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The local ecological knowledge (LEK) of fishers may help to fill the knowledge gaps about migration of tropical fish. We investigated fishers’ LEK on migratory patterns of seven fish species along 550 km of the Tapajos River, in the Brazilian Amazon. We interviewed 270 fishers individually in four stretches of this river. The interviewed fishers indicated that three fish species do not migrate over long distances, while four fish species perform migrations, usually longitudinal migration from downstream to upstream reaches. Fishers also mentioned an increase in size of a large catfish species in the upstream stretches of the studied river, indicating the potential occurrence of spawning adults there. These results from fishers’ knowledge indicated that planned dams in the upstream reaches of the Tapajos River will threaten migratory fishes, small-scale fisheries and food security. Fishers’ LEK on fish migration contributed to raise testable biological hypotheses about fish spawning and feeding grounds, as well as the occurrence of distinct populations along the river. The analysis of the LEK of many fishers at several sites over a broad range is a cost-effective source of information on fish migration, supporting environmental impact assessment, fisheries management, and conservation in this and other tropical rivers.

There has been much written about the negative social and environmental impacts of large hydropower dams, particularly the impacts on people and the environment caused by flooding linked to the creation of large reservoirs. There has also long been recognition of the importance of Indigenous and local knowledge for understanding ecological processes and environmental impacts. In this paper, however, we focus on a topic that has received insufficient consideration: the downstream impacts of dams, and the role of Indigenous and local knowledge in assessing and addressing these impacts. Using examples from three river basins in different parts of the world: the Peace–Athabasca in Canada, the Mekong in mainland Southeast Asia, and the Amazon in Brazil, we demonstrate that the downstream impacts of hydropower dams are often neglected due to the frequently long distances between dams and impacted areas, jurisdictional boundaries, and the less obvious nature of downstream impacts. We contend that Indigenous or local knowledge, if applied consistently and appropriately, has important roles to play in understanding and addressing these impacts, with the goal of avoiding, reducing, and appropriately compensating for the types of environmental injustices that are frequently associated with the downstream impacts of dams.

This opinion piece, written by ethnobiologists from different parts of the world, emphasizes the importance of ethnobiology research in advancing contemporary biology, natural resource management, biodiversity conservation, sustainable development, and, especially, contributing to the ecological transition and more just and inclusive world. To achieve these goals, it is essential to develop research and collaborate with social groups that live in close relationship with nature in research activities, such as Indigenous Peoples and Local Communities (IPLC), as well as Afro-descendants and other Marginalized, Minority or Minoritized Communities (AMMC). Ethnobiology can identify and provide locally appropriate solutions to local problems, enabling sustainable resource management at the landscape level. The text explores important aspects that need to be considered to guide the future of ethnobiology in the next 20 years, aiming to integrate and amplify previous discussions held in the discipline and identify points that demand ongoing attention. This paper highlights reflections from diverse researchers, emphasizing how ethnobiology can embrace different perspectives and employ rigorous analysis of complex phenomena toward effective policies and practices. This approach holds the potential to address the challenges the planet is currently facing in the coming decades.

In the Tapajós River, Brazilian Amazon, fishing is an important activity, especially for low-income riverine populations. Unfortunately, the Tapajós River fish diversity and abundance are threatened by several anthropogenic drivers, including deforestation and overfishing. We modeled the lower Tapajós River's food web and simulated changes in biomass compartments as a response to increases in deforestation (loss of floodplain forest habitat) and on artisanal fishing pressure over 30 years. According to our simulations, the large-bodied species could be reduced drastically while small-bodied and fast-growing species could be favored by fishing effort increasing. The loss of floodplain forest is expected to cause a general decline (23%) of the total standing fish biomass. This reduction could reflect greater losses on species that are directly dependent on resources from the floodplain forests, such as fruits and seeds. These results indicated that the food web of the lower Tapajós River is structurally characterized by bottom-up control, through the use of basal resources, such as detritus (mostly from decomposing plants), fruits, seeds, terrestrial, and aquatic invertebrates. Furthermore, the simulations’ results highlight that the protection of the floodplain forest through the existing protected areas will be of essential importance in the future to maintain fish biomass, sustainable artisanal fishing, and improve the food security of Amazonian riverine inhabitants.

Gillnets, which are frequently used in small-scale fisheries (SSF), raise concerns regarding bycatch and conservation. This study evaluates the context of use and the potential advantages of gillnets alone compared to mixed (gillnet and other gear) and other gears in multi-species freshwater SSF, based in analyses of 36,869 fishing events in four rivers in the Brazilian Amazon (Tapajós, Negro, Madeira and Xingu) and considering five indicators: catch per unit of effort (CPUE), fish value (average income per kg), diversity and composition of fish caught, and variance of catches. Gillnets were used in most (~ 61%) of fish landings in three of the studied rivers, and the frequency of use matches the observed advantages from gillnets in each river. Overall, the probability of gillnet use was statistically (P < 0.05) lower inside protected areas and higher when fishing in group, being also influenced by season. The highest advantages of using gillnets were to reduce variance (hence unpredictability) on the biomass of fish caught in the Tapajós River (L.Ratio = 9.59, P = 0.008), to increase CPUE (t(15220) = − 2.94, P = 0.005 and t(15230) = − 4.73, P < 0.001, compared to mixed and other gears, respectively) and to reduce variance (L.Ratio = 29.26, P < 0.001) in the Madeira River, and to catch more valuable fishes in the Xingu River (t(13160) = − 3.68, P < 0.001 and t(13160) = − 3.67, P < 0.001, compared to mixed and other gears), whereas in the Negro River, both gillnets and mixed gears increased CPUE (t(1073) = − 2.89, P = 0.004). The gillnets could also allow fishers to harvest a higher diversity of fish. However, mixed gears provided similar or better outcomes when compared to gillnets alone. These results can inform policies aiming to manage use of gillnets while considering fishers’ needs in the Brazilian Amazon and other multispecies SSF.

Mercury is one of the most dangerous contaminants on the planet. In recent years, evidence of mercury contamination in the Amazon has significantly increased, notably due to gold-mining activities. Although mercury contamination in fish has consistently been documented, little is known about the risk associated with fish consumption by populations in urban areas of the Amazon. We sampled 1010 fish sold in public markets in six state capitals and 11 additional cities. Mercury levels were determined for each specimen, and the evaluation of the health risks associated with consuming mercury-contaminated fish was conducted according to the methodology proposed by the World Health Organization (WHO). Our study reveals that more than one-fifth (21.3%) of the fish sold in urban centers had mercury levels above the safe limits (≥0.5 µg/g) established by the Brazilian Health Surveillance Agency (ANVISA). The prevalence of Hg contamination ≥0.5 µg/g was approximately 14 times higher in carnivorous than in noncarnivorous fish. The analysis of the risk attributable to fish consumption reveals that daily mercury intake exceeded the reference dose recommended by the U.S. EPA in all population groups analyzed, reaching up to 7 and 31 times in women of childbearing age and children from 2 to 4 years old, respectively. However, these risks are diverse depending on the type of fish consumed and must be considered to formulate appropriate nutritional guidelines for safe fish consumption by the local community.

A Correction to this paper has been published: https://doi.org/10.1007/s00267-021-01440-7

Empirical data are needed to show the efficacy of co-management, which is regarded as a promising approach to achieve conservation goals. In this study, we addressed the potential influence of fisheries co-management to increase fish abundance and fishing yields in the lower Tocantins River Basin (Brazilian Amazon), downstream from a large dam. We analyzed 590 fish landings (6.7 t of fish) from five fishing villages and 48 fish samples obtained using gillnets (10,378 fish from 101 species) in 12 floodplain lakes in four regions: two with incipient co-management and two unmanaged. The fish species richness did not differ among the regions, but the lakes in the regions that were co-managed had higher fish abundance (biomass and number of individuals) and a higher mean proportion of fish reproducing during the high water season. Fishers had higher catches per unit of effort in the co-managed regions than fishers in the non-managed regions. These results were also influenced by geographic factors (distance and accessibility of lakes), as fish biomass was higher in lakes that were distant from the main river and from the main city in the region. Managers should thus consider strategic selection of the geographic locations of managed sites, even in remote areas. However, the fish biomass sampled in lakes was more related to region than to the lakes’ geographical location. Therefore, co-management has at least partially contributed to increased fish abundance and fishing yields in the studied region, through the protection of an important fish habitat (lakes). We provide empirical evidence that co-management can contribute to the maintenance of fish abundance, sustainability of fisheries, and food security in large tropical rivers impacted by damming.

Frugivory and seed dispersal by fish is an important mutualistic interaction in complex and species-rich tropical rivers. The local ecological knowledge (LEK) held by fishers can provide new information on relationships between fishes and plants in less studied rivers. This study aims to investigate the feeding interactions between frugivorous fish and plants through interaction networks based on the fishers' LEK in three rivers in the Brazilian Amazon (Negro, Tapajós and Tocantins). A total of 418 fishers were interviewed in 24 communities (eight in each river). The studied fishes were tambaqui (Colossoma macropomum), matrinxã (Brycon spp.), pacu (Myloplus spp.), pacu manteiga (Mylossoma duriventre), pirapitinga (Piaractus brachypomus), and jaraqui (Semaprochilodus spp.). The interviewed fishers cited a total of 92 plants consumed by the six frugivorous fishes in the three rivers. The interaction networks showed a higher nestedness in the Tocantins, greater connectance and modularity in the Tapajós and more specialization in the Negro, where the protected areas may have contributed to a more complex and specialized interaction network. The more nested network in the Tocantins River indicated the loss of specialized interactions in disturbed communities. The Tapajós River network showed the highest number of interactions between fish and plants, but this river has been threatened by environmental changes. Fishers' LEK associated to network analyses can advance our understanding on ecological interactions. This approach can be also useful to evaluate and mitigate ecological effects from anthropic changes in the Amazon and other high diverse tropical rivers.

Freshwater small-scale fisheries sustain millions of livelihoods worldwide, but a lack of monitoring makes it difficult to check the sustainability of these fisheries. We aim to compare and describe participatory research methods used in studies with fishers in the Tapajos River, a poorly known tropical river in the Brazilian Amazon. We address three interview approaches, two ways to do fisheries monitoring and two approaches for georeferenced mapping based on fishers’ knowledge, which can provide data about at least 16 topics related to fisheries. We highlight major advantages and shortcomings of these methods and illustrate their potential with examples of results on fisheries and fish biology of Peacock bass (Cichla spp. tucunaré in Brazil), an important commercial fish in the Brazilian Amazon. The interviews, participatory monitoring and mapping revealed which fish are more valued by local communities, how fish abundance and sizes varied over time, when fish are more often caught and show reproductive activity, and which sites or habitats fish need to reproduce. In addition to providing useful data from many sites in a cost-effective way, participatory methods can bring the additional benefit of including local stakeholders in the monitoring, management, and research activities.