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Dynamic of fish trophic guilds in the plateau-plain gradient in the Paraguay River, Northern Pantanal

Abstract

This study aims to evaluate the dynamics of fish trophic guilds according to the longitudinal gradient of the Paraguay River, northern Pantanal, Brazil. Three river segments were sampled: plateau, confluence and plain. These segments have different physical and biological characteristics, with high water flow in forest areas in plateau and slow flow in meanders, with Pantanal typical vegetation. In total, 26,542 individuals distributed in 130 fish species were collected. The sampled species were characterized in seven trophic guilds. From the seven trophic guilds identified, only three were statistically related to the type of the environment; herbivores were more abundant in the plateau, piscivores in the confluence, and invertivores in the plain. According to values of corrected Akaike Information Criteria, the environmental variable that best explains the abundance of piscivorous fishes in the segments sampled in the Paraguay River was water transparency. For herbivores, the model that explained the variation in abundance was composed by temperature, altitude and dense forest proportion. The variable altitude best represented the abundance of invertivores. Water transparency, temperature, altitude, river width and dense forest proportion were determining factors for the distribution of piscivorous, herbivorous and invertivorous fishes as a response to an environmental gradient that meets its ecological requirements. Understanding the trophic relationships is fundamental for management actions, contributing to the maintenance of ecosystem services of different species. Therefore, future research must be taken into account regarding management and ecological relationships.

Keywords.
Feeding habits; Ichthyofauna; Longitudinal gradient; Riverine landscape; Wetland

INTRODUCTION

The concept of the trophic guild is defined as a group of species that exploit the same class of food resources in a similar manner (Root, 1967Root, R.B. 1967. The Niche exploitation pattern of the Blue-Gray Gnatcatcher. Ecological Monographs, 37(4): 317-350.). To obtain these resources, the organisms need to search, detect, capture, manipulate and ingest the item (Wootton, 1999Wootton, R.J. 1999. Ecology of teleost fish. Dordrecht, Springer.). In this process, the different eating behaviours of species interfere with the use of the wide diversity of food resources available to fish in the aquatic environment (Hahn et al., 1997Hahn, N.S.; Agostinho, A.A. & Goiten, R. 1997. Feeding ecology of curvina Plagioscion squamosissimus (Hechel, 1840) (Osteichthyes, Perciformes) in the Itaipu reservoir and Porto Rico floodplain. Acta Limnologica Brasiliensia, 9: 11-22.).

In addition to the species behaviour, changes in its diets are driven by seasonal and spatial habitat modifications (Abelha et al., 2001Abelha, M.C.F.; Agostinho, A.A. & Goulart, E. 2001. Plasticidade trófica em peixes de água doce. Acta Scientiarum: Biological Sciences, 23(2): 425-434. https://doi.org/10.4025/actascibiolsci.v23i0.2696.
https://doi.org/10.4025/actascibiolsci.v...
). In rivers that are seasonally influenced by flooding events, such as Brazilian ones, most of the fish species switch the food items consumption as hydrological periods change (Dary et al., 2017Dary, E.P.; Ferreira, E.; Zuanon, J. & Röpke, C.P. 2017. Diet and trophic structure of the fish assemblage in the mid-course of the Teles Pires river, Tapajós river basin, Brazil. Neotropical Ichthyology, 15(4): 1-14. https://doi.org/10.1590/1982-0224-20160173.
https://doi.org/10.1590/1982-0224-201601...
; Muniz et al., 2019Muniz, C.C.; Flamini, A.C.; Kantek, D.L.Z.; Lázaro, W.L.; Souza, A.R. & Oliveira-Junior, E.S. 2019. Stress hídrico determina a dieta de Tetragonopterus argenteus (Cuvier, 1816) no Pantanal Norte. Revista Ibero-Americana de Ciências Ambientais, 10(4): 209-218. https://doi.org/10.6008/cbpc2179-6858.2019.004.0016.
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), being sometimes related to the riparian vegetation phenology (Furlan et al., 2017Furlan, A.O.; Muniz, C.C. & Carniello, M.A. 2017. Análise do componente vegetal na alimentação de peixes e da relação com a dispersão de sementes no Pantanal Mato-grossense. Revista Brasileira de Ciências Ambientais, 45: 61-70. https://doi.org/10.5327/z2176-947820170176.
https://doi.org/10.5327/z2176-9478201701...
).

Regarding spatial changes, the feeding habits of the ichthyofauna are influenced by the spatial availability of habitats, altitude, and the order of rivers and streams (Bistoni & Hued, 2002Bistoni, M.A. & Hued, A.C. 2002. Patterns of fish species richness in rivers of the central region of Argentina. Brazilian Journal of Biology, 62(4B): 753-764. https://doi.org/10.1590/S1519-69842002000500004.
https://doi.org/10.1590/S1519-6984200200...
; Da Silva et al., 2014Da Silva, A.D.R.; Santos, R.B.; Bruno, A.M.S.S.; Gentelini, A.L.; Silva, A.H.G. & Soares, E.C. 2014. Biofilter efficiency of water hyacinth on limnological variables in irrigation channels used for tambaqui farming [Eficiência do aguapé sobre variáveis limnológicas em canais de abastecimento utilizados no cultivo de tambaqui]. Acta Amazonica, 44(2): 255-261. https://doi.org/10.1590/S0044-59672014000200011.
https://doi.org/10.1590/S0044-5967201400...
). These characteristics change along the river continuum, forming a gradient of trophic interactions, that change from headwaters to floodplains (Vannote et al., 1980Vannote, R.L.; Minshall, G.W.; Cummins, K.W.; Sedell, J.R. & Cushing, C.E. 1980. The River Continuum Concept. Canadian Journal of Fisheries and Aquatic Sciences, 37(1): 130-137.; Wolff et al., 2013Wolff, L.L.; Carniatto, N. & Hahn, N.S. 2013. Longitudinal use of feeding resources and distribution of fish trophic guilds in a coastal Atlantic stream, southern Brazil. Neotropical Ichthyology, 11(2): 375-386. https://doi.org/10.1590/S1679-62252013005000005.
https://doi.org/10.1590/S1679-6225201300...
; Curtis et al., 2018Curtis, W.J.; Gebhard, A.E. & Perkin, J.S. 2018. The river continuum concept predicts prey assemblage structure for an insectivorous fish along a temperate riverscape. Freshwater Science, 37(3): 618-630. https://doi.org/10.1086/699013.
https://doi.org/10.1086/699013...
).

Studies assessing trophic guilds, based on the differences between allochthonous resource input and primary productivity, have shown changes in the distribution of generalist (such as insectivores) and specialist (detritivores, planktophagous, and piscivores) species in longitudinal gradients (Schlosser, 1982Schlosser, I.J. 1982. Fish community structure and function along two habitat gradients in a headwater stream. Ecological monographs, 52(4): 395-414. https://doi.org/10.2307/2937352.
https://doi.org/10.2307/2937352...
; Angermeier & Karr, 1986Angermeier, P.L. & Karr, J.R. 1986. Fish communities along environmental gradients in a system of tropical streams. Environmental Biology of Fishes, 9(1): 117-135.; Silva et al., 2014Silva, M.R.; Fugi, R.; Carniatto, N. & Ganassim, M.J.M. 2014. Importance of allochthonous resources in the diet of Astyanax aff. fasciatus (Osteichthyes: Characidae) in streams: a longitudinal approach. Biota Neotropica, 14(3): e20130016. https://doi.org/10.1590/1676-06032014001613.
https://doi.org/10.1590/1676-06032014001...
). Thus, the composition of trophic groups of aquatic communities is primarily structured according to the gradient produced by the variation of physical and biotic parameters (river discharge, channel width, and vegetation cover) and by the input and processing of organic matter and yield along the river continuum (Vannote et al., 1980Vannote, R.L.; Minshall, G.W.; Cummins, K.W.; Sedell, J.R. & Cushing, C.E. 1980. The River Continuum Concept. Canadian Journal of Fisheries and Aquatic Sciences, 37(1): 130-137.).

Therefore, temporal and spatial modifications lead to changes in the richness and abundance of trophic fish guilds, considering that distinct periods and environments present different abiotic conditions and food offerings (Abelha et al., 2001Abelha, M.C.F.; Agostinho, A.A. & Goulart, E. 2001. Plasticidade trófica em peixes de água doce. Acta Scientiarum: Biological Sciences, 23(2): 425-434. https://doi.org/10.4025/actascibiolsci.v23i0.2696.
https://doi.org/10.4025/actascibiolsci.v...
). While the fish assemblages change according to the environmental characteristics (Súarez et al., 2011Súarez, Y.R.; Souza, M.M.; Ferreira, F.S.; Pereira, M.J.; Silva, E.A., Ximenes, L.Q.L.; Azevedo, L.G.; Martins, O.C. & Lima-Júnior, S.E. 2011. Patterns of species richness and composition of fish assemblages in streams of the Ivinhema River basin, Upper Paraná River. Acta Limnologica Brasiliensia, (23)2: 177-188. https://doi.org/10.1590/s2179-975x2011000200008.
https://doi.org/10.1590/s2179-975x201100...
), in non-transformed Amazonian wetlands, the well-structured riparian vegetation along the streams did not affect the diet of a fish assemblage, showing that the continuity of the marginal vegetation reduces the impact on fish diet (Soares et al., 2020Soares, B.E.; Benone, N.L.; Rosa, D.CO. & Montag, L.F.A. 2020. Do local environmental factors structure the trophic niche of the Splash Tetra, Copella arnoldi? A test in an Amazonian stream system. Acta Amazonica, 50(1): 54-60. https://doi.org/10.1590/1809-4392201802681.
https://doi.org/10.1590/1809-43922018026...
). Studies focusing on the distribution of fish trophic guilds help to understand the behaviour of these organisms in the face of environmental transformations and food availability in natural environments (Silva et al., 2012Silva, D.A.; Pessoa, E.K.R.; Costa, S.A.G.L.; Chellappa, N.T. & Chellappa, S. 2012. Ecologia Reprodutiva de Astyanax lacustris (Osteichthyes: Characidae) na Lagoa do Piató, Assú, Rio Grande do Norte, Brasil. Biota Amazônia, 2(2): 54-61. https://doi.org/10.18561/2179-5746/biotaamazonia.v2n2p54-61.
https://doi.org/10.18561/2179-5746/biota...
).

The Paraguay River - one of the most important water sources for the Pantanal - has being highly modified, and this environmental transformation highly impact the riparian vegetation (Damasceno-Junior et al., 2005Damasceno-Junior, G.A.; Semir, J.; Santos, F.A.M.S. & Leitão-Filho, H.F. 2005. Structure, distribution of species and inundation in a riparian forest of Rio Paraguai, Pantanal, Brazil. Flora, 200(2): 119-135. https://doi.org/10.1016/j.flora.2004.09.002.
https://doi.org/10.1016/j.flora.2004.09....
), as well as the water availability (Lázaro et al., 2020Lázaro, W.L.; Oliveira-Júnior, E.S.; Da Silva, C.J.; Castrillon, S.K.I. & Muniz, C.C. 2020. Climate change reflected in one of the largest wetlands in the world: an overview of the Northern Pantanal water regime. Acta Limnologica Brasiliensia, 32: 8. https://doi.org/10.1590/S2179-975X7619.
https://doi.org/10.1590/S2179-975X7619...
). These transformations also change other environmental functionalities, such as food and habitat provision for fish under the macrophyte beds (Da Silva et al., 2010Da Silva, H.P.; Petry, A.C. & Da Silva, C.J. 2010. Fish communities of the Pantanal wetland in Brazil: Evaluating the effects of the upper Paraguay river flood pulse on baía Caiçara fish fauna. Aquatic Ecology, 44(1): 275-288. https://doi.org/10.1007/s10452-009-9289-9.
https://doi.org/10.1007/s10452-009-9289-...
) and may consequently change the fish assemblages and the guilds.

Due to environmental changes in the river longitudinal gradient, we hypothesized that: 1) herbivorous fish are prone to be more abundant in the midreaches regions due to its dependence on allochthonous resources, and 2) while insectivores, carnivores and omnivores guilds are more abundant in the floodplain regions, mainly due to environmental heterogeneity, which enables a wide variety of food resources. Considering these aspects, this research aimed to evaluate the dynamics of trophic guilds according to the longitudinal gradient of the Paraguay River, Northern Pantanal, Brazil.

MATERIAL AND METHODS

Study site

The samplings were performed from July to November/2017 and August/2018 in the dry period in the upper reaches of the Paraguay River, in six sampling sites divided into two sampling areas in each segment (Fig. 1). Three segments were evaluated: 1) plateau: composed by the sampled areas in the municipalities of Barra do Bugres and Porto Estrela. These sites have similar environmental characteristics, including altitude (± 150 m above sea level), river width (± 50 m) and vegetation composed by forest. About 70 km downstream the plateau is the region of confluence 2) between Sepotuba, Cabaçal and Jauru Rivers with the Paraguay River, where the altitude is lower (± 120 m above sea level) and the river is wider (± 125 m), and) 3) plain: composed by areas (hills region and Taiamã Ecological Station) located in regions where the altitude was the lowest (± 100 m above sea level) and the river was the widest (± 185 m). Table 1 shows the mean values for altitude and river width among the sampling sites and the geographic coordination of each segment. The slow water flow in this last segment allows the colonization of the littoral area by aquatic macrophytes, occurring mainly the species Eichhornia azurea (Sw.) Kunth and E. crassipes (Mart.) Solms.

Figure 1
The sampling sites along the Paraguay River, state of Mato Grosso, Brazil.

Table 1
Geographic coordinates, mean altitude (m) and mean river width (m) among the sampled segments in the Paraguay River, Northern Pantanal.

Data collection

To collect the fish specimens two sampling methods were used. A dragging net with 4 m height × 25 m width and 5 mm mesh size and a net, armed in a metallic structure of 2 m length × 1 m width × 25 cm of depth were used. In each point the limnological variables such as water transparency (cm), water temperature (℃), water conductivity (µS/cm) and dissolved oxygen (mg/L) were measured using a multiparameter probe Hach HQ40D. The environment variables selected were the altitude, river width and dense forest proportion (1 km buffer), considering that this ratio directly contributes to habitat quality to the aquatic organisms.

Fish specimens’ standard length (SL hereafter) (cm) and weight (g) were measured and identified following the identification keys found in Britski et al. (2007Britski, H.A.; de Silimon, K.Z. & Lopes, B.S. 2007. Peixes do Pantanal. Manual de Identificação. Brasília, EMBRAPA.). The species valid names were checked according to Fricke et al. (2022Fricke, R.; Eschmeyer, W.N. & Van der Laan, R. (Eds.). 2022. Eschmeyer’s Catalog of Fishes: genera, species, references. Available: Available: http://researcharchive.calacademy.org/research/ichthyology/catalog/fishcatmain.asp . Access: 18/03/2022.
http://researcharchive.calacademy.org/re...
). Specimens were fixed with formalin 10% and preserved in alcohol 70% and deposited in the collection of the Laboratório de Ictiologia do Pantanal Norte (LIPAN; see supplementary material SUPPLEMENTARY MATERIAL Table 1 List of trophic guilds with species. PLT: plateau CON: confluence e PLA: plain. Voucher Trophic guilds, species Authors PLT % CON % PLA % Length range (cm) Weight range (g) Piscivores 0.85 2.86 2.01 0.9-80.4 0.01-13300 LIPAN052 Acestrorhynchus pantaneiro Menegalezes, 1992 Resende et al. (1996) 0.212 0.371 0.042 8.5-22.1 5.8-202.8 LIPAN265 Ageneiosus ucayalensis Castelnau, 1855 Corrêa et al. (2009) - - 0.010 3.5 0.74 LIPAN272 Ageneiosus valenciennesi Bleeker, 1864 Hahn et al. (2002) - 0.018 0.010 19-27.8 71.5-156.1 LIPAN012 Catathyridium jenynsii (Günther, 1862) Hahn et al. (2002) - 0.009 - 5.6 6.8 LIPAN294 Cichla sp Block & Schneider, 1801 Hahn et al. (2002) - 0.009 - 21 277.95 LIPAN266 Galeocharax humeralis (Valenciennes, 1834) Corrêa et al. (2009) - 0.168 0.366 1.9-4.2 1.18-1.12 LIPAN149 Hemisorubim platyrhynchos (Valenciennes, 1840) Hahn et al. (2002) 0.018 0.415 0.031 16.8-52.1 67.2-2142 LIPAN050 Hoplias malabaricus Bloch, 1794 Resende et al. (2016) 0.460 1.042 0.418 0.9-10.8 0.01-1090 LIPAN267 Pinirampus pirinampu (Spix & Agassiz, 1829) Hahn et al. (2002) - - 0.052 33.1-68.4 22-4390 LIPAN047 Plagioscion ternetzi Boulenger, 1895 Resende et al. (1996) - 0.035 - 18.1-18.5 111.17-118.2 LIPAN117 Potamotrygon falkneri Castex & Maciel, 1963 Lonardoni et al. (2006) 0.071 0.018 - 20.5-53 35.42-1770 LIPAN293 Pseudoplatystoma corruscans (Spix & Agassiz, 1829) Resende et al. (1996) - 0.026 0.105 25.8-108.1 31.10-13300 LIPAN120 Pseudoplatystoma fasciatum (Linnaeus, 1766) Resende et al. (1996) - 0.062 0.052 14.6-77.8 153.6-6410 LIPAN053 Pygocentrus nattereri Kner, 1858 Resende et al. (1996) 0.018 0.238 0.324 10.5-29.5 49.34-860 LIPAN268 Roeboides bonariensis Steindachner, 1879 Resende et al. (2016) 0.018 0.009 0.094 2.5-7 0.19-6.11 LIPAN292 Salminus brasiliensis (Cuvier, 1816) Corrêa et al. 2009 - 0.009 - 50.5 2210 LIPAN051 Serrasalmus maculatus Kner, 1858 Corrêa et al. (2009) 0.018 0.177 0.157 1.2-23.5 0.03-506.3 LIPAN177 Serrasalmus marginatus Valenciennes, 1837 Corrêa et al. (2009) 0.035 0.124 0.167 1.5-19.9 0.06-480 LIPAN125 Serrasalmus spilopleura Kner, 1858 Resende et al. (2016) - 0.035 0.105 9.6-23.7 33-638 LIPAN123 Sorubim lima (Bloch & Schneider, 1801) Hahn et al. (2002) - 0.026 0.031 14.5-47.8 20-1040 Detritivores 11.84 17.34 10.27 1-55 0.01-810 LIPAN133 Apareiodon affinis (Steindachener, 1879) Hahn et al. (2002) 0.478 - 0.209 2.7-9 0.20-12.5 LIPAN073 Curimatella dorsalis (Eigenmann & Eigenmann, 1889) Polaz et al. (2017) 1.026 1.625 0.178 3.1-8.1 0.68-7.1 LIPAN063 Curimatopsis myersi Vari, 1982 Resende et al. (2016); Polaz et al. (2017) 0.035 1.042 0.679 1.2-4 0.03-1.81 LIPAN036 Cyphocharax gillii (Eigenmann & Kennedy, 1903) Resende et al. (2016); Polaz et al. (2017) - 0.088 0.031 2.5-10 0.36-28.59 LIPAN144 Farlowella paraguayensis Retzer & Page, 1997 Polaz et al. (2017) - - 0.021 5-6.2 0.14-0.20 LIPAN151 Hypoptopoma inexspectatum (Holmberg, 1893) Resende et al. (2016) 7.415 11.171 2.759 1.7-7 0.14-7.6 LIPAN273 Hypostomus cochliodon Kner, 1854 Polaz et al. (2017) - - 0.052 1.5-4.9 0.07-3.9 LIPAN056 Hypostomus sp (Lacepède 1803) Polaz et al. (2017) 0.867 0.433 2.822 1-7.5 0.01-7.5 LIPAN269 Liposarcus anisitsi (Eigenmann & Kennedy, 1903) Resende et al. (2016) 0.071 0.052 20.5-34.5 223.9-770 LIPAN274 Loricaria sp Linnaeus, 1758 Polaz et al. (2017) - - 0.010 1.6-8.7 0.09-6.9 LIPAN124 Loricariichthys labialis (Boulenger, 1895) Resende et al. (2016); Polaz et al. (2017) - - 0.010 15 18.4 LIPAN156 Loricariichthys platymetopon Isbrücker & Nijssen, 1979 Resende et al. (2016); Polaz et al. (2017) 0.035 0.035 0.031 7.2-26 0.23-127.5 LIPAN270 Megalancistrus aculeatus (Perugia, 1891) Hahn et al. 2002 0.026 3-4.3 0.85-2.04 LIPAN004 Potamorhina squamoralevis (Braga & Azpelicueta, 1983) Polaz et al. (2017) 0.018 0.062 0.512 1.2-23.7 0.04-369 LIPAN091 Prochilodus lineatus (Valenciennes, 1836) Polaz et al. (2017) 0.274 0.010 8.7-33 19-810 LIPAN069 Psectrogaster curviventris (Eigenmann & Kennedy, 1903) Polaz et al. (2017) 0.194 0.449 4.2-55 1.62-226 LIPAN130 Rineloricaria parva (Boulenger, 1895) Resende et al. (2016); Polaz et al. (2017) 0.301 2.119 1.829 1.6-8.7 0.09-6.9 LIPAN018 Satanoperca papaterra (Heckel, 1840) Sampaio & Goulart 2011 0.053 0.071 0.010 8-16.5 26.8-167 LIPAN178 Spatuloricaria evansii (Boulenger, 1892) Polaz et al. (2017) 0.212 0.018 0.010 3-31 0.11-227.2 LIPAN179 Steindachnerina brevipinna (Boulenger, 1902) Resende et al. (2016) 1.363 0.018 0.439 2.5-10.7 0.24-38.2 LIPAN076 Steindachnerina conspersa (Holmberg, 1891) Resende (2000) 0.053 0.021 3.2-10.5 0.73-41.4 LIPAN116 Sturissoma barbatum (Kner, 1853) Polaz et al. (2017) 0.035 0.044 0.136 5.8-11.4 0.55-71.73 Herbivores 53.91 1.31 2.0 1.3-46.7 0.07-3720 LIPAN132 Abramites hypselonotus (Günther, 1868) Polaz et al. (2017) 0.177 0.044 0.073 1.7-9.3 0.09-21.52 LIPAN005 Mesonauta festivus (Heckel, 1840) Polaz et al. (2017) - 0.088 0.125 2.5-10 0.20-56.1 LIPAN287 Myloplus levis (Eigenmann & McAtee, 1907) Polaz et al. (2017) - 0.044 0.167 7.5-25.2 19.2-270 LIPAN286 Mylossoma paraguayensis (Norman, 1928) Resende et al. (2016) - - 0.010 14.5 102.3 LIPAN071 Mylossoma duriventre (Cuvier, 1818) Polaz et al. (2017) - 0.009 0.659 2.8-22.2 3.9-490 LIPAN161 Otocinclus vittatus Regan, 1904 Polaz et al. (2017) 53.61 0.318 0.042 1.3-5 0.07-0.60 LIPAN074 Piabucus melanostoma Holmberg, 1991 Polaz et al. (2017) 0.088 0.283 0.303 4.9-8.5 0.39-3.9 LIPAN164 Piaractus mesopotamicus (Holmberg, 1887) Polaz et al. (2017) 0.035 0.009 0.366 12-40.8 72.6-2420 LIPAN288 Schizidon borelli (Boulenger, 1900) Polaz et al. (2017) 0.230 0.240 4.6-15.5 0.56-702.1 LIPAN092 Schizidon isognathus Kner, 1858 Polaz et al. (2017) 0.283 0.010 3.2-22.8 0.68-250.2 Invertivores 11.43 34.01 44.06 0.09-9.7 0.01-9.01 LIPAN279 Apistogramma trifasciata (Eigenmann & Kennedy, 1903) Resende et al. (2016) - 0.645 1.850 0.8-4.6 0.01-1.4 LIPAN037 Aphyocharax anisitsi Eigenmann & Kennedy, 1903 Resende et al. (2016) 1.380 0.406 1.087 1.3-3 0.03-7.13 LIPAN289 Aphyocharax paraguayensis Eigenmann, 1915 Resende et al. (2016) - 0.009 0.387 1.5-3 0.03-0.47 LIPAN087 Apistogramma commbrae (Regan, 1906) Polaz et al. (2017) 0.425 0.115 0.136 1.2-3.1 0.04-1.93 LIPAN070 Apteronotus albifrons (Linnaeus, 1766) Polaz et al. (2017) - 0.009 0.261 4.3-19.8 0.34-19.9 LIPAN097 Astronotus crassipinis Heckel, 1840 Polaz et al. (2017) - 0.044 - 16.5-22 125.5-461 LIPAN021 Auchenipterus osteomystax (Miranda Ribeiro, 1918) Hahn et al. (2002) - 0.009 - 3.9 0.91 LIPAN111 Brachyhypopomus sp Mago Leccia, 1994 Resende et al. (2016) 0.088 0.636 2.216 4.7-22.1 0.05-22 LIPAN028 Bryconamericus exodun Eigenmann, 1907 Polaz et al. (2017) 0.088 0.177 0.125 1.3-5.7 0.08-2.76 LIPAN136 Bryconamericus stramineus Eigenmann, 1908 Polaz et al. (2017) 0.991 0.009 0.031 1.2-4.8 0.05-2 LIPAN011 Bujurquina vittata (Heckel, 1840) Resende et al. (2016) 0.053 0.159 0.063 1.1-6.9 0.03-14.5 LIPAN030 Characidium aff. zebra Eigenmann, 1909 Resende et al. (2016) 2.442 2.464 1.725 1.2-4.7 0.02-2.32 LIPAN290 Characidium laterale (Boulenger, 1895) Polaz et al. (2017) - 0.018 0.125 2.2-3.3 0.21-0.53 LIPAN072 Charax leticiae Lucena, 1987 Resende et al. (2016) 0.053 0.062 0.157 4.9-9.4 1.38-15.95 LIPAN112 Crenicichla lepidota Heckel, 1840 Resende et al. (2016) - 0.062 0.021 11-33.1 35-525.6 LIPAN140 Corydoras aeneus (Gill, 1858) Brandão-Gonçalves et al. (2010) 0.230 - - 2-3.5 0.40-1.9 LIPAN027 Corydoras hastatus Eigenmann & Eigenmann, 1888 Polaz et al. (2017) 0.124 0.265 0.084 1.2-2 0.04-0.46 LIPAN055 Crenicichla vittata Heckel, 1840 Resende et al. (2016) 0.319 2.181 2.028 2-16.8 0.12-113.7 LIPAN024 Eigenmania virescens (Valenciennes, 1847) Polaz et al. (2017) - 0.106 5.676 1.7-18.7 0.03-11 LIPAN025 Eigenmania trilineata Lopes & Castello, 1996 Resende et al. (2016) 0.018 5.175 10.996 2.5-31.5 0.03-39.6 LIPAN46 Entomocorus benjamini Eigenmman, 1917 Resende et al. (2016) - 0.026 1.631 2-4.9 0.08-12.9 LIPAN145 Gasteropelecus sternicla (Linnaeus, 1758) Polaz et al. (2017) 0.672 0.212 0.010 2-6 0.23-3.7 LIPAN060 Gymnotus inaequilabiatus (Valenciennes, 1839) Polaz et al. (2017) 0.053 0.168 0.345 4.4-90 0.34-1370 LIPAN107 Gymnotus paraguensis Albert & Crampton, 2003 Polaz et al. (2017) - 0.018 - 9.9-11.7 2.1-3.7 LIPAN022 Hemigrammus ulrey (Boulenger, 1895) Resende et al. (2016) 0.566 6.323 2.530 1.5-3.4 0.06-1.1 LIPAN146 Hemigrammus marginatus Ellis, 1911 Brandão-Gonçalves et al. (2010) 0.035 - - 2.5-2.7 0.27-0.33 LIPAN058 Hemiodontichthys acipenserinus (Kner, 1853) Polaz et al. (2017) - 0.009 - 9 3.26 LIPAN032 Hyphessobrycon eques (Steindachner, 1882) Polaz et al. (2017) 0.672 7.144 6.930 0.9-9.7 0.01-2.2 LIPAN034 Ituglanis herberti (Miranda Ribeiro, 1940) Polaz et al. (2017) 0.177 2.874 1.3-3.2 0.01-12 LIPAN291 Ituglanis eichorniarum (Miranda Ribeiro, 1912) Polaz et al. (2017) 0.009 0.010 1.5-3.4 0.05-0.45 LIPAN081 Ossancora eigenmanni (Boulenger, 1895) Polaz et al. (2017) 0.238 0.146 3.3-6 0.87-5.67 LIPAN045 Pimelodella mucosa Eigenmann & Ward, 1907 Resende et al. (2016); Polaz et al. (2017) 1.274 1.872 1.913 2.7-9 0.35-15.8 LIPAN167 Poptella paraguayensis (Eigenmann, 1907) Polaz et al. (2017) 1.345 3.214 0.157 2-6.9 0.38-8.7 LIPAN061 Potamorrhaphis eigenmanni Miranda Ribeiro, 1915 Ibañez et al. (2007) - - 0.021 11.3-24 15.1-24.88 LIPAN122 Potamotrygon motoro (Müller & Henle, 1841) Lonardoni et al. (2006) - 0.018 - 30.5-50.7 158-278.63 LIPAN105 Pseudotylosurus angusticeps (Günther, 1866) Resende et al. (2016) 0.106 - - 21-26 11.4-21.2 LIPAN029 Pyrhulina australis Eigenmann & Kennedy, 1903 Resende et al. (2016); Polaz et al. (2017) 0.035 0.971 2.216 1.4-4.2 0.06-2.47 LIPAN285 Rhamphicthys hahni (Meinken, 1937) Resende et al. (2016); Polaz et al. (2017) - 0.035 0.178 1.5-44.3 0.02-177.1 LIPAN049 Sternopygus macrurus (Bloch & Schneider, 1801) Resende et al. (2016) 0.018 0.742 1.056 2.4-52.5 0.60-380 LIPAN106 Synbranchus marmoratus Bloch, 1795 Polaz et al. (2017) 0.142 0.026 0.052 4.5-22.9 0.11-10.60 LIPAN019 Tatia neivai (Ihering, 1930) Polaz et al. (2017) - 0.009 - 2.7 0.49 LIPAN065 Tetragonopterus argenteus Cuvier, 1816 Resende et al. (2016) - 0.212 0.314 4.5-11.1 0.73-57.3 LIPAN180 Thoracocharax stellatus (Kner, 1858) Resende et al. (2016); Polaz et al. (2017) 0.142 0.424 0.031 2.9-10.8 0.56-4.7 LIPAN119 Trachelyopterus galeatus (Linnaeus, 1766) Resende et al. (2016) 0.159 0.353 0.199 4.9-16.4 0.70-123 Lepidophagous 0.33 0.28 0.52 2.5-20.5 0.19-6.11 LIPAN176 Roeboides prognathus Boulenger, 1895 Sazima & Machado (1983) 0.336 0.380 0.523 2.5-20.5 0.19-6.11 Omnivores 21.63 43.43 37.66 1-57.8 0.01-2600 LIPAN066 Aequidens plagiozonatus Kullander, 1984 Polaz et al. (2017) - 0.795 0.188 1.1-6.7 0.02-13.1 LIPAN101 Anadoras weddellii (Castelnau, 1855) Resende et al. (2016); Polaz et al. (2017) 0.142 0.751 0.324 2.5-9.5 0.06-19 LIPAN283 Aphyocharax dentatus Eigenmann e Kennedy, 1903 Polaz et al. (2017) 3.840 1.210 0.575 1.5-6.7 0.07-6.4 LIPAN282 Apteronotus caudimaculosus de Santana, 2003 Polaz et al. (2017) - - 0.042 9.4-12.7 2.40-5.93 LIPAN102 Astyanax asuncionensis Géry, 1972 Resende et al. (2016); Polaz et al. (2017) - 0.362 0.251 5-18.3 2.72-12.10 LIPAN001 Brycon hilarii (Valenciennes, 1850) Polaz et al. (2017) - 0.035 0.084 18.7 136.1-630 LIPAN110 Ctenobrycon alleni (Eigenmann & Mcate, 1907 Polaz et al. (2017) - 0.026 0.105 6.5-8.9 5.8-16.9 LIPAN033 Gymnocorimbus ternetzi Boulenger, 1895 Resende et al. (2016) 0.018 0.018 0.220 2-4 0.17-2.2 LIPAN083 Gymnogeophagus balzanii (Perugia, 1891) Resende et al. (2016) 0.142 0.018 0.063 3.3-13.5 1.34-131.9 LIPAN148 Hemiodus orthonops Eigenmann & Kennedy, 1903 Polaz et al. (2017) 0.088 0.124 0.669 3-26.6 0.28-242.3 LIPAN062 Jupiaba acantogaster (Eigenmann, 1911) Polaz et al. (2017) 3.327 0.442 6.345 1.1-3.5 0.01-0.46 LIPAN093 Laetacara dorsigera (Heckel, 1840) Polaz et al. (2017) 0.035 0.026 0.084 2-7.3 0.37-20.1 LIPAN100 Leporinus friderici (Bloch, 1794) Resende et al. (2016); Polaz et al. (2017) 0.035 0.424 0.491 3.5-25.5 0.48-380 LIPAN020 Leporinus lacustris Campos, 1945 Polaz et al. (2017) - 0.185 0.178 3.5-25.5 0.79-244 LIPAN154 Leporinus macrocephalus Garavello & Britski, 1988 Resende et al. (2016); Polaz et al. (2017) 0.088 0.009 0.010 20-48 210-2600 LIPAN155 Leporinus striatus Kner, 1858 Resende et al. (2016); Polaz et al. (2017) 0.018 0.318 0.376 1.7-7.4 0.12-7.6 LIPAN157 Moenkhausia dichroura (Kner, 1858) Polaz et al. (2017) 4.229 12.390 12.899 1-4.6 0.01-2.33 LIPAN035 Moenkhausia sanctaefilomenae (Steindachner, 1907) Resende et al. (2016) 1.522 2.746 2.080 1.6-5.7 0.07-5.3 LIPAN280 Odontostilbe calliura (Boulenger, 1900) Resende et al. (2016) 5.256 5.961 4.097 1-3 0.01-4.81 LIPAN281 Odontostilbe pequira (Steindachner, 1882) Polaz et al. (2017) 2.265 14.509 6.042 1-4.1 0.01-0.86 LIPAN075 Oxydoras kneri Bleeker, 1862 Polaz et al. (2017) - 0.026 0.021 31-38.8 458.9-980 LIPAN284 Pimelodella gracilis (Valenciennes, 1835) Resende et al. (2016) - - 0.010 8.1 7.7 LIPAN65 Pimelodus argenteus Perugia, 1891 Resende et al. (2016); Polaz et al. (2017) - 0.009 - 17 100.68 LIPAN042 Pimelodus maculatus Lacépède, 1803 Lolis & Andrian (1996) 0.406 0.220 3.9-31.3 0.5-550 LIPAN166 Pimelodus ornatus Kner, 1858 Hahn et al. (2002) 0.018 0.009 0.031 22-39.3 180-1810 LIPAN275 Platydoras armatulus (Valenciennes, 1840) Polaz et al. (2017) - 0.088 0.073 4-6.1 2.1-7.36 LIPAN026 Prionobrama paraguayensis (Eigenmann, 1914) Polaz et al. (2017) 0.248 0.627 0.042 2.2-3.9 0.13-1.78 LIPAN006 Psellogrammus kennedyi (Eigenmann, 1903) Polaz et al. (2017) 0.053 0.565 0.941 1.4-5.8 0.05-2.46 LIPAN276 Pterodoras granulosus (Valenciennes, 1821) Hahn et al. 2002 - - 0.021 28-57.8 5.3-22.8 LIPAN278 Rhamdia aff. quelen (Quoy & Gaimard, 1824) Polaz et al. (2017) - 0.026 0.010 2.3-23.5 0.12-241.5 LIPAN277 Trachelyopterus coriaceus Valenciennes, 1840 Polaz et al. (2017) - - 0.073 5.5-12.5 5.9-65.5 LIPAN064 Trachydoras paraguayensis (Eigenmann & Ward, 1907) Resende et al. (2016) 0.283 0.450 0.470 3.7-7.6 1.4-28.4 LIPAN003 Triportheus paranensis (Günther, 1874) Resende et al. (2016) 0.018 0.874 0.627 3.5-20.5 2.5-172.4 Total 100 100 100 ) at the Centro de Limnologia, Biodiversidade e Etnobiologia do Pantanal (CELBE-UNEMAT).

The guilds were attributed according to the most expressive behaviour reported by the specialized bibliographies (Sazima & Machado, 1983Sazima, I. & Machado, F.A. 1983. Hábitos e comportamento de Roeboides prognathus, um peixe lepidófago (Osteichthyes, Characoidei). Boletim de Zoologia, 7(7): 37-56. https://doi.org/10.11606/issn.2526-3358.bolzoo.1983.122032.
https://doi.org/10.11606/issn.2526-3358....
; Lolis & Andrian, 1996Lolis, A. & Andrian, I.F. 1996. Alimentação de Pimelodus maculatus Lacépède, 1803 (Siluriformes, Pimelodidae) na planície de inundação do Alto rio Paraná, Brasil. Boletim do Instituto de Pesca, 23: 187-202.; Resende et al., 1996Resende, E.K.; Pereira, R.A.C.; Almeida, V.D. & Silva, A.D. 1996. Alimentação de peixes carnívoros da planície inundável do rio Miranda, Pantanal, Mato Grosso do Sul, Brasil. Corumbá-MS, EMBRAPA-CPAP.; Resende, 2000Resende, E.K. 2000. Trophic structure of fish assemblages in the lower Miranda river, Pantanal, Mato Grosso do Sul State, Brazil. Revista brasileira de Biologia, 60(3): 389-403.; Hahn et al., 2002Hahn, N.S.; Fugi, R.; Peretti, D.; Russo, M.R. & Loureiro-Crippa, V.E. 2002. Estrutura trófica da ictiofauna da planície de inundação do alto rio Paraná. In: A Planicie de Inundação do Alto rio Paraná. Maringá, Area de Pesquisas Ecológicas de Longa Duração, Núcleo de Pesquisas em Limnologia, Ictiologia e Aqüicultura-Nupelia, Universidade Estadual de Maringá. p. 123-126.; Lonardoni et al., 2006Lonardoni, A.P.; Goulart, E.; de Oliveira, E.F. & Abelha, M.C.F. 2006. Hábitos alimentares e sobreposição trófica das raias Potamotrygon falkneri e Potamotrygon motoro (Chondrichthyes, Potamotrygonidae) na planície alagável do alto rio Paraná, Brasil. Acta Scientiarum. Biological Sciences, 28(3): 195-202. https://doi.org/10.4025/actascibiolsci.v28i3.208.
https://doi.org/10.4025/actascibiolsci.v...
; Ibañez et al., 2007Ibañez, C.; Tedesco, P.A.; Bigorne, R.; Hugueny, B.; Pouilly, M.; Zepita, C.; Zubieta, J. & Oberdorff, T. 2007. Dietary-morphological relationships in fish assemblages of small forested streams in the Bolivian Amazon. Aquatic Living Resources, 20(2): 131-142. https://doi.org/10.1051/alr:2007024.
https://doi.org/10.1051/alr:2007024...
; Corrêa et al., 2009Corrêa, C.E.; Petry, A.C. & Hahn, N.S. 2009. Influência do ciclo hidrológico na dieta e estrutura trófica da ictiofauna do rio Cuiabá, Pantanal Mato-Grossense. Iheringia . Série Zoologia, 99(4): 456-463. https://doi.org/10.1590/S0073-47212009000400018.
https://doi.org/10.1590/S0073-4721200900...
; Brandão-Gonçalves et al., 2010Brandão-Gonçalves, L.; Oliveira, S.A.D. & Lima-Junior, S.E. 2010. Hábitos alimentares da ictiofauna do córrego Franco, Mato Grosso do Sul, Brasil. Biota Neotropica, 10(2): 21-30. https://doi.org/10.1590/S1676-06032010000200001.
https://doi.org/10.1590/S1676-0603201000...
; Sampaio & Goulart, 2011Sampaio, A.L.A. & Goulart, E. 2011. Ciclídeos neotropicais: ecomorfologia trófica. Oecologia Australis, 15(4): 775-798. https://doi.org/10.4257/oeco.2011.1504.03.
https://doi.org/10.4257/oeco.2011.1504.0...
; Prado, 2015Prado, A.V.R. 2015. Ecomorfologia e uso de recursos alimentares: relações inter e intraespecíficas da ictiofauna associada a bancos de macrófitas aquáticas. (Doctoral Thesis). Universidade Estadual de Maringá, Maringá.; Resende et al., 2016Resende, E.K.; Ferreira, L.; Mônaco, I.D.A. & Cruz, L.D.S. 2016. Aspectos bio-ecológicos dos peixes associados à macrófitas aquáticas na Baía Tuiuiú, Rio Paraguai, Pantanal Sul. Corumbá-MS, EMBRAPA Pantanal. (Boletim de Pesquisa e Desenvolvimento INFOTECA-E).; Polaz et al., 2017Polaz, C.N.M.; Ferreira, F.C. & Petrere-Junior, M. 2017. The protected areas system in Brazil as a baseline condition for wetlands management and fish conservancy: the example of the Pantanal National Park. Neotropical ichthyology, 15(3): e170041. https://doi.org/10.1590/1982-0224-20170041.
https://doi.org/10.1590/1982-0224-201700...
; Lopes et al., 2022Lopes, T.M.; Ganassin, M.J.; Oliveira, A.G.D.; Affonso, I.P. & Gomes, L.C. 2022. Feeding strategy of the introduced Astronotus crassipinnis (Cichlidae) in upper Paraná river floodplain. Iheringia. Série Zoologia, 112: e2022001. https://doi.org/10.1590/1678-4766e2022001.
https://doi.org/10.1590/1678-4766e202200...
). The invertivores’ guild was composed by invertivorous, insectivorous, invertivorous/insectivorous and zooplanktophagous species. When the species showed feeding habit alteration depending on river seasonality, it was considered the habit coincident to the period sampled in this study (i.e., dry period).

Data analysis

Assumptions of normality of data distribution were assessed using the Shapiro-Wilk test and the Levene’s test was used to assess the homogeneity of variances. The one-way ANOVA was used to evaluate the difference between guild richness and abundance in the studied areas. A post hoc Tukey’s test was applied to verify multiple comparisons between the sampled areas. When the data did not follow the assumptions Kruskal Wallis’ test and post hoc Dunn’s test were used.

A Principal Component Analysis (PCA) was performed with limnological and environmental variables and to evaluate these variables’ influence on the abundance of trophic guilds the Generalized Linear Models (GLM) analysis was used. The multicollinearity between the predictor variables was verified by the Variance Inflation Factor (VIF) of each variable, excluding those with VIF ≥ 10 (Lin, 2008Lin, F.J. 2008. Solving multicollinearity in the process of fitting regression model using the nested estimate procedure. Quality and Quantity, 42(3): 417-426. https://doi.org/10.1007/s11135-006-9055-1.
https://doi.org/10.1007/s11135-006-9055-...
). Thus, for each model, the values of K, AICc, ΔAICc and weight of evidence were calculated (this measure indicates that the support level (or weight of evidence)), in favour of a given model, is the most parsimonious among the set of candidate models. Statistical analyses were conducted in the software R (R Core Team, 2019R Core Team. 2019. R: A language and environment for statistical computing. Vienna, R Foundation for Statistical Computing. URL https://www.R-project.org.
https://www.R-project.org...
), together with the MuMIn (Bartoń, 2017Bartoń, K. 2017. MuMIn: Multi-model inference . Version 1.40.0. URL https://cran.r-project.org/web/packages/MuMIn/index.html.
https://cran.r-project.org/web/packages/...
), modEvA (Barbosa et al., 2013Barbosa, A. M.; Real, R.; Muñoz, A.R., & Brown, J.A. 2013. New measures for assessing model equilibrium and prediction mismatch in species distribution models. Diversity and Distributions, 19(10): 1333-1338. https://doi.org/10.1111/ddi.12100.
https://doi.org/10.1111/ddi.12100...
) and vegan (Oksanen et al., 2019Oksanen, J.; Guillaume Blanchet, F.; Friendly, M.; Kindt, R.; Legendre, P.; McGlinn, D.; Minchin, R.; O’Hara, R.B.; Simpson, G.L.; Solymos, P.; Stevens, M.H.H.; Szoecs, E. & Wagner, H. 2019. vegan: Community Ecology Package. R package version 2.5-6. https://CRAN.R-project.org/package=vegan.
https://CRAN.R-project.org/package=vegan...
) packages.

RESULTS

The environmental variation between segments

The water temperature in the sampled segments was different (F = 4.59, P < 0.05) and Tukey’s test indicated higher temperature in the plateau (29.1 ± 1.49) than in the plain (25.4 ± 4.88) (P < 0.05) (Fig. 2A). The water transparency variable was statistically different in the sampled segments (P < 0.05) (Fig. 2B). Tukey’s test indicated that the transparency in the plateau was lower than in confluence and plain (P < 0.05).

Figure 2
Limnological characteristics in the sampled segments in the Paraguay River, Mato Grosso, Brazil, from July to November 2017 and August 2018. (A) temperature. (B) water transparency. * indicates statistical difference at the 0.05 significance level.

Among the environmental variables, the altitude was different in the three sampled segments (F = 136.7, P < 0.05) and Tukey’s test indicated higher values in the plateau than in confluence and plain (P < 0.05) (Fig. 3A). The segments have different river width (F = 239.1, P < 0.05), being narrow in the plateau and wider in the plain (P < 0.05) (Fig. 3B). The proportion of dense forest did not vary among the sampled segments (P < 0.05).

Figure 3
The difference of environmental characteristics in the segments sampled in the Paraguay River, from July to November 2017 and August 2018. (A) altitude. (B) river width. * indicates statistical difference at the 0.05 significance level.

The PCA showed that the components explained 63.22% of the data variation, contributing with 41.22% and 22% of the total variance. The variables dissolved oxygen and altitude had major contribution on data variation whereas water conductivity had the minor contribution (Fig. 4A). Altitude was positively related to the Plateau and dissolved oxygen to the Plain. Temperature and proportion of dense forest had a mild contribution on data variation and were positively associated to confluence and plain (Fig. 4B).

Figure 4
Principal Component Analysis (PCA) of the limnological and environmental variables in the segments in the Paraguay River, from July to November 2017 and August 2018. (A) variables contribution. (B) sampled areas ordination.

Trophic guilds distribution along the longitudinal gradient

During the study period, 26,542 individuals distributed in 130 fish species were collected. The sampled species were characterized in six trophic guilds (Table 1 - Supplementary material SUPPLEMENTARY MATERIAL Table 1 List of trophic guilds with species. PLT: plateau CON: confluence e PLA: plain. Voucher Trophic guilds, species Authors PLT % CON % PLA % Length range (cm) Weight range (g) Piscivores 0.85 2.86 2.01 0.9-80.4 0.01-13300 LIPAN052 Acestrorhynchus pantaneiro Menegalezes, 1992 Resende et al. (1996) 0.212 0.371 0.042 8.5-22.1 5.8-202.8 LIPAN265 Ageneiosus ucayalensis Castelnau, 1855 Corrêa et al. (2009) - - 0.010 3.5 0.74 LIPAN272 Ageneiosus valenciennesi Bleeker, 1864 Hahn et al. (2002) - 0.018 0.010 19-27.8 71.5-156.1 LIPAN012 Catathyridium jenynsii (Günther, 1862) Hahn et al. (2002) - 0.009 - 5.6 6.8 LIPAN294 Cichla sp Block & Schneider, 1801 Hahn et al. (2002) - 0.009 - 21 277.95 LIPAN266 Galeocharax humeralis (Valenciennes, 1834) Corrêa et al. (2009) - 0.168 0.366 1.9-4.2 1.18-1.12 LIPAN149 Hemisorubim platyrhynchos (Valenciennes, 1840) Hahn et al. (2002) 0.018 0.415 0.031 16.8-52.1 67.2-2142 LIPAN050 Hoplias malabaricus Bloch, 1794 Resende et al. (2016) 0.460 1.042 0.418 0.9-10.8 0.01-1090 LIPAN267 Pinirampus pirinampu (Spix & Agassiz, 1829) Hahn et al. (2002) - - 0.052 33.1-68.4 22-4390 LIPAN047 Plagioscion ternetzi Boulenger, 1895 Resende et al. (1996) - 0.035 - 18.1-18.5 111.17-118.2 LIPAN117 Potamotrygon falkneri Castex & Maciel, 1963 Lonardoni et al. (2006) 0.071 0.018 - 20.5-53 35.42-1770 LIPAN293 Pseudoplatystoma corruscans (Spix & Agassiz, 1829) Resende et al. (1996) - 0.026 0.105 25.8-108.1 31.10-13300 LIPAN120 Pseudoplatystoma fasciatum (Linnaeus, 1766) Resende et al. (1996) - 0.062 0.052 14.6-77.8 153.6-6410 LIPAN053 Pygocentrus nattereri Kner, 1858 Resende et al. (1996) 0.018 0.238 0.324 10.5-29.5 49.34-860 LIPAN268 Roeboides bonariensis Steindachner, 1879 Resende et al. (2016) 0.018 0.009 0.094 2.5-7 0.19-6.11 LIPAN292 Salminus brasiliensis (Cuvier, 1816) Corrêa et al. 2009 - 0.009 - 50.5 2210 LIPAN051 Serrasalmus maculatus Kner, 1858 Corrêa et al. (2009) 0.018 0.177 0.157 1.2-23.5 0.03-506.3 LIPAN177 Serrasalmus marginatus Valenciennes, 1837 Corrêa et al. (2009) 0.035 0.124 0.167 1.5-19.9 0.06-480 LIPAN125 Serrasalmus spilopleura Kner, 1858 Resende et al. (2016) - 0.035 0.105 9.6-23.7 33-638 LIPAN123 Sorubim lima (Bloch & Schneider, 1801) Hahn et al. (2002) - 0.026 0.031 14.5-47.8 20-1040 Detritivores 11.84 17.34 10.27 1-55 0.01-810 LIPAN133 Apareiodon affinis (Steindachener, 1879) Hahn et al. (2002) 0.478 - 0.209 2.7-9 0.20-12.5 LIPAN073 Curimatella dorsalis (Eigenmann & Eigenmann, 1889) Polaz et al. (2017) 1.026 1.625 0.178 3.1-8.1 0.68-7.1 LIPAN063 Curimatopsis myersi Vari, 1982 Resende et al. (2016); Polaz et al. (2017) 0.035 1.042 0.679 1.2-4 0.03-1.81 LIPAN036 Cyphocharax gillii (Eigenmann & Kennedy, 1903) Resende et al. (2016); Polaz et al. (2017) - 0.088 0.031 2.5-10 0.36-28.59 LIPAN144 Farlowella paraguayensis Retzer & Page, 1997 Polaz et al. (2017) - - 0.021 5-6.2 0.14-0.20 LIPAN151 Hypoptopoma inexspectatum (Holmberg, 1893) Resende et al. (2016) 7.415 11.171 2.759 1.7-7 0.14-7.6 LIPAN273 Hypostomus cochliodon Kner, 1854 Polaz et al. (2017) - - 0.052 1.5-4.9 0.07-3.9 LIPAN056 Hypostomus sp (Lacepède 1803) Polaz et al. (2017) 0.867 0.433 2.822 1-7.5 0.01-7.5 LIPAN269 Liposarcus anisitsi (Eigenmann & Kennedy, 1903) Resende et al. (2016) 0.071 0.052 20.5-34.5 223.9-770 LIPAN274 Loricaria sp Linnaeus, 1758 Polaz et al. (2017) - - 0.010 1.6-8.7 0.09-6.9 LIPAN124 Loricariichthys labialis (Boulenger, 1895) Resende et al. (2016); Polaz et al. (2017) - - 0.010 15 18.4 LIPAN156 Loricariichthys platymetopon Isbrücker & Nijssen, 1979 Resende et al. (2016); Polaz et al. (2017) 0.035 0.035 0.031 7.2-26 0.23-127.5 LIPAN270 Megalancistrus aculeatus (Perugia, 1891) Hahn et al. 2002 0.026 3-4.3 0.85-2.04 LIPAN004 Potamorhina squamoralevis (Braga & Azpelicueta, 1983) Polaz et al. (2017) 0.018 0.062 0.512 1.2-23.7 0.04-369 LIPAN091 Prochilodus lineatus (Valenciennes, 1836) Polaz et al. (2017) 0.274 0.010 8.7-33 19-810 LIPAN069 Psectrogaster curviventris (Eigenmann & Kennedy, 1903) Polaz et al. (2017) 0.194 0.449 4.2-55 1.62-226 LIPAN130 Rineloricaria parva (Boulenger, 1895) Resende et al. (2016); Polaz et al. (2017) 0.301 2.119 1.829 1.6-8.7 0.09-6.9 LIPAN018 Satanoperca papaterra (Heckel, 1840) Sampaio & Goulart 2011 0.053 0.071 0.010 8-16.5 26.8-167 LIPAN178 Spatuloricaria evansii (Boulenger, 1892) Polaz et al. (2017) 0.212 0.018 0.010 3-31 0.11-227.2 LIPAN179 Steindachnerina brevipinna (Boulenger, 1902) Resende et al. (2016) 1.363 0.018 0.439 2.5-10.7 0.24-38.2 LIPAN076 Steindachnerina conspersa (Holmberg, 1891) Resende (2000) 0.053 0.021 3.2-10.5 0.73-41.4 LIPAN116 Sturissoma barbatum (Kner, 1853) Polaz et al. (2017) 0.035 0.044 0.136 5.8-11.4 0.55-71.73 Herbivores 53.91 1.31 2.0 1.3-46.7 0.07-3720 LIPAN132 Abramites hypselonotus (Günther, 1868) Polaz et al. (2017) 0.177 0.044 0.073 1.7-9.3 0.09-21.52 LIPAN005 Mesonauta festivus (Heckel, 1840) Polaz et al. (2017) - 0.088 0.125 2.5-10 0.20-56.1 LIPAN287 Myloplus levis (Eigenmann & McAtee, 1907) Polaz et al. (2017) - 0.044 0.167 7.5-25.2 19.2-270 LIPAN286 Mylossoma paraguayensis (Norman, 1928) Resende et al. (2016) - - 0.010 14.5 102.3 LIPAN071 Mylossoma duriventre (Cuvier, 1818) Polaz et al. (2017) - 0.009 0.659 2.8-22.2 3.9-490 LIPAN161 Otocinclus vittatus Regan, 1904 Polaz et al. (2017) 53.61 0.318 0.042 1.3-5 0.07-0.60 LIPAN074 Piabucus melanostoma Holmberg, 1991 Polaz et al. (2017) 0.088 0.283 0.303 4.9-8.5 0.39-3.9 LIPAN164 Piaractus mesopotamicus (Holmberg, 1887) Polaz et al. (2017) 0.035 0.009 0.366 12-40.8 72.6-2420 LIPAN288 Schizidon borelli (Boulenger, 1900) Polaz et al. (2017) 0.230 0.240 4.6-15.5 0.56-702.1 LIPAN092 Schizidon isognathus Kner, 1858 Polaz et al. (2017) 0.283 0.010 3.2-22.8 0.68-250.2 Invertivores 11.43 34.01 44.06 0.09-9.7 0.01-9.01 LIPAN279 Apistogramma trifasciata (Eigenmann & Kennedy, 1903) Resende et al. (2016) - 0.645 1.850 0.8-4.6 0.01-1.4 LIPAN037 Aphyocharax anisitsi Eigenmann & Kennedy, 1903 Resende et al. (2016) 1.380 0.406 1.087 1.3-3 0.03-7.13 LIPAN289 Aphyocharax paraguayensis Eigenmann, 1915 Resende et al. (2016) - 0.009 0.387 1.5-3 0.03-0.47 LIPAN087 Apistogramma commbrae (Regan, 1906) Polaz et al. (2017) 0.425 0.115 0.136 1.2-3.1 0.04-1.93 LIPAN070 Apteronotus albifrons (Linnaeus, 1766) Polaz et al. (2017) - 0.009 0.261 4.3-19.8 0.34-19.9 LIPAN097 Astronotus crassipinis Heckel, 1840 Polaz et al. (2017) - 0.044 - 16.5-22 125.5-461 LIPAN021 Auchenipterus osteomystax (Miranda Ribeiro, 1918) Hahn et al. (2002) - 0.009 - 3.9 0.91 LIPAN111 Brachyhypopomus sp Mago Leccia, 1994 Resende et al. (2016) 0.088 0.636 2.216 4.7-22.1 0.05-22 LIPAN028 Bryconamericus exodun Eigenmann, 1907 Polaz et al. (2017) 0.088 0.177 0.125 1.3-5.7 0.08-2.76 LIPAN136 Bryconamericus stramineus Eigenmann, 1908 Polaz et al. (2017) 0.991 0.009 0.031 1.2-4.8 0.05-2 LIPAN011 Bujurquina vittata (Heckel, 1840) Resende et al. (2016) 0.053 0.159 0.063 1.1-6.9 0.03-14.5 LIPAN030 Characidium aff. zebra Eigenmann, 1909 Resende et al. (2016) 2.442 2.464 1.725 1.2-4.7 0.02-2.32 LIPAN290 Characidium laterale (Boulenger, 1895) Polaz et al. (2017) - 0.018 0.125 2.2-3.3 0.21-0.53 LIPAN072 Charax leticiae Lucena, 1987 Resende et al. (2016) 0.053 0.062 0.157 4.9-9.4 1.38-15.95 LIPAN112 Crenicichla lepidota Heckel, 1840 Resende et al. (2016) - 0.062 0.021 11-33.1 35-525.6 LIPAN140 Corydoras aeneus (Gill, 1858) Brandão-Gonçalves et al. (2010) 0.230 - - 2-3.5 0.40-1.9 LIPAN027 Corydoras hastatus Eigenmann & Eigenmann, 1888 Polaz et al. (2017) 0.124 0.265 0.084 1.2-2 0.04-0.46 LIPAN055 Crenicichla vittata Heckel, 1840 Resende et al. (2016) 0.319 2.181 2.028 2-16.8 0.12-113.7 LIPAN024 Eigenmania virescens (Valenciennes, 1847) Polaz et al. (2017) - 0.106 5.676 1.7-18.7 0.03-11 LIPAN025 Eigenmania trilineata Lopes & Castello, 1996 Resende et al. (2016) 0.018 5.175 10.996 2.5-31.5 0.03-39.6 LIPAN46 Entomocorus benjamini Eigenmman, 1917 Resende et al. (2016) - 0.026 1.631 2-4.9 0.08-12.9 LIPAN145 Gasteropelecus sternicla (Linnaeus, 1758) Polaz et al. (2017) 0.672 0.212 0.010 2-6 0.23-3.7 LIPAN060 Gymnotus inaequilabiatus (Valenciennes, 1839) Polaz et al. (2017) 0.053 0.168 0.345 4.4-90 0.34-1370 LIPAN107 Gymnotus paraguensis Albert & Crampton, 2003 Polaz et al. (2017) - 0.018 - 9.9-11.7 2.1-3.7 LIPAN022 Hemigrammus ulrey (Boulenger, 1895) Resende et al. (2016) 0.566 6.323 2.530 1.5-3.4 0.06-1.1 LIPAN146 Hemigrammus marginatus Ellis, 1911 Brandão-Gonçalves et al. (2010) 0.035 - - 2.5-2.7 0.27-0.33 LIPAN058 Hemiodontichthys acipenserinus (Kner, 1853) Polaz et al. (2017) - 0.009 - 9 3.26 LIPAN032 Hyphessobrycon eques (Steindachner, 1882) Polaz et al. (2017) 0.672 7.144 6.930 0.9-9.7 0.01-2.2 LIPAN034 Ituglanis herberti (Miranda Ribeiro, 1940) Polaz et al. (2017) 0.177 2.874 1.3-3.2 0.01-12 LIPAN291 Ituglanis eichorniarum (Miranda Ribeiro, 1912) Polaz et al. (2017) 0.009 0.010 1.5-3.4 0.05-0.45 LIPAN081 Ossancora eigenmanni (Boulenger, 1895) Polaz et al. (2017) 0.238 0.146 3.3-6 0.87-5.67 LIPAN045 Pimelodella mucosa Eigenmann & Ward, 1907 Resende et al. (2016); Polaz et al. (2017) 1.274 1.872 1.913 2.7-9 0.35-15.8 LIPAN167 Poptella paraguayensis (Eigenmann, 1907) Polaz et al. (2017) 1.345 3.214 0.157 2-6.9 0.38-8.7 LIPAN061 Potamorrhaphis eigenmanni Miranda Ribeiro, 1915 Ibañez et al. (2007) - - 0.021 11.3-24 15.1-24.88 LIPAN122 Potamotrygon motoro (Müller & Henle, 1841) Lonardoni et al. (2006) - 0.018 - 30.5-50.7 158-278.63 LIPAN105 Pseudotylosurus angusticeps (Günther, 1866) Resende et al. (2016) 0.106 - - 21-26 11.4-21.2 LIPAN029 Pyrhulina australis Eigenmann & Kennedy, 1903 Resende et al. (2016); Polaz et al. (2017) 0.035 0.971 2.216 1.4-4.2 0.06-2.47 LIPAN285 Rhamphicthys hahni (Meinken, 1937) Resende et al. (2016); Polaz et al. (2017) - 0.035 0.178 1.5-44.3 0.02-177.1 LIPAN049 Sternopygus macrurus (Bloch & Schneider, 1801) Resende et al. (2016) 0.018 0.742 1.056 2.4-52.5 0.60-380 LIPAN106 Synbranchus marmoratus Bloch, 1795 Polaz et al. (2017) 0.142 0.026 0.052 4.5-22.9 0.11-10.60 LIPAN019 Tatia neivai (Ihering, 1930) Polaz et al. (2017) - 0.009 - 2.7 0.49 LIPAN065 Tetragonopterus argenteus Cuvier, 1816 Resende et al. (2016) - 0.212 0.314 4.5-11.1 0.73-57.3 LIPAN180 Thoracocharax stellatus (Kner, 1858) Resende et al. (2016); Polaz et al. (2017) 0.142 0.424 0.031 2.9-10.8 0.56-4.7 LIPAN119 Trachelyopterus galeatus (Linnaeus, 1766) Resende et al. (2016) 0.159 0.353 0.199 4.9-16.4 0.70-123 Lepidophagous 0.33 0.28 0.52 2.5-20.5 0.19-6.11 LIPAN176 Roeboides prognathus Boulenger, 1895 Sazima & Machado (1983) 0.336 0.380 0.523 2.5-20.5 0.19-6.11 Omnivores 21.63 43.43 37.66 1-57.8 0.01-2600 LIPAN066 Aequidens plagiozonatus Kullander, 1984 Polaz et al. (2017) - 0.795 0.188 1.1-6.7 0.02-13.1 LIPAN101 Anadoras weddellii (Castelnau, 1855) Resende et al. (2016); Polaz et al. (2017) 0.142 0.751 0.324 2.5-9.5 0.06-19 LIPAN283 Aphyocharax dentatus Eigenmann e Kennedy, 1903 Polaz et al. (2017) 3.840 1.210 0.575 1.5-6.7 0.07-6.4 LIPAN282 Apteronotus caudimaculosus de Santana, 2003 Polaz et al. (2017) - - 0.042 9.4-12.7 2.40-5.93 LIPAN102 Astyanax asuncionensis Géry, 1972 Resende et al. (2016); Polaz et al. (2017) - 0.362 0.251 5-18.3 2.72-12.10 LIPAN001 Brycon hilarii (Valenciennes, 1850) Polaz et al. (2017) - 0.035 0.084 18.7 136.1-630 LIPAN110 Ctenobrycon alleni (Eigenmann & Mcate, 1907 Polaz et al. (2017) - 0.026 0.105 6.5-8.9 5.8-16.9 LIPAN033 Gymnocorimbus ternetzi Boulenger, 1895 Resende et al. (2016) 0.018 0.018 0.220 2-4 0.17-2.2 LIPAN083 Gymnogeophagus balzanii (Perugia, 1891) Resende et al. (2016) 0.142 0.018 0.063 3.3-13.5 1.34-131.9 LIPAN148 Hemiodus orthonops Eigenmann & Kennedy, 1903 Polaz et al. (2017) 0.088 0.124 0.669 3-26.6 0.28-242.3 LIPAN062 Jupiaba acantogaster (Eigenmann, 1911) Polaz et al. (2017) 3.327 0.442 6.345 1.1-3.5 0.01-0.46 LIPAN093 Laetacara dorsigera (Heckel, 1840) Polaz et al. (2017) 0.035 0.026 0.084 2-7.3 0.37-20.1 LIPAN100 Leporinus friderici (Bloch, 1794) Resende et al. (2016); Polaz et al. (2017) 0.035 0.424 0.491 3.5-25.5 0.48-380 LIPAN020 Leporinus lacustris Campos, 1945 Polaz et al. (2017) - 0.185 0.178 3.5-25.5 0.79-244 LIPAN154 Leporinus macrocephalus Garavello & Britski, 1988 Resende et al. (2016); Polaz et al. (2017) 0.088 0.009 0.010 20-48 210-2600 LIPAN155 Leporinus striatus Kner, 1858 Resende et al. (2016); Polaz et al. (2017) 0.018 0.318 0.376 1.7-7.4 0.12-7.6 LIPAN157 Moenkhausia dichroura (Kner, 1858) Polaz et al. (2017) 4.229 12.390 12.899 1-4.6 0.01-2.33 LIPAN035 Moenkhausia sanctaefilomenae (Steindachner, 1907) Resende et al. (2016) 1.522 2.746 2.080 1.6-5.7 0.07-5.3 LIPAN280 Odontostilbe calliura (Boulenger, 1900) Resende et al. (2016) 5.256 5.961 4.097 1-3 0.01-4.81 LIPAN281 Odontostilbe pequira (Steindachner, 1882) Polaz et al. (2017) 2.265 14.509 6.042 1-4.1 0.01-0.86 LIPAN075 Oxydoras kneri Bleeker, 1862 Polaz et al. (2017) - 0.026 0.021 31-38.8 458.9-980 LIPAN284 Pimelodella gracilis (Valenciennes, 1835) Resende et al. (2016) - - 0.010 8.1 7.7 LIPAN65 Pimelodus argenteus Perugia, 1891 Resende et al. (2016); Polaz et al. (2017) - 0.009 - 17 100.68 LIPAN042 Pimelodus maculatus Lacépède, 1803 Lolis & Andrian (1996) 0.406 0.220 3.9-31.3 0.5-550 LIPAN166 Pimelodus ornatus Kner, 1858 Hahn et al. (2002) 0.018 0.009 0.031 22-39.3 180-1810 LIPAN275 Platydoras armatulus (Valenciennes, 1840) Polaz et al. (2017) - 0.088 0.073 4-6.1 2.1-7.36 LIPAN026 Prionobrama paraguayensis (Eigenmann, 1914) Polaz et al. (2017) 0.248 0.627 0.042 2.2-3.9 0.13-1.78 LIPAN006 Psellogrammus kennedyi (Eigenmann, 1903) Polaz et al. (2017) 0.053 0.565 0.941 1.4-5.8 0.05-2.46 LIPAN276 Pterodoras granulosus (Valenciennes, 1821) Hahn et al. 2002 - - 0.021 28-57.8 5.3-22.8 LIPAN278 Rhamdia aff. quelen (Quoy & Gaimard, 1824) Polaz et al. (2017) - 0.026 0.010 2.3-23.5 0.12-241.5 LIPAN277 Trachelyopterus coriaceus Valenciennes, 1840 Polaz et al. (2017) - - 0.073 5.5-12.5 5.9-65.5 LIPAN064 Trachydoras paraguayensis (Eigenmann & Ward, 1907) Resende et al. (2016) 0.283 0.450 0.470 3.7-7.6 1.4-28.4 LIPAN003 Triportheus paranensis (Günther, 1874) Resende et al. (2016) 0.018 0.874 0.627 3.5-20.5 2.5-172.4 Total 100 100 100 ). The richest guilds were invertivores (44 species), omnivores (33 species), piscivores (20 species), detritivores (22 species) and herbivores (10). The lepidophagous guild had only one species (Roeboides prognathus Boulenger, 1895).

The piscivores made up the group with the largest mean standard length and mean weight among the sampled segments (Fig. 5). Species such as Pseudoplatystoma corruscans (Spix & Agassiz, 1829), P. fasciatum (Linnaeus, 1766) and Salmimus brasiliensis (Cuvier, 1816), contributed to the longest standard length (over 100 cm) and consequently the biggest weight of these guilds.

Figure 5
Trophic guilds mean weight and mean length in the sampled segments in the Paraguay River, from July to November 2017 and August 2018. PIS = piscivores; LEP = lepidophagous; DET = detritivores; INV = invertivorous; OMN = omnivores and HER = herbivores.

The occurrence of the species Piaractus mesopotamicus (Holmberg, 1887) (herbivores) and Brycon hilarii (Valenciennes, 1850) (omnivores), which reach approximately 50 cm SL, were relevant for the larger length and average weight of the respective guilds. However, the invertivores and lepidophagous trophic guilds had the lowest mean weight and mean length, since these guilds are mainly represented by small species that live associated with aquatic macrophyte beds. Small fish species composed around 90% of the sample collected in confluence and plain segments and 96% in the plateau.

The Kruskal-Wallis test showed that the abundance of fish was distributed differently between the trophic guilds (X² = 76.26, P < 0.05). Omnivorous and invertivorous fish abundance was higher than the other guilds (9,670 and 9,228 individuals respectively). The number of guilds per sampled site varied from four to six in plateau, the mean number of collected trophic guilds was 5.66 (± 0.81) while in the confluence and plain the mean number of trophic guilds was slightly higher (6 ± 0 both).

Along the longitudinal gradient, one-way ANOVA indicated that some trophic guilds are distributed differently. Among these, the herbivores guild (F = 6.17, P < 0.05), which abundance in the plateau is greater than in confluence and in the plain (P < 0.05) (Fig. 6A). Piscivores, in turn, are more abundant in confluence (F = 5.54, P < 0.05) (Fig. 6B), differing from the plateau (P < 0.05), but with similar abundance to the plain. Invertivorous species occurred in greater abundance in the plain compared to the plateau (X² = 6.75, P < 0.05), although not showing a difference with the confluence (P > 0.05) (Fig. 6C). Albeit we hypothesised that some trophic guilds would be more abundant in plain regions due to the food availability, detritivores, lepidophagous and omnivores trophic guilds showed no significant difference between the sampled segments, which might indicate food availability for these guilds along all the river segments.

Figure 6
Difference between trophic guilds in the sampled segments in Paraguay River. (A) herbivores. (B) piscivores. (C) invertivores. * indicates statistical difference at the 0.05 significance level.

According to AICc values, the environmental variable that best explains the abundance of piscivorous fish in the segments sampled in the Paraguay River was water transparency (ΔAICc = 0.00, weight = 0.71). For herbivores, the model that explained the variation in abundance was composed by temperature, altitude, and proportion of dense forest (ΔAICc = 0.00, weight = 0.85). The variable altitude best represented the abundance of invertivorous (ΔAICc = 0.00, weight = 0.78) along the longitudinal gradient (Table 2).

Table 2
AICc models ranking of predictor variables with the abundance of trophic guilds sampled in the Paraguay River. Predictor variables were dissolved oxygen (mg/L), temperature (℃), transparency (cm), altitude (m), river width (m) and proportion of dense forest.

All variables showed a positive relationship with the abundance of the guilds except for dense forest proportion. Even though this variable did not show significant difference among the sampled segments, the plain region is less forested due to the floodplain characteristics, with strong effect of Taiamã Ecological Station, showing a negative relation with invertivorous fish abundance, since this guild was more abundant in the plain region.

DISCUSSION

As this research was conducted from the plateau to the plain region, we expected to find morphophysiological environmental differences, which could result in different trophic guilds along the river corridor. Our results indicate that there is a clear trophic guilds distribution pattern along the longitudinal gradient in Paraguay River, where the guilds were more abundant. Herbivores occur mostly in the plateau region, piscivores in confluence and invertivores in the plain region. Thus, such as happens in species sorting (Leibold et al., 2004Leibold, M.A.; Holyoak, M.; Mouquet, N.; Amarasekare, J.M.; Chase, J.M.; Hoopes, M.F.; Holdt, R.D.; Shurin, J.B.; Law, D.; Tilman, D.; Loreau, M. & Gonzalez, A. 2004. The metacommunity concept: A framework for multi-scale community ecology. Ecology Letters, 7(7): 601-613. https://doi.org/10.1111/j.1461-0248.2004.00608.x.
https://doi.org/10.1111/j.1461-0248.2004...
), trophic guilds are also filtered by interactions and environmental characteristics in each locality in a gradient.

For instance, we found that in the plateau the water transparency is lower than in the plain, which corroborate the higher abundance of piscivorous fish in the confluence and plain that needs more visual acuity to find the prey. Turbid environments are characterized as reducers of visual efficiency, due to light dispersion by the presence of suspension particles in the water column (Utne-Palm, 2002Utne-Palm, A.C. 2002. Visual feeding of fish in a turbid environment: Physical and behavioural aspects. Marine and Freshwater Behaviour and Physiology, 35(1-2): 111-128. https://doi.org/10.1080/10236240290025644.
https://doi.org/10.1080/1023624029002564...
). This fact demonstrated that foraging by piscivores on prey is highest under clearwater conditions, whilst in turbid water, it is greatly reduced (Shoup & Wahl, 2009Shoup, D.E. & Wahl, D.H. 2009. The Effects of Turbidity on Prey Selection by Piscivorous Largemouth Bass. Transactions of the American Fisheries Society, 138(5): 1018-1027. https://doi.org/10.1577/t09-015.1.
https://doi.org/10.1577/t09-015.1...
).

Predator fishes presented a bigger standard length and heavier weight mean than other trophic guilds, and the fish size also influences its visual acuity, wherein bigger eyes imply an image size increasing facilitating the predation (Blaxter, 1980Blaxter, J.H.S. 1980. Vision and the feeding of fishes. In: Bardarch, J.E.; Magnusson, J.J.; May, R.C. & Reinhart, J.M. Fish behaviour and its use in the capture and culture of fishes. Manila, ICLARM.). Moreover, pelagic predators, such as the piscivores, are capable to swallow the entire prey due to higher body mass proportion (Nakazawa et al., 2013Nakazawa, T.; Ohba, S.Y. & Ushio, M. 2013. Predator-prey body size relationships when predators can consume prey larger than themselves. Biology Letters, 9: 1-5. https://doi.org/10.1098/rsbl.2012.1193.
https://doi.org/10.1098/rsbl.2012.1193...
), corroborating to the fact that lower percentage between big fish (bigger than 10 cm SL) and small fish (smaller than 10 cm SL) was found in the confluence and the plain, indicating a prey relaxation in the plateau due to fewer piscivores (as shown in Table 1 - Supplementary material SUPPLEMENTARY MATERIAL Table 1 List of trophic guilds with species. PLT: plateau CON: confluence e PLA: plain. Voucher Trophic guilds, species Authors PLT % CON % PLA % Length range (cm) Weight range (g) Piscivores 0.85 2.86 2.01 0.9-80.4 0.01-13300 LIPAN052 Acestrorhynchus pantaneiro Menegalezes, 1992 Resende et al. (1996) 0.212 0.371 0.042 8.5-22.1 5.8-202.8 LIPAN265 Ageneiosus ucayalensis Castelnau, 1855 Corrêa et al. (2009) - - 0.010 3.5 0.74 LIPAN272 Ageneiosus valenciennesi Bleeker, 1864 Hahn et al. (2002) - 0.018 0.010 19-27.8 71.5-156.1 LIPAN012 Catathyridium jenynsii (Günther, 1862) Hahn et al. (2002) - 0.009 - 5.6 6.8 LIPAN294 Cichla sp Block & Schneider, 1801 Hahn et al. (2002) - 0.009 - 21 277.95 LIPAN266 Galeocharax humeralis (Valenciennes, 1834) Corrêa et al. (2009) - 0.168 0.366 1.9-4.2 1.18-1.12 LIPAN149 Hemisorubim platyrhynchos (Valenciennes, 1840) Hahn et al. (2002) 0.018 0.415 0.031 16.8-52.1 67.2-2142 LIPAN050 Hoplias malabaricus Bloch, 1794 Resende et al. (2016) 0.460 1.042 0.418 0.9-10.8 0.01-1090 LIPAN267 Pinirampus pirinampu (Spix & Agassiz, 1829) Hahn et al. (2002) - - 0.052 33.1-68.4 22-4390 LIPAN047 Plagioscion ternetzi Boulenger, 1895 Resende et al. (1996) - 0.035 - 18.1-18.5 111.17-118.2 LIPAN117 Potamotrygon falkneri Castex & Maciel, 1963 Lonardoni et al. (2006) 0.071 0.018 - 20.5-53 35.42-1770 LIPAN293 Pseudoplatystoma corruscans (Spix & Agassiz, 1829) Resende et al. (1996) - 0.026 0.105 25.8-108.1 31.10-13300 LIPAN120 Pseudoplatystoma fasciatum (Linnaeus, 1766) Resende et al. (1996) - 0.062 0.052 14.6-77.8 153.6-6410 LIPAN053 Pygocentrus nattereri Kner, 1858 Resende et al. (1996) 0.018 0.238 0.324 10.5-29.5 49.34-860 LIPAN268 Roeboides bonariensis Steindachner, 1879 Resende et al. (2016) 0.018 0.009 0.094 2.5-7 0.19-6.11 LIPAN292 Salminus brasiliensis (Cuvier, 1816) Corrêa et al. 2009 - 0.009 - 50.5 2210 LIPAN051 Serrasalmus maculatus Kner, 1858 Corrêa et al. (2009) 0.018 0.177 0.157 1.2-23.5 0.03-506.3 LIPAN177 Serrasalmus marginatus Valenciennes, 1837 Corrêa et al. (2009) 0.035 0.124 0.167 1.5-19.9 0.06-480 LIPAN125 Serrasalmus spilopleura Kner, 1858 Resende et al. (2016) - 0.035 0.105 9.6-23.7 33-638 LIPAN123 Sorubim lima (Bloch & Schneider, 1801) Hahn et al. (2002) - 0.026 0.031 14.5-47.8 20-1040 Detritivores 11.84 17.34 10.27 1-55 0.01-810 LIPAN133 Apareiodon affinis (Steindachener, 1879) Hahn et al. (2002) 0.478 - 0.209 2.7-9 0.20-12.5 LIPAN073 Curimatella dorsalis (Eigenmann & Eigenmann, 1889) Polaz et al. (2017) 1.026 1.625 0.178 3.1-8.1 0.68-7.1 LIPAN063 Curimatopsis myersi Vari, 1982 Resende et al. (2016); Polaz et al. (2017) 0.035 1.042 0.679 1.2-4 0.03-1.81 LIPAN036 Cyphocharax gillii (Eigenmann & Kennedy, 1903) Resende et al. (2016); Polaz et al. (2017) - 0.088 0.031 2.5-10 0.36-28.59 LIPAN144 Farlowella paraguayensis Retzer & Page, 1997 Polaz et al. (2017) - - 0.021 5-6.2 0.14-0.20 LIPAN151 Hypoptopoma inexspectatum (Holmberg, 1893) Resende et al. (2016) 7.415 11.171 2.759 1.7-7 0.14-7.6 LIPAN273 Hypostomus cochliodon Kner, 1854 Polaz et al. (2017) - - 0.052 1.5-4.9 0.07-3.9 LIPAN056 Hypostomus sp (Lacepède 1803) Polaz et al. (2017) 0.867 0.433 2.822 1-7.5 0.01-7.5 LIPAN269 Liposarcus anisitsi (Eigenmann & Kennedy, 1903) Resende et al. (2016) 0.071 0.052 20.5-34.5 223.9-770 LIPAN274 Loricaria sp Linnaeus, 1758 Polaz et al. (2017) - - 0.010 1.6-8.7 0.09-6.9 LIPAN124 Loricariichthys labialis (Boulenger, 1895) Resende et al. (2016); Polaz et al. (2017) - - 0.010 15 18.4 LIPAN156 Loricariichthys platymetopon Isbrücker & Nijssen, 1979 Resende et al. (2016); Polaz et al. (2017) 0.035 0.035 0.031 7.2-26 0.23-127.5 LIPAN270 Megalancistrus aculeatus (Perugia, 1891) Hahn et al. 2002 0.026 3-4.3 0.85-2.04 LIPAN004 Potamorhina squamoralevis (Braga & Azpelicueta, 1983) Polaz et al. (2017) 0.018 0.062 0.512 1.2-23.7 0.04-369 LIPAN091 Prochilodus lineatus (Valenciennes, 1836) Polaz et al. (2017) 0.274 0.010 8.7-33 19-810 LIPAN069 Psectrogaster curviventris (Eigenmann & Kennedy, 1903) Polaz et al. (2017) 0.194 0.449 4.2-55 1.62-226 LIPAN130 Rineloricaria parva (Boulenger, 1895) Resende et al. (2016); Polaz et al. (2017) 0.301 2.119 1.829 1.6-8.7 0.09-6.9 LIPAN018 Satanoperca papaterra (Heckel, 1840) Sampaio & Goulart 2011 0.053 0.071 0.010 8-16.5 26.8-167 LIPAN178 Spatuloricaria evansii (Boulenger, 1892) Polaz et al. (2017) 0.212 0.018 0.010 3-31 0.11-227.2 LIPAN179 Steindachnerina brevipinna (Boulenger, 1902) Resende et al. (2016) 1.363 0.018 0.439 2.5-10.7 0.24-38.2 LIPAN076 Steindachnerina conspersa (Holmberg, 1891) Resende (2000) 0.053 0.021 3.2-10.5 0.73-41.4 LIPAN116 Sturissoma barbatum (Kner, 1853) Polaz et al. (2017) 0.035 0.044 0.136 5.8-11.4 0.55-71.73 Herbivores 53.91 1.31 2.0 1.3-46.7 0.07-3720 LIPAN132 Abramites hypselonotus (Günther, 1868) Polaz et al. (2017) 0.177 0.044 0.073 1.7-9.3 0.09-21.52 LIPAN005 Mesonauta festivus (Heckel, 1840) Polaz et al. (2017) - 0.088 0.125 2.5-10 0.20-56.1 LIPAN287 Myloplus levis (Eigenmann & McAtee, 1907) Polaz et al. (2017) - 0.044 0.167 7.5-25.2 19.2-270 LIPAN286 Mylossoma paraguayensis (Norman, 1928) Resende et al. (2016) - - 0.010 14.5 102.3 LIPAN071 Mylossoma duriventre (Cuvier, 1818) Polaz et al. (2017) - 0.009 0.659 2.8-22.2 3.9-490 LIPAN161 Otocinclus vittatus Regan, 1904 Polaz et al. (2017) 53.61 0.318 0.042 1.3-5 0.07-0.60 LIPAN074 Piabucus melanostoma Holmberg, 1991 Polaz et al. (2017) 0.088 0.283 0.303 4.9-8.5 0.39-3.9 LIPAN164 Piaractus mesopotamicus (Holmberg, 1887) Polaz et al. (2017) 0.035 0.009 0.366 12-40.8 72.6-2420 LIPAN288 Schizidon borelli (Boulenger, 1900) Polaz et al. (2017) 0.230 0.240 4.6-15.5 0.56-702.1 LIPAN092 Schizidon isognathus Kner, 1858 Polaz et al. (2017) 0.283 0.010 3.2-22.8 0.68-250.2 Invertivores 11.43 34.01 44.06 0.09-9.7 0.01-9.01 LIPAN279 Apistogramma trifasciata (Eigenmann & Kennedy, 1903) Resende et al. (2016) - 0.645 1.850 0.8-4.6 0.01-1.4 LIPAN037 Aphyocharax anisitsi Eigenmann & Kennedy, 1903 Resende et al. (2016) 1.380 0.406 1.087 1.3-3 0.03-7.13 LIPAN289 Aphyocharax paraguayensis Eigenmann, 1915 Resende et al. (2016) - 0.009 0.387 1.5-3 0.03-0.47 LIPAN087 Apistogramma commbrae (Regan, 1906) Polaz et al. (2017) 0.425 0.115 0.136 1.2-3.1 0.04-1.93 LIPAN070 Apteronotus albifrons (Linnaeus, 1766) Polaz et al. (2017) - 0.009 0.261 4.3-19.8 0.34-19.9 LIPAN097 Astronotus crassipinis Heckel, 1840 Polaz et al. (2017) - 0.044 - 16.5-22 125.5-461 LIPAN021 Auchenipterus osteomystax (Miranda Ribeiro, 1918) Hahn et al. (2002) - 0.009 - 3.9 0.91 LIPAN111 Brachyhypopomus sp Mago Leccia, 1994 Resende et al. (2016) 0.088 0.636 2.216 4.7-22.1 0.05-22 LIPAN028 Bryconamericus exodun Eigenmann, 1907 Polaz et al. (2017) 0.088 0.177 0.125 1.3-5.7 0.08-2.76 LIPAN136 Bryconamericus stramineus Eigenmann, 1908 Polaz et al. (2017) 0.991 0.009 0.031 1.2-4.8 0.05-2 LIPAN011 Bujurquina vittata (Heckel, 1840) Resende et al. (2016) 0.053 0.159 0.063 1.1-6.9 0.03-14.5 LIPAN030 Characidium aff. zebra Eigenmann, 1909 Resende et al. (2016) 2.442 2.464 1.725 1.2-4.7 0.02-2.32 LIPAN290 Characidium laterale (Boulenger, 1895) Polaz et al. (2017) - 0.018 0.125 2.2-3.3 0.21-0.53 LIPAN072 Charax leticiae Lucena, 1987 Resende et al. (2016) 0.053 0.062 0.157 4.9-9.4 1.38-15.95 LIPAN112 Crenicichla lepidota Heckel, 1840 Resende et al. (2016) - 0.062 0.021 11-33.1 35-525.6 LIPAN140 Corydoras aeneus (Gill, 1858) Brandão-Gonçalves et al. (2010) 0.230 - - 2-3.5 0.40-1.9 LIPAN027 Corydoras hastatus Eigenmann & Eigenmann, 1888 Polaz et al. (2017) 0.124 0.265 0.084 1.2-2 0.04-0.46 LIPAN055 Crenicichla vittata Heckel, 1840 Resende et al. (2016) 0.319 2.181 2.028 2-16.8 0.12-113.7 LIPAN024 Eigenmania virescens (Valenciennes, 1847) Polaz et al. (2017) - 0.106 5.676 1.7-18.7 0.03-11 LIPAN025 Eigenmania trilineata Lopes & Castello, 1996 Resende et al. (2016) 0.018 5.175 10.996 2.5-31.5 0.03-39.6 LIPAN46 Entomocorus benjamini Eigenmman, 1917 Resende et al. (2016) - 0.026 1.631 2-4.9 0.08-12.9 LIPAN145 Gasteropelecus sternicla (Linnaeus, 1758) Polaz et al. (2017) 0.672 0.212 0.010 2-6 0.23-3.7 LIPAN060 Gymnotus inaequilabiatus (Valenciennes, 1839) Polaz et al. (2017) 0.053 0.168 0.345 4.4-90 0.34-1370 LIPAN107 Gymnotus paraguensis Albert & Crampton, 2003 Polaz et al. (2017) - 0.018 - 9.9-11.7 2.1-3.7 LIPAN022 Hemigrammus ulrey (Boulenger, 1895) Resende et al. (2016) 0.566 6.323 2.530 1.5-3.4 0.06-1.1 LIPAN146 Hemigrammus marginatus Ellis, 1911 Brandão-Gonçalves et al. (2010) 0.035 - - 2.5-2.7 0.27-0.33 LIPAN058 Hemiodontichthys acipenserinus (Kner, 1853) Polaz et al. (2017) - 0.009 - 9 3.26 LIPAN032 Hyphessobrycon eques (Steindachner, 1882) Polaz et al. (2017) 0.672 7.144 6.930 0.9-9.7 0.01-2.2 LIPAN034 Ituglanis herberti (Miranda Ribeiro, 1940) Polaz et al. (2017) 0.177 2.874 1.3-3.2 0.01-12 LIPAN291 Ituglanis eichorniarum (Miranda Ribeiro, 1912) Polaz et al. (2017) 0.009 0.010 1.5-3.4 0.05-0.45 LIPAN081 Ossancora eigenmanni (Boulenger, 1895) Polaz et al. (2017) 0.238 0.146 3.3-6 0.87-5.67 LIPAN045 Pimelodella mucosa Eigenmann & Ward, 1907 Resende et al. (2016); Polaz et al. (2017) 1.274 1.872 1.913 2.7-9 0.35-15.8 LIPAN167 Poptella paraguayensis (Eigenmann, 1907) Polaz et al. (2017) 1.345 3.214 0.157 2-6.9 0.38-8.7 LIPAN061 Potamorrhaphis eigenmanni Miranda Ribeiro, 1915 Ibañez et al. (2007) - - 0.021 11.3-24 15.1-24.88 LIPAN122 Potamotrygon motoro (Müller & Henle, 1841) Lonardoni et al. (2006) - 0.018 - 30.5-50.7 158-278.63 LIPAN105 Pseudotylosurus angusticeps (Günther, 1866) Resende et al. (2016) 0.106 - - 21-26 11.4-21.2 LIPAN029 Pyrhulina australis Eigenmann & Kennedy, 1903 Resende et al. (2016); Polaz et al. (2017) 0.035 0.971 2.216 1.4-4.2 0.06-2.47 LIPAN285 Rhamphicthys hahni (Meinken, 1937) Resende et al. (2016); Polaz et al. (2017) - 0.035 0.178 1.5-44.3 0.02-177.1 LIPAN049 Sternopygus macrurus (Bloch & Schneider, 1801) Resende et al. (2016) 0.018 0.742 1.056 2.4-52.5 0.60-380 LIPAN106 Synbranchus marmoratus Bloch, 1795 Polaz et al. (2017) 0.142 0.026 0.052 4.5-22.9 0.11-10.60 LIPAN019 Tatia neivai (Ihering, 1930) Polaz et al. (2017) - 0.009 - 2.7 0.49 LIPAN065 Tetragonopterus argenteus Cuvier, 1816 Resende et al. (2016) - 0.212 0.314 4.5-11.1 0.73-57.3 LIPAN180 Thoracocharax stellatus (Kner, 1858) Resende et al. (2016); Polaz et al. (2017) 0.142 0.424 0.031 2.9-10.8 0.56-4.7 LIPAN119 Trachelyopterus galeatus (Linnaeus, 1766) Resende et al. (2016) 0.159 0.353 0.199 4.9-16.4 0.70-123 Lepidophagous 0.33 0.28 0.52 2.5-20.5 0.19-6.11 LIPAN176 Roeboides prognathus Boulenger, 1895 Sazima & Machado (1983) 0.336 0.380 0.523 2.5-20.5 0.19-6.11 Omnivores 21.63 43.43 37.66 1-57.8 0.01-2600 LIPAN066 Aequidens plagiozonatus Kullander, 1984 Polaz et al. (2017) - 0.795 0.188 1.1-6.7 0.02-13.1 LIPAN101 Anadoras weddellii (Castelnau, 1855) Resende et al. (2016); Polaz et al. (2017) 0.142 0.751 0.324 2.5-9.5 0.06-19 LIPAN283 Aphyocharax dentatus Eigenmann e Kennedy, 1903 Polaz et al. (2017) 3.840 1.210 0.575 1.5-6.7 0.07-6.4 LIPAN282 Apteronotus caudimaculosus de Santana, 2003 Polaz et al. (2017) - - 0.042 9.4-12.7 2.40-5.93 LIPAN102 Astyanax asuncionensis Géry, 1972 Resende et al. (2016); Polaz et al. (2017) - 0.362 0.251 5-18.3 2.72-12.10 LIPAN001 Brycon hilarii (Valenciennes, 1850) Polaz et al. (2017) - 0.035 0.084 18.7 136.1-630 LIPAN110 Ctenobrycon alleni (Eigenmann & Mcate, 1907 Polaz et al. (2017) - 0.026 0.105 6.5-8.9 5.8-16.9 LIPAN033 Gymnocorimbus ternetzi Boulenger, 1895 Resende et al. (2016) 0.018 0.018 0.220 2-4 0.17-2.2 LIPAN083 Gymnogeophagus balzanii (Perugia, 1891) Resende et al. (2016) 0.142 0.018 0.063 3.3-13.5 1.34-131.9 LIPAN148 Hemiodus orthonops Eigenmann & Kennedy, 1903 Polaz et al. (2017) 0.088 0.124 0.669 3-26.6 0.28-242.3 LIPAN062 Jupiaba acantogaster (Eigenmann, 1911) Polaz et al. (2017) 3.327 0.442 6.345 1.1-3.5 0.01-0.46 LIPAN093 Laetacara dorsigera (Heckel, 1840) Polaz et al. (2017) 0.035 0.026 0.084 2-7.3 0.37-20.1 LIPAN100 Leporinus friderici (Bloch, 1794) Resende et al. (2016); Polaz et al. (2017) 0.035 0.424 0.491 3.5-25.5 0.48-380 LIPAN020 Leporinus lacustris Campos, 1945 Polaz et al. (2017) - 0.185 0.178 3.5-25.5 0.79-244 LIPAN154 Leporinus macrocephalus Garavello & Britski, 1988 Resende et al. (2016); Polaz et al. (2017) 0.088 0.009 0.010 20-48 210-2600 LIPAN155 Leporinus striatus Kner, 1858 Resende et al. (2016); Polaz et al. (2017) 0.018 0.318 0.376 1.7-7.4 0.12-7.6 LIPAN157 Moenkhausia dichroura (Kner, 1858) Polaz et al. (2017) 4.229 12.390 12.899 1-4.6 0.01-2.33 LIPAN035 Moenkhausia sanctaefilomenae (Steindachner, 1907) Resende et al. (2016) 1.522 2.746 2.080 1.6-5.7 0.07-5.3 LIPAN280 Odontostilbe calliura (Boulenger, 1900) Resende et al. (2016) 5.256 5.961 4.097 1-3 0.01-4.81 LIPAN281 Odontostilbe pequira (Steindachner, 1882) Polaz et al. (2017) 2.265 14.509 6.042 1-4.1 0.01-0.86 LIPAN075 Oxydoras kneri Bleeker, 1862 Polaz et al. (2017) - 0.026 0.021 31-38.8 458.9-980 LIPAN284 Pimelodella gracilis (Valenciennes, 1835) Resende et al. (2016) - - 0.010 8.1 7.7 LIPAN65 Pimelodus argenteus Perugia, 1891 Resende et al. (2016); Polaz et al. (2017) - 0.009 - 17 100.68 LIPAN042 Pimelodus maculatus Lacépède, 1803 Lolis & Andrian (1996) 0.406 0.220 3.9-31.3 0.5-550 LIPAN166 Pimelodus ornatus Kner, 1858 Hahn et al. (2002) 0.018 0.009 0.031 22-39.3 180-1810 LIPAN275 Platydoras armatulus (Valenciennes, 1840) Polaz et al. (2017) - 0.088 0.073 4-6.1 2.1-7.36 LIPAN026 Prionobrama paraguayensis (Eigenmann, 1914) Polaz et al. (2017) 0.248 0.627 0.042 2.2-3.9 0.13-1.78 LIPAN006 Psellogrammus kennedyi (Eigenmann, 1903) Polaz et al. (2017) 0.053 0.565 0.941 1.4-5.8 0.05-2.46 LIPAN276 Pterodoras granulosus (Valenciennes, 1821) Hahn et al. 2002 - - 0.021 28-57.8 5.3-22.8 LIPAN278 Rhamdia aff. quelen (Quoy & Gaimard, 1824) Polaz et al. (2017) - 0.026 0.010 2.3-23.5 0.12-241.5 LIPAN277 Trachelyopterus coriaceus Valenciennes, 1840 Polaz et al. (2017) - - 0.073 5.5-12.5 5.9-65.5 LIPAN064 Trachydoras paraguayensis (Eigenmann & Ward, 1907) Resende et al. (2016) 0.283 0.450 0.470 3.7-7.6 1.4-28.4 LIPAN003 Triportheus paranensis (Günther, 1874) Resende et al. (2016) 0.018 0.874 0.627 3.5-20.5 2.5-172.4 Total 100 100 100 ).

Piscivores naturally require more energy quantity compared to herbivores and omnivores despite the protein catabolism (Halver, 1972Halver, J.E. 1972. Fish nutrition. New York, Academic Press.) and are mostly present in plain, where the high environmental heterogeneity increase the chance to find more prey diversity (Winemiller et al., 2000Winemiller, K.O.; Tarim, S.; Shormann, D. & Cotner, J. B. 2000. Fish Assemblage Structure in Relation to Environmental Variation among Brazos River Oxbow Lakes. Transactions of the American Fisheries Society, 129(2): 451-468. https://doi.org/10.1577/1548-8659(2000)129<0451:fasirt>2.0.co;2.
https://doi.org/10.1577/1548-8659(2000)1...
). The high abundance of piscivores in this segment happen once the connectivity between the main channel and its tributaries (Sepotuba, Cabaça and Jauru rivers) and bays (e.g., Caiçara bay) creates refuge habitats, such as oxbow lakes and lateral bays, promoting the fish viability and diversity (Shao et al., 2019Shao, X.; Fang, Y.; Jawitz, J.W.; Yan, J. & Cui, B. 2019. River network connectivity and fish diversity. Science of the Total Environment, 689: 21-30. https://doi.org/10.1016/j.scitotenv.2019.06.340.
https://doi.org/10.1016/j.scitotenv.2019...
).

The morphological characteristics of the Paraguay River change from straight in the plateau to meandric next to the confluence with other rivers (Cabaçal and Sepotuba rivers) (Silva et al., 2008Silva, A.; Souza Filho, E. & Cunha, S.B. 2008. Padrões de canal do rio Paraguai na região de Cáceres (MT). Revista Brasileira de Geociências, 38(1): 167-177. https://doi.org/10.25249/0375-7536.2008381167177.
https://doi.org/10.25249/0375-7536.20083...
). This morphologic change is also seen in the river shore vegetation diversity, which increases in the downstream reaches (Naiman & Décamps, 1997Naiman, R.J. & Décamps, H. 1997. The ecology of interfaces: Riparian zones. Annual Review of Ecology and Systematics, 28(102): 621-658. https://doi.org/10.1146/annurev.ecolsys.28.1.621.
https://doi.org/10.1146/annurev.ecolsys....
; Ward et al., 2002Ward, J.V.; Tockner, K.; Arscott, D.B.; & Claret, C. 2002. Riverine landscape diversity. Freshwater Biology, 47(4): 517-539. https://doi.org/10.1046/j.1365-2427.2002.00893.x.
https://doi.org/10.1046/j.1365-2427.2002...
). The vegetation along the shore results in lower water temperature (Leach et al., 2012Leach, J.A.; Moore, R.D.; Hinch, S.G. & Gomi, T. 2012. Estimation of forest harvesting-induced stream temperature changes and bioenergetic consequences for cutthroat trout in a coastal stream in British Columbia, Canada. Aquatic Sciences, 74(3): 427-441. https://doi.org/10.1007/s00027-011-0238-z.
https://doi.org/10.1007/s00027-011-0238-...
), affecting the fish composition.

The higher temperature observed in the plateau was considered a predictor variable for herbivorous fish. However, we consider this result due to the period of collection, where the plain region indicated a lower temperature than the plateau. Moving through a longitudinal gradient from mid-reaches to downstream, the wider channels separate the riparian vegetation, increasing the sunlight incidence, consequently, resulting in higher temperature and algae productivity (Power & Dietrich, 2002Power, M.E. & Dietrich, W.E. 2002. Food webs in river networks. Ecological Research, 17(4): 451-471. https://doi.org/10.1046/j.1440-1703.2002.00503.x.
https://doi.org/10.1046/j.1440-1703.2002...
).

The distribution of fish is rarely driven by only one factor (Angermeier et al., 2002Angermeier, P.L.; Krueger, K.L. & Dolloff, C.A. 2002. Discontinuity in Stream-fish Distributions: Implications for Assessing and Predicting Species Occurrence. In: Scott, J.M.; Heglund, P.J.; Morrison, M.L.; Haufler, J.B.; Raphael, M.G.; Wall, W.A. & Samson, F. (Eds.). Predicting species occurences: issues of accuracy and scale. Washington, Island Press. p. 519-527.), and is also associated with physiographic characteristics and biotic interactions (Leclerc & Desgranges, 2005Leclerc, J. & Desgranges, J.L. 2005. Exploratory multiscale analysis of the fish assemblages and habitats of the lower St. Lawrence River, Québec, Canada. Biodiversity and Conservation, 14(5): 1153-1174. https://doi.org/10.1007/s10531-004-7839-y.
https://doi.org/10.1007/s10531-004-7839-...
). The high abundance of herbivorous fish in the plateau was attributed by a species that do not depend directly on riparian vegetation as a food source. Otocinclus vittatus Regan, 1904 represented more than 90% of the herbivores sampled and 51% of fish abundance in this river segment. Despite occurring in all segments O. vittatus showed dominance in the plateau segment. The lack of piscivores, the fast-running water and the submerged vegetation, like Alchornea castaneifolia (Willd.) A. Juss., in this segment composed a favourable environment for O. vittatus dominance, where it feeds on wood debris or algae (Axenrot & Kullander, 2003Axenrot, T.E. & Kullander, S.O. 2003. Corydoras diphyes (Siluriformes: Callichthyidae) and Otocinclus mimulus (Siluriformes: Loricariidae), two new species of catfishes from Paraguay, a case of mimetic association. Ichthyological Exploration of Freshwaters, 14(3): 249-272.).

Altitude was the main predictor for the higher invertivores abundance in the confluence and plain. The result indicate that the abundance of this trophic guild increases towards the plain. In the plain region, the water flow is slow due to the larger river width, allowing the aquatic macrophytes colonization, and favouring the invertivorous fish.

The aquatic macrophyte bed, abundant in the confluence and plain, provide a detritus substrate and periphytic algae (Dudley, 1988Dudley, T.L. 1988. The roles of plant complexity and epiphyton in colonization of macrophytes by stream insects. Internationale Vereinigung fuer Theoretische und Angewandte Limnologie Verhandlungen, 23(2): 1153-1158. (SIL Proceedings, 1922-2010). https://doi.org/10.1080/03680770.1987.11899786.
https://doi.org/10.1080/03680770.1987.11...
) that feed associated invertebrates. Zooplanktons, for instance, occur in greater biomass in the littoral region of a watercourse, following the development of aquatic macrophytes (Estlander et al., 2009Estlander, S.; Nurminen, L.; Olin, M.; Vinni, M. & Horppila, J. 2009. Seasonal fluctuations in macrophyte cover and water transparency of four brown-water lakes: Implications for crustacean zooplankton in littoral and pelagic habitats. Hydrobiologia, 620(1): 109-120. https://doi.org/10.1007/s10750-008-9621-8.
https://doi.org/https://doi.org/10.1007/...
), floodplain important elements.

The piscivores, herbivores and omnivores represented the bigger and heavier fishes. However, the species with major length in these three trophic guilds are migratory, thus indicating more energetic requirements due to the necessity of energy storage for migration and reproduction (Resende et al., 1996Resende, E.K.; Pereira, R.A.C.; Almeida, V.D. & Silva, A.D. 1996. Alimentação de peixes carnívoros da planície inundável do rio Miranda, Pantanal, Mato Grosso do Sul, Brasil. Corumbá-MS, EMBRAPA-CPAP.).

The distribution of trophic guilds varied in the sampled segments in the Paraguay River, with more piscivores and invertivorous fish in the confluence and plain regions of the river, consequently herbivores were more abundant in the plateau region. The environmental variable water transparency is important for piscivorous fish once its predation is associated with visual acuity. The altitude, water temperature and proportion of dense forest are important to herbivores, and it was more abundant in the plateau region, where these characteristics showed higher values. Altitude is an important variable for invertivorous fish, as its abundance is higher in lower regions.

The piscivorous species are characterized as key species in aquatic ecosystems as they have an important role in energy flow in trophic webs. Although not showing big differences in SL, piscivorous fish weight in plain was the highest among the segments, indicating a good provision of food in these areas. On the other hand, forager fish such as herbivores often rely solely on the riparian vegetation as a food source, eating fruits and leaves. Both predators and foragers are favored by the hydrographic extension that allows their colonization in different environments. However, studies including the temporal scale with the spatial scale should be conducted, once it is the main drivers of feeding behavior of fish species in seasonally inundated environments.

This study provided the evidence of different distribution of trophic guilds along a portion of the Paraguay River, in Northern Pantanal. However, this area is susceptible to damming of tributaries rivers, forest loss by cattle raising, grain production, long period droughts and fires that might cause temporal and spatial alterations in local food webs, changing the dynamics along the longitudinal gradient. Therefore, future research monitoring the environment and evaluating the effect of several dangerous anthropogenic activities in the ichthyofauna must be taken into account in the Pantanal.

ACKNOWLEDGMENTS

The authors are thankful to the collaboration of Estação Ecológica de Taiamã, in special to the coordinator Daniel Kantek, Recanto do Dourado Hotel, Fazenda Morrinhos and the fishermen Jânio and Reis for the field support.

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  • FUNDING INFORMATION: The first author is grateful to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for the scholarship to support this research project (number process: 1709830). Field research in Mato Grosso was also funded by Fundação de Amparo à Pesquisa do Estado de Mato Grosso (FAPEMAT - concession term number 0589188/2016).
  • Published with the financial support of the “Programa de Apoio às Publicações Científicas Periódicas da USP”.

SUPPLEMENTARY MATERIAL

Table 1
List of trophic guilds with species. PLT: plateau CON: confluence e PLA: plain.

Edited by

Edited by: Murilo Nogueira de Lima Pastana.

Publication Dates

  • Publication in this collection
    03 Oct 2022
  • Date of issue
    2022

History

  • Received
    16 Nov 2021
  • Accepted
    27 June 2022
  • Published
    02 Aug 2022
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