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

**1. Rivers of the Neotropical region**

250 Biological Resources of Water

tions, sea-level fluctuations, local dryness and local uplifts [2, 3].

The Amazon drainage basin covers 7.05 million km<sup>2</sup>

a 2.8 × 10<sup>6</sup> km<sup>2</sup>

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 migra-

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].

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

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

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

drainage area. It comprises 10.5% of the total area of Brazil and flows by the

channels and floodplains where they migrate and spawn in lotic areas [10].

*Astyanax altiparanae* and *Serrasalmus* spp., present in the basin [12].

occupying approximately 39% of the South

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 species between basins and their unequal distribution are considerable. This is largely due to the formation of lakes, puddles, streams, rapids, rivers and floodplains that have become determining factors for the high diversity of fish species that exist today in the Neotropical region [1, 21].

level is not entirely clear. The increase in water level combined with changes in temperature, photoperiod and water conductivity may lead to physiological changes in the individual that stimulate gonad maturation, migratory movements, spawning, egg fertilisation and offspring development [36]. Although migratory movements are due to reproductive needs, they have seasonal, trophic and ontogenetic characteristics that are all associated with the hydrological

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*S. brasiliensis*, commonly known as "dourado," is a large and carnivorous species found mainly in the Prata Basin. They are vulnerable to indiscriminate fishing and have high commercial value. Their reproductive migration for maintenance of their population stretches over 400 km [39] and can reach 1000 km [40]. In addition, large catfish are known for their remarkably long-distance migrations in rivers such as the Amazon, Paraguay, Paraná and Orinoco [6]. Most of these species have high commercial value due to the taste of their meat and large size. However, stocks of catfish have declined due mainly to overfishing and habitat fragmentation. In the Amazon, goliath catfish such as *Brachyplatystoma rousseauxii*, known as "dourada," cross the entire South American continent from east to west to lay eggs at the headwaters of the rivers near the Andes mountain range, and their migration is considered the longest of any freshwater fish. The larvae, which are less than 6 mm in length, develop along the route that can reach 11,600 km. Their destination is the rich ecosystem of the Amazon River estuary [41]. Neotropical fish may also be responsible for the proliferation of plant species due to their migratory behaviour. The pacu (*P. mesopotamicus*) is a migratory and omnivorous species that travels up to 300 km [42] and plays an important role as a seed disperser for many fleshyfruited plants of the Pantanal, contributing to seed dispersion for 27% of the tree community

In addition to large migratory species that require long stretches of river and seasonal stimuli to exercise their life strategies, there are also sedentary species that carry out their vital activities in a restricted area and are more influenced by local environmental variations. Due to their small size, they spend their lives associated to a substrate, such as trunks, rocks and aquatic plants where they find protection, food and a suitable surface for egg deposition. The displacement of these species is generally short and occurs in more lentic environments, where there may be occasional variations. The reproductive period of some species occurs during lower levels of precipitation so that eggs and larvae are not dragged to stretches of the river where they will be unable to find suitable conditions for development. On the other hand, some species of sedentary fish such as the lambari (*Astyanax scabripinnis paranae*) are able to prolong their reproductive period throughout the year and overcome the unpredict-

Some sedentary species also show seasonality in reproduction. *Oligosarcus jenynsii*, popularly known as the "dog fish," is one of the most abundant species in the coastal lagoons of southern Brazil. Spawning starts in the winter months where the water temperature is very low. This strategy can lead to high mortality in the larvae but, on the other hand, it can guarantee food for juvenile individuals since in the months following winter they would be more developed compared with other species that reproduce in spring [44]. Variations in reproductive strategy exist in the same population and between different populations of the same species

regime of the river [21, 37, 38].

existing in the gallery forests [43].

ability of floods in short streams [21].

There are estimates that the number of freshwater fish species in the Neotropical region exceeds 8000 [20]; however, the total number remains unknown. There are many factors that make it difficult to study biodiversity in this region, in addition to problematic taxonomic issues [1] like cryptic species [22]. Furthermore, few institutions [United Nations Food and Agriculture Organisation (FAO) and the Brazilian Institute of Geography and Statistics (IBGE)] provide fishing data statistics that ultimately affects the management of these species.

Despite the low number of fish species described, there was an increase following the advent of molecular biology and cytogenetic techniques. Many single species were re-described as a complex of cryptic species after genetic analyses [23].

Efforts to describe new species of Neotropical fish have focused mainly on the Amazon Basin, with more than half of the fish species described found in this region [5]. Of these, there are around 200 poorly known species that are exploited by commercial and subsistence fishing, and this number may be even greater due to misidentification errors. This makes it difficult to implement fishery management and conservation policies [24, 25].

The diversity of fish species found in the Neotropical basins reflects variations in life-history strategies and exhibition of particular morphological, physiological and ecological attributes. Such attributes are related to the different forms of feeding, life maintenance and reproduction [26–28]. Research into the pattern of life strategies may have practical applications in conservation and is fundamental to fishery management in identifying appropriate measures to reduce the impact from reservoirs and other anthropogenic activities [29].

Neotropical fish species range from colonising and opportunist species to periodic and equilibrium species. Colonising populations, such as those belonging to the genera *Astyanax* and *Phalloptychus*, mostly require energy for reproduction and are generally represented by small, rapidly maturing and short-lived species. Therefore, their numbers are more likely to recover on anthropic impacts. Periodic and equilibrium species are larger, longer-living individuals with few offspring requiring greater parental investment [30]. They use a large part of their energy for somatic growth to ensure their survival to age-at-maturity [31, 32] and generally show late maturation and slow growth that would take longer to recover from high levels of overfishing. The main examples in this group are large, freshwater catfish, such as "piraíba," that belong to the *Brachyplatystoma* genus, followed by other Siluriformes [33]. Neotropical fish have a wide diversity of life histories, with respect to reproduction, migratory patterns and spatial behaviours that can make their classification very difficult [34].

The high degree of variability in reproductive strategies is closely related to their environment and the selective forces present over the life history of the species. As a result, reproductive strategies can be expressed in different ways, including the type of egg fertilisation, differences in age of maturation, parental care, spawning and migratory patterns [35]. Seasonal periods of flooding and dryness influence reproductive strategies, especially in migratory species. However, the connection between reproductive events and fluctuations in hydrometric level is not entirely clear. The increase in water level combined with changes in temperature, photoperiod and water conductivity may lead to physiological changes in the individual that stimulate gonad maturation, migratory movements, spawning, egg fertilisation and offspring development [36]. Although migratory movements are due to reproductive needs, they have seasonal, trophic and ontogenetic characteristics that are all associated with the hydrological regime of the river [21, 37, 38].

species between basins and their unequal distribution are considerable. This is largely due to the formation of lakes, puddles, streams, rapids, rivers and floodplains that have become determining factors for the high diversity of fish species that exist today in the Neotropical region [1, 21]. There are estimates that the number of freshwater fish species in the Neotropical region exceeds 8000 [20]; however, the total number remains unknown. There are many factors that make it difficult to study biodiversity in this region, in addition to problematic taxonomic issues [1] like cryptic species [22]. Furthermore, few institutions [United Nations Food and Agriculture Organisation (FAO) and the Brazilian Institute of Geography and Statistics (IBGE)] provide

Despite the low number of fish species described, there was an increase following the advent of molecular biology and cytogenetic techniques. Many single species were re-described as a

Efforts to describe new species of Neotropical fish have focused mainly on the Amazon Basin, with more than half of the fish species described found in this region [5]. Of these, there are around 200 poorly known species that are exploited by commercial and subsistence fishing, and this number may be even greater due to misidentification errors. This makes it difficult to

The diversity of fish species found in the Neotropical basins reflects variations in life-history strategies and exhibition of particular morphological, physiological and ecological attributes. Such attributes are related to the different forms of feeding, life maintenance and reproduction [26–28]. Research into the pattern of life strategies may have practical applications in conservation and is fundamental to fishery management in identifying appropriate measures

Neotropical fish species range from colonising and opportunist species to periodic and equilibrium species. Colonising populations, such as those belonging to the genera *Astyanax* and *Phalloptychus*, mostly require energy for reproduction and are generally represented by small, rapidly maturing and short-lived species. Therefore, their numbers are more likely to recover on anthropic impacts. Periodic and equilibrium species are larger, longer-living individuals with few offspring requiring greater parental investment [30]. They use a large part of their energy for somatic growth to ensure their survival to age-at-maturity [31, 32] and generally show late maturation and slow growth that would take longer to recover from high levels of overfishing. The main examples in this group are large, freshwater catfish, such as "piraíba," that belong to the *Brachyplatystoma* genus, followed by other Siluriformes [33]. Neotropical fish have a wide diversity of life histories, with respect to reproduction, migratory patterns

The high degree of variability in reproductive strategies is closely related to their environment and the selective forces present over the life history of the species. As a result, reproductive strategies can be expressed in different ways, including the type of egg fertilisation, differences in age of maturation, parental care, spawning and migratory patterns [35]. Seasonal periods of flooding and dryness influence reproductive strategies, especially in migratory species. However, the connection between reproductive events and fluctuations in hydrometric

fishing data statistics that ultimately affects the management of these species.

complex of cryptic species after genetic analyses [23].

252 Biological Resources of Water

implement fishery management and conservation policies [24, 25].

to reduce the impact from reservoirs and other anthropogenic activities [29].

and spatial behaviours that can make their classification very difficult [34].

*S. brasiliensis*, commonly known as "dourado," is a large and carnivorous species found mainly in the Prata Basin. They are vulnerable to indiscriminate fishing and have high commercial value. Their reproductive migration for maintenance of their population stretches over 400 km [39] and can reach 1000 km [40]. In addition, large catfish are known for their remarkably long-distance migrations in rivers such as the Amazon, Paraguay, Paraná and Orinoco [6]. Most of these species have high commercial value due to the taste of their meat and large size. However, stocks of catfish have declined due mainly to overfishing and habitat fragmentation. In the Amazon, goliath catfish such as *Brachyplatystoma rousseauxii*, known as "dourada," cross the entire South American continent from east to west to lay eggs at the headwaters of the rivers near the Andes mountain range, and their migration is considered the longest of any freshwater fish. The larvae, which are less than 6 mm in length, develop along the route that can reach 11,600 km. Their destination is the rich ecosystem of the Amazon River estuary [41]. Neotropical fish may also be responsible for the proliferation of plant species due to their migratory behaviour. The pacu (*P. mesopotamicus*) is a migratory and omnivorous species that travels up to 300 km [42] and plays an important role as a seed disperser for many fleshyfruited plants of the Pantanal, contributing to seed dispersion for 27% of the tree community existing in the gallery forests [43].

In addition to large migratory species that require long stretches of river and seasonal stimuli to exercise their life strategies, there are also sedentary species that carry out their vital activities in a restricted area and are more influenced by local environmental variations. Due to their small size, they spend their lives associated to a substrate, such as trunks, rocks and aquatic plants where they find protection, food and a suitable surface for egg deposition. The displacement of these species is generally short and occurs in more lentic environments, where there may be occasional variations. The reproductive period of some species occurs during lower levels of precipitation so that eggs and larvae are not dragged to stretches of the river where they will be unable to find suitable conditions for development. On the other hand, some species of sedentary fish such as the lambari (*Astyanax scabripinnis paranae*) are able to prolong their reproductive period throughout the year and overcome the unpredictability of floods in short streams [21].

Some sedentary species also show seasonality in reproduction. *Oligosarcus jenynsii*, popularly known as the "dog fish," is one of the most abundant species in the coastal lagoons of southern Brazil. Spawning starts in the winter months where the water temperature is very low. This strategy can lead to high mortality in the larvae but, on the other hand, it can guarantee food for juvenile individuals since in the months following winter they would be more developed compared with other species that reproduce in spring [44]. Variations in reproductive strategy exist in the same population and between different populations of the same species due to environmental conditions. For example, the "traíra" (*Hoplias malabaricus*), which has the ability to reproduce throughout the year or to follow a defined seasonal cycle depending on their environment [21].

There is a particularly high concentration of hydroelectric dams in the Upper Paraná and São Francisco rivers (many of the rivers in South America are so heavily dammed they become a chain of reservoirs) [6]. The largest dams in Latin America are Itaipu (Paraná River, Paraguay-Brazil), Guri (Rio Caroni, Venezuela), Tucuruí (Rio Tocantins, Brazil) and Yacyretá (Paraná River, Argentina-Brazil). Currently, 90% of the energy consumed in Brazil originates from hydroelectric plants, with an annual output of 78,000 MW [6]. Dams have been built in almost all hydrographic basins with consequent formation of reservoirs. These reservoirs alter the natural distribution of seasonal flows and nutrients, leading to the formation of new ecosystems with specific structures and functioning [56]. These new ecosystems have several factors that affect the local ichthyofauna have a serious impact on the life cycle of the fish [57]. Dams act as barriers to the natural flow of rivers. They are built mainly to produce electricity, but also to supply water to residential, agricultural and industrial areas. The change and/or loss in water flow impacts the distribution of aquatic life biodiversity [58]. Dams also affect the watershed and lower the water quality, impacting not only the river itself but also its tributaries. This may also be harmful to native species by destabilising the ecosystem and the living communities [57]. For example, migratory fish suffer due to the interruption to their migratory routes and require a different habitat to complete their life cycle. These species generally migrate upstream to spawn during the wet season producing numerous small eggs. These eggs and larvae are transported with the current to nurseries downstream without any parental care, where they find ideal conditions for initial development and protection from predators [59, 60]. The consequences of blocking migratory fish routes is observed in their reproductive cycle

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for years, leading to the depletion of natural stocks and extinction of the species [61].

of some species and eliminate others, causing collapse of the ecosystem [66].

water so that fish can climb and pass through the dam [21].

The new ecosystem formed modifies the structure of fish communities that inhabit the river, and the establishment of new communities depends on the physical, chemical, hydrological and geomorphological changes as a result of the spatial and temporal redistribution of the river flow [62–65]. Changes in species composition and abundance can increase the numbers

In order to mitigate such effects, management measures have been put in place to preserve the Neotropical ichthyofauna [67]. Until the 1950s, the main objective of management programmes was to ensure that species could migrate through the reservoirs to complete their life cycle. Transposition mechanisms (fish ladders) were created in the main Brazilian dams. In the 1990s, dozens of fish transposition systems were constructed, even with few studies into the efficacy of the method and despite the costs and effort required [68]. Most of these mechanisms are based on ladders, structures that reduce the velocity and gradient of the

However, these mechanisms have species selectivity and allow the movement of only some species of fish. This divergence between species can cause dramatic imbalances in the population and the Neotropical ecosystem [59]. The main process of passage is recognition of the entrance [69]. If the fish cannot recognise the entrance to the passage, they remain where they are, which delays migration and spawning and interferes significantly with their reproductive process [70]. Storage and repopulation of fish are alternative methods to mitigate the impacts of hydroelectric dams [57]. Several breeding programmes were implemented aimed at the production of fish to restock the reservoirs, mainly to improve fishing activities. Some non-native species were
