Impacts on fisheries assessed by local ecological knowledge in a reservoir cascade in the lower São Francisco River, northeastern Brazil

Abstract

Our study assessed the fishers’ perception (local ecological knowledge, LEK) concerning environmental impacts on fisheries and fish species in the final portion of Sub-Middle and Lower São Francisco. The river was divided into four areas where 107 fishers from 22 locations (4 states) were surveyed. The dam was the most significant impact on fisheries. The loss of connectivity, changing the environmental complexity, and its secondary effects (e.g., flow control, interruption of migratory routes) were determining factors. Other impacts were pointed out, such as pollution, macrophytes, overfishing, non-native species, and aquaculture. Among migratory species, Pseudoplatystoma corruscans and Salminus franciscanus have been absent from fishery catches for decades. Prochilodus argenteus records in fisheries are likely associated with migration routes to small tributaries. The environmental disturbance favored the establishment of non-native species such as Oreochromis niloticus, Cichla monoculus, and Metynnis lippincottianus along the studied section. Over the final portion of the São Francisco River, the more significant seawater intrusion changed the target species for fisheries activities. Thus, our data validate the importance of fishers’ LEK and reinforce the adverse effects of the reservoir cascade on fishing and migratory fishes.

Keywords:
Anthropogenic impacts; Ethnoecology; Migratory fishes; Non-native species; Small-scale fisheries

Resumo

Nosso estudo avaliou a percepção dos pescadores (conhecimento ecológico local, CEL) em relação aos impactos ambientais sobre a pesca e peixes no trecho final do Submédio e Baixo São Francisco. O rio foi dividido em quatro áreas onde foram entrevistados 107 pescadores de 22 localidades (4 estados). Barragem foi o impacto mais significativo sobre a pesca. A perda da conectividade alterando a complexidade ambiental e seus efeitos secundários (e.g., controle da vazão, interrupção da rota migratória) foram fatores determinantes. Outros impactos foram apontados, como poluição, macrófitas, sobrepesca, espécies introduzidas e piscicultura. Dentre as espécies migradoras, Pseudoplatystoma corruscans e Salminus franciscanus estão ausentes da pesca há décadas. Os registros de Prochilodus argenteus nas pescarias provavelmente estão associados às rotas migratórias para pequenos tributários. A perturbação ambiental favoreceu o estabelecimento de espécies introduzidas como Oreochromis niloticus, Cichla monoculus e Metynnis lippincottianus ao longo do trecho estudado. No trecho final do rio São Francisco, a maior intrusão de água do mar mudou as espécies-alvo da pesca. Dessa forma, nossos dados validam a importância do conhecimento ecológico local dos pescadores e reforçam os efeitos adversos das cascatas de reservatórios sobre a pesca e peixes migratórios.

Palavras-chave:
Espécies não-nativas; Etnoecologia; Impactos antrópicos; Peixes migradores; Pesca em pequena escala

INTRODUCTION

Dams are responsible for alterations in morphology and hydrology of rivers (Agostinho et al., 2007). Their effects on freshwater biodiversity are expressed by the shifts in aquatic productivity, ecosystem services, ecological process, composition and abundance of species, and even local extinction (Agostinho et al., 2007; Santos, 2010; Hallwass et al., 2013; Winemiller et al., 2016). The alterations caused by dams transform a dynamic system with high structural complexity into a homogeneous and less productive environment (Agostinho et al., 2007). These changes contribute to increased sedentary, generalist, and tolerant species and the decline of migratory species (Santos, 2010; Pelicice et al., 2018). The river fragmentation process also interferes in flood pulses, reducing river connectivity and access to floodplains that correspond to fundamental nurseries and feeding grounds (Agostinho et al., 2007). Blocking migratory routes is another impact that, combined with the previous ones, can generate failures in recruiting and maintaining viable populations (Sato, Godinho, 2003; Pompeu et al., 2012; Pelicice et al., 2015).

These issues are intensified in rivers with reservoir cascades. The restructuring of fish communities in these reservoirs has shown the prevalence of small-medium-sized species (Santos, 2010; Petesse et al., 2014; Loures, Pompeu, 2018; Pelicice et al., 2018) with trophic plasticity to explore the new areas, using their reproductive strategies for rapid colonization (Petesse et al., 2014). In these new environments, non-native piscivores also find favorable conditions for their establishment (Lockwood et al., 2007) and follow the same pattern of population increase (Santos, 2010; Pelicice et al., 2018). On the other hand, the decline in migratory fish populations is increasingly evident, mainly in the downstream direction (Santos, 2010; Petesse et al., 2014; Pelicice et al., 2018), except for reservoirs with lotic stretches above the flooded area or tributary (Loures, Pompeu, 2018). The consequences are also observed in social and economic dimensions since small-scale fishing represents the primary source of income and food for the riverine population (Hallwass et al., 2013; Doria et al., 2021).

The São Francisco River (~2,800 km in length) has large hydroelectric plants throughout its main course, and a reservoir cascade has caused numerous socio-environmental impacts in the final portion of the basin (Godinho, Godinho, 2003; Brito, Magalhães, 2017). Significant water flow restrictions over the past decades (2,800 m³/s – 550 m³/s) have also favored the establishment of invasive species such as the Peacock bass Cichla monoculus Spix & Agassiz, 1831, Nile tilapia Oreochromis niloticus (Linnaeus, 1758), and Spotted silver dollar Metynnis lippincottianus (Cope, 1870) (Assis et al., 2017; Brito, Magalhães, 2017). The absence of flood pulses has hampered connections of lagoons and floodplains to the main river channel, with adverse effects on the subsistence agriculture (e.g., rhiziculture) and the economy of the riverine population (Sato, Godinho, 2003; Martins et al., 2011; CBHSF, 2016; Santana et al., 2016; Brito, Magalhães, 2017). The reduction of river discharge has also led to more significant seawater intrusion, increasing salinity close to the river mouth, and intensifying the problems associated with a water supply and public health issues (Brito, Magalhães, 2017).

Implications over artisanal fisheries after the construction of reservoirs have been recurrent throughout the São Francisco River Basin (SFRB). In the Upper-Middle São Francisco, Três Marias hydroelectric power plant (HPP) construction caused changes in fishing productivity and the floodplains (Thé, 2012; Lopes et al., 2019). Sobradinho dam in the Sub-Middle São Francisco (SMSF) river altered the river discharge and its original functionality and importance for fish migration (Sato, Godinho, 2003), becoming more complex in the final portion with the construction of reservoirs from the Apolônio Sales and Luís Gonzaga HPPs. In the Lower São Francisco (LSF) portion, water control was intensified with the Paulo Afonso Hydroelectric Complex (I, II, III, IV) and the Xingó HPP (CHESF, 2020). The impacts over artisanal fisheries have been irreparable, causing changes in the structure of the ichthyofauna and the disappearance of important migratory and commercial species (Assis et al., 2017; Brito, Magalhães, 2017; D’avilla et al., 2017).

Studies developed with artisanal fishers have provided relevant data for the management of fishing resources, making it possible to meet the demands of a given area more effectively with local ecological knowledge (LEK) (Marques, 2001; Thé et al., 2003; Costa-Neto, Marques, 2008; Azevedo-Santos et al., 2010; Silvano, Begossi, 2012; Hallwass et al., 2013; Ramires et al., 2015). Artisanal fishers provide theoretical and practical information, which they have observed concerning the behavior, food habits, reproduction, and ecology of fish species (Costa-Neto, 2000). Also, fishers are aware of the relationships between biotic and abiotic environments, understanding where, when, and why organisms are found in specific environments (Marques, 2001).

LEK can support the study of environmental changes caused by human actions, as these changes are perceived by the actors who carry out subsistence and recreational fishing activities in impacted environments. The survey of information from ethnoecological studies makes it possible to understand the magnitude and evolution of social and biological impacts, especially in data-deficient and dammed areas (Hallwass et al., 2013). In this sense, we address the fishers’ perception (LEK) concerning environmental impacts on fisheries and fish species in the highly modified final portion of Sub-Middle and Lower São Francisco.

MATERIAL AND METHODS

Sampling sites. The SFRB has an area of 645,000 km², and drains the states of Minas Gerais, Goiás, Bahia, Pernambuco, Sergipe, Alagoas, also including the Distrito Federal (MMA, 2006). The basin is divided into four regions: Upper, Middle, Sub-Middle, and Lower São Francisco. The SMSF and part of the LSF are inserted in the Caatinga Biome (semi-arid climate), and the final portion of the LSF is situated in the Atlantic Forest Biome (humid tropical climate). The rainfall precipitation mainly influences the regional climate of the basin (400–1500 mm/year), with an average annual temperature varying between 20–26.5 °C (Silva, Clarke, 2004).

The study was carried out with artisanal fishing communities in the final portion of the SMSF and the LSF, between Luís Gonzaga HPP and the mouth of the São Francisco River. Four areas were defined according to river morphology and hydrology in order to assess if the impacts on fisheries would differ among areas based on their features (Fig. 1):

Area 1. Downstream of Luís Gonzaga HPP to the upstream area of Xingó HPP (~90 km). It is characterized by the reservoirs of Apolônio Sales and Paulo Afonso HPPs, and the lotic portion between the Paulo Afonso Hydroelectric Complex and the reservoir of Xingó HPP. The area is composed of remobilized massifs from the flattened soil surfaces of Paulo Afonso (BA) municipality, which expands into waterfalls and rocky beds of approximately 100 km in length (Cavalcante, 2011). Visited communities: Glória (BA) – Nova Glória, Povoado Quixaba, Povoado Queimadas; Paulo Afonso (BA) – Paulo Afonso, Povoado Rio do Sal, and Povoado Xingozinho; Jatobá (PE) – Jatobá.

Area 2. Downstream from Xingó HPP to Poço Redondo municipality (SE) (~35 km). It is characterized by a rocky and lotic portion that runs through a narrow channel (ca. 50 m) widening as the river follows its course (600 m). Visited communities: Piranhas (AL) – Piranhas Nova, Piranhas Velha, and Povoado Entremontes; Poço Redondo (SE) – Povoado Angico, Povoado Cajueiro, Povoado Curralinho, and Povoado Bom Sucesso.

Area 3. Pão de Açúcar (AL) to Gararu (SE) (~50 km). Floodplain area, with an increase of river width (over 600 m). The margin slope is reduced with the presence of meanders, sandy banks, and sedimentary banks on the river channel (Silva, 2009; Cavalcante, 2011). Visited communities: Pão de Açúcar (AL) – Pão de Açúcar; Porto da Folha (SE) – Povoado Niterói, and Povoado Ilha do Ouro; Gararu (SE) – Povoado Jenipatuba.

Area 4. Penedo (AL) to the mouth of the river in the Atlantic Ocean (~40 km). It is a coastal plain formed by the interaction between the river and the sea. Major influence of the sea with small islands close to its mouth. The area has flat surfaces of recent sediments, alluvial, wind, and beaches (Medeiros et al., 2007). Visited communities: Penedo (AL) – Penedo, and Povoado Ponta Mufina; Brejo Grande (SE) – Brejo Grande; Piaçabuçu (AL) – Piaçabuçu, and Pontal do Peba.

Four expeditions (75 days) were carried out between March and August/2017. The respondents were selected using the Snowball method (Bailey, 1982), which identifies a referenced specialist recognized by the community, who, after being interviewed, recommends another specialist, and so on (Albuquerque et al., 2010a). To survey the ethnoecological data, free and semi-structured interviews (Tab. S1), visual stimulus, and guided tours were conducted.

Free interviews consist of informal conversations with the studied population by establishing proximity and detecting relevant information to the research (Bernard, 2006). Semi-structured interviews involve asking targeted questions with a survey, leading to an effective method for gathering specific information (Huntington, 1998; Silvano et al., 2008). The semi-structured interviews were recorded with a SONY ICD PX333 recorder and later transcribed for content analysis. Personal information of fishers (e.g., age, time of fishing activity, and birthplace) and the status of fishes (e.g., distribution, introduction, and disappearance) were registered in interviews. Furthermore, additional information on environmental impacts was obtained to verify if changes observed in the ichthyofauna were related to the impacts mentioned above.

Visual stimulus occurred by displaying a manual of photographs of fishes from the SFRB to identify ethno-species and obtain ethnoecological information. The guided tour is a method that relies on the help of a local guide with extensive knowledge to elucidate information in the field (Albuquerque et al., 2010b) through tours conducted with fishers during the fishery activity (Marques, 1991). The specimens caught by the fishers were identified using identification keys, and taxonomical status following Fricke et al., (2020).

FIGURE 1 |
Fishers’ communities on the four areas visited on the Sub-Middle and on Lower São Francisco, Northeastern Brazil. PE = Pernambuco State; BA = Bahia State; AL = Alagoas State; SE = Sergipe State.

Statistical analyses. We analyzed all data by “Union of all individual competencies” (Hays, 1976) even if containing discrepancies. Tests to verify the consistency and validity of the information were performed through interviews in synchronous and diachronic situations. The first consists of making the same question to different respondents over a short time, and the second is applied when the same question is asked to the same respondent over a longer interval (Marques, 1991).

The Frequency, Word Cloud and Similitude analyses were performed with the software IRAMUTEQ version 0.7 alpha 2 (Ratinaud, 2009), using the R environment (R Core Team, 2019). A Frequency test demonstrated the expression of the impacts mentioned by the fishers according to the number of citations. Then, the Word Cloud analysis was performed, positioning the lexical groups according to repetitions (Camargo, Justo, 2013). This analysis exhibits the weight of the words mentioned by respondents, showing them in more extensive and centralized positions according to the highest frequency of citations.

To verify the relationship between the impacts mentioned by the fishers, the Similitude Analysis was carried out, which identifies co-occurrences between words and graphically demonstrates their connection through the presentation of trees — groups of words that are related to each other —, and the intensity of their connections illustrated by the line thickness (increased thickness = higher intensity) (Marchand, Ratinaud, 2012; Camargo, Justo, 2013). The difference from mentioned impacts between the surveyed areas was observed through a multivariate analysis of Non-Metric Multidimensional Scaling (NMDS) with Jaccard distribution and the Similarity Analysis (ANOSIM) using the free software Past version 21.7c (Hammer et al., 2001). P values < 0.05 were considered significant.

The status of fishes was defined regarding the frequency of mentions and the recognition of species by fishers during interviews and visual stimulus. The categories were classified in: Frequent = most mentioned and recognized species (51–100%), with occurrence confirmed by the fishers; Rare = species with intermediate mention and identification frequency (26–50%), with occurrence related to reducing stocks or local disappearance; Imprecise = species with low mention and identification frequency (0–25%), whose status was imprecise due to insufficient data; Probably extinct = species recognized and reported for the study area, but missing.

RESULTS

A total of 107 fishers were interviewed from 22 locations over 11 municipalities in the states of Alagoas, Bahia, Pernambuco, and Sergipe. The average age was 53.2±10.44 years, and the time of experience performing the fishery activity was 40.3±11.68 years. Most of the fishers (68%) were born and have lived in the communities where they were surveyed. A portion of them (18%) reported having migrated from adjacent locations when they were younger, while 14% of respondents migrated from locations that were flooded during the construction of reservoirs of Luís Gonzaga and Xingó HPPs.

Impacts on the ichthyofauna. The surveyed fishers have pointed out 13 impacts responsible for altering the ichthyofauna and reducing fish stocks in the region. The frequency analysis of the mentioned impacts in all sampled areas showed that the dam construction (93%) was the most significant one (Fig. 2A), corroborated by its prevalence in the Word Cloud analysis (Fig. 2B).

The analysis of impacts by area also highlighted dams as the significant impact (Fig. 3). For example, at areas 1 and 2, the shift in river dynamics caused by the construction of reservoirs (Figs. 3A,B) eliminated lotic environments that included waterfalls and rapids. Fishers reported that dams also interfere on migratory routes of essential species for artisanal fisheries, such as Pseudoplatystoma corruscans (Spix & Agassiz, 1829), Salminus franciscanus Lima & Britski, 2007, Prochilodus argenteus Spix & Agassiz, 1829, and Conorhynchus conirostris (Valenciennes, 1840), claiming that after its construction there was a reduction of their populations culminating in the disappearance of some of them.

Fishers from Area 1 also mentioned impacts such as aquaculture and overgrowth of macrophytes (Egeria densa and Eichornia crassipes) (Fig. 3A). Statements made by the fishers associated these macrophytes with introduced Amazonian fish C. monoculus and Colossoma macropomum (Cuvier, 1816) after the construction of dams. Macrophyte overgrowth was associated with the formation of lakes (lentic section), low river flow (lotic section), as well as pollution from the cultivation of O. niloticus in aquaculture cages (Area 1) and domestic and industrial effluents.

Overfishing was also among the most representative impacts (Fig. 3B), with the harpoon device identified as predominantly used to capture breeding individuals. However, the target species reported were non-native primarily species, such as O. niloticus, C. monoculus, C. macropomum, and Astronotus ocellatus (Agassiz, 1831), though P. argenteus was also mentioned as a target.

FIGURE 2 |
Frequency (A) and Word Cloud (B) analyses regarding the impacts over the ichthyofauna, according to fishers from the four sampled areas on the Sub-Middle and on Lower São Francisco. a. Dam; b. Macrophytes; c. Pollution; d. Overfishing; e. Absence of rain; f. Aquaculture; g. Deforestation; h. Silting; i. Water transfer; j. Non-native species; k. Irrigation; l. Pesticide; m. Salinization.

FIGURE 3 |
List of main impacts mentioned in A. Area 1, B. Area 2, C. Area 3, and D. Area 4, according to fishers from the final portion of the Sub-Middle and Lower São Francisco. Dam: dams; Mac: macrophytes; Aqu: aquaculture; Pol: pollution; Ovf: overfishing; Abr: absence of rain; Def: deforestation; Sil: silting; Salt: salinity.

The synergy of factors such as reduced rainfall, intense water use, water flow restriction through reservoirs, and diversion was listed as responsible for the compromised flow of the São Francisco River. These factors have contributed to the increase in the salinity of Area 4 with the increased intrusion of seawater over freshwater habitats (Fig. 3D). Fishers from Brejo Grande (SE) and Piaçabuçu (AL) highlighted the reduction and disappearance of some freshwater species, the increase of marine species in the region, and problems associated with saline water capture in communities near to the river mouth.

Area 1 impacts differed significantly (Fig. 4, ANOSIM; R = 0.151; p <0.001) from the other areas, due to the more significant number of citations of the impacts from aquaculture, pollution, and macrophytes overgrowth, enhanced by the transformation from lotic to lentic waters due to the construction of dams (Fig. 4).

The maximum similarity tree exhibited three lexical islands, where “Dam” played a central role in the impacts over fisheries, which was associated with the lexical islands “Silting” and “Macrophytes” (Fig. 5). The lexical island “Macrophytes” showed greater linkage power with the central island. The relationships were associated with the proliferation of macrophytes mainly due to water pollution and aquaculture of O. niloticus in the region.

Occurrence status. Fishers recognized 82 species (Tab. S2) and stated that some migratory and economically important ones from the São Francisco River have disappeared from the study area, such as C. conirostris (±20 years), P. corruscans (±10 years), and S. franciscanus (±10 years). Other migratory species were restricted to some areas with a reduced population. Brycon orthotaenia Günther, 1864 was poorly recognized (27%) and cited as missing in areas 1 and 4. On areas 2 and 3, some fishers have recognized B. orthotaenia, stating that the species has disappeared. However, it was captured one time over recent years. During the data collection expedition, a specimen of B. orthotaenia was registered from the artisanal fisheries at Povoado Entremontes (Area 2). Megaleporinus obtusidens (Valenciennes, 1837) is restricted to the lotic sections on the canyons (Area 1), with specific capture mentions on areas 2 and 3. Despite the low catches, two Prochilodontidae (P. argenteus and Prochilodus costatus Valenciennes, 1850) seem to be the only migratory species that still occur, possibly in all areas, considering their low abundance. Fishers highlighted the increase of catches during the rainy season and associated their occurrence with the presence of tributaries around the region, such as the rivers Moxotó (Area 1), Capivara, and Ipanema (Area 3), Itiúba, and Marituba (Area 4). The remaining presence of Anostomidae populations was also associated with the tributaries.

Currently, the most common native species in catches is Serrasalmus brandtii Lütken, 1875, along with other fish considered in the past as secondary or low value as Hoplias malabaricus (Bloch, 1794), Pygocentrus piraya (Cuvier, 1819), Trachelyopterus galeatus (Linnaeus, 1766), Hypostomus sp., P. etentaculatus, and now the commercially important R. aspera (Areas 1-2). Non-native species as Plagioscion squamosissimus (Heckel, 1840) (Area 1), C. monoculus, O. niloticus, A. ocellatus, C. macropomum, and M. lippincottianus have established populations in the final section of the SMSF and LSF. Fishers informed about the increase of occurrence of the non-native species, as well as the confirmation on reproductive interactions by finding spawning sites (e.g., nests with eggs and juveniles), and description of the behavior displayed during parental care (e.g., O. niloticus and C. monoculus). The most abundant species in fisheries activity were M. lippincottianus and S. brandtii, despite having low commercial value. Catches of P. squamosissimus have also been frequent but restricted to the lentic section of the reservoirs (Area 1).

Euryhaline species such as Caranx spp., Megalops atlanticus Valenciennes, 1847, Diapterus rhombeus (Cuvier, 1829), Mugil curema Valenciennes, 1836, Centropomus parallelus Poey, 1860, and Centropomus undecimalis (Bloch, 1792) have become more frequent in catches on areas 2, 3, and 4. The higher occurrence of euryhaline species has been associated with river regulation by dams, leading to an increase of salinity in Area 4 locations.

FIGURE 4 |
Non-Metric Multidimensional Scaling (NMDS) analysis concerning impacts over the ichthyofauna in the four sampled areas on the Sub-Middle and Lower São Francisco.

FIGURE 5 |
Maximum Similitude Tree showing the expression and relationship of impacts over fisheries in the four areas sampled on the Sub-Middle and on Lower São Francisco, based on fishers’ testimonies (n = 107).

DISCUSSION

The majority of fishers pointed out the impact caused by dams on the São Francisco River as the main responsible for changes in fisheries and fish species, as observed in other large Neotropical rivers (Hallwass et al., 2013; Santos et al., 2018; Runde et al., 2020). Dams interrupt the river connectivity, changing its environmental complexity and triggering a series of side effects that lead to environmental simplification. The formation of a lentic environment upstream, the controlled water flow downstream of the dam, changes in the morphology of the river channel (e.g., silting), limnological variations, and the interruption of migratory routes affect mainly rheophilic and migratory species (Reid et al., 2019; Zambaldi, Pompeu, 2020). These species depend on accessible routes to migrate and complete their life cycle (Sato, Godinho, 2003; Godinho et al., 2007; Andrade-Neto, 2008). Thus, the construction of reservoirs may lead to consecutive recruitment failures resulting in the disappearance of migratory species, as reported in the present study for P. corruscans, C. conirostris, and S. franciscanus, and the reduction in stocks of P. argenteus, P. costatus, and M. obstusidens.

This environmental change has created an invasion window (Lockwood et al., 2007) and enabled the establishment of non-native fish, such as C. monoculus, O. niloticus, A. ocellatus, P. squamosissimus, and M. lippincottianus. Invasive species characteristics, such as rapid colonization due to reproduction occurring almost year-round (Magalhães et al., 1996; Assis et al., 2017), parental care, and a generalist diet have enabled their establishment (Pérez et al., 2004; Agostinho et al., 2007). Besides, propagule pressure should not be ruled out since the introduction of species in Northeastern Brazil is of common practice (Brito et al., 2020), as well as escapes from aquaculture cages (e.g., O. niloticus) (Azevedo-Santos et al., 2010; Lima Junior et al., 2018). The absence of an anti-predatory response from the native species to foraging strategies of introduced piscivores (prey naiveté, Cox, Lima, 2006), as C. monoculus, may have favored their establishment and resulted in the reduction of small-size species as reported by fishers. The decline of small-size fauna seems inevitable in these cases (Pelicice, Agostinho, 2009). On the other hand, the herbivorous species M. lippincottianus was favored by the river regulation that resulted in the growth of filamentous algae (Assis et al., 2017), which corresponds to the main item of its diet.

The macrophytes E. densa and E. crassipes were negatively correlated to fisheries, given their impacts on navigability and equipment handling for fisheries activities. These macrophytes found favorable conditions for establishment in the reservoir lentic environment and the downstream section with controlled water flow. The overgrowth obstructed the water surface in some sections, with greater intensity on small branches of the reservoirs. At Area 1, macrophyte overgrowth was associated with residues from O. niloticus farming in aquaculture cages. This region has become one of the leading aquaculture sites for O. niloticus farming in Brazil, producing almost 33,000 tons in 2014 (Ribeiro et al., 2015). The high solar incidence (Araújo, 2011) and the significant incorporation of phosphorus, nitrogen, and carbon into the system (Kubitza, 1999) contributed to plant propagation. Moreover, macrophyte beds act as reproductive sites, foraging areas, and shelters for the first developmental stages (Delariva et al., 1994), which seem to have also collaborated in the establishment of non-native species pre-adapted to lentic environments (Assis et al., 2017).

The absence of sewage systems and waste processing are recurrent issues over the study area and the São Francisco basin (Figueiredo et al., 2011). In some locations, sewage is released in natura into the river channels, affecting water quality while prompting public health implications (Figueiredo et al., 2011). The improper disposal of oil from vessels has been another concern. Over the last decade, an investment in tourism in the canyons region (Area 1), visited by ca. 46,000 people in 2008 (Braghini et al., 2009), increased boat traffic in the region, possibly affecting water quality with pollution from oil by-products (Freitas et al., 2015).

The reduction in flow resulting from dams has significantly restricted the flood pulses and led to the more significant seawater intrusion over the region of the mouth of the São Francisco River (Souza, Leitão, 2000). This process was enhanced after constructing the Xingó dam on the main channel of the São Francisco River (Araújo et al., 2016). Also, a section of the river’s water volume on the final portion has been diverted to irrigate crops (Souza, Leitão, 2000) and shrimp farming, the latter activity having removed an essential portion of the mangrove areas (Carvalho, Fontes, 2007). The circumstances worsened with the ongoing reductions of river flow reaching 550 m³/s (Brito, Magalhães, 2017). Thus, the capture of marine and euryhaline species became more frequent, with records of M. atlanticus, Mugil spp., Cynoscion spp., C. parallelus, Bagre marinus (Mitchill, 1815), Eugerres brasilianus (Cuvier, 1830), and D. rhombeus (Souza, Leitão, 2000; D’avilla et al., 2017).

The construction of a reservoir cascade in the final portion of the São Francisco River began in the 1950s with Paulo Afonso I HPP and finished in 1994, with the start of operations of Xingó HPP (CHESF, 2020). As a result, populations of commercial migratory species were driven to decline, and some have disappeared. On the other hand, non-native species (e.g., C. monoculus, A. ocellatus, O. niloticus, P. squamosissimus, M. lippincottianus), and native species with higher tolerance to environmental variations (e.g., S. brandtii, L. piau, S. knerii, Hypostomus spp.) and affinity to lentic and semi-lentic environments (Santos, 2010) encountered optimal conditions to maintain their populations. The long period without records of P. corruscans and S. franciscanus attests to the harmful effect of dams on large-distance migratory species since they demand long lotic sections connected to tributaries and marginal lagoons (Godinho et al., 2007; Andrade-Neto, 2008). The living area of S. franciscanus, for example, can reach 243 km (Andrade-Neto, 2008). Recent records of B. orthotaenia in artisanal fisheries from the present study and Brito et al., (2016) are due to the stocking actions carried out by Companhia de Desenvolvimento dos Vales do São Francisco e Parnaíba (CODEVASF) in the LSF over the last decade. Similarly, the capture of C. conirostris in May/2020 (CBHSF, 2020) is a direct result of fish stocking performed by CODEVASF in 2017 (MFGB, pers. obs.; Video S3).

Prochilodus argenteus has been pointed out in the study area among the native commercial species. Recent records highlight the capture of sexually mature fish (Silva et al., 2018), suggesting that the species uses tributaries as spawning areas in the region. This migratory behavior has been observed in the upper São Francisco by species of Prochilodus (Godinho, Kynard, 2006; Rosa et al., 2018) that spawn in the tributaries and the drifting larvae reach the main river (Rosa et al., 2018; Lopes et al., 2019). We believe that tracing the migratory route of P. argenteus over this area can be a source of essential data for future investigations. Nevertheless, the population increase through artificial stocking is not ruled out, as CODEVASF conducts fish stocking with these native species (MFGB, pers. obs.). The native Loricariidae R. aspera is a secondary species that has stood out in local cuisine (Area 2) due to the collapse of medium-large commercial fishes. The increase of tourism over the region (Braghini et al., 2009) may lead to higher fisheries pressure and depletion of populations of this species in the future.

Integrative planning is essential to balance the potential of electric energy and the sustainability of natural resources to minimize the loss of biodiversity and other environmental, social, and economic effects (Winemiller et al., 2016). An old project for constructing another dam on the main channel of Middle São Francisco has been discussed and represents a new threat (Gomes et al., 2020). Maintaining the longitudinal connectivity and natural flow regimes of rivers (Zambaldi, Pompeu, 2020), combined with revitalization actions, must be effectively implemented to maintain biodiversity and ecosystem dynamics (Azevedo-Santos et al., 2021).

Fishers’ LEK represents a reliable data source for investigating biological, ecological, and socio-environmental regards, especially in areas with insufficient data. Our study provides evidence that signals the incidence of anthropic impacts on artisanal fishing in the final stretch of the SFRB in areas under the influence of the reservoir cascade. The data indicate that changes in the environment caused alterations in the ichthyofauna composition, with probable local extinction of migratory species of economic importance, resulting in losses on fishing activities. The information presented here aims to collaborate with management and supervisory agencies and provide subsidies to assist in regulating measures to conserve of fishery resources. The integration of academic knowledge and fishers’ knowledge favors a contextualized analysis connected with the reality of these social actors, which can result in management practices that are appropriate to the local fishing resource (Carlsson, Berkes, 2005).

ACKNOWLEDGEMENTS

We want to thank the artisanal fisheries communities of the Sub-Middle and Lower São Francisco, who contributed uniquely to this study. Thanks to Carlos B. M. Alves for the availability of fish pictures used on the visual stimulus manual. To the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for granting student funding (TD), and to the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the research grant (CNPq – 306851/2018-0, MFGB). We also thank the anonymous referees for various constructive comments on the earlier version of the manuscript.

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ADDITIONAL NOTES

• HOW TO CITE THIS ARTICLE

  D’avilla T, Costa-Neto EM, Brito MFG. Impacts on fisheries assessed by local ecological knowledge in a reservoir cascade in the lower São Francisco River, northeastern Brazil. Neotrop Ichthyol. 2021; 19(3):e200156. https://doi.org/10.1590/1982-0224-2020-0156

Edited-by

Fernando Pelicice

Publication Dates

• Publication in this collection
  08 Oct 2021
• Date of issue
  2021

History

• Received
  31 Dec 2020
• Accepted
  6 July 2021