One of the most outstanding features of the Neotropics is the presence of many large river basins, some of which harbour spectacular ecosystems, such as the Amazon River, the Orinoco River, and the Paraná River, each draining South America in different directions. Particularly noteworthy is the diversity of fish species found in these river systems, which is greater than those found in other continents, with estimates for the Amazon basin alone being over 2000 species, nearly half of which only occur in this region. Amongst this diversity, characins (Characiformes) and catfishes (Siluriformes) generally represent the highest species diversity and abundance.
This plethora of fish species includes some that undertake amazing migrations. So far, migrations have been described for over 70 species in South America, a number that is expected to increase as more research is conducted within a region with long rivers and high seasonality. The aquatic environments of the Amazon basin are very varied not only due to its large coverage (over six million sq. kilometres), but also due to differential rainfall resulting in seasonal flooding. Hence, some fish species migrate up- and downstream to complete their life cycle, whereas others migrate between seasonally flooded plains and the main river channels. Combined with a genetic disposition, the main factors that modulate migration are rainfall patterns, light, and temperature. Fish migration is a complex phenomenon strongly associated with breeding behaviour. As a case in point, the goliath catfishes generally spawn in the western Amazon basin, closer to the Andes and up to 5000 kilometres away from the Atlantic Ocean. Eggs, larvae and juvenile fish drift downstream to the lower reaches of the basin, with some even settling in the Amazon River estuary, from where they undertake upstream migrations once they reach adulthood.
Some of these migratory species are very important to the people that inhabit the Amazon basin, as they are a source of food and economic activity. It is estimated that about 80 percent of the annual commercial fisheries catch within this region corresponds to migratory fish. Examples include the pacú (Colossoma macropomum), which migrates between river channels and flooded plains, and the piramutaba (Brachyplatystoma vaillantii), a species of goliath catfish that migrates up- and downstream. This phenomenon is known as ‘Piracema’, an indigenous word for fish migration that means ‘river ascent’ (pira: fish; cema: uphill).
The future of these seemingly abundant migratory fish, however, is far from secure. Threats to fish populations in the Amazon basin are numerous—deforestation, overfishing, alteration of river courses, siltation, pollution, and introduction of exotic species, amongst others. However, the main threat to migratory fish is undoubtedly the interruption of water courses by hydroelectric dams, not only transforming free-flowing waters into still waters, but also interrupting fish migrations, such as the ones undertaken by the goliath catfishes. These dams change the flood pulse regime of rivers, cause artificial daily water level fluctuations, alter the interior natural circulation of the water body and temperature, bring biogeochemical changes, and increase turbidity, thereby reducing light penetration, with knock-on effects on food webs as primary productivity is compromised due to reduced photosynthetic activity. As the energy grid of South America is largely fed by river dams, with plans for expansion, disruption of migration will only worsen.
Although the construction of dams in Brazilian rivers—which account for a large proportion of the Amazon basin—has been regulated by a policy framework that includes mitigation of threats to migratory fish, there remains uncertainty about the actual effectiveness of such policies for conserving these species. For instance, the design of some dams has included so-called fish passages, which indeed allow upstream movement of fish, but with little evidence of downstream movement. Additionally, the hatching and survival rates of fish born upstream of the reservoirs are unknown. Because the movements up- and downstream need to be completed in a cycle, dams are likely disrupting the life cycle of some migratory species, such as the goliath catfishes. A quick and superficial observation of the effectiveness of mitigation measures used in dam design is often illusory. For example, following the construction of the Salvajina dam, surviving adult fish provided food to local communities for some time in a section of the Cauca River in Colombia; however, the fish populations were ultimately depleted as there was no replenishment through breeding.
Looking ahead, the conservation of migratory fish in the Amazon basin is not only being hampered by human activities, but also by a general lack of knowledge. First, our understanding of the fish diversity of the Amazon basin is still growing, as species continue to be discovered, which means we could be potentially losing species without knowing. Second, those species already described generally lack data on population status and trends, two pieces of information that are vitally important to inform conservation priorities and specific actions. And third, the migratory patterns of fish in the Amazon basin continue to be described, meaning that there are still large gaps in our knowledge that need to be filled, before we can truly understand how dams and other human activities can impact their survival. Migratory fish are a very complex and fragile evolutionary group due to the heterogeneity of environments associated with their life cycles. Importantly, migratory fish could be considered as an umbrella species for conserving freshwater biodiversity, and thus have the potential to drive international conservation policies across the Amazon basin and other large river basins in the Neotropics and beyond.
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