**1. Rivers of the Neotropical region**

The distribution of freshwater fish around the world was mediated by historical climatic and geological events at different time points. Today, each global region has distinct patterns of distribution due to physical barriers obstructing species dispersion, representing different tolerances to environmental variables [1]. The tropics of the American continent are well known for their high biodiversity. This is due to habitat heterogeneity and a complex geological history. The Neotropical region is a biogeographic region that comprises Central America (including the southern part of Mexico and the peninsula of Baja California), the south of Florida, the Caribbean and South America. The origin and evolution of the Neotropical region arose through a process of synergism between its fauna that experienced local rainfall variations and gradual climate change resulting in a mosaic of habitats controlled by river migrations, sea-level fluctuations, local dryness and local uplifts [2, 3].

The Paraná-Paraguay Basin covers most of the southeastern region of Brazil and other countries such as Paraguay, eastern Bolivia and northern Argentina. Together with the Uruguay River, it covers most of central South America. The hydrographic basin covers 2.8 million km<sup>2</sup> and is considered the second biggest Brazilian basin. In contrast to the Amazon Basin, the climate in the Paraná-Paraguay Basin is drier, and the basin oscillates between harsh dryness and shallowness, with rains from October to March. The annual precipitation rate is 800–1200 mm leading to the formation of significant seasonal floodplains [13]. One of the biggest and most important wetlands of the world is the Pantanal located in the Upper Paraguay Basin. The complex hydrological cycle of the Pantanal wetland creates selective pressures on the adaptive and diversified traits of fish species. The Pantanal wetland consists of 5% of all existing Neotropical species [14, 15], but surprisingly few studies into diversity, structure and the population dynamics of fish populations have been carried out. Most of the fish in the Paraguay-Paraná River are economically important and are migratory, such as *Salminus brasiliensis*, *P. mesopotamicus*, *Brycon orbignyanus* and large catfish such as *P. corruscans*, *Pseudoplatystoma* 

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The Uruguay River Basin system is located in the temperate latitudes near the southern coast of Brazil with altitudes reaching 1800 m. It runs along the border between the Santa Catarina and Rio Grande do Sul states of Brazil until the Paraná River where it forms the estuary of the Plata River in Argentina. As a result of its sloping profile and abundance of rapids, the Uruguay River is hard to navigate compared to other rivers. With faster water, there are a considerable number of hydropowered dams in the basin that can affect the reproduction of

The São Francisco River covers 7.4% of Brazil and represents a large number of reservoirs, representing the second highest source of hydropower in the country [18]. The headwaters rise in the southern region of Minas Gerais state, and run through Bahia, Pernambuco, Sergipe and Alagoas states to then empty into the Atlantic Ocean. The altitude reaches 1600 m above sea level, and there are diverse climate conditions raging from humid tropical to semi-arid, with temperatures from 18 to 27°C and high evaporation rates (2300–3000 mm/year). The São Francisco River is rich in floodplains and marginal lagoons that are used by fish species as a habitat for feeding, reproduction and refuge. Around 8% of the species migrate to reproduce and are considered important commercial fish. These include some Characiformes (*Brycon lundii, S. brasiliensis*, *Leporinus elongatus, Prochilodus affinis* and *Prochilodus marggravii*) and

Neotropical fish comprise approximately 30% of all fish species in the world (5160 species) and are found in only 0.003% of all the freshwater on the planet [20]. The Neotropical region has some of the highest numbers of fish families, and unlike other zoogeographic zones where Cypriniformes predominate, there is a high proportion of endemic families belonging to the Characiformes (~1200 species) and Siluriformes (~1300 species) orders. Despite the predominance of Characiformes and Siluriformes species in the Neotropical basins, the heterogeneity of

*fasciatum* and *Paulicea luetkeni* [16].

migratory species and their eggs and larvae drift [17].

Siluriformes (*Conorhynchos conirostris* and *P. corruscans*) [19].

**2. Diversity and biology of Neotropical fish**

Regional geographical formations can affect the local hydrography and species distribution by forming distinct biogeographic barriers and allowing speciation of some isolated populations. Consequently, large basins separated by physical barriers with heterogeneous distribution across thousands of river systems, tend to have distinct species, with behaviours relating to environmental characteristics [4, 5]. The main hydrographic basins covering the Neotropical region are concentrated in South America, including the Amazon Basin, which covers the Colombian and Brazilian hydrographic regions, the Upper Paraná River Basin, the Paraguay-Paraná Basin, the São Francisco River Basin and the Uruguay River Basin [6].

The Amazon drainage basin covers 7.05 million km<sup>2</sup> occupying approximately 39% of the South American land mass. Around 72% of the basin is concentrated in Brazil, but it covers almost the whole continent from the Andes Mountains in the west to the Atlantic Ocean in the east. The mean water temperature in the basin is 27–29°C and reaches up to 34°C. Rainfall is the main source of water for the Amazon Basin, with about 50% of water originating from precipitation, being 6% of the basin area continuously flooded by large and medium rivers [7]. Several fish species of economical relevance in the Amazon River are migratory, such as catfishes of the Siluriformes order that can migrate for thousands of kilometres, and seed dispersers, such as *Brycon* spp., *Colossoma macropomum* and *Piaractus brachypomus* [8, 9]. The major habitats used by migratory fish are large channels and floodplains where they migrate and spawn in lotic areas [10].

The Upper Paraná-River Basin is formed by the junction of the Grande and Parnaiba rivers in the south-central region of Brazil. It is one of the longest rivers in the world at 4695 km with a 2.8 × 10<sup>6</sup> km<sup>2</sup> drainage area. It comprises 10.5% of the total area of Brazil and flows by the region that has the greatest population density of the country, subject to dam construction and agricultural, industrial and urban pollution. The Upper Paraná region has a tropical and subtropical climate with an average temperature of 22°C and 140 cm of precipitation per year [11]. There is a large floodplain located between the Porto Primavera and Itaipu dams, with a 230 km dam-free stretch, and is a region considered important for the conservation of local fish fauna. There are large migrators such as *Piaractus mesopotamicus* and *Pseudoplatystoma corruscans*, and a wide variety of short-distance migratory species and sedentary species, such as *Astyanax altiparanae* and *Serrasalmus* spp., present in the basin [12].

The Paraná-Paraguay Basin covers most of the southeastern region of Brazil and other countries such as Paraguay, eastern Bolivia and northern Argentina. Together with the Uruguay River, it covers most of central South America. The hydrographic basin covers 2.8 million km<sup>2</sup> and is considered the second biggest Brazilian basin. In contrast to the Amazon Basin, the climate in the Paraná-Paraguay Basin is drier, and the basin oscillates between harsh dryness and shallowness, with rains from October to March. The annual precipitation rate is 800–1200 mm leading to the formation of significant seasonal floodplains [13]. One of the biggest and most important wetlands of the world is the Pantanal located in the Upper Paraguay Basin. The complex hydrological cycle of the Pantanal wetland creates selective pressures on the adaptive and diversified traits of fish species. The Pantanal wetland consists of 5% of all existing Neotropical species [14, 15], but surprisingly few studies into diversity, structure and the population dynamics of fish populations have been carried out. Most of the fish in the Paraguay-Paraná River are economically important and are migratory, such as *Salminus brasiliensis*, *P. mesopotamicus*, *Brycon orbignyanus* and large catfish such as *P. corruscans*, *Pseudoplatystoma fasciatum* and *Paulicea luetkeni* [16].

The Uruguay River Basin system is located in the temperate latitudes near the southern coast of Brazil with altitudes reaching 1800 m. It runs along the border between the Santa Catarina and Rio Grande do Sul states of Brazil until the Paraná River where it forms the estuary of the Plata River in Argentina. As a result of its sloping profile and abundance of rapids, the Uruguay River is hard to navigate compared to other rivers. With faster water, there are a considerable number of hydropowered dams in the basin that can affect the reproduction of migratory species and their eggs and larvae drift [17].

The São Francisco River covers 7.4% of Brazil and represents a large number of reservoirs, representing the second highest source of hydropower in the country [18]. The headwaters rise in the southern region of Minas Gerais state, and run through Bahia, Pernambuco, Sergipe and Alagoas states to then empty into the Atlantic Ocean. The altitude reaches 1600 m above sea level, and there are diverse climate conditions raging from humid tropical to semi-arid, with temperatures from 18 to 27°C and high evaporation rates (2300–3000 mm/year). The São Francisco River is rich in floodplains and marginal lagoons that are used by fish species as a habitat for feeding, reproduction and refuge. Around 8% of the species migrate to reproduce and are considered important commercial fish. These include some Characiformes (*Brycon lundii, S. brasiliensis*, *Leporinus elongatus, Prochilodus affinis* and *Prochilodus marggravii*) and Siluriformes (*Conorhynchos conirostris* and *P. corruscans*) [19].
