A Review of Rice-Field Fisheries, Aquatic Animal Production, and Pollution Governance in the Lower Mekong Basin: Evidence from the Vietnamese Mekong Delta

1. Introduction

Freshwater fisheries or inland capture fisheries are important components of global food systems, particularly in low- and middle-income countries where fish and other aquatic animals provide affordable animal protein, essential micronutrients, employment, income, and livelihood security. Their contribution is often underestimated because subsistence harvests, informal trade, and dispersed small-scale fisheries are inadequately represented in conventional production statistics. Household-based estimates indicate that freshwater catches may be substantially higher than officially reported, highlighting the importance of inland fisheries to populations that have limited access to alternative sources of nutritious food [1,2]. At the same time, freshwater biodiversity is particularly vulnerable to interacting pressures from overexploitation, pollution, flow modification, habitat degradation, and other forms of river-basin development [3].

The Lower Mekong Basin exemplifies these close relationships among hydrology, aquatic biodiversity, fisheries, and human well-being. Its fisheries depend strongly on seasonal flood pulses, longitudinal and lateral connectivity, fish migration, and access to floodplain feeding, spawning, and nursery habitats [4]. Seasonal inundation connects rivers with wetlands, canals, ponds, and rice fields, transferring water, sediments, nutrients, organisms, and organic matter across the river–floodplain landscape. This connectivity supports not only fish but also a diverse assemblage of other aquatic animals, including crustaceans, molluscs, amphibians, and aquatic insects. Historically, these resources have made major contributions to food consumption and rural livelihoods throughout the Lower Mekong Basin [3,5,6]. Long-term Mekong River Commission (MRC) monitoring further demonstrates the basin's exceptional aquatic diversity, recording 617 fish species across monitored sites during 2007–2018.

At the downstream terminus of this river system, the Mekong Delta region of Vietnam is both a highly productive food-producing region and a strongly modified river–floodplain environment. Rice cultivation, freshwater aquaculture particularly Pangasius catfish and brackish-water shrimp (including white legs shrimp: Litopenaeus vannamei) farming have become economically important production sectors. However, wild fish and other aquatic organisms harvested from rivers, canals, wetlands, ponds, ditches, and seasonally flooded rice fields remain important for household food supply, informal markets, and livelihood diversification. Rice landscapes should therefore not be considered exclusively as crop-production areas. When connected to surrounding aquatic habitats, they form seasonally dynamic ecosystems that support naturally recruited fish and other aquatic organisms. In this review, rice-field fisheries refer to wild or naturally recruited aquatic organisms harvested from rice fields and associated canals, ditches, ponds, wetlands, and floodplains, distinguishing them from deliberately stocked rice–aquatic animal systems and conventional pond or cage aquaculture.

The ecological functioning of these landscapes has changed substantially with agricultural modernization and increasingly intensive water management. Since the late twentieth century, high-dyke construction and controlled irrigation have enabled the expansion of double- and triple-rice cropping by reducing seasonal inundation [7]. Although these interventions have increased the predictability and intensity of rice production, they have also restricted river–floodplain connectivity, reduced sediment and nutrient deposition, and limited fish access to seasonally inundated feeding and nursery habitats. Intensive rice production has simultaneously increased dependence on fertilizers, pesticides, pumping, and other external inputs [8,9,10]. Recent farmer-based evidence from An Giang Province similarly indicates perceived reductions in sediment and nutrient inflows, water quality, wild-fish yields, and fish diversity following high-dyke development, together with increased use of synthetic fertilizers [11,12].

These landscape transformations have occurred alongside reported declines in inland fishery yields in several parts of the Mekong Delta region. Importantly, such declines should not be interpreted as evidence of a single causal mechanism. Analyses of long-term fishery trends indicate that changes in catches are associated with combinations of fishing pressure, altered flood magnitude and duration, habitat loss, reduced connectivity, agricultural intensification, and broader hydrological change [4]. Upstream hydropower development and sediment trapping can further modify flows and sediment delivery, while groundwater extraction, land subsidence, channel modification, and salinity intrusion increase environmental pressures on the Delta [13,14]. Fishery responses therefore emerge from cumulative processes acting at different spatial and temporal scales rather than from pollution, hydrological alteration, or exploitation in isolation.

Water pollution and contaminant exposure nevertheless represent increasingly important components of this cumulative pressure. The Mekong Delta’s dense canal network simultaneously receives agricultural drainage, aquaculture effluent, domestic wastewater, and industrial discharges while supplying water for irrigation, aquaculture, capture fisheries, household activities, and navigation. Intensive agriculture introduce nutrients and pesticide residues into receiving waters, whereas aquaculture generate localized loads of suspended solids, organic matter, nitrogen, phosphorus, veterinary pharmaceuticals, and disinfectants. Pangasius catfish production, for example, may contribute a relatively small proportion of total basin-scale nutrient loads while still producing substantial localized pollution where untreated pond wastewater enters small canals or receiving waters [15]. Pesticide residues have been detected in surface water, sediments, and drinking-water sources, and recent studies have documented microplastic contamination in both surface water and sediments [4,16,17,18].

Pollution exposure is also spatially and temporally heterogeneous. Monitoring across the Mekong Delta of Vietnam has demonstrated substantial variation in microbial contamination, salinity, metals, organic pollution, and other water-quality parameters among seasons and locations, with smaller waterways influenced by intensive land use and settlements potentially experiencing greater degradation than major channels [19]. This spatial variability is particularly important for fisheries because contaminant effects may interact with temperature, dissolved oxygen, salinity, habitat quality, food availability, and hydrological connectivity [4,20]. Consequently, detection of a contaminant does not itself demonstrate a population-level effect on fisheries. Robust attribution requires evidence linking pollution sources to exposure, biological responses, recruitment, population dynamics, and ultimately fishery yields. Such causal evidence remains comparatively limited for many pollutant mixtures in the Mekong Delta.

Aquaculture further complicates this relationship. Its expansion has increased aquatic-food production, employment, and commercial value, but farmed production cannot necessarily substitute for all functions of wild fisheries. Wild and cultured aquatic foods differ in species diversity, accessibility, nutritional composition, ownership, market dependence, and livelihood functions. At the same time, integrated systems can provide opportunities to reconcile production with ecological processes. Comparative research in the Mekong Delta indicates that rice–fish systems combined with reduced agrochemical dependence can provide economically and environmentally attractive alternatives to intensive high-dyke rice monoculture [21]. Such findings emphasize that maintaining aquatic production requires management of both production practices and the ecological connectivity on which aquatic organisms depend.

A further challenge lies in governance. Water, fisheries, rice production, aquaculture, pesticides, wastewater, hydropower, and environmental protection are managed through different institutions and regulatory frameworks, whereas water, sediments, fish, and contaminants move across sectoral and administrative boundaries. Research on Mekong water governance has identified mismatches between the spatial scale of environmental processes and existing institutional arrangements, including policy fragmentation and insufficient integration of transboundary hydrological change into regional planning [22,23]. Effective pollution governance therefore requires more than regulatory standards; it requires coordinated monitoring, source identification, data sharing, enforcement, ecological assessment, and institutional cooperation across farms, canals, provinces, and the wider basin.

Despite extensive research on Mekong’s fisheries, rice intensification, dyke development, aquaculture, water quality, individual contaminants, and hydrological change, these topics have largely developed as separate fields of inquiry. Few syntheses have explicitly connected the transformation of rice-field fisheries with aquaculture development, contaminant pathways, basin-scale hydrological pressures, and pollution governance. This review addresses this gap by conceptualizing the Mekong Delta of Vietnam as a coupled agri-aqua-culture system embedded within the wider Lower Mekong Basin. The review aims to: (i) examine the historical transformation of rice landscapes, rice-field fisheries, and aquatic animal production; (ii) critically evaluate evidence linking hydrological alteration, agricultural intensification, water quality contamination and pollution to rice-field fisheries/inland fisheries and aquatic ecosystem responses; (iii) assess the coherence and implementation of relevant fisheries, water, environmental, and” pollution-governance instruments; and (iv) identify management transitions and research priorities capable of reconciling food production with aquatic biodiversity, environmental quality, livelihood security, and climate resilience.

2. Review Approach and Analytical Framework

This study employed a systematic critical review combining systematic evidence mapping [24] with structured narrative synthesis [25]. The review built on the evidence compiled in the IPEN technical reports on chemical pollution and fisheries in the Mekong River, Vietnam [26,27], and subsequently expanded, updated, and critically reassessed this evidence base.

Four questions guided the synthesis: (i) how rice intensification and hydraulic modification have transformed rice-field fisheries and aquatic production; (ii) which pollutants, contaminants, and exposure pathways are most strongly supported by available evidence; (iii) how effectively existing governance instruments address these pressures; and (iv) which production and governance transitions could improve ecological and livelihood resilience. The Mekong Delta was the primary study area, while evidence from Cambodia, Lao PDR, and Thailand was included where relevant to transboundary hydrology, sediment transport, fish migration, water quality, pollution, or governance.

The evidence base comprised peer-reviewed articles, books, official statistics, legislation and policy documents, Mekong River Commission reports, and publications from international organizations both English and Vietnamese Language. Searches were conducted in Web of Science, Scopus, CAB Abstracts, and Aquatic Sciences and Fisheries Abstracts, supplemented by ScienceDirect, Google Scholar, publisher databases, and institutional repositories. Search terms combined geographic descriptors with keywords related to “rice fields and floodplains”, “fisheries”, “rice–fish and rice–shrimp systems”, “nutrients-sediments”, “pesticides”, “antibiotics”, “wastewater”, “plastics”, “acid-sulfate soils”, “water quality”, “regulation”, “monitoring”, and “climate adaptation”.

Sources were critically appraised for methodological quality, geographic relevance, temporal resolution, transparency, and data traceability. Meta-analysis was not undertaken because of substantial ecological and methodological heterogeneity among studies. Consequently, relationships among hydrological alteration, agricultural intensification, contaminant exposure, and fishery change were interpreted as interacting cumulative pressures rather than single-factor causal effects.

3. Results and Discussions

3.1. Prevalence and Productivity of Rice-Field Fisheries and Their Contribution to National Fisheries in the Lower Mekong Basin Countries

Rice-field fisheries are a widespread yet poorly enumerated component of food production in the Lower Mekong Basin. They comprise connected aquatic landscapes of seasonally inundated paddies, canals, ditches, ponds, and permanent refuges, rather than harvests taken only within cropped fields. These habitats support finfish and other aquatic animals, including crustaceans, molluscs, amphibians, and aquatic insects. Because catches are dispersed, seasonal, and used largely for household consumption, exchange, or small-scale sale, their contribution is commonly obscured in production and employment statistics [6,28,29,30]. Evidence from Cambodia, where rice-field fisheries have been most extensively quantified, illustrates this statistical invisibility. In a survey of 37 villages, 58% of households whose members fished did not identify fishing as a primary or secondary occupation [31]. Similarly, during three years and 19 sampling rounds, 35% of surveyed households never reported selling their catch [32]. Occupational statistics indicating that only 20–26% of Cambodian households participated in fishing therefore excluded many occasional, subsistence, and landless fishers [33].

Catch-based estimates also demonstrate substantial productivity. Freed, Kura, Sean, Mith, Cohen, Kim, Thay and Chhy [32] estimated annual finfish yields of 66.8 ± 10.7 to 104 ± 16.0 kg/ha/year, depending on assumptions about household participation, and an upper-bound other aquatic animals yield of 34 ± 7.9 kg/ha/year. These values are consistent with a synthesis estimate of 112 kg fish/ha/year and a wider range of 25–209 kg fish/ha/year for rainfed rice landscapes in Cambodia and Thailand [34]. In Battambang, monitored wet-season catch averaged 119 kg/ha, comprising approximately 76% finfish and 24% other aquatic animals [35]. These figures describe harvested yield, not ecological production, and are sensitive to fishing effort, sampled area, gear coverage, and inclusion of dry-season catch.

Estimates of the national contribution of Cambodian rice-field fisheries consequently range from about 30% in official accounts to 60–70% in field-based extrapolations [32,34]. This divergence reflects alternative definitions of rice-field habitat, uncertain inundated areas, incomplete recording of subsistence catch, and spatially variable yield. Sites near Tonle Sap and well-connected Community Fish Refuges may also be unusually productive, whereas drought during the 2013–2015 assessment probably depressed catch. Cambodian values should therefore not be extrapolated uncritically across the Lower Mekong Delta. Nevertheless, evidence from Lao PDR confirms the importance of rice-based aquatic biodiversity to household diets [36], while earlier Vietnamese Mekong Delta studies reported contrasting yields in acid-affected and non-acid floodplain rice landscapes (summarized by Hortle, Troeung and Lieng [35]). Later research links rice intensification, high dikes, reduced habitat connectivity, and greater agrochemical use with declining availability of wild aquatic foods [10,37]. The principal governance implication is that omitting subsistence harvests causes assessments to undervalue losses associated with pollution and hydrological modification. National monitoring should therefore include landless and occasional fishers; year-round catch-and-effort diaries; finfish and other aquatic animals identification; standardized habitat areas; hydrological conditions; and separation of wild, stocked, and cultured production. Linking these observations with contaminant measurements in water, sediment, and biota would provide a defensible basis for evaluating fishery productivity, food-safety risks, and the benefits of pollution control.

3.2. Regional and Global Consumption of Rice, Rice-Field Fisheries (Inland Fisheries), and Aquatic Animal Foods

Apparent consumption refers to food available for human consumption at the national or regional level, rather than the amount actually eaten. Food balance sheets estimate it from production, trade, stocks, and competing uses. Although useful for comparing broad trends, they inadequately capture household waste, unequal access, informal trade, subsistence harvesting, or intra-household allocation [38]. This is especially problematic for inland and rice-field fisheries, whose catches are often consumed directly or exchanged informally. Moreover, rice is reported in milled-rice equivalent and aquatic foods in live-weight equivalent, so their quantities are not directly comparable.

Rice remains the principal staple across most of Asia and is increasingly important in Africa. It supplies dietary energy for more than half of the global population and may provide up to 50% of caloric intake among rice-dependent, low-income groups [39]. The OECD–FAO baseline for 2023–2025 estimated global consumption at 50.7 kg per person per year. Consumption was highest in Asia (72.2 kg), followed by Africa (26.0 kg), Latin America and the Caribbean (25.4 kg), Oceania (19.0 kg), North America (11.5 kg), and Europe (6.8 kg) (OECD–FAO, 2026). These averages conceal substantial variation. Consumption remains high in mainland Southeast and South Asia but has declined in wealthier East Asian economies as urbanization and rising incomes encourage dietary diversification. Global demand will nevertheless rise mainly through population growth. By 2035, per-capita consumption is projected to remain nearly unchanged globally at 50.8 kg, decline marginally in Asia to 72.1 kg, and increase in Africa to 29.6 kg [40].

Aquatic animal food availability has grown more rapidly, from 9.1 kg per person in 1961 to 20.7 kg in 2022 and 21.1 kg in 2023. Aquaculture now supplies approximately 59% of aquatic animal foods available for direct consumption [41]. In 2023, availability averaged 26.3 kg in Asia, 22.1 kg in North America, 21.2 kg in Europe, 20.6 kg in Oceania, 10.1 kg in Latin America and the Caribbean, and 9.1 kg in Africa. Asia accounted for nearly three-quarters of global availability and approximately 89% of aquaculture production. Increasing supply does not necessarily imply equitable access or better diets. South and Southeast Asia are shifting from diverse wild freshwater fisheries towards farmed species and commercial value chains. Aquaculture has increased the volume and stability of supplies but cannot automatically replace the ecological, nutritional, cultural, and livelihood functions of capture fisheries. Wild and farmed foods differ in species, price, nutrient density, acceptability, and accessibility; rising production may therefore conceal declining dietary diversity or access to common-pool resources [42].

Rice-field fisheries occupy an important but poorly quantified position within this rice–aquatic food nexus. They include naturally recruited fish, prawns, crabs, molluscs, frogs, snakes, aquatic insects, and edible plants harvested from seasonally flooded rice fields and connected aquatic habitats. Unlike rice–fish aquaculture, they depend mainly on natural recruitment, floodplain connectivity, habitat diversity, and access to harvesting grounds. Neither FAOSTAT nor most national food balance sheets record these catches separately. Substantial quantities are omitted because they are consumed within households, processed domestically, exchanged locally, or sold informally. Surveys defining fishing as a formal occupation may also exclude rice farmers, women, children, and seasonal fishers. The absence of a reliable global time series is therefore a systematic measurement gap, not evidence of low importance [40].

Southeast Asian studies demonstrate the magnitude of this “hidden harvest.” In Cambodia’s Tonle Sap region, 158 wild aquatic species were recorded in rice-field landscapes, and these fisheries supplied approximately 60% of the fish and other aquatic animals consumed by surveyed households during both flood and dry seasons [32,43]. Official estimates attribute about 30% of Cambodian inland fishery production to rice-field systems, although field evidence suggests a larger contribution. In northern Lao PDR, most rice-field aquatic production is retained for household consumption, emphasizing its importance as food, supplementary income, and a buffer against seasonal shocks [44].

The trajectory has been less favourable in intensively cultivated areas of the Mekong Delta of Vietnam. High-dyke systems, controlled irrigation, triple-rice cropping, agrochemical use, and reduced floodplain connectivity have increased the predictability of rice production while diminishing aquatic habitats and fish recruitment. Farmers in An Giang province perceived declines of 68–83% in wild fish yield and 75–81% in fish diversity after high-dyke construction, together with reduced sediment inflows and a 26–37% increase in synthetic fertilizer use [11,12]. These findings illustrate a critical trade-off: rice intensification may increase crop production while reducing wild foods, natural soil-fertility processes, dietary diversity, and livelihood resilience [43,45]. For the Lower Mekong Basin, Hortle [6], Hortle and Bush [46] synthesis of 20 household surveys estimated annual consumption of inland fish and other aquatic animals at 33.7 kg/person: 36.8 kg in Cambodia, 28.6 kg in Lao PDR, 29.0 kg in the Thai portion of the basin, and 39.0 kg in the Vietnamese portion. These foods supplied an estimated 47–80% of animal protein. Although based largely on conditions around 2000 and therefore unsuitable as current estimates, the findings remain an important benchmark showing how official statistics may underestimate artisanal catches, preserved products, other aquatic animals, and subsistence consumption.

Rice and aquatic foods are nutritionally complementary rather than interchangeable. Rice supplies dietary energy but relatively few essential micronutrients, whereas aquatic animals provide high-quality protein, essential fatty acids, iron, zinc, calcium, selenium, and vitamins A, D, and B₁₂. Small indigenous fish eaten whole are often particularly nutrient-dense because their bones, heads, eyes, skin, and viscera are consumed [47,48]. Modelled evidence suggests that inland small-scale fisheries potentially provide about 24% of micronutrient intake for 360 million people in Asia (Basurto et al., 2025). Replacing diverse wild catches with equivalent quantities of rice or a few large farmed species may therefore maintain aggregate food supply while reducing micronutrient density, affordability, and access.

Food-security assessments should consequently integrate rice, aquaculture, inland capture fisheries, and rice-field biodiversity within a single food-system framework. Monitoring should combine apparent availability with household intake, species and nutrient composition, seasonal access, prices, informal trade, subsistence harvesting, and socioeconomic inequality. Hydrological connectivity, flood duration, biodiversity, and access to fishing habitats should also be treated as determinants of food security. Otherwise, increases in rice and aquaculture production may obscure simultaneous losses of wild aquatic foods and the ecosystem services sustaining rural nutrition and resilience [45].

3.3. The production Trends of Rice-Field Fisheries and Aquatic Animal Foods in Lower Mekong River

The Lower Mekong Basin supports one of the world’s largest and most diverse inland fisheries. Mekong River Commission monitoring conducted between 2007 and 2018 recorded 617 fish species across 25 monitoring sites, demonstrating considerable spatial and temporal variation in fish abundance, diversity, and catch [49]. These fisheries include migratory river species, resident floodplain fishes, and numerous fish and other aquatic animals harvested from rice fields, canals, ponds, wetlands, estuaries, and coastal habitats [6,50,51]. In the Vietnamese Mekong Delta, these resources have historically supplied animal protein, micronutrients, income, and seasonal employment, particularly for land-poor households with limited access to commercial aquaculture [4,6,50]. The contribution of small-scale fisheries is not adequately represented in official production statistics. Government records generally capture commercial landings more effectively than subsistence harvesting, household consumption, informal exchange, or catches sold through dispersed local markets. Rice-field harvests may include fish, eels, shrimp, crabs, molluscs, frogs, snakes, insects, and edible aquatic plants. Their dietary and livelihood value may therefore be considerably greater than indicated by reported fish tonnage. This under-recording also complicates the interpretation of long-term trends: reductions in official catch may reflect declining fish abundance, decreasing fishing participation, changes in market channels or reporting practices, or a combination of these processes.

Since the 1990s, total aquatic production in the Vietnamese Delta has increased substantially, driven principally by the development of Pangasius catfish and brackish-water shrimp aquaculture. These industries have generated employment, processing capacity, export revenue, and more predictable commodity supplies. However, their expansion contrasts sharply with the trajectory of inland capture fisheries. In An Giang, Dong Thap, Can Tho, Vinh Long, Hau Giang, and other freshwater provinces, catches generally increased during the late 1990s before declining after the early 2000s. Interviews with 1,020 fishers in the Mekong Delta showed that 68–90% perceived declining catch rates, although trends differed among locations and fishing practices [4]. Aquaculture growth should therefore not be interpreted as evidence that wild fisheries have remained stable or that farmed commodities have fully replaced their nutritional and livelihood functions.

Basin-wide evidence reinforces this interpretation. Long-term monitoring reveals substantial geographical and temporal heterogeneity, indicating that fishery changes cannot be represented by a single basin-wide trajectory. Nevertheless, reductions in catch per unit effort and the increasing dominance of smaller fishes have been reported in several areas. Uy, et al. [52] also documented declining body sizes across 257 species, with particularly pronounced reductions among large-bodied and threatened fishes. Declining body size is consistent with cumulative fishing and environmental pressures because it can indicate the selective removal of large individuals, impaired growth, altered recruitment, or habitat degradation. However, it is not diagnostic of any single causal mechanism. Rice-field fisheries provide the clearest example of the trade-offs accompanying agricultural transformation. Rice intensification and flood-control infrastructure may have increased crop production but have reduced seasonal connectivity between rivers, floodplains, canals, and fields. Household research demonstrates that increased rice production too often coincide with declining collection and consumption of wild fish, other aquatic animals, and vegetables, with the greatest losses experienced by poorer households [10,12]. The contrasting trends therefore reflect a transition from hydrologically connected, multispecies food systems towards increasingly specialized and commercially oriented rice and aquaculture production. Future assessments should integrate official production data with household-consumption surveys, fishing participation, catch per unit effort, species composition, body-size distributions, and nutritional indicators. Such an approach is essential for determining whether growth in marketed production is being achieved at the expense of aquatic biodiversity, equitable food access, and the resilience of floodplain-dependent livelihoods.

3.4. Land-Use Change of Rice Production and Agrochemical Pressures on Rice-Field Fisheries (Inland Fisheries) and Aquatic Animal in the Lower Mekong Basin

Rice fields, floodplains, wetlands, canals, ponds, and rivers form a hydrologically interconnected aquatic landscape throughout the Lower Mekong Basin. During the monsoon, rising water connects permanent channels with seasonally inundated rice fields, enabling wild fish and other aquatic organisms including crustaceans, molluscs, amphibians, reptiles, and aquatic insects to migrate, reproduce, feed, and use floodplains as nursery and refuge habitats. Rice-growing landscapes therefore function not only as agricultural areas but also as seasonal wetlands that support aquatic biodiversity, household nutrition, and rural livelihoods. Consequently, changes in rice production cannot be evaluated independently of alterations in floodplain connectivity, water quality, habitat accessibility, and the availability of wild aquatic foods [32].

National production indices for 2000–2024 reveal contrasting trajectories among Cambodia, Lao PDR, Thailand, and Viet Nam [40,53] (Figure 1). Cambodia’s rice-production index increased to more than three times its 2000 level, while its reported inland-capture-fisheries index fluctuated but remained above the baseline. Lao PDR also recorded increases in rice production and reported inland catches. By contrast, inland-fisheries indices declined substantially in Thailand and Viet Nam, although rice production remained close to or above its initial level. Fertilizer use followed similarly divergent patterns, increasing markedly in Cambodia and Lao PDR, rising more moderately in Viet Nam, and declining in Thailand. These trends provide no evidence of a uniform basin-wide relationship among rice production, fertilizer use, and inland-fishery output (Figure 1).

Instead, they demonstrate that comparable agricultural changes can coexist with different fishery trajectories, depending on infrastructure, habitat condition, fishing effort, climate variability, markets, and statistical reporting. The comparative data for 2000–2024 reveal two contrasting patterns among the four Lower Mekong Basin countries (Table 1). Reported inland catch increased substantially in Cambodia and Lao PDR, whereas it declined in Thailand and Viet Nam. Meanwhile, rice production and fertilizer use increased in all four countries, although changes in harvested area differed. Cambodia recorded the largest inland catch in 2024, at 467,344 tonnes, representing an increase of 90.3% since 2000 and a 35.3% increase when the first and last five-year averages are compared. During the same period, rice production increased by 252.7%, harvested area by 104.8%, and fertilizer use by 157.8%. Nevertheless, the mean official-data share for the fishery series was 0%, giving Cambodia a “very low” quality classification (Table 1). The entire catch trajectory was therefore derived from estimated or imputed observations rather than consistently reported official statistics. Cambodia’s increasing catch should not be interpreted as evidence of fish-stock recovery or a positive fertilizer–fishery relationship. This statistical limitation is especially important because Cambodian rice-field fisheries are highly diffuse and subsistence-oriented. In the Tonle Sap region, 158 wild aquatic species were documented, and rice-field catches supplied approximately 60% of the fish and other aquatic animals consumed by surveyed households [32,43]. Much of this production does not pass through formal landing sites and can therefore be omitted or modelled in national statistics. Lao PDR displayed an even larger reported increase: inland catch reached 78,795 tonnes in 2024, 169.4% above the 2000 levels, while the last-five-year average exceeded the first-five-year average by 142.2%. Rice production, harvested area, and fertilizer use increased by 70.4%, 26.9%, and 252.6%, respectively. However, only 4% of the fishery observations were classified as official, resulting in a “low” quality signal (Table 1). The unusually smooth co-movement of catch and agricultural inputs may consequently reflect estimation procedures rather than a genuine ecological response. Field research in southern Lao PDR shows that irrigation effects are more complex: weirs reduced household catches, whereas dams redistributed fishing effort and catches from rivers towards reservoirs without producing a significant overall decline [62]. National totals may therefore conceal substantial changes in fishing locations, habitat composition, species assemblages, and household access.

The declining trends in Thailand and Viet Nam are supported by considerably stronger reporting coverage. Thailand’s inland catch declined by 46.4% between 2000 and 2024 and by 44.5% between the first and last five-year periods, reaching 107,869 tonnes in 2024. Over the same period, rice production increased by 34.5%, harvested area by 15.8%, and fertilizer use by 72.7% (Table 1). Because the official-data share was 100%, the downward fishery trend is more credible than the increases reported for Cambodia and Lao PDR.

Nevertheless, it cannot be attributed exclusively to rice intensification because fishing effort, reservoir development, hydropower infrastructure, habitat conversion, climate variability, urbanization, and statistical classification remain uncontrolled.

Viet Nam recorded 155,569 tonnes of inland catch in 2024, representing declines of 25.9% from the 2000 levels and 31.4% between the first and last five-year averages. Rice production increased by 33.6% despite a 7.9% reduction in harvested area, indicating rising yield and cropping intensity, while fertilizer use increased by 24.8%. With 93.3% of fishery observations classified as official, the series received a “moderate–higher” quality rating. The trend is consistent with evidence from the Vietnamese Mekong Delta, where high dikes, multiple rice cropping, flood exclusion, and reduced river–field connectivity have diminished wild-fish habitat and household access to aquatic foods [9,61]. In An Giang Province, farmers perceived substantial declines in wild-fish yield and diversity following high-dike construction, together with greater dependence on synthetic fertilizer [4,12,27]. However, the national series covers all Vietnamese inland waters and cannot specifically identify conditions in the Mekong Delta or rice-field habitats. In general, the Table 1 provides stronger evidence of divergent trends than of a causal relationship. Fertilizer use should therefore be interpreted as a proxy for agricultural intensification rather than as an independent driver of fishery change, because increased fertilizer application often coincides with greater pesticide use, intensified water control, shortened fallow periods, and multiple cropping. Evidence from the Vietnamese Mekong Delta indicates that intensive pesticide use can reduce fish survival and undermine farm profitability, whereas integrated rice–fish systems with lower agrochemical inputs may deliver better economic and ecological outcomes [21,63,64]. Future analyses should integrate official national statistics with subnational catch assessments, catch-per-unit-effort estimates, household consumption surveys, species-level data, hydrological indicators, and transparent documentation of imputed observations. Such an integrated approach is necessary to distinguish genuine ecological change from variations in fishing effort, habitat use, and statistical reporting.

3.5. Transformation and Decline of Rice-Field Fisheries and Aquatic Animal Production in the Vietnamese Mekong Delta

Rice-field fisheries occupy the least-managed end of the rice–fish production continuum. They encompass rice paddies and hydrologically connected habitats, including canals, streams, ponds, ditches, and seasonally inundated wetlands [43]. Wild fish are generally more abundant where seasonal flooding is maintained, water-control infrastructure is limited, and agrochemical use remains low [4,43,65,66]. Small canals, ponds, and adjacent floodplain water bodies function as dry-season refuges, while trap ponds concentrate fish as floodwaters recede. Fishers use traps, fence systems, nets, hooks, and other small-scale gears to harvest fish migrating between rivers, wetlands, and rice fields. The resulting catch includes wild fish and other aquatic animals, such as eels, shrimp, freshwater crabs, molluscs, frogs, and snakes. Edible aquatic plants are also collected from these landscapes. These rice-field aquatic resources make important contributions to household nutrition, income diversification, and ecosystem functioning, including nutrient cycling, biological pest regulation, and the maintenance of aquatic biodiversity. Nevertheless, rice-field fisheries remain poorly represented in agricultural planning, fishery statistics, and aquaculture-oriented policy frameworks because much of the catch is consumed within households or traded informally [4,67]. Historically, these fisheries have supported food and nutrition security and rural livelihoods across Asian low- and middle-income countries, including Bangladesh, Cambodia, Lao PDR, Myanmar, Thailand, and Viet Nam [36,43,62].

The Vietnamese Mekong Delta contains diverse freshwater, brackish-water, and coastal habitats, including rivers, floodplains, wetlands, mangroves, mudflats, seagrass beds, riparian zones, rice paddies, and peatlands [68,69]. Although rice cultivation and aquaculture are now dominant livelihood activities, inland capture fisheries and rice-field fisheries remain important supplementary sources of food and income, particularly for land-poor and resource-dependent households. During the 1970s and much of the 1980s, the delta was less intensively modified than it is today; extensive high-dyke networks, widespread triple-rice cropping, and large-scale commercial aquaculture had not yet reached their present extent. Seasonal floods therefore maintained greater connectivity among rivers, canals, wetlands, and rice fields, supporting diverse and productive wild fisheries [49,70,71,72].

These fisheries have subsequently declined as the delta has been transformed by intensive rice production, hydraulic engineering, aquaculture expansion, and wider land-use change. High-yielding rice varieties and double- and triple-cropping systems have increased rice output, but their implementation has frequently required high dykes, sluice gates, irrigation canals, rapid drainage, and increased agrochemical inputs. At the same time, freshwater fish ponds and coastal shrimp farms have expanded to supply domestic and export markets. These changes have altered rural employment patterns, with many households moving from seasonal fishing and aquatic-resource collection towards wage labour, intensive agriculture, or commercial aquaculture [4,12,49,70,71].

The production series presented in Figure 2 illustrate these contrasting trajectories. Between 1995 and 2024, recorded inland wild-fish catch declined from approximately 165,000 to 30,000 tonnes, an 82% reduction. Over the same period, aquaculture production including fish and shrimp increased more than eightfold, from approximately 78,000 to 650,000 tonnes, while rice production rose by 117%, from about 3.5 to 7.6 million tonnes. Aquaculture surpassed recorded inland catch around 2000 and, by 2024, its reported output was more than 20 times greater. Rice and aquaculture production expanded most rapidly between 2000 and 2015. Rice production subsequently stabilized, aquaculture continued to increase more moderately, and inland catch declined further. Because rice production is plotted on a separate vertical axis, however, the relative distances among the three series should be interpreted cautiously. Moreover, these trends demonstrate temporal association rather than direct causation.

Nevertheless, the decline in inland catch is consistent with evidence linking fishery productivity to the Mekong flood pulse. Seasonal flooding historically connected rivers, canals, wetlands, ponds, and rice fields, providing essential spawning, feeding, and nursery habitats for migratory and floodplain-dependent species. High dykes, sluice gates, drainage systems, and intensive irrigation have fragmented these habitats and reduced the extent and duration of seasonal inundation. Although such infrastructure has enabled more predictable double- and triple-rice cropping, it has also restricted fish migration, reduced access to floodplain habitats, and limited the exchange of water, sediment, and nutrients. Long-term datasets and interviews with 1,020 fishers showed that fish yields were positively associated with flood magnitude but negatively associated with rice intensification, particularly in high-dyke areas supporting three annual rice crops [4]. Similar effects on wild-food availability, household consumption, and rural livelihoods have been reported elsewhere in the delta [9,10,61].

Hydrological modification interacts with multiple additional pressures. Intensive rice cultivation commonly involves herbicides, fungicides, insecticides, and chemical fertilizers that may directly affect aquatic organisms or indirectly alter water quality, habitat conditions, and food-web structure [9,12,64,73]. Fishing pressure, destructive or illegal gears, declining spawning populations, hydropower development, sediment trapping, sand mining, and altered seasonal flows further contribute to fishery degradation [4,13]. High-dyke systems may also reduce natural sediment deposition and soil-fertility renewal, increasing dependence on purchased fertilizers and other agricultural inputs [21,45,74]. The decline in wild fisheries should therefore be understood as the cumulative outcome of interacting hydrological, ecological, agricultural, and fishery pressures rather than being attributed to a single driver.

Aquaculture expansion represents a parallel shift towards controlled, input-dependent, and commercially oriented aquatic production. This growth has been supported by hatchery development, improved seed and feed supplies, processing infrastructure, technological intensification, export markets, and investment in Pangasius catfish and whiteleg shrimp (Litopenaeus vannamei) production [75,76,77]. Although aquaculture generates employment, export earnings, and a more predictable supply of aquatic products, farmed fish and shrimp cannot fully replace the ecological, nutritional, and livelihood functions of wild fisheries. Wild aquatic resources comprise diverse species harvested for household consumption and informal exchange, whereas commercial aquaculture is concentrated on relatively few species and is often oriented towards higher-value markets. Poorly managed intensive aquaculture may also increase nutrient discharge, organic sludge, disease risks, chemical use, and competition for land and water [78,79].

The observed trends indicate a transition from hydrologically connected, multispecies, and partly common-pool food systems towards increasingly specialized rice and aquaculture production. Official catch statistics should nevertheless be interpreted cautiously because subsistence harvests, informal trade, small-scale fishing, and household consumption remain substantially under-recorded [6,55]. Management should therefore integrate rice cultivation, aquaculture, inland fisheries, and floodplain biodiversity within a unified food-system framework. Priorities include restoring seasonal connectivity, retaining low-dyke and controlled-flooding areas, incorporating fish-friendly structures into irrigation systems, protecting dry-season aquatic refuges, reducing agrochemical risks, and promoting integrated rice–fish and rice–shrimp systems. Future monitoring should distinguish freshwater aquaculture from brackish-water shrimp production and incorporate household catches, fishing effort, and catch per unit effort.

3.6. Pollutants, Exposure Pathways, and Mechanisms of Fishery Decline

3.6.1. Multiple Mechanisms of Fishery Decline

Evidence indicates that declining catches in Mekong fisheries cannot be attributed to a single driver. Rather, they arise from interacting pressures operating at basin, floodplain, and local scales. Overfishing and small-mesh or destructive gears remove juveniles and spawning adults, while habitat conversion, invasive species, and declining broodstock reduce recruitment. Hydropower dams impose an additional basin-scale constraint by obstructing migration between feeding, refuge, and spawning habitats and altering downstream flows, sediment transport, and seasonal cues [60,80]. Development scenarios consequently project substantial losses of fish biomass, biodiversity, protein supply, and associated ecosystem services [81,82]. Sediment trapping and flow regulation interact with high-dyke and irrigation systems in the Mekong Delta, where diminished floodplain connectivity restricts nursery habitat, nutrient exchange, and fish migration [9,13].

Rice intensification reinforces these pressures by shortening inundation and ecological recovery periods while increasing agrochemical applications. Pesticides may cause mortality or sublethal impairment of fish growth, behaviour, enzyme function, reproduction, and prey availability; repeated or sequential applications may prolong these effects [83,84,85]. Low-flow conditions can simultaneously contract aquatic habitat and reduce pollutant dilution. Declining catches may therefore reflect reduced abundance and recruitment, altered catchability, lower fishing effort, and incomplete statistical reporting. The most defensible interpretation is cumulative causation: hydrological fragmentation, habitat degradation, exploitation, pollution, and livelihood change jointly undermine fishery resilience [4].

3.6.2. Impacts of Water-Infrastructure Development as an Intensifier of Pollution on Inland and Rice-Field Fisheries

Large-scale water infrastructure including hydropower and irrigation dams, reservoirs, diversion structures, high-dyke polders, sluice gates, pumps, and engineered canals has fundamentally altered the hydrological functioning of the Lower Mekong Basin. Although these structures are designed to provide electricity, irrigation, flood protection, navigation, and water supply, their planning and operation also reflect political and economic priorities that determine how environmental benefits and risks are distributed. Infrastructure does not necessarily generate pollutants directly; instead, it modifies their dilution, transport, retention, transformation, and biological availability. It may therefore reduce pollution under some operating conditions but intensify contaminant exposure where water exchange and flushing are restricted [86,87].

At the basin scale, reservoirs transform free-flowing rivers into slower, deeper water bodies with longer residence times. These conditions can promote thermal stratification, oxygen depletion, nutrient transformation, and algal proliferation. Deep-water releases may subsequently alter downstream temperature and dissolved-oxygen regimes [88]. Regulation of discharge also changes the river’s capacity to dilute agricultural, industrial, and domestic effluents. During reservoir filling or periods of restricted downstream release, reduced flow may increase contaminant concentrations; conversely, controlled dry-season releases may improve dilution. Consequently, pollution outcomes depend on reservoir operating rules, season, hydrological conditions, and pollutant properties. Basin-wide monitoring nevertheless indicates that hydropower development, sediment trapping, and expanding point and non-point pollution are associated with deteriorating water quality in several Lower Mekong Delta reaches [20].

Reservoirs also retain suspended sediment, nutrients, organic matter, and particle-associated contaminants. Kondolf, et al. [89] estimated that a modeled full-development scenario without sediment-management measures could trap approximately 96% of the Mekong’s historical sediment load. Although this represents a scenario rather than the current condition, it illustrates the potential magnitude of downstream sediment starvation. Reduced sediment delivery can diminish floodplain fertility and aquatic productivity while promoting channel incision, bank erosion, and habitat degradation in Cambodia and the Vietnamese Mekong Delta [89]. These effects interact with the direct obstruction of fish migration and the alteration of flood-pulse cues that regulate spawning, larval drift, dispersal, and recruitment [60,80,90,91].

Within the Vietnamese Mekong Delta, high dykes and sluice-controlled canals enable triple-rice cultivation but reduce seasonal flooding, sediment deposition, fish movement, and water exchange. In An Giang, concentrations of COD, ammonium, nitrate, phosphate, electrical conductivity, and turbidity were significantly higher inside full-dyke compartments than in semi-dyke or unprotected areas [92]. Restricted flushing can prolong exposure to fertilizers, pesticides, organic matter, pathogens, and aquaculture effluent. Surveyed farmers similarly perceived declining water quality and estimated reductions of 68–83% in wild-fish yield and 75–81% in fish diversity following high-dyke development; however, these results represent farmer assessments rather than controlled ecological measurements [12].

For rice-field fisheries, these combined pressures reduce access to spawning grounds, nursery habitats, natural foods, and dry-season refuges. Inland fisheries may experience altered species composition, reduced recruitment, and localized fish mortality, while aquaculture and other aquatic-animal production systems face poorer intake-water quality, increased disease risk, hypoxia, and greater aeration or treatment costs. Management should therefore integrate pollution-source control with environmental flows, sediment passage, fish-friendly infrastructure, seasonal opening of floodplain compartments, coordinated reservoir–sluice operation, and event-based monitoring of contaminants, fish recruitment, and aquaculture performance. Basin-wide cumulative-impact assessments are essential because environmental costs commonly occur far downstream from the infrastructure generating the benefits.

3.6.3. Pollution Pressures on Inland Fisheries/ Rice-Field Fisheries and Aquatic Ecosystems

Regional monitoring indicates that much of the Mekong mainstem maintains generally good water quality, whereas secondary canals and densely populated or intensively farmed areas experience greater pollution pressure. Conditions vary spatially and seasonally in nutrients, organic matter, dissolved oxygen, salinity, acidity, microbial indicators, and pesticide residues [19,20,93,94]. The principal concern is not a single contaminant, but the interaction of hydrological stress, habitat modification, and contaminant mixtures.

Table 2 identifies seven interrelated pressures on wild fisheries and aquatic ecosystems: hydrological and sediment alteration, nutrient and organic enrichment, pesticides, antibiotics and pharmaceuticals, industrial and domestic effluent, micro-plastics, and acid-sulfate-soil drainage. Because these pressures frequently interact, the evidence should be interpreted as a synthesis of exposure pathways, not as a ranking of individual causes of fishery decline.

Altered Flow, Sediment Deficit and Hydromorphological Change Impact on Rice-Field Fisheries and Aquatic Systems

Hydrological alteration and sediment depletion are major system-level pressures affecting aquatic ecosystems throughout the Lower Mekong Basin, with particularly important consequences for the Vietnamese Mekong Delta. The ecological productivity of the Mekong has historically depended on the seasonal flood pulse, which connects rivers, floodplains, wetlands, canals, ponds, and rice fields and facilitates the transport of water, sediment, nutrients, fish, and other aquatic organisms. Increasing flow regulation by upstream reservoirs, together with sand extraction, channel modification, groundwater abstraction, and high-dike construction, has progressively altered these processes. These changes affect flood magnitude and duration, sediment deposition, channel morphology, floodplain inundation, and fish access to spawning, nursery, feeding, and refuge habitats [95,96]. Sediment trapping by hydropower reservoirs is central to this transformation. Long-term monitoring indicates that upstream dam development reduced the sediment budget reaching the Mekong Delta by approximately 74.1%, while contributing to riverbed incision [95,97,98]. This reduction is further intensified by sand mining. Hackney, et al. [99] estimated that only about 6.18 ± 2.01 million tonns/year of sand naturally enters the Delta, compared with extraction rates of approximately 50 million tonns year⁻¹during the study period. The resulting sediment imbalance can deepen river channels, destabilize riverbanks, alter water levels and tidal propagation, and increase susceptibility to salinity intrusion. These effects interact with groundwater-induced land subsidence, further increasing vulnerability to flooding, coastal erosion, and relative sea-level rise [100,101,102]. High-dike systems impose additional ecological constraints by reducing lateral exchanges between rivers and floodplains. Although such infrastructure supports intensive rice cultivation and flood protection, it also limits seasonal inundation, sediment deposition, and hydrological connectivity among canals, wetlands, rice fields, and permanent water bodies [103]. These changes are particularly important for rice-field fisheries because many migratory and floodplain-dependent species rely on seasonally inundated habitats for feeding, growth, reproduction, and refuge. Reduced sediment replenishment may also decrease natural soil fertility and increase dependence on external fertilizer inputs. However, quantitative relationships between sediment depletion, floodplain disconnection, and fish recruitment or fishery yields remain poorly defined.

Nutrient Enrichment, Organic Pollution, and Eutrophication

Intensification of rice cultivation and aquaculture has increased nutrient and organic-matter inputs to aquatic environments in the Vietnamese Mekong Delta. Basin-scale assessments indicate increasing inorganic nitrogen fluxes in the Mekong River, with agriculture constituting an important source [128]. Fertilizer losses from rice fields nevertheless vary with soil characteristics, hydrological conditions, fertilizer rate and timing, rainfall, cropping intensity, and water management. Evidence from a triple-rice system in the VMD showed that ammonium, orthophosphate, and total phosphorus concentrations increased immediately after fertilizer application. These nutrients were exported predominantly through surface runoff rather than subsurface leaching, demonstrating that drainage timing is a major determinant of nutrient transfer from rice fields to canals [129].

Aquaculture represents an additional, spatially concentrated source of nutrients and oxygen-demanding organic matter. These wastes originate from uneaten feed, faeces, metabolic excretion, dead plankton, pond sludge, and water exchange. Nutrient budgets for integrated agriculture–aquaculture ponds showed that only approximately 5–6% of nitrogen, organic carbon, and phosphorus inputs were recovered in harvested fish. Approximately 29% of nitrogen, 81% of organic carbon, and 51% of phosphorus accumulated in pond sediments, while substantial residual fractions were exported through water exchange [106]. High-exchange ponds discharged 10–14 times more nutrients than low-exchange systems. Pangasius catfish production also generates substantial nitrogen and phosphorus loads, although actual riverine emissions depend on feed efficiency, sedimentation, sludge removal, and reuse in crop production [105].

The relationship between nutrient enrichment and fisheries productivity is nonlinear. Moderate nutrient availability may enhance phytoplankton, periphyton, zooplankton, and benthic production, thereby increasing natural food availability for fish and other aquatic animals [66,130]. Beyond the assimilative capacity of the ecosystem, however, nutrient enrichment combined with organic loading, warm temperatures, prolonged retention, and restricted flushing accelerates eutrophication. Algal respiration and microbial decomposition then generate pronounced diel fluctuations in dissolved oxygen and pH, potentially culminating in hypoxia or anoxia.

Shallow rice fields and enclosed canals may be particularly vulnerable because their high temperatures increase metabolic oxygen demand while limited water depth and connectivity reduce access to oxygenated refuges. Hypoxia can impair feeding, growth, reproduction, egg and larval survival, and predator–prey interactions; prolonged events may cause fish displacement or mortality and shift communities toward pollution-tolerant species [131]. Consequently, eutrophication may reduce rice-field fish recruitment, aquatic-invertebrate availability, inland-fishery catch rates, and species diversity. In aquaculture ponds, cultured animals cannot readily escape deteriorating conditions. Low dissolved oxygen suppresses feed intake and growth, increases physiological stress and disease susceptibility, and may produce mass mortality when oxygen demand exceeds natural reaeration and mechanical aeration capacity [132,133].

Comparative monitoring in the Saigon–Dong Nai system documented nutrient enrichment, high phytoplankton biomass, and severe oxygen depletion associated with anthropogenic inputs [134]. Although this system lies outside the Mekong Delta, the underlying processes are relevant to nutrient-enriched water bodies throughout southern Vietnam. Cyanobacterial blooms and microcystins have also been detected in regional fish ponds, canals, rivers, lakes, and reservoirs, indicating additional risks to fish health, aquaculture production, and food safety [135]. Management should integrate fertilizer optimization, controlled field drainage, improved feed conversion, reduced unnecessary pond-water exchange, solids and sludge recovery, and beneficial effluent reuse [136,137]. Biological treatment, constructed wetlands, biofloc and recirculating systems may further reduce nutrient emissions [109]. However, farm-level interventions must be complemented by landscape-scale nutrient budgets because agricultural drainage, aquaculture effluent, domestic wastewater, and industrial discharges converge in the same canal–river network. Long-term monitoring should therefore combine nutrient and oxygen measurements with fish recruitment, community composition, mortality, and catch-per-unit-effort data to establish whether eutrophication is producing population-level fishery effects.

Pesticides and Mixture Toxicity

Agricultural pesticides enter rice fields, canals, wetlands, and rivers through surface runoff, field drainage, spray drift, irrigation return flows, and inappropriate disposal of unused chemicals and containers. Monitoring in the Vietnamese Mekong Delta has detected insecticides, herbicides, fungicides, and their transformation products in surface water, drinking-water sources, soils, and sediments. The year-round occurrence and co-occurrence of several active ingredients demonstrate that aquatic organisms may experience repeated or chronic exposure rather than isolated contamination events [16]. Legacy organochlorine pesticides and other persistent contaminants have also been reported in deltaic sediments and aquatic environments [110]. Moreover, pesticide residues occur in permanent-rice, alternating rice–shrimp, and permanent-shrimp systems, indicating contaminant transfer among interconnected agricultural and aquaculture landscapes (Braun et al., 2019).

Pesticide use in Vietnam expanded markedly alongside agricultural intensification. Imports increased from approximately 20,300 tonnes in the late 1980s to 72,560 tonnes by 2010, while average application rates in the Vietnamese Mekong Delta rose from 0.3 kg active ingredient (a.i.)/ha/year in 1981–1986 to 2.54 kg a.i./ha/year in 2001–2010 [16,138]. Farmer surveys further indicate substantial overuse, with >60–80% of respondents applying pesticides above recommended rates [112]. Monitoring in An Giang and Can Tho detected widespread contamination of water sources by isoprothiolane, fenobucarb, fipronil, and other pesticides, raising concerns for drinking-water safety and aquatic organisms [85,112]. Regulatory controls have subsequently removed several hazardous compounds, including carbendazim, paraquat, chlorpyrifos ethyl, fipronil, and glyphosate. MARD [139] reported that Vietnam have registered 1,847 active-ingredient entries and 4,229 trade names across agricultural pesticide categories in 2020 (Table 3).

Exposure is strongly influenced by pesticide formulation, application frequency, rainfall, drainage timing, water depth, sediment adsorption, and hydraulic connectivity. Fixed-interval monitoring may therefore underestimate short-lived concentration peaks occurring immediately after spraying or field drainage. In An Giang and Can Tho, isoprothiolane, fenobucarb, and fipronil were detected in almost all surface-water samples, with canal water generally containing the highest concentrations [112]. Risk modelling based on pesticide-use patterns in Can Tho and Tien Giang identified several pyrethroid, organophosphate, carbamate, and pyrazole insecticides as potential hazards to fish and aquatic arthropods. Predicted risks were generally lower in integrated pest management (IPM) and rice–fish systems than in conventional rice monocultures [85].

Biological effects extend well beyond acute mortality. Organophosphate and carbamate insecticides inhibit acetylcholinesterase (AChE), disrupting neural transmission, swimming, feeding, predator avoidance, and other behaviours essential for survival. Field exposure to diazinon caused substantial brain cholinesterase inhibition in snakehead (Channa striata) [140]. Similarly, chlorpyrifos applied at rates commonly used by rice farmers significantly inhibited brain AChE activity in climbing perch (Anabas testudineus). Although chlorpyrifos concentrations declined below analytical detection limits within three days, biomarker effects persisted for more than 12 days [84]. Thus, the apparent disappearance of a chemical from water does not necessarily indicate biological recovery. Mixture toxicity further complicates assessment because compounds with similar or different modes of action may produce additive, synergistic, or antagonistic responses. Sequential field applications of fenobucarb and chlorpyrifos caused prolonged AChE inhibition in climbing perch [84]. Laboratory experiments showed that their combined effect was greater than that of fenobucarb alone but less persistent than that of chlorpyrifos alone [141]. This response demonstrates that mixture effects cannot be inferred reliably from individual-compound toxicity values [142].

Antibiotics, Pharmaceuticals, and Resistance

Antibiotics are used in Vietnamese aquaculture to treat and, less appropriately, prevent bacterial diseases, particularly in intensive fish-production systems. A global review reported that Vietnam used 39 of the 67 antibiotic compounds documented among major aquaculture-producing countries during 2008–2018; this indicates a high diversity of compounds rather than per-capita consumption [119]. A national farm survey found antibiotic use in 64% of surveyed fish farms and 24% of shrimp farms, including fluoroquinolones, sulfonamides, tetracyclines, and combination treatments [114,119]. In An Giang Province, small-scale tilapia, striped-catfish, and giant-river-prawn farmers reported non-prescription access and prophylactic treatment prompted by weather changes or disease outbreaks on neighbouring farms [115]. Following administration, unabsorbed antibiotics and their metabolites enter pond water and sediments through uneaten medicated feed, faeces, urinary excretion, water exchange, and sludge disposal. Sulfamethoxazole, sulfadiazine, trimethoprim, and enrofloxacin have been detected in aquaculture-affected canals in Can Tho and An Giang, including canals receiving direct farm effluent and wastewater first discharged through rice fields [17]. Enrofloxacin and ciprofloxacin also accumulated in sediments downstream from a Pangasius farm, demonstrating that sediments can prolong exposure after concentrations in the water column decline [143].

The principal concern is antibiotics resistance (AMR). Repeated exposure selects resistant bacteria and maintains resistance genes in ponds, sediments, effluent canals, and receiving rivers. Resistant Pseudomonas and Aeromonas isolated from Mekong Delta catfish carried mobile genetic elements capable of facilitating resistance transfer [144]. Metagenomic analysis of a river receiving wastewater from small-scale aquaculture also detected antibiotic residues, resistance genes, and comparatively low bacterial diversity [145]. For aquaculture, AMR can reduce treatment effectiveness, prolong disease outbreaks, increase mortality and production costs, and encourage escalation to broader-spectrum or critically important antibiotics. Residues in harvested fish and shrimp additionally create food-safety and market-access risks [116,117].

Antibiotics may also alter microbial communities responsible for organic-matter decomposition, nutrient cycling, and primary production. However, one the Mekong Delta risk assessment found only minor predicted ecological risks from measured enrofloxacin and ciprofloxacin concentrations, illustrating that resistance selection may be more important than direct toxicity at environmentally observed concentrations [143]. Direct population-level effects on rice-field and inland fisheries remain poorly quantified.

Evidence for non-antibiotic pharmaceuticals and personal-care products in the VMD is still limited, despite their widespread detection and biological activity in freshwater environments globally [118]. Priorities include prescription-based antibiotic access, diagnostic testing, vaccination and biosecurity, treated effluent and sludge management, withdrawal-period enforcement, and integrated monitoring of residues, resistant bacteria, resistance genes, microbial functions, fish health, and fishery recruitment.

Industrial and Domestic Wastewater

Industrial and domestic wastewater constitutes an important, although spatially uneven, pressure on aquatic ecosystems in the Vietnamese Mekong Delta. Wastewater-treatment infrastructure has historically lagged behind urban and industrial development. A regional assessment published in 2019 identified only eight municipal wastewater-treatment plants, providing capacity equivalent to approximately 6.9% of estimated demand; these figures should be interpreted as a historical baseline rather than the current situation [120]. Field studies similarly show that urban and lower-order canals receiving household and commercial discharges generally exhibit poorer water quality than well-flushed main channels, with pronounced seasonal and tidal variability in organic matter, ammonium, dissolved oxygen, salinity, and microbial contamination [19,146,147].

Industrial effluent varies markedly among sectors. Seafood and food-processing wastewater typically contains high concentrations of biodegradable organic matter, suspended solids, fats, nutrients, and sometimes salts, whereas textile, chemical, metal-processing, and related industries may release dyes, solvents, metals, and other persistent or toxic compounds. Where treatment is inadequate, microbial decomposition of organic matter increases biochemical and chemical oxygen demand, causing oxygen depletion in receiving canals. Ammoniacal nitrogen presents an additional hazard because the proportion occurring as toxic unionized ammonia increases with temperature and pH, potentially impairing fish gill function, feeding, growth, reproduction, and survival [148,149].

These effects are particularly relevant to rice-field fisheries because fields, drainage channels, floodplains, and rivers form a seasonally connected habitat network. Contaminated irrigation or drainage water can expose fish, crustaceans, molluscs, and aquatic insects during critical spawning, nursery, and feeding periods. Hypoxia, ammonia toxicity, sediment contamination, and loss of pollution-sensitive invertebrates may reduce recruitment and alter food-web structure. In inland capture fisheries, severely polluted canal reaches may function as ecological barriers, restrict fish movement, cause localized mortality, and reduce catchability or catch per unit effort. Nevertheless, direct attribution of population-level fishery decline to wastewater remains limited because wastewater effects interact with hydrological modification, pesticides, nutrient enrichment, fishing pressure, and habitat loss.

For aquaculture and other aquatic-animal production, contaminated intake water may increase disease risk, physiological stress, mortality, aeration requirements, treatment costs, and food-safety concerns. Metal and bacterial contamination reported in cultivated fish from the Delta illustrates the need to monitor both environmental and product quality [150]. Priority actions include industrial pretreatment, expanded municipal sewerage and treatment, improved effluent enforcement, protection of aquaculture intakes, and monitoring that integrates dissolved oxygen, BOD, COD, ammonia, pathogens, priority toxicants, fish biomarkers, recruitment, and fishery yields.

Plastics and Microplastics

Plastic pollution is an emerging pressure on aquatic ecosystems in the Vietnamese Mekong Delta. A national review estimated that Vietnam generated more than 1.8 million tonnes of plastic waste annually and ranked seventh globally at the time of assessment; however, such rankings depend on reference year and estimation method [151]. Important regional sources include inadequately collected household waste, wastewater-borne fibres, agricultural packaging, fertilizer and pesticide containers, aquaculture feed bags, pond liners, ropes, nets, cages, and discarded fishing gear.

Riverbed surveys in the Delta recorded an average macroplastic burden of 33.4 g/100m², increasing to 923.2 g/100m² near major urban centres. Accumulation was significantly greater in shallow, low-flow channels and during the low-water season; during flooding, riverbed loads decreased approximately 2.5-fold. Plastic mass was positively associated with the abundance of several fish and decapod species, suggesting that debris may be used as substrate or shelter [122]. This association should not be interpreted as an ecological benefit, because plastic debris may also cause ingestion, entanglement, habitat modification, and obstruction of water-control structures.

Microplastics are also present in regional water, sediment, and aquatic organisms. In the Saigon River, outside the Mekong Delta but relevant as comparative evidence, anthropogenic fibres reached 172,000–519,000 items/m³, whereas fragments ranged from 10 to 223 items/m³ [123]. In the northern Delta, mean concentrations were 53.8 ± 140.7 items/m³ in Tien River surface water and 6.0 ± 2.0 items/g dry weight in sediment, with fibres predominating [18]. The large differences among reported concentrations partly reflect differences in sampling mesh, particle-size limits, fibre classification, and polymer-confirmation methods.

For rice-field fisheries, plastics transported through irrigation and drainage networks may obstruct fish movement, degrade seasonal nursery habitats, and expose fish, crustaceans, molluscs, and aquatic insects through ingestion. For inland fisheries and aquaculture, experimental syntheses indicate possible reductions in feeding, growth, development, reproduction, and survival, although responses vary substantially among taxa, polymers, particle shapes, and exposure concentrations [152,153]. Microplastics have been detected in Vietnamese fish, shrimp, and cultivated clams, but evidence linking environmentally realistic exposure to fishery recruitment, farm productivity, or human-health outcomes remains insufficient.

Management should prioritize source reduction, extended producer responsibility, collection and recovery of agricultural and aquaculture plastics, retrieval of lost fishing gear, and improved wastewater filtration. Decision 1316/QD-TTg [154]provides a national framework, but implementation requires sector-specific targets and standardized monitoring incorporating polymer identification, contamination controls, biological endpoints, and field-realistic chronic exposures.

Acid-Sulfate-Soil Drainage

Acid sulfate soils occupy approximately 1.6 million ha, equivalent to about 40–41% of the Vietnamese Mekong Delta [155]. Drainage, excavation, canal construction, and raised-bed reclamation expose sulfide-bearing soil layers to oxidation, generating acidity and mobilising aluminium, iron, and other solutes into adjacent waterways [125,126,156]. During a floodplain fishery survey conducted in the 1992 monsoon, the estimated minimum fish yield was 42 kg/ha/year at Phu Thanh, which lies within a moderate-to-severe acid-sulfate area, compared with 80 kg/ha/year at Binh Long [157]. However, because the study compared two observational sites, the yield difference cannot be attributed exclusively to acidity.

Evidence from acid-sulfate-soil-affected aquatic systems indicates that acidic drainage and mobilised aluminium can damage fish gills, disrupt ion regulation, reduce embryonic and larval survival, alter aquatic invertebrate communities, and contribute to fish-kill events [158,159,160]. These effects depend on pH, metal concentrations and speciation, water hardness, and salinity. Management measures include minimising unnecessary soil disturbance, maintaining appropriate groundwater and surface-water levels, applying controlled or staged flushing, retaining or treating acidic drainage, and monitoring pH and dissolved metals during early-rainfall or first-flush events [156,161,162]. An important trade-off remains: flushing may improve root-zone conditions for crops while transferring acidity and dissolved metals to canals, floodplain fisheries, and aquaculture systems [126,161].

3.7. Evolution of the Policy Framework for Pollution and Fisheries Governance in Vietnam

Vietnamese fisheries policy has evolved from production expansion toward a broader emphasis on resource protection, licensing, ecosystem management, and co-management. Earlier policies promoted offshore capacity and export growth, while the 2003 Fisheries Law provided a more comprehensive legal foundation for aquaculture and capture-fisheries management. Analyses of small-scale fisheries nevertheless identified persistent tensions between production objectives, livelihood needs, and enforcement capacity [163,164]

The Law on Fisheries No. 18/2017/QH14, effective from 2019 and consolidated with relevant amendments in 2024, covers aquatic-resource protection, aquaculture, capture fisheries, co-management, environmental monitoring, traceability, and responsibilities across levels of government [165]. The Law on Environmental Protection No. 72/2020/QH14, effective from 2022, strengthened environmental licensing, impact assessment, waste management, monitoring, disclosure, and liability [166]. The Law on Water Resources No. 28/2023/QH15 adds integrated management of water quantity and quality, river-basin planning, minimum flows, pollution restoration, water reuse, and interoperable water information [167] (Table 3). Together, these laws provide authority to address many pressures identified in this review, although their practical integration remains incomplete.

Strategic instruments further reorient development. Resolution No. 120/NQ-CP of 2017 promotes sustainable and climate-resilient development in the Mekong Delta and emphasizes adaptation based on natural conditions [168,171]. Decision No. 287/QD-TTg of 2022 approved the regional plan for 2021-2030 with a vision to 2050 [169]. Decision No. 339/QD-TTg of 2021 approved Vietnam's fisheries development strategy to 2030 with a vision to 2045 [170]. These instruments create an opportunity to link ecological subregions, aquaculture, rice, fruit production, water management, and livelihood transitions.

3.8. Evidence Gaps and Research Priorities

Despite expanding research on fisheries, aquaculture, hydrological change, and pollution in the Lower Mekong Basin, causal attribution remains weak. Most studies are spatially restricted, short-term, or address single stressors, making it difficult to separate the effects of fishing, altered flows, habitat fragmentation, agricultural intensification, contamination, and climate variability [172]. Long-term monitoring should therefore integrate fishing effort, catch and recruitment, hydrology, floodplain connectivity, sediment transport, land use, water quality, and contaminant exposure. Before–after–control–impact and other quasi-experimental designs are needed to evaluate dike opening, habitat reconnection, community fish refuges, and pollution-control interventions.

Contaminant surveillance requires improved temporal and chemical resolution. Sampling should target pesticide application, rainfall, field drainage, and flood recession, when routine monitoring may miss exposure peaks [26]. Analyses should include transformation products, sediment and biota residues, mixture-risk models, and bioassays. Pesticide and antibiotic residues across rice, rice–shrimp, and shrimp systems demonstrate potential cross-system transfer [16,173]. Antibiotic-resistance research should apply a One Health framework linking farm use with residues, resistant bacteria and genes, receiving waters, aquatic foods, and human exposure [145,174]. Standardized baselines are also required for pharmaceuticals and microplastics [18,123].

A further priority is to connect exposure with population- and production-level consequences. Laboratory tests should be combined with field measurements of biomarkers, survival, reproduction, recruitment, community composition, catch per unit effort, and farm yield under realistic mixtures and interacting salinity, temperature, and hypoxia. Aquaculture research should quantify full-cycle balances of nutrients, antibiotics, water, and sludge and compare treatment, reuse, constructed wetlands, and integrated farming systems.

Social and governance research must make subsistence fisheries and implementation outcomes visible. Household studies should quantify informal harvests, nutrition, food-safety risks, gendered labour, income, and access to land and water [32]. Finally, licensing, discharge standards, certification, producer organizations, and community management should be evaluated using indicators of pollution reduction, habitat condition, fish stocks, compliance, livelihood distribution, and cost-effectiveness. Studies of Vietnamese aquaculture and Cambodian fish refuges show that institutional capacity strongly conditions outcomes [175,176] (Ha & Bush, 2010; Anh et al., 2011; [176] et al., 2019).

6. Conclusions and Recommendations

6.1. Conclusions

This review demonstrates that rice-field fisheries are integral components of the Lower Mekong Basin’s food systems rather than incidental by-products of rice cultivation. Seasonally connected rice fields, canals, wetlands, ponds, and floodplains provide habitats for fish and other aquatic animals while supporting nutrition, income, and biodiversity. In the Vietnamese Mekong Delta, agricultural modernization has increased rice and aquaculture production. However, high-dyke construction, intensified cropping, habitat conversion, and reliance on external inputs have simplified aquatic landscapes, nutrient-alluvial water and restricted seasonal connectivity.

The decline of rice-field and inland fisheries cannot be attributed to pollution alone. It reflects the interacting effects of altered flow and sediment regimes, migration barriers, floodplain disconnection, fishing pressure, salinity intrusion, climate variability, and contaminant exposure. Nutrient enrichment, pesticides, antibiotics, untreated wastewater, plastics, and acid-sulfate-soil drainage has the potential to degrade habitats and increase stress, disease susceptibility, and mortality. Nevertheless, evidence directly linking these exposures to population-level fishery decline remains limited. Aquaculture is increasingly important, but it cannot fully replace the nutritional, livelihood, and ecosystem functions of wild fisheries. Although Vietnam has strengthened its environmental, water-resource, and fisheries frameworks, implementation remains constrained by fragmented responsibilities, inconsistent enforcement, limited source disclosure, and inadequate ecological monitoring.

6.2. Recommendations

Future management should establish a basin-linked monitoring system integrating hydrology, sediment transport, habitat connectivity, water quality, contaminant mixtures, antibitic resistance, fish recruitment, fishing effort, catch per unit effort, and household aquatic-food use.. Pollution control should prioritize integrated pest management, fertilizer and feed optimization, antibitic stewardship, wastewater and sludge treatment, effluent reuse, and plastic reduction.

Seasonal connectivity should be restored where feasible through modified dykes and sluices, protected migration corridors, and maintained nursery and dry-season refuges. Context-appropriate floating-rice, rice–fish, and rice–shrimp systems should be supported through extension, farmer organizations, finance, and markets rewarding verified environmental performance. Finally, cross-sector institutions should adopt shared indicators, transparent reporting, community co-management, and enforceable corrective measures. Together, these actions provide a practical pathway for reconciling food production with aquatic biodiversity, livelihood security, and climate resilience in the Mekong Delta.