Fish assemblages in Neotropical reservoirs: Colonization patterns, impacts and management
Introduction
Impoundments lead to extreme changes in fluvial habitats, transforming rivers into semi-lentic systems. Animals and plants for which these new conditions are restrictive will have their populations drastically reduced. However, species that can complete their life cycle in the new environment and take advantage of the available food resources will achieve their full potential for proliferation (Agostinho et al., 2007a). The nature of and intensity with which the fluvial biota is altered by impoundments are highly variable among reservoirs and must be studied case by case.
The literature demonstrates that even reservoirs arranged in series in the same river, with unidirectional interactions from upstream to downstream, show distinct peculiarities in relation to the colonization process and the organization of assemblages (Agostinho and Gomes, 1997, Petesse and Petrere, 2012). The degree of alteration in the structure and dynamics of the local biota depends on several local and regional factors, such as morphometry of the catchment, discharge, patterns of water circulation, depth, habitat structure, species pool, surface area, the design of the dam and its operational procedures. Thus, a detailed understanding of the context of a particular reservoir is paramount for effective mitigation measures and/or management actions for the conservation of fish populations (Weithman and Haas, 1982). A manager should, based on local and regional studies, identify any alterations in the structure of the local fish assemblage and take action to avoid irreversible losses of regional biological diversity and/or natural resources as a consequence of river damming.
In general, the fish species most affected by impoundments are large in size, migrate and have high longevity (k-strategist). In contrast, a massive proliferation of primarily small-sized sedentary species (i.e. those that do not migrate) occurs, which have a high reproductive potential and short longevity (r-strategists) and for which the availability of food resources is high (Agostinho et al., 1999, Agostinho et al., 2008a, Hoeinghaus et al., 2009). Yet, sedentary species are also affected by hydrological alterations and tend to redistribute along the river/reservoir gradient (Araújo et al., 2013). In the inner areas of large reservoirs, fish assemblages are profoundly altered and composed of a few species with pre-adaptations to live in semi-lentic environments (Gomes and Miranda, 2001, Agostinho et al., 2007a).
Reservoirs are present in the main river basins in Brazil, and the principal purpose is the production of electricity. Although reservoirs are widespread in the country, their distribution is not homogeneous, e.g. the Upper Paraná River has half of the total impounded area and is one of the most regulated rivers in the world (Agostinho et al., 2008a). Even considering the specificity of the response of the biota to the impacts generated by each reservoir, some patterns can be described based on studies of dozens of reservoirs in Brazil. Therefore, the objective of this paper is to review the patterns of fish fauna once a reservoir is formed. First, we described the variation in fish assemblages over time, from the filling of the reservoir to the periods in which environmental and biotic conditions are rearranged and more stable. We categorized these variations into phases (heterotrophic, post-heterotrophic and trophic equilibrium), considering predicted alterations in productivity. Then, considering the phases, we described broad trends in fish abundance, species richness, pre-adaptations to pelagic environments, and variations in size and reproductive strategies. Finally, we evaluated management measures presently implemented to mitigate impacts caused by reservoirs on the Neotropical fish fauna, and we discuss opportunities for improvement as well as the existing knowledge gaps. As the Upper Paraná River basin is the most dammed in South America as well as the most studied, we used it as a model to achieve our goals every time an example was necessary.
Section snippets
Reservoirs and fish diversity
It is estimated that the number of large reservoirs (dams higher than 15 m; World Commission on Dams, 2000) in South America is greater than one thousand, and around 50% of them are located within Brazilian territory (Fig. 1). Thirty-seven percent of these reservoirs produce electricity. Although hydroelectric production in dams started in Brazil at the end of the XIX century (Marmelos Dam; Paraíba do Sul River; 1889), most of the dams were constructed in the second half of the XX century. With
Variation in fish abundance
The large release of nutrients resulting from the decomposition of organic matter in the flooded area during a reservoir's early years and the subsequent reduction of nutrients result in wide fluctuations in production throughout a reservoir's history. The nutrient input increases the production of all trophic levels during a period known as the “trophic upsurge period” (Kimmel and Groeger, 1986, Kimmel et al., 1990). This heterotrophic period begins in the filling phase, which is marked by
Fish colonization during the filling phase
During reservoir filling, the patterns of vertical colonization of fish are associated with thermal stratification, an increase in depth and a sharp decrease or even the virtual absence of dissolved oxygen. All these factors impose changes on fish distribution patterns. The increase in water volume and the reduction in water flow lead to an increase in the area available for colonization (Agostinho et al., 2008a, Wang et al., 2013). The lack of oxygen in the deeper strata (in the bathypelagic
Heterotrophic and trophic equilibrium phases
There is evidence from several Neotropical reservoirs that the species richness increases immediately after the filling phase (Fig. 5a and b). This increase in species richness is followed by an increase in the abundance of fish (Fig. 5c), which is common during the trophic upsurge period. However, the magnitude of the increase in abundance varies among species in a new reservoir, and the dominance of certain species with regard to abundance (low evenness) causes a continuous decreasing in the
Constraints and pre-adaptations
The virtual absence of natural lakes in Brazil (excepting those associated with fluvial corridors) and the consequent scarcity of species with pre-adaptations to occupy open areas of reservoirs, allied with the longitudinal gradients related to the processes of transport and deposition (e.g. transparency and nutrient loads), leads to a heterogeneous pattern of the occupation of the new environment. The most important characteristics of truly pelagic species are their short food chains, high
Fish assemblage stabilization
The time span after reservoir closure that the fish community structure requires for a certain degree of stability varies widely, and no consensus concerning this time span exists in the literature (Petrere, 1996). There are evidences of stabilization of the fish abundance and species richness between 15 and 40 years after a reservoir is formed (Mol et al., 2007, Orsi and Britton, 2014). Several factors may influence this time, such as latitude, hydraulic retention time, morphometry, fish fauna
Management and impact mitigation
The search for measures to mitigate impacts caused by dams and associated reservoirs on fish diversity and fish stocks in Brazil began with the construction of the first hydroelectric reservoirs. The first action taken was the construction of the fish ladder in the Itaipava Dam (Pardo River, Upper Paraná River basin) at the beginning of the last century. After that initiative, the history of management in Brazil encompassed several phases, with emphasis on fish stocking, fish farming in cages
Final considerations
Large reservoirs are noticeable landscape features in most of the main hydrographic basins in Brazil. Long-distance migratory fish are the most impacted by impoundments, as a result of three fundamental characteristics inherent to their life history strategy: (i) the huge home range these species occupy in the hydrographic basin, including migration routes to areas critical for completion of life cycles, such as spawning areas and nurseries; (ii) a strong dependency of this life strategy on the
Acknowledgments
The authors are grateful to the power companies (Itaipu, Copel, Furnas Centrais Elétricas, Eletrobrás, and CESP) that financed several projects and Nupelia for the infrastructure that enabled the development of the projects. The authors also thank CNPq for the fellowships awarded to JCGO and NCLS. AAA, LCG and FMP are “Bolsista de Produtividade” of CNPq. Jaime Luiz Lopes Pereira made all the figures.
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