Trace Metal Contamination in Commercial Fish from the Ecuadorian Amazon: Preliminary Health Risk Assessment in a Local Market

Abstract

Trace metal pollution in tropical freshwater ecosystems poses growing public health concerns, particularly in regions where fisheries are central to food security; however, little is known about metal exposure risks in the Western Amazon. This study presents the first assessment of trace metal concentrations in fish sold at the main market in El Coca, a rapidly growing city in the Ecuadorian Amazon. We analyzed 11 trace metals in 17 commercially important species and estimated seven health risk indices based on two fish consumption scenarios and international reference dose standards. Our results show that all species exceeded recommended thresholds for arsenic, mercury, and lead, while one species surpassed guidelines for aluminum. Metal concentrations varied by species and river of origin: small catfish from the Payamino River had elevated cadmium, chromium, copper, and manganese levels, potentially linked to upstream gold mining, whereas larger catfish showed higher mercury and arsenic accumulation. Monte Carlo simulations of risk indices suggested overall some disease risk, but the lack of local demographic data limits accurate assessments for vulnerable groups. Despite sampling limitations, our findings offer the first baseline for monitoring trace metal exposure in the northern Ecuadorian Amazon and underscore the need for targeted public health strategies in this understudied region.

1. Introduction

Fish represents a main source of animal protein for humans worldwide and provides essential nutrients such as high levels of lipids, vitamins, minerals, proteins, and calcium [1,2]. However, the intensification of industrial, agricultural, and urban activities has led to increasing contamination of aquatic ecosystems with heavy metals and other trace elements [3]. These trace pollutants can be bioaccumulated and biomagnified across the aquatic food webs, especially threatening top predators [4], thereby posing a chronic health risk to human populations relying on fish consumption.

Heavy metals, such as mercury (Hg), cadmium (Cd), and lead (Pb), as well as metalloids like arsenic (As), have been proven to cause adverse effects on human health when ingested over prolonged periods [5]. For instance, chronic exposure to mercury affects the central nervous system, causing tremors and memory loss [6]. Long-term cadmium exposure can cause kidney damage, particularly tubular dysfunction [7]. Lead has been associated with hypertension and cardiovascular disease [8]. Arsenic has been linked with an increased risk of skin cancer [9]. To prevent these effects, organizations such as the FAO and OMS have established oral reference doses for most heavy metals and metalloids in foods [10]. Likewise, the US EPA has established an Integrated Risk Assessment program that provides daily reference doses and guidelines to prevent exposure to different pollutants through diverse sources [11]. However, differences in these guidelines may lead to varying health risk assessments, underscoring the importance of comparative analyses across standards.

Numerous studies have documented trace element contamination in fish sold in local markets worldwide, including countries such as Nigeria [12], Turkey [13], Poland [14], Bangladesh [15], and the United States [16], with different degrees of risk to human health. Even though marine fish accumulate higher concentrations of heavy metals than freshwater fish [17], studies indicate potential long-term risks associated with mercury intake in the latter [18]. Similarly, spatial variation in contaminant concentrations is influenced by local environmental and anthropogenic factors, making region-specific evaluations critical [18,19]. In the Amazon Drainage, water bodies have been impaired by the expansion of agriculture and cattle ranching [20], the development of the oil industry [21], and gold mining [22]. Their presence has been linked to pollution with certain trace elements such as mercury (Hg) [23,24].

Trace elements can be bioaccumulated in different organisms through oral ingestion of contaminated prey, and concentrations can increase from one trophic level to the next, causing their biomagnification in the higher trophic levels, usually represented by carnivorous and piscivorous species [25]. Recent studies have documented the biomagnification of mercury across the fish food webs in floodplain lakes of this drainage [26]. Furthermore, smaller species can accumulate high rates of cadmium (Cd) or copper (Cu) [27]. For these reasons, researchers across the Amazon are examining daily intakes of heavy metals and other metalloids from fish meat [28] and their potential effects on human health [29]. A study in the Tapajós River Basin demonstrated that mercury exposure is linked to lower activity or levels of antioxidant enzymes, which reduces the body’s ability to fight oxidative stress, particularly in women [30]. The evidence indicates that in the Amazon Drainage, species of high commercial value, particularly large migratory catfish, accumulate high mercury concentrations and other heavy metals [31,32]. One of the most comprehensive risk assessments of mercury pollution in the human population of the Brazilian Amazon, spanning six states and six age groups, revealed that women of reproductive age and children aged between two and four years are at a higher risk of developing adverse health effects [32]. Nevertheless, research remains scarce in several national contexts of the basin, particularly in Ecuador, where Indigenous vulnerability and regulatory gaps in oral reference doses and the monitoring of fisheries, and weak environmental monitoring of activities such as oil extraction, mining, and agriculture, further complicate exposure scenarios.

The Northern Ecuadorian Amazon is a critical region for such assessments. Decades of oil exploitation [33] and gold mining [34] have resulted in recurrent spills and contamination events, disproportionately affecting Indigenous and rural communities, whose inhabitants show higher cancer incidence [35]. At least 69 fish species are part of the subsistence fishery, and 73 are part of the commercial fishery [36]. While recent evidence has revealed the presence of mercury (Hg), aluminum (Al), cadmium (Cd), and arsenic (As) in fish consumed by Indigenous communities of the Napo, Aguarico, and Pastaza rivers at levels above standard recommendations [27], there is still limited understanding of the factors that drive contaminant accumulation in fish sold in local markets, or how consumption patterns intersect with health risks under different regulatory thresholds.

Given these knowledge gaps, the objectives of this study were twofold: (1) to assess human health risks associated with the consumption of commercially important fish species from markets in the Northern Ecuadorian Amazon under different intake scenarios and reference dose guidelines; and (2) to identify environmental and biological drivers associated with trace element concentrations in these species. By integrating multiple risk standards and focusing on a historically underrepresented region, our study seeks to contribute to regional environmental health monitoring, support evidence-based public health decisions, and advance understanding of contaminant dynamics in Amazonian fisheries.

2. Materials and Methods

2.1. Sample Collection and Preparation

We visited the fish market in El Coca, Orellana Province, Ecuador, which is part of the Western Amazon, twice. Orellana is part of the Northern Ecuadorian Amazon and the Napo Basin. The first visit occurred in March 2024, during the rising waters season, and the second in August 2024, during the high waters season. During the visits, we bought different specimens of commercial fish species. These specimens were caught and distributed to the market’s vendors by commercial fishers. Tissues of muscle located in the dorsal region of the body were then excised from each specimen using a ceramic-bladed knife, which was wiped clean between samples to reduce metal and cross-contamination. Tissues were preserved in a 97% alcohol solution. Sample weights ranged from 0.5 to 3 g. Analyses were conducted for 54 fish tissue samples.

2.2. Element Analysis

This study analyzed 11 trace elements, including nine heavy metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn, and Hg), one metalloid (As), and Al. Samples for metal concentration analysis were dried, and a 0.5 g portion was digested in 10 mL of Trace Metal-grade HNO3 following the Animal Tissue protocol [37] using a CEM 360 microwave. The digested samples were filtered in 50 mL of type I water, and the concentrations of heavy metals were determined using an ICP 7400 ICP-OES Duo spectrophotometer with the inductively coupled plasma technique. Typical LODs ranged from 0.001 to 0.1 mg/L, depending on the element and the matrix. For example, the LOD for Cd was 0.001 mg/kg, whereas for Hg, it was around 0.005 mg/kg. Certified reference materials, ERM-BB422 Fish Muscle and ERM-CE464 Tuna Fish [38,39], were employed to evaluate the procedure’s accuracy. The average recovery rate for trace elements was 90.82%, reflecting reliable measurement accuracy (Table S1). To estimate the concentrations in the tissues’ wet weights, we first calculated the moisture content (%) of each tissue sample as follows:

Moisture content (%) = (Wet weight − Dry weight)/Wet weight × 100

where Wet Weight is the weight of the tissue sample before drying (gr) and Dry Weight is the weight of the tissue sample after lyophilization (gr). We then calculated wet weight concentrations as follows:

C_wet = ((100 − Moisture content (%))/100) × C_dry

where C_wet (mg/kg) is the trace element concentration in the wet tissue and C_dry (mg/kg) is the concentration in the dry tissue. We used wet-weight concentrations to calculate all the indices encompassed in the health risk assessment. Values below the detection curves were considered NAs and were not included in the health risk assessment or the statistical analysis. These NAs represented 13% of the sample size for Fe, 37% for Mn, and 15% for Hg.

2.3. Health Risk Assessment

To assess the health risk of exposure to the 11 selected trace metals, we calculated indices that provide information on the risk of developing noncarcinogenic and carcinogenic diseases based on short and long-term exposure to these elements. We used two daily fish intake rates for the health risk assessment to establish an ingestion threshold. For the lower limit, we calculated the mean daily fish intake reported for Indigenous communities in the Ecuadorian Amazon [40], which was equal to 0.183 mg/kg/day. For the upper limit, we used a rate of 0.46 mg/kg/day reported in the Brazilian Amazon [41]. Likewise, we considered two oral reference doses for each trace element (Table 1). The first standard included the oral reference doses suggested by the FAO and OMS (2011, 2019) [10,42], the US National Research Council [43], the USFDA [44], and the Ecuadorian norms for tuna [45]. The second standard included the oral reference doses established by the US Environmental Protection Agency [46]. All the indices were calculated for the two daily fish intake rates and standards. The US EPA has not established an oral reference dose for lead, arguing that the presence of this metal is not safe at any concentration.

Table 1. Oral reference doses used for the health risk assessment of trace elements in commercial fish.

Metal	Standard 1		Standard 2
	Oral Reference Dose mg/kg/Day	References	Oral Reference Dose mg/kg/Day	References
Aluminum (Al)	0.29	[42]	1	[47]
Arsenic (As)	0.002	[10]	0.00006	[48]
Cadmium (Cd)	0.0008	[10]	0.001	[49]
Chromium (Cr)	0.05	[43]	0.0009	[50]
Copper (Cu)	10	[45]	0.04	[51]
Iron (Fe)	100	[43]	0.7	[52]
Manganese (Mn)	1	[10]	0.14	[53]
Nickel (Ni)	70	[43]	0.02	[54]
Lead (Pb)	0.3	[44]	0.00	[55]
Zinc (Zn)	100	[45]	0.3	[56]
Mercury (Hg)	0.0012	[43]	0.0003	[57]

Table 1. Oral reference doses used for the health risk assessment of trace elements in commercial fish.

Metal	Standard 1		Standard 2
	Oral Reference Dose mg/kg/Day	References	Oral Reference Dose mg/kg/Day	References
Aluminum (Al)	0.29	[42]	1	[47]
Arsenic (As)	0.002	[10]	0.00006	[48]
Cadmium (Cd)	0.0008	[10]	0.001	[49]
Chromium (Cr)	0.05	[43]	0.0009	[50]
Copper (Cu)	10	[45]	0.04	[51]
Iron (Fe)	100	[43]	0.7	[52]
Manganese (Mn)	1	[10]	0.14	[53]
Nickel (Ni)	70	[43]	0.02	[54]
Lead (Pb)	0.3	[44]	0.00	[55]
Zinc (Zn)	100	[45]	0.3	[56]
Mercury (Hg)	0.0012	[43]	0.0003	[57]

We calculated the Daily Intake Index (DII) of each trace element, which measures the exposure of an individual to a substance through food consumption, as follows:

DII = C_wet × FI/BW

where FI is the daily fish intake rate, and BW is the average body weight of Ecuadorians, for which we used a value of 67.9 kg [58]. Then, we estimated the Health Risk Index (HRI), which compares the estimated intake of contaminants to established safety thresholds:

HRI = DII/RfD

where RfD is the oral reference dose for each trace element. HRI below 1 indicates that the exposure level is below the RfD, suggesting a minimal risk of adverse health effects, while values greater than 1 indicate that the exposure exceeds the RfD, suggesting a potential health risk [59]. To assess potential risks associated with long-term exposure to trace elements through ingestion, we first calculated the Chronic Daily Intake (CDI) using the following formula:

CDI = (C × FI × EF × ED)/BW × AT

where C is the trace element concentration; EF is the exposure frequency, established as 365 days/year; ED is the exposure duration, established as 70 years; and AT is the averaging time for non-carcinogens, typically 365 days × 70 years [60]. Using the CDI of each element trace, we calculated the respective Target Hazard Quotient (THQ) as follows:

THQ = (C × FI × EF × ED)/(BW × AT × RfD)

A THQ value of less than 1 suggests that the exposure is unlikely to cause adverse health effects, while a value equal to or greater than 1 indicates a potential health risk [61]. We added the THQ values of all the trace elements to estimate the Hazard Index (HI). An HI above 1 indicated the need for mitigation measures to decrease the risk of noncarcinogenic effects in the population [59]. Since the RfD for Pb by the USEPA is 0 [55], it was not possible to calculate the THQ index for this metal for Standard 2. Finally, to evaluate the potential carcinogenic risks associated with exposure to trace elements, we estimated the Target Cancer Risk (TCR) from arsenic (As), chromium (Cr), and lead (Pb) using the formula below:

TCR = CDI × CSF

where CSF refers to the Cancer Slope Factor. For arsenic, we used a value of 32 mg/kg/day [48], for chromium, 0.27 mg/kg/day [46], and for lead, 0.0085 mg/kg/day [62]. This index represents the probability that a person will develop cancer due to exposure to a carcinogen throughout their life [60]. The sum of the three TCRs yielded the Cumulative Cancer Risk (CCR), which evaluates the combined cancer risk from exposure to multiple carcinogenic substances. CCR values above 1 are considered very high risk and demand urgent mitigation measures [59].

2.4. Statistical Analysis

We calculated Pearson’s correlations between all trace element pairs to explore associations between pollutants. We conducted a Principal Component Analysis to identify patterns of trace metal accumulation across species and rivers and determine the significance of the effects of these factors with a two-way PERMANOVA [63]. Because of the small sample size for several species, we did not include the interaction between river and species in the PERMANOVA. To explore temporal and spatial variations, we compared total trace metal concentrations between hydrological phases and among rivers using the Kruskal–Wallis test. All statistical analyses were conducted in R [64]. A significance level of 0.05 was considered in all statistical analyses.

To assess the uncertainty associated with the non-carcinogenic (HI) and carcinogenic (CCR) risk indices derived from trace metal concentrations in fish species, a sensitivity analysis was conducted using a Monte Carlo simulation. The behavior of the indices for each species and fish consumption rate combination and reference standard was simulated by introducing 10% variability to the original index values (HI and CCR) to model measurement errors, natural variability, and uncertainty in fish intake rates. Simulations were run for 10,000 iterations for each species and consumption rate combination to generate probability distributions for both indices. Percentiles P5, P50, and P95 were calculated to provide an uncertainty interval for each value, to encompass different scenarios for the risk assessment. These analyses were conducted using the R package mc2d v.0.2.1 [65].

3. Results

We analyzed 54 fish tissues from 17 species of commercial fish. Of these, only one species, Prochilodus nigricans, belonged to the order Characiformes. The other 16 species belonged to the Siluriformes order and the Pimelodidae family. The fish specimens included in our analyses were collected from the rivers Napo, Aguarico, Payamino, and Putumayo in the Northern Ecuadorian Amazon. The most sold species were Pimelodus jivaro and Zungaro zungaro, which contributed eight specimens each, followed by Calophysus macropterus with six specimens. We analyzed four specimens of Brachyplatystoma rousseauxii, Platystomatichthys sturio, and Pseudoplatystoma punctifer, three of Pinirampus pirinampu and Prochilodus nigricans, two of Brachyplatystoma platynema, Phractocephalus hemioliopterus, Pimelodina flavipinnis, Platynematichthys notatus, and Sorubim lima. Finally, Brachyplatystoma juruense, Duopalatinus peruanus, Pseudoplatystoma tigrinum, and Sorubimichthys planiceps were rare, contributing only one specimen each. Large catfish species were scarce in general, and sales were frequently based on one or two very large specimens. Species sold in the market varied between hydrological seasons. For instance, P. flavipinnis, and P. sturio were only found during high waters, while P. jivaro and S. lima were only found during rising waters.

Al, Zn, Fe, and Cu showed the highest concentrations, with means of 15.08 (±13.02), 4.63 (±2.73), 3.85 (±2.3), and 3.37 (±0.23) mg/kg, respectively. The species varied considerably in their trace metal concentrations (

Table 2. Trace metal concentrations (mg/kg) in wet weight for 17 commercial fish species sold in El Coca Market.

Species	River	N		Al	As	Cd	Cr	Cu	Fe	Mn	Ni	Pb	Zn	Hg
Brachyplatystoma juruense	Napo	1	Value	5.44	0.14	0.01	0.04	0.60	4.79	0.09	0.00	0.03	6.88	1.49
Brachyplatystoma platynema	Napo	1	Value	19.94	0.04	0.01	0.02	0.35	4.76	0.12	0.00	0.01	4.52	3.44
Brachyplatystoma platynema	Putumayo	1	Value	12.32	0.17	0.03	0.01	0.09	0.02	0.01	0.01	0.07	0.21	0.20
Brachyplatystoma rousseauxii	Napo	4	Mean	8.54	0.08	0.01	0.02	0.35	2.49	0.08	0.00	0.04	5.78	2.08
			SD	5.67	0.04	0.00	0.01	0.08	1.22	0.01	0.00	0.02	1.08	1.63
Calophysus macropterus	Napo	3	Mean	11.09	0.06	0.01	0.02	0.55	3.91	0.09	0.00	0.03	5.13	1.36
			SD	11.86	0.01	0.00	0.01	0.15	2.80	0.04	0.00	0.03	1.16	0.57
Calophysus macropterus	Putumayo	3	Mean	22.96	0.04	0.03	0.01	0.14	0.01	0.01	0.01	0.09	0.15	0.62
			SD	2.10	0.01	0.01	0.01	0.03	0.00	0.00	0.01	0.02	0.06	0.35
Duopalatinus peruanus	Napo	1	Value	21.44	0.08	0.04	0.03	0.15	NA	0.01	0.02	0.11	0.24	NA
Phractocephalus hemioliopterus	Napo	2	Mean	7.46	0.10	0.01	0.02	0.45	3.47	0.09	0.00	0.07	6.67	1.10
			SD	2.14	0.04	0.01	0.00	0.01	0.51	0.02	0.00	0.07	0.82	0.34
Pimelodina flavipinnis	Putumayo	2	Mean	51.40	0.05	0.02	0.01	0.15	0.14	0.02	0.03	0.06	0.22	0.89
			SD	50.16	0.02	0.00	0.01	0.01	0.09	0.00	0.02	0.00	0.04	0.42
Pimelodus jivaro	Payamino	8	Mean	20.44	0.05	0.01	0.04	0.77	6.81	0.18	0.01	0.08	4.56	0.06
			SD	6.93	0.03	0.00	0.01	0.18	1.86	0.03	0.01	0.05	0.59	0.04
Pinirampus pirinampu	Napo	3	Mean	12.93	0.03	0.02	0.02	0.41	2.06	0.08	0.02	0.04	4.79	1.58
			SD	8.73	0.02	0.03	0.01	0.31	2.46	0.06	0.01	0.04	4.47	2.23
Platynematichthys notatus	Aguarico	1	Value	7.90	0.05	0.01	0.02	0.68	2.49	0.17	0.00	0.04	6.04	1.03
Platynematichthys notatus	Napo	1	Value	6.94	0.08	0.01	0.02	0.49	3.01	0.09	0.00	0.04	3.98	0.61
Platystomatichthys sturio	Napo	4	Mean	27.26	0.03	0.04	0.02	0.23	NA	0.02	0.17	0.16	0.16	NA
			SD	9.38	0.03	0.01	0.01	0.08	NA	0.00	0.28	0.09	0.03	NA
Prochilodus nigricans	Napo	3	Mean	11.46	0.06	0.01	0.02	0.63	5.50	0.41	0.01	0.04	6.09	NA
			SD	2.37	0.02	0.00	0.01	0.05	1.67	0.31	0.00	0.03	1.30	NA
Pseudoplatystoma punctifer	Napo	4	Mean	10.01	0.07	0.01	0.01	0.43	3.07	0.11	0.00	0.08	6.77	0.38
			SD	7.32	0.03	0.00	0.00	0.08	0.93	0.04	0.00	0.05	2.09	0.22
Pseudoplatystoma tigrinum	Aguarico	1	Value	4.83	0.07	0.01	0.02	0.70	5.27	0.11	NA	NA	9.56	0.84
Sorubim lima	Napo	2	Mean	13.80	0.03	0.01	0.02	0.62	4.53	0.14	0.00	0.07	4.84	0.26
			SD	5.21	0.03	0.00	0.00	0.02	1.20	0.00	0.00	0.00	0.16	0.05
Sorubimichthys planiceps	Napo	1	Value	24.48	0.05	0.01	0.02	0.39	4.95	0.12	0.01	NA	10.8	0.42
Zungaro zungaro	Aguarico	2	Mean	5.24	0.04	0.01	0.02	0.47	3.03	0.08	0.00	NA	4.64	0.75
			SD	2.99	0.00	0.00	0.01	0.18	1.64	0.02	0.00	0.04	0.33	0.59
Zungaro zungaro	Napo	6	Mean	10.73	0.07	0.01	0.02	0.49	3.73	0.11	0.01	0.04	5.03	0.67
			SD	6.57	0.01	0.00	0.01	0.22	1.47	0.03	0.00	0.03	0.81	0.32

NAs indicate measurements below the detection curve.

). Two large catfish species, Brachyplatystoma rousseauxii and Pinirampus pirinampu, had Hg concentrations above 2 mg/kg, while B. juruense showed the highest As concentration. In contrast, the mid-sized catfish, Platystomatichthys sturio, showed the highest Pb concentration, and the smaller Pimelodus jivaro showed the highest Cd concentration (Table 2).

3.1. Health Risk Assessment Results

DII varied greatly among trace elements. Al, Cu, and Fe showed the highest DII (

Table 3. Daily intake index (DII) and chronic daily intake (CDI) (mg/kg/day) range for eleven trace elements. Min and max values were based on the lower and upper daily fish intake rates, respectively. The values above RfDs are highlighted in bold.

Trace Element	DII		CDI
	Min	Max	Min	Max
Al	0.0130	0.3482	0.00980	0.34824
As	0.0001	0.0010	0.00006	0.00096
Cd	0.00001	0.0003	0.00001	0.00028
Cr	0.00002	0.0003	0.00002	0.00027
Cu	0.00039	0.0051	0.00030	0.0052
Fe	0.00037	0.0461	0.00028	0.04617
Mn	0.000032	0.0027	0.00002	0.00279
Ni	0.0000013	0.0011	0.000001	0.00112
Pb	0.000085	0.00105	0.00006	0.00105
Zn	0.00044	0.07339	0.00033	0.07339
Hg	0.00016	0.01411	0.00012	0.01411

), although these did not surpass the RfDs in the two standards. On the contrary, As and Hg showed DII above the lower RfDs provided by the EPA and included in Standard 2, and for both fish intake rates (Table S2). CDI showed the same trends, and values were above these RfDs for all species, indicating intakes above EPA’s short- and long-term recommendations (Table 2). The species with the highest As DII and CDI were the large catfishes Brachyplatystoma juruense, B. platynema, and Phractocephalus hemioliopterus, while B. rousseauxii, B. platynema, and B. juruense had the highest Hg DII and CDI (Table S3).

Similarly, the HRI only showed values above one for As and Hg, although for the upper fish intake rate, one species, Pimelodina flavipinnis, had an Al HRI of 1.20 (±0.94) for Standard 1 (Table S4). The values of all other trace elements were below 1 for all species and fish intake rates. For As, HRI were above 1 only for Standard 2, where the highest values were exhibited by catfishes Brachyplatystoma juruense, B. platynema, and Phractocephalus hemioliopterus, all of which scored above 10. The highest Hg HRIs for Standard 2 were observed in B. rousseauxii, B. platynema, and B. juruense, with values above 20 for Standard 2 and the higher daily fish intake (Table S4).

For the lower fish intake rate, all THQ values, with the exception of Hg, were below 1 at Standard 1, and the latter ranged from 0.1 to 3.54 (Table S5), whereas for Standard 2, THQ values of Cd and Hg were above 1. The highest value of As THQ at the higher fish intake rate was 9.417 for Pseudoplatystoma punctifer. For the higher fish intake rate, values of THQ for As and Hg were well above 1, indicating a potential risk of developing non-carcinogenic diseases. The species with the highest HI were the three catfishes of the genus Brachyplatystoma and Pinirampus pirinampu in all scenarios (Figure 1). The contribution of metals varied between standards. For Standard 1, Hg and, to a lesser extent, As, contributed the most to the HI scores. For Standard 2, As, Zn, and Hg contributed the most to the HI scores.

TCR values for As and Cr were above 1.10⁻⁴ for all species, except for Pimelodina flavipinnis for Cr (Table S6). The CCR showed the same trend. These results indicate a potential risk of developing carcinogenic diseases through the ingestion of these species. For the lower fish intake rate, Brachyplatystoma juruense, B. platynema, and Phractocephalus hemioliopterus showed the highest TCR, but for the higher fish intake rate, these were B. platynema, Pseudoplatystoma punctifer, and Zungaro zungaro (Figure 2).

The Monte Carlo simulations indicated that between 35% and 0% of species had HI values below 1 for percentiles 5 and 95, respectively, for the lower fish intake rate, but all species had HI values above 1 for the higher fish intake rate (Table S7). Thus, the risk of developing noncarcinogenic diseases through the ingestion of fish is moderate to high. For the CCR index, all values were above 10⁻⁴ for all species, except for Sorubimichthys planiceps for the higher fish intake rate (Table S8), indicating a high risk of developing carcinogenic diseases through the ingestion of fish.

3.2. Factors Driving Trace Metal Concentrations

Pearson’s correlation analysis of trace elements revealed that Cd, Cr, Fe, Mn, and Zn were all positively correlated with each other and negatively correlated with Cu (p ≤ 0.05) (Figure 3). Aluminum was positively correlated with Ni and negatively correlated with Cu, Fe, and Mn. Pb and As were negatively correlated with each other. Hg was not correlated with other trace elements (Figure 3).

The PERMANOVA indicated significant interspecific differences in trace element composition (F = 2.73, R² = 0.53, p = 0.001) and among rivers (F = 2.15, R² = 0.052, p = 0.029). The PCA showed that the fish trace element composition from the Putumayo River was different from fish from the Payamino and Aguarico Rivers (Figure 4A). The fish species from the Napo and Putumayo Rivers—Platystomatichthys sturio, Duopalatinus peruanus, Pimelodina flavipinnis, and Calophysus macropterus—showed higher Cd, Ni, Al, and Pb concentrations (Figure 4B). The large catfishes from the rivers Napo and Aguarico—Brachyplatystoma rousseauxii, B. platynema, Phractocephalus hemioliopterus, Platynematichthys notatus, Pseudoplatystoma punctifer, and Zungaro zungaro—showed higher Hg and As concentrations and lower Pb and Fe concentrations. Smaller catfishes, such as Pimelodus jivaro and Sorubim lima, and the Characiformes, Prochilodus nigricans, from the Payamino and Napo Rivers, showed higher Fe, Cr, Cu, and Mn concentrations.

Finally, we found no differences in total trace elements in fish between rising and high waters (KW Chi² = 0.31, p = 0.57). Conversely, there were significant differences among rivers of origin (KW Chi² = 10.23, p = 0.016), where the Payamino and Putumayo Rivers showed the highest total concentrations of trace elements (Figure 5). In the Putumayo River, the outlier represented one specimen of Pimelodina flavipinnis, with 86.87 mg/kg of Al (Figure 5). The higher total trace metal concentrations in the fish from the Payamino River resulted from higher Fe, Cr, and Cu concentrations.

4. Discussion

4.1. Health Risk Assessment

This is the first assessment of health risks associated with heavy metal concentrations in fish sold in the El Coca Market. Unlike many health-risk assessments of exposure to heavy metals in the Amazon, which rely on a single standardized intake rate, our research offers a multi-scenario framework that incorporates a threshold of fish intake rates to account for dietary variability in human populations. Likewise, by applying two different reference standards and intake rates, we demonstrate how estimated health risks can shift from negligible to substantial depending on the exposure context. This approach not only provides a more accurate risk characterization for populations in the Ecuadorian Amazon but also offers a flexible tool applicable to other tropical regions where freshwater fish is a dietary staple and sociocultural heterogeneity influences exposure levels. Our framework can thus inform public health strategies in a range of freshwater-dependent communities beyond the Amazon, especially since fish sold in El Coca Market can be distributed in other cities in Ecuador, including Quito, Ambato, Guayaquil, Esmeraldas, Loja, and Machala [66].

Our study detected the presence of 11 trace elements, including heavy metals and metalloids, in fish sold at El Coca Market, one of the largest cities in the Ecuadorian Amazon, which is a direct threat to the region’s food security. Through health risk assessment, we described four risk scenarios based on two different fish intake rates and RfD standards of trace element exposure in the short and long term. We observed the presence of arsenic (As), mercury (Hg), and lead (Pb) in concentrations above the standards proposed by the OMS, FAO [10] and USEPA [11] for short and long-term exposure in all species, as indicated by the Daily Intake (DII), the Chronic Daily Intake, and Health Risk indices. Likewise, Pimelodina flavipinnis had an Al HRI above the RfD for our Standard 1 and a higher fish intake rate. Furthermore, the Target Hazard Quotient (THQ), the Hazard Index (HI), the Target Cancer Risk (TCR), and the Cumulative Cancer Risk (CCR) indices suggest that the risks of developing noncarcinogenic and carcinogenic diseases due to chronic exposure to these trace elements from the fish sold at the El Coca Market exist and might be high. However, this assessment is based on exposure estimates at a single point in time. Considering the prolonged consumption of fish containing detectable levels of elements such as arsenic (As), mercury (Hg), lead (Pb), cadmium (Cd) and chromium (Cr), even at low concentrations, cumulative exposure over 10 to 20 years could lead to progressive bioaccumulation and further increase the risk of chronic health effects [67]. Thus, our results should be considered a preliminary assessment and an early warning that highlights the urgency of establishing long-term biomonitoring programs and integrated management strategies for the aquatic ecosystems, fisheries, and human populations in the Western Amazon.

However, some limitations to our study should be considered when interpreting our results. First, our calculations were based on Ecuadorian adults’ average weight of 67.9 kg, published over 10 years ago [58], since no recent statistics on health and nutrition are available in Ecuador. Second, the daily fish intake rate for the Ecuadorian Amazon (0.183 kg/day) was based on a study published in 2011 [40]. These factors add a level of uncertainty to our results. Given the potential shifts in nutritional status and public health conditions in Ecuador during this period, actual average body weights may have changed, affecting dose estimates and risk characterization. For instance, undernutrition seems to be prevalent in rural contexts of the northern Ecuadorian Amazon, with an accompanying increasing tendency towards obesity in the last ten years [68,69]. Likewise, our study does not rule out the possibility of a higher risk for women of reproductive age and children, who are target groups that could not be assessed due to a lack of available statistics. In addition, for some species, the sample size was very limited, meaning that inferences about the health risks they pose should be considered with caution, and further studies are needed. Children and women of reproductive age have been identified as facing the highest risk in studies of mercury, arsenic, and other heavy metal concentrations in fish and other foods worldwide, including the Brazilian Amazon [32,70], India [61], Thailand [69], and Bosnia and Herzegovina [17], to provide a few examples. Furthermore, the synergistic effects of malnutrition and exposure to trace metals should be investigated. In addition, further studies encompassing the lower water season are needed, since heavy metal concentrations can increase during this period [27].

Considering that women and children might be frequently below the average adult body weight of 67.9 kg used in this study, they might face significantly higher exposure risks. For instance, a child weighing 15 kg would receive a contaminant dose per unit of body weight more than four times greater than that of an adult under the same fish intake levels. In Madre de Dios, in the Peruvian Amazon, higher levels of mercury were found in women of reproductive age with higher fish intake rates [71]. This is concerning, since mercury, arsenic, lead, and cadmium can be transferred to children through breastfeeding [72]. While we could not directly assess these vulnerable groups due to a lack of disaggregated fish consumption rates and health data in Ecuador, these findings highlight the urgent need to establish updated, sex- and age-specific dietary and exposure profiles. Future research and public health policies must prioritize women of reproductive age as well as children, as their vulnerability is often underestimated in general population risk assessments.

In our study, aluminum, arsenic, zinc, and mercury concentrations were the main contributors to the Hazard Index (HI) across all species, with arsenic contributing the most to the Cumulative Cancer Risk (CCR). These elements were consistently found in concentrations exceeding international safety thresholds, particularly in large catfish species such as Brachyplatystoma spp., and in small catfish such as Pimelodus jivaro, caught in the Payamino River. Elevated aluminum concentrations in rivers might be associated with the upstream agricultural practices involving pesticide use, resulting in neurological and bone diseases [73]. The main source of mercury exposure is via inhalation [74], and the USEPA RfD is based on respiratory exposure [57]. Acute mercury exposure has been linked to nervous system diseases, nonischemic heart diseases, and pneumonia [75]. The main source of oral exposure is the ingestion of polluted fish, seashells, and other wild fauna, which has been associated with inflammatory bowel disease, destruction of intestinal microbiota, kidney damage, reduced pituitary and thyroid function, and other immunological, reproductive, and hematological effects [76]. In contrast, the main route of arsenic intake is through oral exposure, which has been associated with cardiovascular disorders, fetal mortality, neurological damage, and a variety of carcinogenic diseases, including liver, prostate, lung, and skin cancer, as well as leukemia [77].

We also found the presence of lead in all the fish species we analyzed (0.01 to 0.09 mg/kg), although this trace element did not determine their Cumulative Cancer Risk (CCR). The USEPA decided in 1985 not to establish an RfD for this heavy metal, alleging that no concentration was found that could not be associated with adverse health effects [55]. For this reason, lead daily (DII) and chronic intake indices (CDI) were above the RfD for all species according to our Standard 2. Lead has been linked to cognitive dysfunction, neurobehavioral alterations, impaired kidney function, anemia, hypertension, and reduced fertility [8]. Even though lead is frequently found in lower concentrations in fish compared with mercury, its presence is still considered a risk to humans, particularly when accompanied by mercury and other trace elements such as cadmium (Cd) or nickel (Ni) [78]. In the Amazon region, even though there is evidence of mercury and lead in blood in humans [71,79], a link between this and the above-mentioned health effects in these populations has not been established; therefore, our results point to the need to intensify efforts to assess the risks of oral lead exposure through fish intake, since studies frequently focus on mercury [29,80,81,82], as well as to explore potential links between neurological, gastrointestinal, immunological, and reproductive diseases and exposure to these trace metals in indigenous communities dependent on fish as a main protein source.

On the other hand, our research shows comparable results for daily intakes (DIs) for mercury (Hg) and lead (Pb) in urban areas of the Brazilian Central Amazon, where risks were determined for children under high fish intake scenarios [70]. In contrast, the target hazard quotient (THQ) for mercury for Standard 2 at the higher fish intake was 244.63, which is significantly higher than the ranges observed in urban areas of Brazil, ranging from 2.36 to 29.01 [83]. The cancer risk values (CRI) ranged from 8.6⁻⁴ for the lower fish intake rate to 1.09⁻² for arsenic (As), which fall within the ranges observed in India, where it has been indicated that these levels pose a moderate cancer risk to adults and a higher risk for children [61,84]. These comparisons imply that the El Coca Market may be an important source of trace element exposure in the Western Amazon and potentially beyond that, emphasizing the need for focused biomonitoring of heavy metals in aquatic ecosystems and fish, and dietary guidance on recommended species and fish intake rates.

In the Amazon Drainage, between 50 and 100% of fish are destined for personal consumption [84]. In the Ecuadorian Amazon, fish is the main source of protein for several indigenous communities [40]. Thus, for these indigenous communities, health risk indices might be underestimated. Factors such as their limited resources and the distance to urban areas hinder the feasibility of reducing their fish intake rates. In such cases, chronic ingestion of trace metals, especially in species with higher concentrations, may pose significant long-term health risks, increasing existing health disparities and underscoring the need for targeted risk assessments and localized policy interventions.

4.2. Drivers of Trace Metal Concentrations in Fish

We found that several trace metals were positively correlated with each other. For instance, cadmium (Cd), manganese (Mn), chromium (Cr), iron (Fe), and zinc (Zn) found in the fish specimens were positively correlated, which suggests that these trace elements share a common source. These trace elements, together with aluminum, mercury, lead, and nickel, have been found in large amounts in water bodies near the Auca oil field and downstream of several mining sites in the Napo River Basin [85]. However, we did not detect any correlation between mercury and other trace elements. A previous study could not link mercury concentrations in fish to the presence of oil fields or mining sites within the Ecuadorian Amazon [27]. Considering that several species included in our analyses are large, migratory catfish, we cannot rule out that the mercury exposure occurred outside Ecuador, since there is increasing evidence of heavy metal and other metalloid pollution across the Amazon Drainage [24,86,87].

The positive correlation between cadmium and chromium found in fish sold in El Coca Market suggests the possibility of simultaneous exposure to both trace elements, which poses a further threat to the health of the region’s population. Studies suggest that the combined exposure to these two heavy metals could be associated with decreased renal function at high exposure levels [88]. While our study did not model interaction effects explicitly, the presence of these elements together reinforces the need for multi-contaminant risk assessments in the Amazon region, where humans are likely exposed to complex mixtures of pollutants rather than individual metals, as our results indicate. Additional toxicological and epidemiological research on the synergistic effects of these trace elements on human health is needed, as well as on the potential effects on malnourished children.

Fish specimens from the Payamino River had a distinct composition and exhibited the highest total concentrations of trace elements. Recent studies have shown that illegal mining is advancing in the Punino River, a tributary of the Payamino, where 142 hectares of land have been affected [89]. This pattern suggests a possible causal link between upstream land use and downstream bioaccumulation of trace metals in fish from the Payamino River. The smaller size of the Payamino River may contribute to the observed higher bioaccumulation of trace elements in fish, specifically iron, cadmium, copper, chrome, and manganese. The deforestation associated with mining activities might mobilize these elements from the soils to the rivers [90]. In comparison, the larger channels of the Napo and Aguarico Rivers cause these elements to dilute, favoring the lower concentrations observed in these rivers. Strikingly, all the samples from the Payamino River belong to Pimelodus jivaro, the smallest species analyzed in this study. This means that small fish species can bioaccumulate and biomagnify potentially dangerous trace elements. For instance, chronic exposure to chromium has been linked to stomach, liver, and kidney cancer [91], and manganese can cause decreased cognitive and verbal function, anxiety, depression, and decreased movement coordination in children, even at small doses [92]. Similarly, cadmium exposure has been linked to kidney damage, decreased bone density, the destruction of the lungs’ mucus membranes, and effects in the reproductive system [7]. Consequently, the communities along the Payamino River that fish regularly in this water body might be at higher risk of developing these adverse health effects.

We observed differences in bioaccumulation patterns of trace elements among species. Large catfish of the genus Brachyplatystoma, Pseudoplatystoma, and the species Phractocephalus hemioliopterus, Pinirampus pirinampu, Sorubimichthys planiceps, and Zungaro zungaro accumulated higher concentrations of arsenic (0.05 to 0.17 mg/kg) and mercury (0.2 to 3.44 mg/kg). Medium-sized catfish such as Calophysus macropterus, Duopalatinus malarmo, Pimelodina flavipinnis, and Platystomatichthys sturio accumulated higher concentrations of cadmium (0.01 to 0.04 mg/kg), nickel (0.01 to 0.28 mg/kg), aluminum (11.09 to 21.44 mg/kg), and lead (0.06 to 0.16 mg/kg). Smaller catfish species Sorubim lima, Pimelodus jivaro, and the Characiformes Prochilodus nigricans accumulated higher concentrations of iron (0.01 to 0.18 mg/kg), chrome (0.01 to 0.04 mg/kg), copper (0.02 to 0.77 mg/kg), and manganese (0.14 to 0.41 mg/kg). These patterns are consistent with previous studies in the Ecuadorian Amazon [27] and indicate that avoiding the intake of mercury or arsenic by eating smaller fish implies exposure to other potentially harmful trace elements. However, for some species, we found only one or two individuals, which implies that these trends should be considered with caution, and further analyses are needed, encompassing a larger sample of individuals and across time. Nevertheless, we coincide with other studies [16] on the need to consume a wider diversity of species at more distant occasions. Furthermore, these findings imply the need to evaluate the risks of different trace elements simultaneously to obtain a realistic perspective of the health risks associated with environmental pollution. Since we only visited El Coca Market on two occasions, during rising and high waters, we do not have data from the dry season. This could have caused the lack of significant differences in total trace metal concentrations between the two encompassed months, which are part of the wet season. Likewise, it is possible that we did not detect peak contamination levels in fish during low waters. For example, other studies have found increased trace metal concentrations in fish collected in low waters [81].

Our results indicate that patterns in risk detection from trace element intake can change drastically according to the fish intake rate and the standard of oral reference doses (RfD) used to analyze the samples. Thus, it is essential to include different scenarios to accurately assess potential threats to human health related to heavy metals and metalloid exposure. Furthermore, the presence of arsenic, mercury, and lead above recommended standards in fish is worrisome and could represent a public health crisis in the long term if remediation and management measures are not taken on time. Likewise, our findings represent further evidence of the increasing pollution in the aquatic ecosystems and their fish worldwide, derived from industrial development and other anthropogenic activities. Further research based on isotopic analyses is necessary to determine the specific sources of trace element pollution in fish, which is difficult on the Amazon due to (1) the high diversity, (2) the existence of species that carry out large migrations across the basin, and (3) the inadequate financial resources and lack of properly equipped laboratories in the Amazonian countries to conduct these types of analyses.

By utilizing multiple exposure scenarios based on varying fish intake rates and oral reference dose standards, our results provide a more accurate understanding of potential health risks. This approach enables decision-makers to identify thresholds of concern under different dietary habits, prioritize interventions for high-risk species, such as large catfishes or Pimelodus jivaro, and watersheds such as the Payamino River, and tailor public health advisories to specific consumption patterns. In doing so, we contribute baseline data and a flexible framework for managing food safety and ecosystem conservation on the Western Amazon. Moreover, by providing reference thresholds for 11 trace elements, unlike many studies that focus on mercury, and for 17 widely consumed fish species, we offer a more comprehensive and actionable framework for policymakers.

5. Conclusions

This is the first study to assess the presence of heavy metals in fish sold at the El Coca Market in the northern Ecuadorian Amazon and the potential risks associated with their consumption. We found that the most highly bioaccumulated metals in fish were Al ˃ Zn ˃ Hg ˃ Cu ˃ Mn ˃ Cr ˃ As. However, As and Hg contributed the most to the health risk indices. The large catfish of the genera Brachyplatystoma and Pseudoplatystoma, and the species Pirinampus pinirampu and Zungaro zungaro, caught in the Napo and Aguarico rivers, pose the highest risks to human health in general. Our results point to the presence of illegal mines in the upper basin of the Payamino River as the driving factor contributing to the heavy metal loads in fish sold at El Coca Market, resulting in the highest total concentrations of heavy metals in fish caught in this river. The sensitivity analyses suggested potential risks of developing non-carcinogenic as well as carcinogenic diseases in adults, emphasizing that the risks for vulnerable groups such as children and women of reproductive age still need to be assessed. Furthermore, pollution from the ingestion of these fish is probably not restricted to the Northern Ecuadorian Amazon, since these are commercialized to other provinces and could be higher during the drier months of the year. Even though our results are preliminary, they provide a baseline for further research on heavy metal pollution in fish and its implications for the Amazonian people of Ecuador.