	2.381	9.864	45.200
Pimelodus ornatus Kner, 1858	LIA 436	0.227	0.000	0.000	0.227	0.453
Pinirampus pirinampu (Spix & Agassiz, 1829)	MPEG 29383	2.759	0.680	0.567	1.814	5.820
Platynematichthys notatus (Jardine, 1841)	MPEG 29083	1.134	0.000	0.227	0.907	2.267
Pseudoplatystoma punctifer (Castelnau, 1855)	LIA 395	0.113	0.000	0.000	0.000	0.113
Sorubim lima (Bloch & Schneider, 1801)	LIA 437	0.529	0.454	0.000	0.000	0.982
Sorubim trigonocephalus Miranda Ribeiro, 1920	LIA 438	0.113	0.000	0.000	0.113	0.226
Pseudopimelodidae
Pseudopimelodus bufonius (Valenciennes, 1840)	LIA 318	0.454	0.454	0.000	0.000	0.907
Trichomycteridae
Henonemus sp.	LIA 439	0.340	0.227	0.000	0.000	0.566
TOTAL		1157.407	460.544	241.383	323.582

Acknowledgements

This research was supported by Norte Energia and LEME. The collection of biological material was authorised by permit 057/2012 from IBAMA, the Brazilian Institute for the Environment and renewable Natural Resources. TAPB and NLB were funded by CNPq, and TG (process: 308278/2012-7), LJ (process: 303252/2013-8) and LFAM (process: 301343/2012-8) receive productivity grants from CNPq. We are grateful to Dr. Stephen Ferrari for his help in correcting the text, particularly with the translation to English. All authors belong to the IctioXingu CNPq Research Group.

• (With 3 figures)

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Publication Dates

• Publication in this collection
  Aug 2015

History

• Received
  25 Apr 2014
• Accepted
  21 Jan 2015