Search

Albeit massive fish mortality has an extraordinary visual impact and is certainly a fatality, we still have rudimentary understanding on how addressing this problem in the Neotropical region. The processes that lead to fish die-off events are complex and sometimes ephemeral, which can lead to incorrect diagnosis. In this review, we discuss these events in Neotropical freshwaters, both in areas impacted by dams and natural environment, with a focus on deaths mediated by the lack of oxygen (anoxia) or the excess of dissolved gases (gas supersaturation). We examine the available knowledge about the related mechanisms, lethal thresholds for dissolved oxygen (DO) and total dissolved gases (TDG) for fish, and the sequelae of gas bubble disease (GBD). An assessment of the main mortality events in Brazil in the last 10 years is also presented, as well as the best practices for monitoring, prevention, and mitigation. Finally, it is concluded that the proliferation of hydroelectric plants in the Neotropical region might contribute to the expansion of these events and, consequently, increasing of impacts on fish conservation. We consider urgent the inclusion of this topic in the licensing processes for new hydropower projects by the environmental agencies. Resumo Embora mortandades massivas de peixes tenham impacto visual extraordinário e sejam certamente uma fatalidade, estamos ainda engatinhando no entendimento e na solução desse problema. Os processos que levam aos eventos de mortes de peixes são complexos e algumas vezes efêmeros, o que explica diagnoses incorretas. Nessa revisão nós discutimos esses eventos em bacias hidrográficas neotropicais, tanto em barragens como em ambientes naturais, porém com o foco nas mortes por falta de oxigênio (anoxia) ou excesso de gases dissolvidos (supersaturação gasosa). Em particular são examinados o conhecimento disponível acerca dos processos que levam a essas condições, os limiares letais de oxigênio dissolvido (OD) e gases totais dissolvidos (GTD) para os peixes e as sequelas da doença das bolhas de gás (DBG). Uma avaliação dos principais eventos de mortandade que ocorreram no Brasil nos últimos 10 anos é também apresentada, bem como discutidas as melhores práticas para o monitoramento, prevenção e mitigação. Conclui-se, finalmente, que a proliferação de hidrelétricas deve contribuir com a expansão desses eventos e consequentemente com mais um fator de ameaça à biodiversidade. Consideramos urgente a inclusão desse tema nos processos de licenciamento de novos empreendimentos pelo órgão ambiental.

Dam construction alters flow regimes and can change the composition of aquatic communities. Using data from three Brazilian hydrographic basins, we tested the hypothesis that reservoir cascades act as environmental filters for fish traits. This dataset included information on different environmental variables and fish traits (diet, migration, fecundation, parental care, position in the water column, and body size), and we used multivariate analysis (partial RLQ) to quantify the relationships between environmental variables, species abundance and traits. We found that the abundance of migratory species declined towards downstream reservoirs, which tend to be smaller and less turbid with a shorter water residence time than upstream reservoirs. We also found evidence of an association between reservoir age and the domination of fish communities by small-sized species with parental care, external fecundation, and benthic habits. Our findings suggest that particular fish traits are selected for across reservoir cascades.

Habitat heterogeneity provided by aquatic macrophytes and water level variations (flood pulse) are essential factors in structuring fish assemblages. This study aimed to describe the fish species that prefers macrophytes covered areas and to evaluate how macrophytes complexities and a flood pulse influence the structure of fish assemblages and selected attributes (viz. fish species density, species richness and evenness). Sampling was performed with seining nets in five floodplain lakes associated with the Baia River, before (November to December 2011) and after (February to July 2012) a flood, considering different degrees of macrophyte complexity (Absent, intermediate, and high). A total of 48 fish species was recorded, with Characiformes the most dominant. Eight species were indicators of high complexities (seven before the flood and only one after). Significant differences among the different degrees of macrophyte complexity and before and after the flood were found for assemblage structure, species density and richness. Fish assemblage attributes were high in higher habitat heterogeneity provided by macrophytes, but all before the flood. Nevertheless, macrophyte stands with high and intermediate complexity were less affected by the flood, suggesting that the structure propitiated by macrophytes favors the persistence of the fish assemblage in floodplain lakes. Therefore, any action towards conservation of fish assemblages in macrophytes should consider dam operation upstream, to ensure seasonality of flood pulses.

Floodplain river systems are extremely dynamic environments, where alternating dry and flooded periods affect the availability of food resources for fish. For piscivores, during drought years, a decreased availability of prey is expected, resulting in diets dominated by fewer items, narrower trophic niches with high dietary overlap. During floods, habitats become more similar, and, combined with increased connectivity, provide a wider diversity of prey to piscivores, decreasing dietary overlap and presenting wider trophic niches. This study aimed to evaluate the potential impacts of long periods of flooding on the trophic ecology of nine piscivorous fish in the upper Parana River basin. In drought years, diets were dominated by small characids, presenting narrower niches with higher dietary overlap. The opposite situation was observed during wet years, with different species responses, likely due to species-specific feeding strategies. High niche overlap was observed between native and non-native species, suggesting competition. The inclusion of non-native species in the native piscivorous diet is a concerning fact, as its causes and possible impacts are still unknown. In conclusion, long flooding pulses affect the diets of piscivorous fish in different ways by either increasing or decreasing niche breadth and overlap according to species-specific characteristics.

The Xingu River, located in northern Brazil, is already under the impacts of one of the most harmful projects in the history of the Amazon – the Belo Monte hydroelectric dam. A new megaproject called the ‘Volta Grande Project’, led by Canadian gold mining company Belo Sun will be situated adjacent to the same stretch of the Xingu that is dewatered by the Belo Monte complex. Various aspects of the project are controversial. There are reports that the mining company acquired public lands illegally, the affected area has high biodiversity and endemism of several taxonomic groups, and mining operations would be approximately 9.5 km from indigenous lands. Synergistic impacts of impoundment and mining operations are expected to dramatically alter the Xingu's biodiversity and ecosystem services that support indigenous people and riverine populations. Moreover, the mining activities will generate tons of toxic waste that will be stored in a high-risk tailings dam, which exposes the Xingu River to catastrophic consequences similar to those of the Doce River disaster.

The number of dams has had a significant increase in the last six decades, altering the physiography of rivers and creating systems of reservoirs in series. Therefore, it is expected that in a cascade of reservoirs, detritivorous fish will be especially affected due to the loss of nutritional quality in these regions, resulted from the high sedimentation of nutrients and organic matter. The objective of this study was to evaluate whether changes in the abundance of detritivorous fish, throughout a reservoir cascade in Brazil, resulted from trophic and limnological variations associated with sediment and nutrient retention. We used the Structural Equation Models approach (SEM) to test hypotheses. The six reservoirs studied are located in the Sao Francisco River, in the Northeast Brazil. The variables used to index latent variable oligotrophication were divided into trophic (total phosphate, nitrate and chlorophyll-a) and limnological variables (turbidity and water residence time accumulated), and the abundance of the detritivore guild was indexed by the catch per unit effort (CPUE). Our results indicate a significant negative impact of the both trophic (total phosphate, chlorophyll-a and nitrate) and limnological indicators (turbidity) on the oligotrophication process. The longer residence times of the water accumulated along the reservoir cascade augment the oligotrophication process, consequently affecting negatively all the trophic and limnological indicators analysed. Finally, the latent variable oligotrophication had a significant negative effect on the latent variable detritivorous fish abundance throughout the cascade. The Curimatidae family was the most affected by oligotrophication along the cascade, followed by Prochilodontidae. These different responses to the detritivorous fish families reflect different patterns in the morphology of the oral apparatus and forms of food apprehension. Our findings are of particular importance for the management of fish species in highly fragmented rivers.

The relationships between the degree of dietary overlap and food availability, and implications for food selectivity of fish species were evaluated at floodplain lakes on the upper Parana River floodplain. The hypothesis tested were: i) species become less selective in lakes with high availability of food resources; ii) species present higher food overlap in conditions of high availability of food resources; and iii) individuals of the species, become less specialized, in this case the food overlap will be high in conditions of more availability of food resources. In general, with the results was observed that species become less selective when the environment provided higher availability of food resources. Interspecific overlap did not show a pattern when evaluating availability of food resources in the lakes. However, intraspecific overlap tended to be more accentuated in conditions of high availability of resource food.

A misguided policy, based not on science but rather on the need to generate profit, threatens the highly diverse native fish communities in the Amazon River Basin. A controversial law—4330/2016 ([ 1 ][1])—signed by the governor of Amazonas State, Brazil, allows the rearing of non-native fish and