**4. Potential effects of global climatic changes and dams on freshwater amazonian fish**

The intensity and direction (positive or negative) of the potential effects of environmen‐ tal changes will vary among populations and species in the Amazonian fish fauna. Some global scenarios are catastrophic [38], proposing that 75% of global freshwater fish will become extinct before the end of the 21st century due to a reduction in river discharge. Nevertheless, the possible effect is local extinction, which would be a critical event for endemic species. Two species of the small fish *Paracheirodon*, which are exploited as orna‐ mental species, exist in the middle to upper Rio Negro in Brazil and in the upper Rio Orinoco in Colombia and Venezuela. A study conducted at an inter-fluvial palm camp of the Middle Rio Negro found that these two species are rarely observed in the same habi‐ tat. The *P. simulans* habitat water temperature ranged from a low of 24.6 to a high of 35.2 ºC, while the *P. axelroldi* habitat temperature varied between 25.1 and 29.9 ºC [39]. The authors propose that because inter-fluvial areas flood as a function of rainfall, a de‐ crease in regional precipitation could alter the hydrologic balance of these wetlands, es‐ pecially during dry periods, which would lower water levels and increase the water temperature. This scenario would be extremely adverse for *P. simulans*, which exists only in very shallow inter-fluvial areas. A decrease in precipitation could dry out these areas completely, ultimately leading to the local extinction of this species.

At the beginning of rising waters season, the adults move down river from tributaries of black and clear waters to spawning in the turbid and rich environment of white water riv‐ ers. After breeding event, these fish move toward the flooded forest for feeding. The larvae are carried by drift toward flooded areas of the floodplain. After six months, when the water starts to recede, large schools of adults and young fish move toward tributaries.

recognizes that periods of strong food production in the ocean can be variable and are often controlled by climate, which depends on the strength of the wind, the frequency of storms and the amount of heating or fresh water supplied to the surface layers in the ocean. The hypothesis examines the timing match or mismatch between when and where food is availa‐ ble and when and where early-stage fish are able to encounter and consume this food. As‐ suming that there is a synchrony between the flood pulse and the spawning season of the Amazonian Characiformes, this cycle can be modified by climatic changes with substantial consequences on species that are the most important sources of protein consumption in the

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185

In fact, some species or groups of species may be positively affected. A study of the repro‐ duction of fish from the Rio Cuiabá in Upper Pantanal indicated that both the reproductive dynamics and the hydrological regime were closely related. The authors of the study showed that the intense events of floods were positively related with gonadal development

A potentially useful approach to develop strategies to study the effects of global climate change could be classifying their effects by the type of relationship between the phenomen‐ on and the impact. [44] classified the range of effects that climate change will have on fresh‐ water, estuarine and marine fish into primary, secondary and tertiary categories. These authors found that the primary impacts are climate-related changes that directly affect the behavior, physiology, fitness and survivorship of fish without intermediary causal drivers. Secondary effects are primarily related to changes in the quality or quantity of habitats. Last‐

In contrast, impacts from dams are generally related to the fragmentation of the area in which the individual members of a species live, creating obstacles for migration. The decline in the abundance of long-distance migratory species is the most distinct consequence of such filters. The community of these species undergoes seasonal migrations toward spawning habitats located upstream and consequently requires free-flowing stretches of river. There‐ fore, recruitment success depends on the presence of and accessibility to spawning areas, which are located in the upstream stretches of the main channel and its tributaries, as well as nursery habitats, which are located in flooded areas downstream [25, 45]. The loss of nursery habitat can critically impact several species, including several species of Characins and Perciforms. Dams also alter water temperature and quality [26, 46], which affects the

In addition, there are predictable changes in the species composition of the fish assemblages above the dam in the altered environment of the reservoir, and pre-adapted species could become abundant in this location. However, an impoverishment in the fish diversity as a whole would be expected [25, 26]. A study developed to analyze the alterations of the fish communities due to pollution and the damming of highly impacted rivers from Southeast Brazil, which were fragmented and polluted in their upper stretches, and also detected a synergic effect due to these two impact sources [47]. These authors observed a noticeable de‐ crease in species richness in the polluted stretches of the river, with one or two species dom‐ inating. However, the artificial control of floods and discharge levels should have direct

ly, the tertiary impacts are related to the interactions between several causal factors.

of species that participate in long-distance migration and parental care [43].

Amazon Basin.

community structure as a whole.

**Figure 3.** A general description of the Characiforms migrations.

The survivorship or abundance of fish species in a dynamic environment is dependent on three factors: the intensity of change, the velocity at which change will occur and the ability of organisms to adapt in the midst of these changes. Plasticity is a characteristic inherent to each species of fish. However, some common characteristics are useful for classifying the fish into groups and for discussing the most probable effects of environmental changes. For example, the impact of changes in the water temperature should be related to the lethal, sub-lethal and optimal thermal limits of each species. Despite of a scarcity of data on the physiology, life history and behavior of the Amazonian species, some information about population dynamics is available and can be used to hypothesize the effects of the environ‐ mental changes resulting from climate change and new dams.

The inverse relationship between temperature and dissolved oxygen in the water may result in an expansion of hypoxia zones. Although the Amazonian fish exhibit a variety of strat‐ egies related to oxygen intake in response to hypoxia [40], there are limits to these adaptive strategies that are a result of a long evolutionary time.

Another example of the types of analyses that can be conducted involves the use of the match-mismatch hypothesis (originally proposed by Cushing [41, 42] to describe the rela‐ tionship between starvation and recruitment), which has clear connections to climate varia‐ bility. This hypothesis recognizes that early-stage fish need food to survive and grow. It also recognizes that periods of strong food production in the ocean can be variable and are often controlled by climate, which depends on the strength of the wind, the frequency of storms and the amount of heating or fresh water supplied to the surface layers in the ocean. The hypothesis examines the timing match or mismatch between when and where food is availa‐ ble and when and where early-stage fish are able to encounter and consume this food. As‐ suming that there is a synchrony between the flood pulse and the spawning season of the Amazonian Characiformes, this cycle can be modified by climatic changes with substantial consequences on species that are the most important sources of protein consumption in the Amazon Basin.

are carried by drift toward flooded areas of the floodplain. After six months, when the water

The survivorship or abundance of fish species in a dynamic environment is dependent on three factors: the intensity of change, the velocity at which change will occur and the ability of organisms to adapt in the midst of these changes. Plasticity is a characteristic inherent to each species of fish. However, some common characteristics are useful for classifying the fish into groups and for discussing the most probable effects of environmental changes. For example, the impact of changes in the water temperature should be related to the lethal, sub-lethal and optimal thermal limits of each species. Despite of a scarcity of data on the physiology, life history and behavior of the Amazonian species, some information about population dynamics is available and can be used to hypothesize the effects of the environ‐

The inverse relationship between temperature and dissolved oxygen in the water may result in an expansion of hypoxia zones. Although the Amazonian fish exhibit a variety of strat‐ egies related to oxygen intake in response to hypoxia [40], there are limits to these adaptive

Another example of the types of analyses that can be conducted involves the use of the match-mismatch hypothesis (originally proposed by Cushing [41, 42] to describe the rela‐ tionship between starvation and recruitment), which has clear connections to climate varia‐ bility. This hypothesis recognizes that early-stage fish need food to survive and grow. It also

starts to recede, large schools of adults and young fish move toward tributaries.

**Figure 3.** A general description of the Characiforms migrations.

184 New Advances and Contributions to Fish Biology

mental changes resulting from climate change and new dams.

strategies that are a result of a long evolutionary time.

In fact, some species or groups of species may be positively affected. A study of the repro‐ duction of fish from the Rio Cuiabá in Upper Pantanal indicated that both the reproductive dynamics and the hydrological regime were closely related. The authors of the study showed that the intense events of floods were positively related with gonadal development of species that participate in long-distance migration and parental care [43].

A potentially useful approach to develop strategies to study the effects of global climate change could be classifying their effects by the type of relationship between the phenomen‐ on and the impact. [44] classified the range of effects that climate change will have on fresh‐ water, estuarine and marine fish into primary, secondary and tertiary categories. These authors found that the primary impacts are climate-related changes that directly affect the behavior, physiology, fitness and survivorship of fish without intermediary causal drivers. Secondary effects are primarily related to changes in the quality or quantity of habitats. Last‐ ly, the tertiary impacts are related to the interactions between several causal factors.

In contrast, impacts from dams are generally related to the fragmentation of the area in which the individual members of a species live, creating obstacles for migration. The decline in the abundance of long-distance migratory species is the most distinct consequence of such filters. The community of these species undergoes seasonal migrations toward spawning habitats located upstream and consequently requires free-flowing stretches of river. There‐ fore, recruitment success depends on the presence of and accessibility to spawning areas, which are located in the upstream stretches of the main channel and its tributaries, as well as nursery habitats, which are located in flooded areas downstream [25, 45]. The loss of nursery habitat can critically impact several species, including several species of Characins and Perciforms. Dams also alter water temperature and quality [26, 46], which affects the community structure as a whole.

In addition, there are predictable changes in the species composition of the fish assemblages above the dam in the altered environment of the reservoir, and pre-adapted species could become abundant in this location. However, an impoverishment in the fish diversity as a whole would be expected [25, 26]. A study developed to analyze the alterations of the fish communities due to pollution and the damming of highly impacted rivers from Southeast Brazil, which were fragmented and polluted in their upper stretches, and also detected a synergic effect due to these two impact sources [47]. These authors observed a noticeable de‐ crease in species richness in the polluted stretches of the river, with one or two species dom‐ inating. However, the artificial control of floods and discharge levels should have direct impacts on recruitment success. An analysis on the influence of the mean annual water level (m), the amplitude (maximum water level of the river in a given year; m) and the flood du‐ ration (number of days above 3.5 m; yearly total and for each season; summer and autumn were considered together) on the recruitment of *Prochilodus scrofa* for the fishery conducted at the Itaipu Reservoir and observed that flood duration is more important than flooding amplitude [48].

**Impacts Effects**

Siluriformes–Pimelodidae: long-distance migrations

out of an optimal specific interval; – Medium size reduction;

out of an optimal specific interval;

out of an optimal specific interval;

– Failure in recruitment due to loss of habitat;

Characiformes– Short-distance migrations

out of an optimal specific interval;

Perciformes–Cichlidae

– Sea level rise Siluriformes–Pimelodidae: long-distance migrations

environment;

environment. Perciformes–Cichlidae

Dams Siluriformes–Pimelodidae: long-distance migrations

flooded);

complete their life cycles.

Perciformes–Cichlidae

Characiformes– Short-distance migrations

**Table 3.** The potential impacts of global climate change and dams and their effects on freshwater Amazonian fish

Characiformes– Short-distance migrations

– Alterations in physiological functions to survive in environmental conditions

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187

– Alterations in physiological functions to survive in environmental conditions

– Failure in recruitment due to the mismatch between young fish and food;

– Alterations in physiological functions to survive in environmental conditions

– Size of adult stock reduced due to reductions in prey abundance.

The Potential Impacts of Global Climatic Changes and Dams on Amazonian Fish and Their Fisheries

– Failure in recruitment due to impacts on reproductive functions.

– Strong recruitment due to expansion of nursery area at estuary.

– Reduction of the abundance of less-adapted species for the altered

– Failure in recruitment due to impacts on reproductive functions.

– Stock abundance reduced due to reductions in the livable area;

– Change in species abundance because of alterations in the habitat; – Loss of important habitats for young fish (e.g., areas that are seasonally

– Failure in recruitment due to the loss of spawning habitat.

– Changes in the community structure as a response to the alterations in the

– Alterations in physiological functions to survive in environmental conditions

– Blockage of fish migrations, which create obstacles for freshwater species to

Global Climate Change – Global warming

Table 3 summarizes the effects of global warming, sea level rise and dams on freshwater Amazonian fish, taking into account our level of knowledge. Fish faced with a changing en‐ vironment must adapt, migrate or perish [19]. In addition to the high level of uncertainty at the species level, some evidence is available to predict that the resulting stress of a tempera‐ ture increase will affect fauna as a whole, including fish. The effects of the higher energy de‐ mand to compensate the stress would start at the physiological level and would include size reduction and reproductive failure. This evolution affects the community structure when the dominant species has more adaptive capacity. Therefore, another possible effect of climate change is the loss of biodiversity through the extinction of specialized or endemic fish spe‐ cies [48]. This pattern of environmental change inducing effects will initiate from a rise in sea level and the introduction of dams.
