Main

Community-based conservation (CBC), in which local communities lead the management and protection of natural resources, is one of the most promising conservation strategies in developing tropical countries1. There are several co-benefits of CBC initiatives2. First, CBC ensures biodiversity conservation by promoting sustainable land use practices and critical habitat protection3. Second, it can generate income and create employment opportunities for local communities through ecotourism, sustainable harvesting of natural resources and other forms of income generation4. Third, it can improve social and economic well-being locally by enhancing food security and providing greater access to social services and infrastructure5,6. Fourth, it promotes participatory decision-making, which can enhance the sense of ownership and responsibility among local communities7. Finally, CBC often contributes to the achievement of global conservation goals by effectively conserving biodiversity, building capacity and facilitating knowledge-sharing among stakeholders8.

CBC is seen as a feasible conservation approach in the Amazon because it combines territorial protection, local welfare and biodiversity conservation, while also generating income and preventing biodiversity loss6,9. A notable CBC initiative in the Neotropics is the co-management of pirarucu, or giant arapaima (Arapaima gigas) fisheries in Amazonia6,10. Territorial protection is crucial within CBC as it supports source–sink dynamics in harvested Amazonian landscapes, aiding the recovery of historically overexploited species9,11.

CBC-mediated territorial protection is ensured through year-round, 24/7 environmental surveillance, deterring poaching by local and external resource users12. Community surveillance restricts outsider access and enforces protection rules through punitive measures, such as confiscating equipment, removing trespassers and notifying government agencies13. Surveillance, initiated in 1995, was key in developing ‘fishing agreements’—formal commitments among communities to comply with management rules—and led to the 2002 consolidation of a community-based environmental protection system12.

The CBC approach in Amazonia has been remarkably successful. For example, wild pirarucu populations increased by 425% along the Juruá River9, mirroring trends in other basins adopting this approach14. Additionally, this approach positively impacted fish community structure and composition, increasing species richness, body mass, abundance and biomass15.

A major motivation for pirarucu fisheries co-management is to generate demographic benefits for resource populations, resulting in both subsistence and direct income16. However, the economic burden of territorial surveillance falls heavily on disenfranchised local communities, potentially threatening the long-term viability of this conservation programme. Pirarucu CBC presents a common-pool resource dilemma: enhanced fish populations allow fishers to secure predictable quotas, but this requires costly monitoring and enforcement to ensure sustainable floodplain management. Beyond monitoring costs, fishing communities face substantial logistical expenses in marketing their fish quotas17. Research suggests that conservation policies should offer sufficient incentives to stimulate local economic interests and mobilize commitments to formalize conservation actions18. Therefore, the disparity between positive large-scale conservation outcomes and low socioeconomic benefits poses a serious challenge to the sustainability of CBC efforts9.

One way to offset these costs is through payment for environmental services (PES) programmes co-designed with communities, providing compensation for resource use and stewardship to ensure environmental services. PES participants can be individuals, enterprises, non-governmental organizations (NGOs), private institutions or the public as direct or indirect beneficiaries of territorial protection19. PES often involves carbon sequestration, water quality, ecotourism and biodiversity protection20. Implementing PES for biodiversity is challenging as a result of indirect, delayed benefits, often requiring legal support21. In Latin America, PES uptake has increased, targeting forest carbon, hydrological catchments and biodiversity conservation. Balancing payments is complex because of socioecological intricacies and history; PES can reduce deforestation, especially under long-term contracts22 and bolster ecosystem resilience23. Even in areas with limited governance, PES can yield notable benefits24.

Successful PES programmes should adapt to diverse conditions and be integrated into flexible, existing governance structures. Long-term effectiveness requires continuous monitoring of costs, production and impacts, with a focus on enhancing natural resources rather than merely maintaining the status quo25. Effective implementation also hinges on creating schemes that enhance social equity and foster intrinsic motivation, which are justified on both normative and instrumental grounds26. These schemes should be managed autonomously, fairly and efficiently to maximize socioenvironmental benefits and ensure resource conservation27.

Here we quantitatively assess the impact of the largest CBC programme in Brazilian Amazonia by examining the full extent of community-led environmental surveillance and their associated costs considering 96 protected lakes located along the Juruá River, a major tributary of the Amazon. Specifically, we estimate the full spatial extent of floodplain and upland forests that local communities can effectively guard as a consequence of CBC and the economic cost incurred by this surveillance, which are currently borne out by the communities. We use these estimates to highlight the enormous effort invested by Amazonian rural communities to ensure the protection of natural ecosystems, arguing for the development of new governance and financial tools to reward strong local conservation measures by legitimate resource users as a cost-effective and socially just approach to ensure forest protection. Finally, we discuss the potential viability of PES approaches to support CBC and reduce the asymmetry between the costs of conservation efforts incurred locally and environmental benefits accrued at much larger spatial scales.

Results

Operational structure of community-led protection

Local guards covering a floodplain area are community members who participate in pirarucu management and may be organized in pairs or small teams of up to eight people. The rotation among teams is determined by the community and is established depending on the physical environment, such as lake location, lake accessibility, distance to the community support base and number of guards available in the community (Fig. 1a). In general, surveillance forays ranged from 6 h to seven consecutive days, but in several communities the guard lived in a strategically located floating house on the lake all year-round. Surveillance costs are mostly paid for by community members themselves, in which household food and fuel supplies are made available to enable travel to the vicinity of each surveillance site. However, a few communities included surveillance activities as part of the total cost of pirarucu management. The most critical surveillance season was when the level of floodwaters was receding. Illegal fishers at this time could rapidly move into a lake and harvest protected stocks of commercially desirable fish species, resulting in the remaining stock to relocate from any given lake in search of safer sites elsewhere (Fig. 2).

Fig. 1: Scales of protection generated by pirarucu co-management in Amazonia.
figure 1

a, Illustration elucidating the landscape where territorial protection and surveillance are implemented. Protection extends far beyond oxbow lakes, covering substantially larger areas. Pirarucu co-management activities have varying impacts at different spatial scales: (1) direct scale of protection—immediate lake areas under surveillance; (2) effective scale of protection—full-time territorial surveillance, intensified during the dry season, protecting areas of management interest; (3) functional scale of protection—estimated on the basis of the movement ecology of pirarucu, considering their ability to sustain ecological interactions; and (4) incidental scale of protection—indirect surveillance of adjacent upland forest areas that are incidentally protected by restricting access to the floodplain by outside users. b, Left, mid-section of the Juruá River, western Brazilian Amazonia. Orange circles represent 14 communities located within two contiguous sustainable-use forest reserves, with a combined area of 886,176 ha. These communities perform territorial surveillance for co-management of pirarucu (A. gigas) fisheries within 96 lakes (indicated by blue dots). Right, shows (1) the effective scale of protection (in yellow), which included to the routes that community rangers patrol to protect lakes and (2) the scale of functional protection (shaded in orange), in which pirarucu stocks are fully protected to move into floodplains during the high-water season. Finally, the wider (3) scale of incidental protection (shaded in grey) represents the adjacent upland (terra firme) forests that are also closed off by restricting access by outsiders into floodplain forests.

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Fig. 2: The Juruá River flood pulse (in metres) over the last 38 years and community-based surveillance efforts.
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Community surveillance efforts are intensified during the period of receding floodwaters in which commercially valuable fish stocks become more concentrated and more vulnerable.

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Community-based protection footprint

A total of 96 protected oxbow lakes under the direct jurisdiction and stewardship of 14 rural communities and hosting a population count of approximately 109,000 adult pirarucu were mapped along the Juruá River (Fig. 1b). These communities were on average spaced by 82.8 km from the nearest town (range 51.78–110.9 km). Each of these communities on average contained 12.6 families (range 2–32), with a total of 177 families participating in community-led lake surveillance. On average, 6.4 lakes (range 1–13) were protected per community, with individual lakes accounting for a mean dry-season area of 47.39 (±82.26) ha. The spatial extent of direct protection was on average 305 ha per community, but the wider territorial protection resulting from effective protection was on average 2,346 ha (Fig. 3a and Supplementary Table 1). In other words, the extent of effectively protected areas was almost eightfold (7.69) larger than the aggregate size of all protected lakes within the jurisdiction of any given community, which corresponds to the actual focal area deriving financial returns through co-management. The functional floodplain area supporting co-management was even larger: on average, this amounted to 11,189 ha per community, an area ~36-fold larger than the directly protected area (analysis of variance (ANOVA), d.f. = 3, F = 93.41, P < 0.001; Supplementary Fig. 1 and Supplementary Table 2). Finally, each local community included in this study incidentally protected an overall additional average upland area of 12,383 ha of terra firme (non-flooded) forests by simply closing off those areas by severing physical access through the floodplain area protected by CBC. This area was on average nearly 40-fold larger than the directly protected area. Combining all four zones of either direct or incidental protection, each community protected a mean total area of floodplain and upland forest nearly 86 times larger than the total dry-season area of lakes sustaining local pirarucu populations.

Fig. 3: Community-based surveillance achieves broad protection at lower costs than alternative models.
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a, The area (in hectares, log10 scale) and spatial scale of protection implemented by Amazonian rural communities in pirarucu co-management along the Juruá River. b, The protection costs (US$ per hectare per year) borne by these communities, including estimated costs under three alternative scenarios: (1) using two lake guards per community at local daily wages, (2) hiring two guards under Brazilian labour regulations and (3) hiring two environmental agents deployed by a government agency. Each boxplot summarizes data from 14 communities protecting a total of 96 lakes. Boxes represent the interquartile range (interquartile range (IQR) 25th to 75th percentiles), the horizontal line inside each box indicates the median and whiskers extend to the most extreme values within 1.5× IQR from the box. Points beyond this range are shown as individual outliers.

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Financial cost of CBC protection

The current community-scale monetary costs of environmental surveillance were calculated on the basis of real-world expenditure information reported by each community. The mean annual cost of territorial surveillance was estimated at ~US$31,271 to ensure the overall effective protection of 32,844 ha of floodplain environments. In other words, on average, ~US$0.95 yr−1 was spent on each hectare of effectively protected area. Surveillance expenditure was conservatively estimated at zero labour costs and based on only fuel and food supplies consumed by lake guards who volunteered to contribute unpaid labour time. These costs are low compared with the estimated community-led costs based on our three potential hypothetical PES scenarios. We found that if two lake guards were to be rewarded by local daily wages, these costs would increase to US$5.30 ha−1 yr−1. Assuming that labour costs for two guards could be met considering the Brazilian minimum wage, these costs would slightly increase to US$5.40 ha−1 yr−1. Finally, considering standard payment rates recently awarded by Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio), the official environmental protection agency, these costs would further increase to US$9.60 ha−1 yr−1 (Fig. 3b).

On average, surveillance costs represented 32% of the overall costs of community-based fisheries management and exerted a negative impact of 21% of the net community income. Our model selection identified three most parsimonious models (Supplementary Table 3). The model-averaging approach revealed that travel distance to the farthest lake, which was typically isolated from the river channel, was the most important predictor across all plausible models explaining protection costs (corrected Akaike information criterion weight (ωAICc) = 1). Additionally, the authorized pirarucu harvest quota (ωAICc = 0.33) and number of protected lakes (ωAICc = 0.32) emerged as additional important variables, each appearing in one of the selected models (Supplementary Fig. 1).

To ensure a fair reward system to local fishers who protect wide-ranging fisheries resources that transcend local jurisdictions, we identified three potential scenarios according to the payment system complying with Brazilian labour regulations. The cost of any PES programme was calculated considering the fishing quota allowed by the regulatory agency, so that PES costs could be equated to a standard unit of fish off-take (kilograms of harvested fish). This facilitates the payment rationale to local fishers, in addition to the fact that large quotas translate into more intensive efforts to protect supporting habitats. In these terms, local payments would range from US$0.94 kg−1 of fish considering local wages to US$1.70 kg−1 of fish considering ICMBio hiring practices (Supplementary Table 4). Considering labour costs in terms of the current minimum salary according to Brazilian labour law, cost estimates would be comparable to those based on local daily wages (US$0.95 kg−1). These cost estimates mean that ensuring the viability of a PES programme covering the entire Central Juruá River basin would require funding in the order of between ~US$1,770,000 and ~US$3,170,000 each year. If we were to project those values to support community-based fisheries management across the entire state of Amazonas, this would require between ~US$50.3 million and ~US$90.1 million in annual payments, which would benefit over 400 rural communities and ensure the socially just protection of approximately 15 million hectares of floodplain forests.

Discussion

The territorial protection and resource surveillance carried out by Amazonian local communities involved in pirarucu co-management has ensured the protection of vast areas of tropical forest, safeguarding the flow of several ecosystem services at different scales6. Beyond the ecological benefits reported to date, our results show that community-led protection of aquatic environments within community-based fisheries arrangements also ensure the added-value protection of much larger aquatic and terrestrial areas compared with only the aggregate lake area where dry-season fishing activities are conducted. Quantifying the enormous effort allocated by Indigenous Peoples and local communities to protect their own territories reinforces the positive role of traditional people in conserving Amazonian environments28.

Local communities are always present, thereby protecting their aquatic environments all year round and 24 h each day. Yet commercially valuable fish stocks become more vulnerable during the receding floodwaters, which renders community protection efforts even more diffuse, more complex and more demanding. Therefore, surveillance requires enormous dedication of time and effort, in addition to incurring a high cost to already low-income families, by limiting their capacity to engage in alternative profitable activities and subsistence food production16. Community-led territorial governance and protection therefore represents a substantial opportunity cost for local households. It then becomes critical to recognize and ideally enhance communal surveillance activities through financial support for local communities if the long-term success of these CBC outcomes are to be maintained22.

Spatial footprint of community-led protection

We show that the scale of effective environmental protection by Juruá communities, considering their routine surveillance routes, is almost eightfold larger than the actual aggregate lake area. This extended protection footprint becomes much larger, considering that guarding floodplain environments during the critical time of the year incidentally precludes physical access to adjacent upland forests that would otherwise be reached. Therefore, by precluding illegal incursions by outside users into the floodplain and its anastomosing channels, local communities also ensure the added-value protection of vast areas of unflooded upland forests. In this context, beyond the strong positive impact on fisheries resources and aquatic biodiversity shown elsewhere6,9, the effective protection of both várzea and terra firme forests during the low-water season clearly delivers strong additional benefits to terrestrial biodiversity conservation by preventing illegal exploitation by fishers, hunters, loggers and other illegal activities. A concrete example of this protection was recently observed when the collective action of fishers and local leaders, supported by local organizations, expelled illegal gold miners who had entered the Juruá River. This sociopolitical initiative facilitated enforcement by the Federal Police and Brazilian environmental agencies, who set fire to a large dredge used for mining sediments the riverbed.

Another key finding reported here is the spatial extent of functional protection, which is an important hidden positive impact of CBC. The Juruá River experiences a flood pulse that can reach depths of up to 11 m for up to 230 days each year5. Pirarucu fish exhibit lateral migration patterns during this prolonged flood pulse, including habitual movements into flooded forests between tributary lakes and perennial streams and the main river channel9. Population recovery of this apex predator is closely associated with lateral migration and replenishment of depleted environments9, which can impact the top-down trophic dynamics across an area ~255-fold larger than the neighbouring lake area, thereby controlling the abundance of other important prey species5. In addition, the spatial contagion of enforcing protection ensures recolonization of previously depleted areas far away from the target lake, reinforcing the importance of co-management activities in promoting food security for Amazonian rural communities29.

Cost of community-based protection in a seasonal environment

Pirarucu population viability is closely linked to the hydrological cycle, including the supra-annually variable seasonal flood pulse, which markedly alters the seasonal fluvial connectivity of the floodplains along major meandering rivers of the Amazon9. During the flood season, pirarucu moves between lakes, the main river channel and the flooded forest, where they gain access to high-quality food sources9. When floodwaters begin to recede, pirarucu shows a high degree of site fidelity, returning to their breeding lakes, particularly when conditions are quiet including low ambient noise9. Our results show a much greater community effort during this period in protecting stocks against human disruptions induced by fishing gear and poaching (Fig. 3b). This leads to a marked peak of labour-intensive surveillance activity that requires substantial logistics, including food supplies, fuel, boats and canoes, and a larger number of volunteers because schools of pirarucu can flee the lakes prematurely if they perceive threats from outside fishers.

Comparing the costs of community-led efforts against alternative scenarios that rely on proactive participation of government agencies or NGOs, we easily reach the conclusion that local community inclusion in conservation arrangements is the cheapest and most cost-effective mechanism to ensure the protection of natural ecosystems, such as the Juruá floodplains. However, we emphasize the glaring lack of social justice behind this strategy given the heavy burden and local opportunity costs considering that the time and effort spent in territorial protection could be allocated to alternative income generation activities. In fact, the substantial asymmetry between large conservation benefits accrued at multiple scales and the local socioeconomic costs incurred locally represents one of the main bottlenecks in implementing community-based arrangements. This distortion thus needs to be addressed to strengthen the CBC model in Amazonia and beyond.

Although the costs of community-led protection can be seen as exceedingly low compared with the typical investments in conservation interventions by most external agencies30, those values are extremely high for disenfranchised local communities, which accept to soldier on because this heavy burden yields many other benefits beyond a simple monetary trade-off5. Our study communities have legitimized their interests through co-management actions, increasingly engaging in conservation practices with intrinsic motivations that are often above economic payoffs. In addition to collective decision-making, there is a collective sense of autonomy and belonging that ensures access to natural resources for both present and future generations7,31. Given little or no action enacted by toothless environmental agencies throughout the Amazon, this local community empowerment has filled the vacuum by successfully protecting their own territories against major threats by external enterprises waging predatory overexploitation32,33.

Strengthening recognition of hidden environmental services

Community-led biodiversity protection through local empowerment can ensure socioenvironmental governance and maintenance of ecosystem services and opportunities for self-development both inside and outside protected areas5, especially when confronting hostile policies dismantling environmental regulations34. However, local communities cannot continue to shoulder the heavy burden of environmental protection continuously without external support. This is vital for the maintenance of CBC, given that biodiversity-based value chains are not sufficiently fair to cover the intrinsic costs of environmental protection. In addition, beyond the financial costs associated with surveillance efforts, there are other secondary opportunity costs incurred by neglecting horticultural investments, which also provide subsistence and income35,36,37. Furthermore, a relentless state of day-and-night surveillance and readiness imposes a substantial physical and psychological toll, given the ever-present possibility of violent hostilities from potential intruders, which in extreme cases can be life-threatening.

PES services have the potential to contribute highly positive conditional incentives for the provision of ecosystem services19. Although this approach is more common in terrestrial conservation, it has recently grown in fisheries management38. In sum, PES is more likely to succeed within fisheries arrangements that show (1) demand for one or more ecosystem services or bottlenecks in the value chain; (2) evidence-based approach with a clear baseline; (3) clear boundaries and property rights; (4) strong local governance; (5) robust monitoring, control and surveillance; and (6) financial sustainability38. Pirarucu co-management in the Brazilian Amazon shows a high level of community organization, in addition to the balanced participation of local institutions, NGOs, academic institutions and government agencies. These conditions provide a solid foundation for the implementation, organization and development of PES programmes involving established CBC arrangements. This is critical because the lack of sociopolitical organization often makes these schemes unworkable24.

Our study clearly underscores an imperative moral challenge of directly compensating local communities providing a wider public good generated by their environmental protection efforts11. A fairer return on their conservation efforts is vital to compensate for their tangible contributions and roles as protagonists of these arrangements, aligning biodiversity protection with local well-being. As such, strengthening and ensuring better surveillance conditions and greater economic returns to local communities can capture the long-term goals of local environmental and socioeconomic sustainability.

A co-designed PES model should be transparent in terms of who pays (the buyers), who benefits (the beneficiaries) and who sells (the providers)39. We advocate that a PES mechanism within the pirarucu CBM programme in Brazil should be supported multilaterally between inter-governmental funds, non-governmental initiatives and international cooperation, considering that the ecosystem services indirectly provided by local communities operate at a global scale32. The Brazilian government has the means to implement a PES programme, which could become a key financial mechanism, strengthening the economic benefits of environmental protection, promoting an increased sense of ownership and engaging new communities into pirarucu management, similarly to other PES programmes such as the Bolsa Floresta40. However, we highlight the importance of securing enough decision-making power for local leaders and community representatives to ensure procedural and representative justice throughout the entire process of implementing and maintaining programmes33.

Community participation is a crucial element in the processes of designing, implementing and monitoring the effectiveness and success of PES activities40. In addition, this must be based on transparency among investors, beneficiaries and providers41,42. Thus, the active participation of community members, together with inter-institutional partnerships, can render bureaucratic and legal processes enforceable in a participatory manner39.

PES programmes have raised substantial ethical and social concerns. Treating natural resources as commodities, subject to transactions, can exacerbate unequal benefit distribution, potentially disadvantaging involved communities40. For initiatives to be effective, equity in PES benefit distribution must be integrated throughout the workflow. Ignoring the interconnected aspects of sociobiodiversity can undermine conservation efforts. Focusing solely on measurable environmental services and oversimplifying ecological processes can undervalue natural resources41,43. A comprehensive resource assessment is necessary to avoid excluding critical operational factors in PES development and maintenance43.

While benefiting from PES, communities can paradoxically become dependent and vulnerable without strategies to mitigate financial and structural risks ensuring programme continuity42. Diversifying funding sources reduces risks associated with interruptions or delays in payments and benefits40. This approach ensures long-term viability for community-based surveillance systems, making them fair activities41. Our findings show that CBM is a highly viable and cost-effective method for implementing PES, allowing for territorial protection across vast Amazonian forest areas with relatively modest investments, greatly enhancing frontline conservation efforts.

Pirarucu co-management in Brazilian Amazonia has emerged as a strong window of opportunity to align biodiversity protection with sustainable and equitable prosperity. However, any socioeconomic gains accrued from sustainable off-takes are still very modest considering the huge positive conservation impact5,6,9,16. Here we spotlighted the hidden added-value of community-based territorial surveillance, which ensures both biodiversity protection and the provision of a wide range of ecosystem services that, in turn, enhances the quality of life of local people. We reinforce the need to both recognize and reward the herculean effort allocated by local communities to protect Amazonian natural ecosystems. It is thus imperative to consolidate this new pathway towards a brighter future for Amazonia, in which local livelihoods and the protection of the largest tropical forest on Earth are inextricably linked.

Methods

Study area

This study was primarily conducted within the ~2.58 million-ha municipal county of Carauari (4° 52′ 58′′ S, 66° 53′ 45′′ W), State of Amazonas, Brazil, along the Juruá River, a major tributary of the Solimões (Amazon) River. This region is strongly influenced by commercial and subsistence activities involving fishing, agriculture and Euterpe (açaí) fruit and oilseed extraction36,37. This region contains two contiguous sustainable-use protected areas: the 632,949-ha Uacari Sustainable Development Reserve (RDS Uacari, 5° 43′ 58′′ S, 67° 46′ 53′′ W; decree no. 25,039 of 1 June 2005) and the 253,227-ha Extractive Reserve Médio Juruá (ResEx Médio Juruá, 5° 33′ 54′′ S, 67° 42′ 47′′ W; decree of 4 March 1997). These reserves were decreed in 1997 and 2005, respectively, and currently contain ~4,000 inhabitants distributed across 74 communities, most of which are near the river channel, along a fluvial distance of 800 km, in addition to communities located along the banks of oxbow lakes and perennial streams (Fig. 1b).

Resource governance resulting from pirarucu co-management

To ensure both economic and food security for rural communities, fishing accords (that is, formal agreements) were widely negotiated in the mid-Juruá region during the 2010s. These accords involved local communities, including those outside protected areas, as well as the Fishers Cooperative of Carauari, the nearest urban centre. The agreements created three different categories of access to lake resources during the dry season, when lakes become clearly discrete geographic features where fish concentrate: (1) subsistence-use lakes, which are intended to supply local subsistence needs, and which are restricted to artisanal fishers from the resident community who are responsible for guarding that lake; (2) protected lakes, which are managed by local communities primarily as pirarucu stock recovery sites, and exclude both commercial and subsistence fishing boats, except for a brief community-led off-take season based on a strict harvest quota predetermined by IBAMA, the Brazilian Natural Resources Agency6; and (3) production lakes, which are open-access to both commercial and subsistence fishers.

A floating wooden watchtower is typically erected at the main strategic entrance to the lake. Equipped with makeshift hunting gear and subsistence supplies, these stationary posts, occupied by a small patrol unit and managed by the resident community, conduct year-round, 24/7 surveillance, often armed with shotguns. During the pirarucu co-management season, some of the protected lakes are harvested by the resident community for a brief period of up to 5 days per year, according to a previously determined proportional harvest quota based on a stock assessment defined as the number of adult and juvenile pirarucu counted at that lake in the previous year6.

Annual pirarucu counts began at several lakes along the mid-Juruá in 2005, and lake management was implemented in 2010 by a partnership between local communities, local associations and federal and state agencies. Pirarucu counts take place during the low-water season at each monitored lake each year and the census data are forwarded to IBAMA. IBAMA then authorizes a lake-specific harvest quota of up to 30% of all adults (>1.5 m in length) counted, depending on the fish processing requirements of the resident community and other extenuating factors.

Data analysis

Quantifying territorial protection

We conducted participatory community mapping through semi-structured interviews with lake guards, community leaders and community residents. First, we asked general questions to describe the surveillance dynamics, including the main actors, surveillance alternation dynamics, impact of seasonality on surveillance dynamics, surveillance pathways, conflict resolution strategies and associated costs. Participatory community mapping occurred interactively using A3-sized hardcopy cartographic maps showing LANDSAT-8 satellite images in RGB (5,4,3) colour composition, with a scale of 1:100,000 for location and identification of each lake where territorial surveillance had been deployed by each community. Each lake management category was identified by outlining locations on the map using colour markers42. Participatory community mapping was carried out with community residents who had extensive previous experience with both spatial landmarks across the waterscape, which is the main form of transport in this region, the overall landscape and indepth knowledge of pirarucu co-management activities44. Floodplain mapping was carried out within the scope of either community meetings or visits to resident households45. Experienced individuals were identified by community leaders.

Pirarucu co-management activities exert varying impacts at different spatial scales of influence (Fig. 3a). First, there is a direct scale of protection, represented by the immediate lake area where actual surveillance takes place. Second, there is an effective scale of protection, which is represented by the total area within the community surveillance boundaries. Third, there is a functional scale of protection, represented by the functional impact zone exerted by spatial exclusion, particularly related to the vagrancy and movement capacity of the target species protected at each lake. Finally, there is an incidental scale of protection at which local communities indirectly protect large portions of upland (terra firme) forests farther inland by simply restricting entry to strategic access points within the more accessible adjacent floodplains. We emphasize that the collective surveillance dynamics are structured by the internal regulations of each community. These regulations establish the local rules that must be followed by both local residents and outsiders who may use the areas protected by the communities. The internal regulations outline the local norms, clearly stating that illegal activities and unregulated resource exploitation are prohibited in the lakes and adjacent forests, including illegal hunting, illegal logging and gold mining, for example.

Direct scale of protection

During the mapping sessions, all lakes protected through surveillance that are managed by any given community were identified and further classed as direct surveillance areas, as they are the focus of management activities and their total area was measured using the MapBiomas Água Project collection 1 dataset, which mapped all open-water bodies across Brazil.

Effective scale of protection

Territorial surveillance for lake protection is a set of actions and adaptive strategies that occur on a full-time basis, but intensified in the dry season, to protect areas of management interest. These areas include subsistence-use, protected and production lakes that are harvested for local subsistence. Surveillance aims to protect lakes from illegal harvesting by either local or external fishers and any other exploitation activities that can disturb the lake and the surrounding forest, such as hunting and timber extraction. Surveillance strategies are continuously adapted according to the needs of each community and depend on the number of managed lakes, number of people available for surveillance, landscape context and geographic accessibility of each lake.

Surveillance is conducted by travelling around the perimeter of each lake by canoe or on foot, depending on the season, searching for any presence or signs of intruders. In several communities, floating wooden houses are placed at strategic entry points of access to lakes to optimize surveillance. During surveillance, lake guards cover a floodplain area much larger than the size of individual lakes, which we refer to as ‘effective scale of protection’, where illegal activities, including poaching, fishing and logging, are excluded. To estimate the effective protection of each lake, we combined GPS tracks and spatial data recovered from interviews to map the daily paths that community guards frequently travelled to protect each lake. The area effectively protected, including seasonally flooded várzea forest and open-water bodies, was estimated, including all reported paths on foot and/or canoes between lakes and all strategic surveillance points that were frequently accessed by outside users attempting illegal fishing. Polygons drawn during participatory mapping were reproduced in QGIS 3.14 at the same scale using the corresponding satellite image to fine-tune estimates of the effective scale of protection.

Functional scale of protection

We also estimated the functional protection area of each lake on the basis of the ranging ecology of giant pirarucu (A. gigas), the conservation target species in this arrangement. We therefore considered pirarucu movement patterns, which had been quantified during a previous telemetry study9, to estimate the capacity of each lake to function as a source area of individuals moving into depleted lakes and the spatial configuration of landscape-scale population gene flow, both of which can sustain ecological interactions and top-down control of food webs mediated by an apex predator5.

This was estimated using a 1,730-m buffer area around the dry-season perimeter of each lake (that is, the direct scale of protection). This threshold value corresponds to the radius of an average circular pirarucu home range area, defined by the minimum convex polygon formed by positional fixes obtained for 12 juveniles and adults. Six of these individuals were tracked in our study area in 2014 and seven in 2015 using conventional VHF telemetry, amounting to 309 locations, 125 and 184 of which were during the dry and wet seasons, respectively9. Individual estimates are available in Supplementary Table 5.

Incidental scale protection

In addition to these three scales of protection, oxbow lake surveillance also incidentally protects all the rear areas of upland forests by closing off the physical accessibility to unauthorized users of the várzea floodplain. This strategy prevents non-resident loggers, hunters and fishers from accessing upland areas, typically to stealthily exploit subcanopy natural resources without the explicit consent of the local community. This scale of protection was estimated by multiplying the total width of várzea floodplains protected at the effective scale by a conservative 10-km length of upland forests that could be potentially affected by illegal extractive activities46,47. To assess differences in spatial extent between different scales of protection, an ANOVA was performed with the response variable on a logarithmic scale. Assumptions of normality of residuals and homogeneity of variances were evaluated using the Shapiro–Wilk and Levene tests, respectively48.

Assessing protection dynamics and costs

To better understand local surveillance priorities according to the flood pulse dynamics, we organized focal group interviews at each community with 45 experienced fishers who had conducted local lake surveillance for at least 15 years. These focal groups were adept at mapping the seasonality of surveillance because of previous experience, and fluctuations in water level change the accessibility to water bodies and their vulnerability. Monetary costs of surveillance were acquired during interviews and encompassed general operational expenditure including fuel, food and butane gas used as fuel to power outboard motors during surveillance routes, according to the unique ways in which each community carried them out. This excludes labour input and expenditure related to purchase and maintenance of wooden or aluminium boats, outboard motors, paddles and infrastructure such as strategically positioned floating houses, which served to accommodate lake guards during surveillance shifts. To supplement our field data, we compiled the annual reports of pirarucu co-management fisheries provided by the Association of Rural Producers from Carauari (ASPROC) produced in 2022. ASPROC is a grassroots smallholder and fisher-led organization leading the pirarucu co-management along the Juruá River. We computed the surveillance expenses associated with all four scales of surveillance and subsequently compared costs under three different scenarios: (1) current expenditure covered by local communities or guards who were community members lacking any labour wage payments, (2) general expenditure and costs considering local daily wages of US$14.30 for two people working all year-round; (3) costs incurred by hiring two individuals receiving a minimum wage of US$442.24 (US$247.30 in wages plus US$194.90 in labour taxes) to conduct surveillance in compliance with Brazilian labour regulations; and (4) potential expenditure of US$852 (US$510.20 in wages plus US$341.80 in welfare taxes) covered by the Brazilian Environmental Agency for two additional environmental agents, according to the hiring notice SEI/ICMBio 15343964 and law 7.957/1989.

We also performed generalized linear models using a Gaussian distribution for continuous data to investigate the community-scale variation in protection costs (response variable) as a function of fluvial distance to the nearest town, number of lakes requiring protection, distance to the farthest lake and the locally authorized harvest quota. We mitigated for collinearity between predictors using the variance inflation factor (<3), excluding variables above this threshold49. We further combined all possible models, from the constant to the full model, using the dredge function of the MuMIn package. Models were selected on the basis of the lowest AIC corrected for small sample sizes (AICc). The ΔAICc value represents the difference between the AICc of a given model and the lowest AICc, whereas ΔAICc < 2 represent the most likely set of parsimonious models49. Finally, we applied a model-averaging approach, which represented the beta average of all predictors included in the set of most parsimonious models, and determined the relative importance of each explanatory variable given their model frequency and cumulative Akaike weight. All analyses were conducted in R v.4.3.1. All monetary costs were standardized and corrected for inflation from October 2021 to July 2023 and converted into US$ using a 4.91 BRL exchange rate.

Last, we estimated the financial imperative of meeting the overall costs of territorial protection through a PES mechanism. In an attempt to estimate values that could cover the costs of territorial protection, we built three alternative scenarios (1) considering all operational costs, including fuel and food requirements and, at least two people hired through daily wages, (2) operational costs and minimum wages following Brazilian labour regulations and (3) operational costs and human resources hired through the standard practices followed by ICMBio, Brazil’s environmental agency responsible for protected areas and environmental management. These compensation mechanisms provide valuable insights into ways of rewarding local dwellers for their role in territorial protection. However, it is essential to recognize that community protection of their environments is a collective effort. Therefore, involving local leaders in programme design is crucial from the outset to identify the most effective ways of rewarding those engaged in territorial protection. Finally, we divided these values by the potential fish catch of each community to calculate monetary expenditure per unit of fish biomass harvested, which can facilitate the rational implementation of a PES programme based on territorial protection and official catch statistics.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.