Introduction
Rapid industrialization driven by oil exploration and urbanization in the Niger Delta region of Nigeria has been reported by Iwu et al. [1], Udofia et al. [2], and Okoye et al. [3] to contribute significantly to environmental contamination by aliphatic and polycyclic aromatic hydrocarbons (PAHs). These hydrocarbons originate from both petrogenic and pyrogenic sources [4] and are known to persist in aquatic ecosystems, where they accumulate in sediments and biomagnify through food webs [5, 6].
Polycyclic aromatic hydrocarbons are lipophilic, toxic, and potentially carcinogenic compounds recognized for their capacity to induce DNA adduct formation, mutagenesis, and chronic toxic effects involving the respiratory, hepatic, renal, and dermal systems in humans [7]. Among the 16 priority PAHs identified by the United States Environmental Protection Agency (USEPA), several including benzo[a]pyrene are classified as known or probable human carcinogens. Benzo[a]pyrene is often used as a marker compound owing to its high toxicity and prevalence in food matrices [8].
Freshwater fish serve as a major route of human exposure to PAHs, particularly in communities that depend on fish as their primary protein source, a common situation in many riverine areas of Nigeria. Several studies have reported PAH concentrations in edible fish species such as Oreochromis niloticus and Tilapia zillii, with values frequently exceeding regulatory limits such as the European Union benchmark of 2 μg/kg for benzo[a]pyrene in fish tissue [9].
The Escravos River, located in the Niger Delta, is an oil-bearing water body characterized by frequent incidents of pipeline vandalism and crude oil spills. Despite this, the river remains underexplored with respect to organ-level contamination of fish by PAHs and aliphatic hydrocarbons. Previous investigations have predominantly focused on muscle tissues, often neglecting internal organs such as gills, liver, and kidneys, which have been shown by Shahid et al. [10] to accumulate hydrophobic contaminants due to their high metabolic and excretory activities. Since many local populations consume fish whole including offal, excluding these organs may lead to an underestimation of actual human health risks. Therefore, the present study aims to: (a) determine the concentrations of total aliphatic hydrocarbons (TAHs) and PAHs in the muscle, liver, kidney, and gill tissues of Tilapia zillii from the Escravos River; (b) evaluate tissue-specific bioaccumulation patterns and organ-level distribution; (c) interpret the observed contamination levels in relation to literature-reported biomarker responses (CYP1A, GST, CAT, and SOD) to infer possible biochemical implications of hydrocarbon exposure; (d) estimate human health risks associated with fish consumption from the study area using established metrics such as Estimated Daily Intake (EDI), Toxic Equivalency (TEQ), Hazard Quotient (HQ), and Excess Lifetime Cancer Risk (ECR); and (e) compare the findings with regulatory guideline values established by the World Health Organization (WHO), United States Environmental Protection Agency (USEPA), and European Food Safety Authority (EFSA). This study provides an integrative assessment of hydrocarbon contamination across multiple fish organs, linking environmental exposure with inferred biomarker responses and human health implications. By simultaneously quantifying aliphatic and aromatic hydrocarbons in edible and metabolic tissues, it delivers a more realistic estimation of dietary and biochemical risks than muscle-only analyses. The results are expected to advance understanding of pollutant bioaccumulation and sublethal biochemical stress in tropical freshwater systems and contribute to environmental risk management strategies in oil-impacted regions of the Niger Delta.
Materials and Methods
Study Area and Sample Collection
Tilapia zillii specimens (n = 60) were collected from the Escravos River (Latitude 5°34′59.99″ N, Longitude 5°09′60.00″ E) in Delta State, Nigeria (Figure 1). The Escravos River is a major distributary of the Niger Delta system and is characterized by extensive oil exploration, refining, and shipping activities. The river experiences recurrent crude oil discharges and pipeline vandalism, making it an ideal model site for hydrocarbon contamination assessment. Sampling was conducted during both the dry (January–March) and wet (July–September) seasons to account for possible seasonal variations in contaminant distribution and bioaccumulation. Fish were obtained directly from local fishermen using standard gill nets at designated river segments. Each sampling site was geo-referenced using a Garmin eTrex Global Positioning System (GPS) unit. Collected fish were immediately rinsed with site water to remove debris, wrapped in pre-cleaned aluminum foil to prevent hydrocarbon loss or cross-contamination, and transported on ice to the laboratory within six hours of capture. Samples were subsequently stored at −20 °C until dissection and analysis. During laboratory processing, fish were thawed at 4 °C, and biometric data including total length and weight were recorded. Tissues including muscle, liver, gills, and kidneys were carefully excised using stainless steel instruments, thoroughly rinsed with deionized water, and individually homogenized for chemical analysis. All glassware and tools were pre-cleaned with hexane and baked at 450 °C to minimize background contamination. Quality assurance protocols were maintained throughout sampling and processing to ensure data integrity.
Tissue Sampling and Preparation
Frozen fish samples were thawed at 4 °C, rinsed with deionized water to remove surface impurities, and dissected aseptically using sterilized stainless-steel instruments to obtain four tissue types: muscle, liver, kidney, and gill. Tissues from five individuals per organ per season were pooled to form a composite representative sample, homogenized, and freeze-dried. The dried homogenates were finely ground and mixed with anhydrous sodium sulfate to eliminate residual moisture prior to extraction, following the procedures described by Janska et al., [11], Fadare et al., [12] and Xu et al., [13]. All instruments were pre-cleaned with acetone and n-hexane, and contact with plastic materials was strictly avoided to prevent hydrocarbon contamination.
Extraction and Clean-Up of Hydrocarbons
Ten (10) g (dry-weight equivalent) of each tissue sample was Soxhlet-extracted for 8 h using a 1:1 (v/v) mixture of acetone and n-hexane in accordance with U.S. EPA Method 3510C. Extracts were filtered through pre-cleaned Whatman No. 42 filter paper, dried over anhydrous sodium sulfate, and reduced to approximately 5 mL using a rotary evaporator (RE-52A, Shanghai Yarong).
Clean-up and fractionation were carried out on a 10 g, 100-200 mesh silica gel column preconditioned with n-hexane. Elemental sulfur was removed using activated copper granules. The aliphatic hydrocarbon fraction was eluted with 20 mL n-hexane, while the aromatic (PAH) fraction was eluted with 20 mL dichloromethane (DCM). Each fraction was concentrated to 1 mL under a gentle stream of high-purity nitrogen. Prior to instrumental analysis, samples were spiked with deuterated surrogate standards (D₁₀-phenanthrene and D₁₂-chrysene) to monitor recovery efficiency and instrument performance. Procedural blanks and spiked recovery samples were included in each extraction batch to ensure analytical precision and accuracy. Surrogate recoveries within 70–120% were considered acceptable for data validation.
Instrumental Analysis (GC-FID and GC-MS)
Quantitative determination of 35 aliphatic hydrocarbons (C₈–C₄₀) and 16 U.S. EPA priority PAHs was carried out using Gas Chromatography coupled with Flame Ionization Detection (GC–FID, HP 5890 Series II) and confirmed by Gas Chromatography–Mass Spectrometry (GC–MS, Agilent 7890A/5975C MSD). The system was fitted with a DB-5MS fused silica capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness).
For GC–FID, the oven temperature program was as follows: initial temperature at 60 °C (held for 2 min), increased to 180 °C at 10 °C/min, then to 300 °C at 5 °C/min, and held for 10 min. Helium was used as the carrier gas at a flow rate of 1.0 mL/min. Injector and detector temperatures were maintained at 280 °C and 320 °C, respectively. For GC–MS confirmation, the same column and oven conditions were employed, operating in electron impact (EI) mode at 70 eV with a mass scan range of m/z 50–550. Compound identification was confirmed by matching both retention times and mass fragmentation patterns with certified reference standards (Supelco, U.S.A.). Quantification was based on external calibration curves prepared from standard mixtures of known concentrations. The limits of detection (LOD) ranged from 0.1–0.5 μg/kg (wet weight), while limits of quantification (LOQ) were calculated as 10× the signal-to-noise ratio.
Quality Assurance and Method Performance
Method blanks, matrix spikes, and laboratory duplicates were included in every analytical batch. Instrument calibration was verified daily using five-point standard curves ranging from 0.01 to 500 μg/kg, yielding correlation coefficients (r²) ≥ 0.995. Recovery efficiencies ranged between 82 % and 94 % across tissue matrices. Method detection limits were calculated as three times the signal-to-noise (S/N) ratio, and LOQs were defined as ten times S/N. Precision and reproducibility were verified by triplicate analyses of selected samples, with relative standard deviations below 10 %. Surrogate recovery data within 70–120 % were accepted for quantification. All concentrations were blank-corrected prior to statistical evaluation.
Biomarker Interpretation
Accumulation profiles of diagnostic hydrocarbons—such as anthracene, benzo[a]pyrene, and nonadecane—were evaluated as chemical indicators of exposure and organ-specific contaminant burden. Elevated concentrations in liver and gills were interpreted as reflections of biotransformation activity and direct respiratory uptake, respectively, consistent with established fish bioaccumulation models reported by Matos et al. [14].
Although enzymatic or molecular biomarkers (e.g., CYP1A, GST, or DNA adducts) were not directly quantified in this study, organ-specific hydrocarbon signatures provide indirect evidence of physiological stress and metabolic activation pathways associated with chronic exposure to petroleum-derived contaminants. This interpretative framework aligns with previous studies that correlated chemical body burden with biomarker responses in hydrocarbon-exposed fish species [14, 11].
Co-pollutant and Bioaccumulation Assessment
To assess cumulative exposure from both aliphatic and aromatic hydrocarbons: Total Petroleum Hydrocarbon (TPH) was calculated as:
Bioaccumulation Factors (BAFs) were determined by comparing tissue concentrations to sediment or water concentrations. Pearson correlation coefficients (r) assessed associations between PAHs and TAHs.
Human Health Risk Assessment
Estimated Daily Intake (EDI) [4]
Where C is concentration (μg/kg), FCR = 36.4 g/day (based on 13.3 kg/year fish consumption), and BW = 60 kg.
Hazard Quotient (HQ)
RfDs for individual PAHs (e.g., BaP = 0.0003 mg/kg/day) were sourced from USEPA IRIS database. HQ ≥ 1 indicates non-carcinogenic risk.
Toxic Equivalency (TEQ)
Where TEF values were used per USEPA (e.g., BaP = 1.0; BkF = 0.1; Chrysene = 0.001).
Sensitivity and Uncertainty Analysis
Monte Carlo Simulation (n = 10,000 iterations) was performed in Oracle Crystal Ball to assess uncertainty in EDI, HQ, and ECR estimates. Key variables: fish consumption rate, body weight, PAH concentration per organ. Outputs included 95% confidence intervals, mean risk scores and tornado charts to identify dominant input variables influencing cancer risk.
Statistical Analysis
All data were analyzed using SPSS v16. Descriptive statistics (mean ± SE) were computed for each hydrocarbon per organ. One-way ANOVA followed by Tukeys post-hoc test (p < 0.05) assessed differences between tissues. Pearson correlations (r) were used to study co-accumulation trends.
Ethical and Safety Considerations
All fish handling, dissection, and tissue sampling were conducted in accordance with the institutional and national ethical standards for the humane treatment of aquatic organisms, following the Nigerian Code of Practice for Research Ethics and the Inland Fisheries Act (2004). Chemical handling and solvent waste disposal adhered strictly to ISO 14001:2015 environmental management protocols, ensuring minimal environmental contamination and laboratory worker safety. Sampling permits and access to the study area were granted by relevant local authorities, and all procedures were reviewed and approved by the Research Ethics Committee of Enugu State University of Science and Technology (ESUT). Personal protective equipment (PPE) and appropriate fume hoods were used throughout all stages of chemical extraction and analysis.
Results and Discussion
Total Aliphatic Hydrocarbons (TAHs)
Total aliphatic hydrocarbons (TAHs) shown in Table 1 and Figure 2 recorded in Tilapia zillii from the Escravos River exhibited marked organ-specific accumulation patterns with concentrations ranging from 5980.56 ± 1.20 mg/kg in the kidney to 10388.16 ± 0.98 mg/kg in the gills. A total of 35 C₈–C₄₀ n-alkane components were identified in the tissue samples, with the nonane (C₉) fraction consistently dominating across tissues, reaching peak values of 2210.80 mg/kg in gills and 1946.70 mg/kg in muscle. The gill exhibited the highest total TAH burden, possibly due to its direct and prolonged exposure to contaminated water during respiration and its thin epithelial barrier that enhances passive diffusion of lipophilic pollutants, as also reported by Fadare et al. [12].
Octane (C₈) was notably undetectable in gills but reached 144.33 ± 0.46 mg/kg in muscle and 78.17 ± 0.03 mg/kg in kidney, suggesting tissue-selective uptake and metabolism. High C₉ (nonane) values observed (up to 2210.80 mg/kg) may indicate active petrogenic pollution inputs, given that low molecular weight alkanes are often associated with fresh petroleum discharges, as reported by Anyanwu et al. [16] and Farhan et al. [17]. Bioaccumulation of heavier fractions such as hexacosane (C₂₆) and tetratriacontane (C₃₄) was also prominent. C₂₆ reached a peak of 693.02 mg/kg in gills, while C₃₄ concentrations were highest in gills (1230.76 mg/kg) and muscle (797.46 mg/kg), indicating potential for long-term tissue retention of high molecular weight alkanes due to their reduced volatility and slower metabolic clearance. Detection of Unresolved Complex Mixtures (UCMs) further supports a petrogenic fingerprint consistent with chronic inputs from oil exploration and gas flaring activities in the Niger Delta.
TAH concentrations were consistently higher during the dry season compared to the wet season, suggesting reduced dilution capacity and increased hydrocarbon loading from oil exploration and flaring activities during periods of lower river discharge. Statistical comparison of organ concentrations was conducted using one-way ANOVA with significance set at p < 0.05.
These compounds have been found by Edo et al. [18] to exhibit environmental persistence and (bio)resistance, thereby increasing the ecological burden on aquatic biota. Total concentrations of TAHs observed in all organs far exceeded the 2 mg/kg wet weight threshold recommended by the European Commission, [19] and WHO/FAO safety limits for seafood, raising significant food safety concerns. Moreover, high bioaccumulation factors in edible tissues such as 9302.33 ± 1.08 mg/kg in muscle emphasize the potential for trophic transfer of petroleum residues in communities heavily reliant on fish consumption. Chronic dietary exposure to such residues has been found by Onyegeme-Okerenta et al. [20] and Emoyoma et al. [21] to be associated with hepatotoxicity, endocrine disruption, and increased carcinogenic risk in both animal models and human populations.
This table reveals significant accumulation of aliphatic hydrocarbons across fish organs, with highest values observed in gill tissues, likely due to direct and prolonged exposure to contaminated water during respiration. Elevated levels of nonane (C9) and long-chain alkanes (C26-C40) point to chronic petrogenic input from the nearby oil-rich environments of the Niger Delta. The values far exceed EU permissible limits for hydrocarbons in fish tissue indicating potential health risks from fish consumption.
Distribution of Polycyclic Aromatic Hydrocarbons (PAHs) in Tilapia zillii
Sixteen priority PAHs were quantified in the muscle, gill, liver, and kidney tissues of Tilapia zillii from Escravos River (Table 3 and Figure 3-4). The muscle exhibited the highest total PAH concentration (ΣPAHs = 313.43 ± 0.67 mg/kg), followed by kidney (266.72 ± 0.36 mg/kg), liver (266.17 ± 0.81 mg/kg), and gill (192.96 ± 0.45 mg/kg). The high burden in muscle reflects the affinity of PAHs for lipid-rich matrices and reduced enzymatic turnover relative to metabolically active tissues.
Anthracene (135.57 ± 0.33 mg/kg) and indeno(1,2,3-cd)pyrene (36.33 ± 0.44 mg/kg) dominated in muscle, suggesting sustained exposure to pyrogenic inputs such as gas flaring and incomplete combustion residues common to the Niger Delta. The gills accumulated more low-molecular-weight (LMW) PAHs—fluorene, acenaphthylene, and naphthalene—absorbed directly from the aqueous phase during respiration. In contrast, high-molecular-weight (HMW) PAHs dominated the liver and muscle, indicating mixed petrogenic–pyrogenic influence, consistent with molecular ratio diagnostics reported by Kim et al [22].
The benzo(a)pyrene concentration in muscle (20.95 ± 0.32 mg/kg) far exceeded the EU/WHO permissible limit of 2 μg/kg (wet weight), signifying potential dietary risk. Chronic ingestion of such contaminated fish may induce mutagenic and carcinogenic effects as reported by Bukowska et al. [23]. The observed PAH pattern underscores the continuous hydrocarbon influx and the persistent nature of these compounds in the aquatic food chain.
Biomarker Evidence of PAH-Induced Oxidative Stress
To complement chemical quantification, biochemical biomarkers were analyzed to evaluate sublethal stress responses as shown in Table 2. Hepatic CYP1A activity, assessed via ethoxyresorufin-O-deethylase (EROD) induction, was significantly elevated (12.64 ± 0.52 pmol resorufin min⁻¹ mg⁻¹ protein) in exposed fish compared to control (4.38 ± 0.19 pmol resorufin min⁻¹ mg⁻¹ protein). This increase confirms metabolic activation of PAHs to reactive intermediates through monooxygenase pathways.
Antioxidant enzyme assays revealed significant inhibition of superoxide dismutase (SOD) and catalase (CAT) activities. Hepatic SOD activity decreased from 38.6 ± 2.5 U mg⁻¹ protein (control) to 22.1 ± 1.9 U mg⁻¹ protein, while CAT declined from 52.3 ± 4.2 μmol H₂O₂ min⁻¹ mg⁻¹ protein (control) to 29.7 ± 3.1 μmol H₂O₂ min⁻¹ mg⁻¹ protein. Similar trends were recorded in kidney tissues, where SOD and CAT activities reduced by 42.7% and 43.2%, respectively. These reductions indicate oxidative imbalance caused by reactive oxygen species generated during PAH biotransformation.
Lipid peroxidation (LPO), measured as malondialdehyde (MDA) equivalents, was markedly increased in liver (3.11 ± 0.12 nmol MDA mg⁻¹ protein) and kidney (3.51 ± 0.18 nmol MDA mg⁻¹ protein) relative to control values (1.42 ± 0.08 nmol MDA mg⁻¹ protein), confirming membrane damage from oxidative stress. The overall biochemical response pattern—CYP1A induction, antioxidant suppression, and elevated LPO correlates positively (r = 0.91, p < 0.05) with tissue ΣPAH concentration, validating these molecular markers as sensitive indicators of hydrocarbon-mediated toxicity in T. zillii.
The biomarker data reveal that PAH metabolism in T. zillii proceeds through CYP1A-mediated activation to electrophilic intermediates that form adducts with nucleic acids and lipids, disrupting membrane integrity and enzyme function. The elevated MDA levels and antioxidant depletion indicate that detoxification capacity was exceeded, leading to oxidative damage. The tissue-specific pattern further reflects metabolic specialization-rapid transformation and accumulation in the liver, filtration-related stress in the kidney, and passive storage in muscle.
These integrated chemical–biochemical findings provide compelling evidence that T. zillii in Escravos River is experiencing chronic PAH exposure, with observable molecular alterations consistent with early warning signals of ecological stress. The dominance of HMW PAHs and their associated biomarker signatures confirm pyrogenic sources, predominantly from gas flaring and combustion emissions along the Escravos industrial corridor hydrocarbons as previously reported by Dimatteo et al., [24]. This integrated approach offers a mechanistic basis for linking contaminant burden to functional impairment in aquatic organisms.
The biomarker dataset demonstrates a coordinated biochemical stress response consistent with chronic PAH exposure. The strong positive correlations between ΣPAHs and oxidative stress markers (CYP1A, MDA) and the inverse correlation with antioxidant defenses (SOD, CAT) confirm that biochemical biomarkers effectively complement chemical residue data in delineating mechanistic toxicity pathways in aquatic bioindicators.
Table 3 show Mean concentrations (± standard error) of 16 priority polycyclic aromatic hydrocarbons (PAHs) in tissues of Tilapia zillii collected from the Escravos River, Nigeria. Values are expressed in mg/kg dry weight for muscle, gill, liver, and kidney tissues. ΣPAHs represents the total PAH burden per tissue. High PAH levels in muscle and liver indicate significant (bio)accumulation potential and suggest chronic exposure to hydrocarbon pollution.
Organ-Specific Bioaccumulation Patterns
Bioaccumulation of petroleum hydrocarbons in Tilapia zillii exhibited distinct organ-specific variations, reflecting differences in exposure routes, lipid composition, and metabolic function as shown in Figure 5 [25, 26, 27]. The gills accumulated the highest concentration of total aliphatic hydrocarbons (ΣTAH: 10,388.16 ± 1.52 mg/kg), whereas muscle tissues retained the greatest total PAH burden (ΣPAHs: 313.43 ± 0.67 mg/kg). The high gill concentration corresponds to their direct water contact and large surface area facilitating passive diffusion of soluble and semi-volatile hydrocarbons. In contrast, the elevated muscle and hepatic PAH levels indicate preferential partitioning of hydrophobic compounds into lipid-rich tissues and metabolic sequestration in detoxifying organs [28, 29].
The liver, an established site of xenobiotic metabolism, showed enrichment of high molecular weight (HMW) PAHs such as benzo(k)fluoranthene and indeno[1,2,3-cd]pyrene, implying cytochrome P4501A-mediated oxidative biotransformation and conjugation via glutathione-S-transferase (GST) pathways [30, 31, 14]. Correlation analysis revealed strong positive relationships between hepatic PAH loads and CYP1A activity (r = 0.86, p < 0.05), as well as MDA levels (r = 0.88, p < 0.05), suggesting that hydrocarbon metabolism triggers lipid peroxidation and oxidative stress (see Table 2).
Gill tissues displayed pronounced EROD induction and increased lipid peroxidation, signifying respiratory uptake as a dominant exposure route. Conversely, kidneys exhibited moderate TAH accumulation but a marked decline in catalase activity, reflecting secondary systemic transport and excretory stress. The muscle’s comparatively higher PAH retention without commensurate enzyme activation implies its role as a passive storage depot rather than a site of metabolic transformation [30].
Integration of chemical and biochemical evidence demonstrates a coordinated organ-specific response to chronic hydrocarbon exposure. The order of total petroleum hydrocarbon (ΣTPH = ΣTAH + ΣPAHs) retention followed the pattern: Gill > Muscle > Kidney > Liver, corroborating both exposure and detoxification dynamics observed in the biomarker profile.
These findings emphasize that hydrocarbon accumulation is not solely concentration-dependent but modulated by organ physiology, metabolic enzyme expression, and oxidative status. The convergence of tissue burdens with biomarker responses establishes a mechanistic linkage between contaminant bioavailability and sublethal biological effects, confirming that T. zillii is a reliable sentinel species for monitoring petroleum hydrocarbon pollution in tropical freshwater systems.
Risk Assessment and Health Implications
Health risk assessment based on PAH concentrations in Tilapia zillii tissues revealed significant non-carcinogenic and carcinogenic concerns as shown in Table 4. Estimated daily intake (EDI) of benzo(a)pyrene via 20.95 mg/kg dry weight muscle consumption far exceeds USEPA reference dose of RfD = 0.0003 mg/kg/day resulting in a hazard quotient (HQ) > 1 indicating potential adverse health effects. Excess lifetime cancer risk (ELCR) calculated using oral slope factor for 0.0073 mg/kg/day⁻¹ benzo(a)pyrene yielded values above USEPA threshold of 1 × 10⁻⁴ for both adults and children signifying elevated lifetime cancer risks. Toxic equivalency (TEQ) assessments using established TEFs also showed that muscle and kidney tissues contributed most to overall carcinogenic burden. These results are consistent with previous findings from Sayed et al., [32] which highlight dietary intake of PAHs in fish as a major public health concern in oil-polluted regions. High exposure risk reinforces need for immediate intervention and monitoring to safeguard fish-dependent populations in the Niger Delta.
Table 4 show Summary of estimated daily intake (EDI), hazard quotient (HQ), toxic equivalency (TEQ), and excess lifetime cancer risk (ELCR) associated with the consumption of PAH-contaminated tissues of Tilapia zillii from the Escravos River, Nigeria. All tissues exceeded the USEPA benchmark for acceptable cancer risk (1 × 10⁻⁴), indicating high dietary exposure risks.
Sensitivity and Monte Carlo Risk Modeling
Probabilistic Estimation of Carcinogenic Risk
A probabilistic health risk model was employed to quantify the carcinogenic potential associated with benzo(a)pyrene (BaP) exposure through consumption of Tilapia zillii. Monte Carlo simulations with 10,000 iterations were executed using Crystal Ball® (Oracle, USA) to account for variability and uncertainty in exposure parameters. Input variables-including PAH concentration, ingestion rate, body weight, exposure duration, and slope factor—were defined as probability distributions derived from field data and established exposure models [33, 34].
The excess lifetime cancer risk (ECR) was computed following the U.S. Environmental Protection Agency (USEPA) protocol [35], expressed as: ECR=CDI×CSF where CDI is the chronic daily intake (mg/kg-day) and CSF represents the BaP cancer slope factor (mg/kg-day)⁻¹. The simulation produced a right-skewed ECR distribution, with a 95th percentile value of 1.28 × 10⁻³, which exceeds the USEPA threshold of 1 × 10⁻⁴. This indicates an elevated probability of carcinogenic risk for high-end fish consumers, emphasizing the toxicological relevance of hydrocarbon-contaminated aquatic systems (Figure 6).
Persistent dietary exposure to BaP and related PAHs can induce metabolic activation through cytochrome P450 (CYP1A) pathways, producing reactive intermediates capable of DNA adduct formation and subsequent carcinogenic effects [35, 30]. The modeled cancer risk thus reflects both environmental contamination intensity and bioactivation capacity in exposed populations.
Sensitivity Analysis of Exposure Parameters
A global sensitivity analysis was conducted to evaluate the relative influence of model inputs on the ECR output variance. Among all exposure parameters, BaP concentration and fish ingestion rate emerged as the most sensitive variables, contributing 47.6% and 38.2% of total variance, respectively. Exposure duration and body weight contributed marginally (<10%) (Figure 7).
The results imply that even modest fluctuations in PAH concentrations or fish consumption rates can substantially alter cancer risk outcomes. This reinforces the need for effective contamination control at emission sources and targeted dietary risk communication in hydrocarbon-impacted communities [36, 37]. Similar probabilistic analyses have demonstrated that controlling contaminant levels and reducing consumption frequency are the most effective mitigation strategies in aquatic exposure scenarios [38, 39].
Model Uncertainty and Interpretation
Uncertainty analysis demonstrated consistent convergence across simulation runs, indicating model stability and reliability. The interquartile range (IQR) of ECR outputs exhibited narrow dispersion, suggesting that variability was primarily driven by environmental rather than computational factors. The probabilistic framework employed provides a more realistic representation of exposure heterogeneity compared to deterministic risk assessments that assume fixed input parameters [40, 41].
Model performance was further supported by its congruence with biomarker responses. Elevated hepatic CYP1A and EROD activities, observed in corresponding biological assays, correlated with modeled BaP concentrations and support the mechanistic linkage between exposure, metabolic activation, and carcinogenic outcome [42, 43]. The combination of biomarker evidence and probabilistic risk modeling thus provides a robust, multi-dimensional assessment of ecological and human health implications in hydrocarbon-impacted freshwater systems
Comparative Assessment with Global Standards
Comparison with Data from Other Oil-Polluted Regions
The magnitude of polycyclic aromatic hydrocarbon (PAH) and aliphatic hydrocarbon (AH) contamination in Tilapia zillii from the Escravos River exceeded concentrations reported from several oil-impacted aquatic ecosystems globally. The mean total PAH (ΣPAHs) burden in muscle tissue (313.43 ± 0.67 mg/kg) and total aliphatic hydrocarbons (ΣTAH: 10,388.16 ± 1.52 mg/kg) substantially surpassed values observed in the Niger Delta’s Forcados River (ΣPAHs: 142 mg/kg) [44], the Kuwait Bay coastal region (ΣPAHs: 98.6 mg/kg) [45], South China Sea reported by Wang et al., [46], and the Persian Gulf reported by Filatova et al., [47, 48] following the 1991 oil spill (ΣPAHs: 110 mg/kg) [45]. Comparable contamination gradients have only been documented in heavily industrialized estuaries such as the Pearl River Delta (ΣPAHs: 275 mg/kg) [49].
The dominance of high molecular weight (HMW) PAHs particularly anthracene, benzo(k)fluoranthene, and indeno(1,2,3-cd)pyrene implies pyrogenic origin likely associated with chronic gas flaring, combustion residues, and petrogenic input from oil installations. This compositional pattern mirrors findings from oilfields in the Amazon basin and the Ecuadorian Oriente region, where sustained hydrocarbon exploration produced similar HMW dominance in benthic fish species [50]. Elevated benzo(a)pyrene levels (20.95 ± 0.32 mg/kg) in Escravos fish tissues were nearly tenfold higher than WHO/FAO permissible limits, confirming an acute toxicological signature of chronic petroleum exposure [51].
Relative Severity of Escravos River Contamination
When benchmarked against global datasets, Escravos River contamination ranks among the most severe in oil-producing tropical regions. The ΣTPH (ΣTAH + ΣPAHs) concentration across tissues exceeded 10⁴ mg/kg, surpassing chronic toxicity thresholds established for aquatic organisms [52]. Bioaccumulation factors (BAF) for PAHs in muscle exceeded 10³, reflecting strong partitioning affinity for hydrophobic compounds and limited depuration potential [53]. However, fish species from temperate regions such as the Baltic Sea or North Sea exhibit lower hydrocarbon burdens due to colder temperatures, reduced volatilization rates, and stricter environmental regulation as shown in Figure 8 [54]. The Escravos systems high hydrocarbon flux, coupled with weak enforcement of effluent controls, amplifies bioavailability and trophic transfer. The elevated ΣPAH levels observed are consistent with long-term chronic inputs rather than episodic contamination, a distinction that holds ecological significance for persistent exposure models.
Figure 8 show Comparative distribution of total PAH concentrations (ΣPAHs, mg/kg) in fish species from oil-impacted aquatic ecosystems worldwide. Escravos River exhibits the highest burden relative to Forcados River (Nigeria), Kuwait Bay, Persian Gulf, and Pearl River Delta, indicating severe hydrocarbon contamination pressure.
Ecotoxicological and Ecological Implications
The high hydrocarbon load in T. zillii poses direct ecotoxicological threats through metabolic activation of PAHs to electrophilic intermediates, leading to oxidative stress, lipid peroxidation, and genotoxic effects [55]. Elevated hepatic CYP1A and EROD activities, together with reduced antioxidant enzyme defense (SOD, CAT), observed in this study, confirm sublethal biochemical stress consistent with hydrocarbon exposure pathways documented elsewhere [43, 30].
Chronic exposure at these concentrations has been linked to reproductive impairment, growth inhibition, and immunosuppression in fish [56], as well as indirect ecosystem-level consequences such as altered trophic interactions and biodiversity loss in benthic communities [57]. The elevated excess lifetime cancer risk (ECR = 1.28 × 10⁻³) modeled from dietary exposure further underscores human health implications, particularly for local populations relying on subsistence fishing. In comparison, populations consuming fish from the Persian Gulf and Pearl River Delta [49] exhibited modeled ECR values of 2.7 × 10⁻⁴ and 3.9 × 10⁻⁴, respectively [47, 48], placing Escravos-derived risk estimates among the highest globally. Such findings reveal an urgent need for ecological restoration and continuous biomonitoring to mitigate long-term exposure risks.
Implications for Public Health and Environmental Policy
Food Safety and Dietary Exposure Concerns
The estimated excess lifetime cancer risk (ECR) values above the USEPA benchmark of 1 × 10⁻⁴ highlight a significant carcinogenic concern for consumers of Tilapia zillii from the Escravos River. The elevated benzo(a)pyrene (BaP) concentrations in fish muscle suggest that chronic dietary intake poses a public health threat [12], especially among subsistence fishers and local consumers relying on fish as a primary protein source. Similar risk magnitudes have been reported in Niger Delta studies [51] and oil-impacted regions in the Gulf of Guinea [58], reinforcing that dietary exposure constitutes a critical pathway of human contamination.
Prolonged ingestion of polycyclic aromatic hydrocarbons (PAHs) can induce genotoxic and mutagenic effects through DNA adduct formation and oxidative stress [59]. Figure 9 illustrates the comparative cancer risk estimates for PAHs in Tilapia zillii relative to global dietary exposure limits, showing that Escravos River fish samples exceed both FAO/WHO and EFSA permissible thresholds.
Implications for Environmental Policy and Regulation
The findings emphasize the need for stronger regulatory oversight on effluent discharge and oil exploration activities in the Niger Delta. Current enforcement under the National Environmental Standards and Regulations Enforcement Agency (NESREA) remains inadequate, leading to chronic contamination of aquatic food chains [60]. Policies should adopt risk-based monitoring approaches incorporating probabilistic modeling tools such as Monte Carlo simulations for dynamic risk assessment [61].
A targeted remediation strategy combining bioremediation and eco-restoration could significantly reduce bioavailable PAH fractions in sediments and fish habitats. Stakeholder collaboration between oil companies, regulatory bodies, and local communities is imperative for implementing sustainable environmental management frameworks.
Need for Continuous Monitoring and Remediation
Regular biomonitoring of aquatic biota using sentinel species like Tilapia zillii is essential to track pollution trends and assess the effectiveness of remediation programs. Continuous environmental surveillance shown in Figure 10 will ensure early detection of carcinogenic compounds and prevent long-term exposure. The deployment of biosensors and early-warning systems for hydrocarbon detection can complement conventional chemical monitoring [51].
Integrated Interpretation
Linking Chemical Burden with Biochemical Responses
Observed bioaccumulation of PAHs in fish tissues shown in Figure 11 correlates strongly with biochemical responses such as elevated lipid peroxidation, depletion of antioxidant enzymes (SOD, CAT, GST), and increased DNA fragmentation [51]. The interdependence of chemical and biological responses emphasizes the role of oxidative stress in PAH-induced toxicity [43].
Mechanistic Pathway of Hydrocarbon Toxicity in Tilapia zillii
Mechanistic toxicity pathway involves enzymatic activation of BaP to reactive diol epoxide intermediates that form DNA adducts, leading to mutations and carcinogenicity [15]. Cytochrome P450 (CYP1A) induction serves as a key biomarker of exposure, while excessive ROS generation disrupts mitochondrial integrity and induces apoptosis. Figure 12 schematically represents the molecular mechanism underlying BaP-induced carcinogenesis in Tilapia zillii.
Ecological and Human Health Continuum
The ecological implications extend beyond individual fish health to population-level declines, altered reproductive success, and disruption of aquatic food webs as shown in Figure 13. Bioaccumulation and biomagnification of PAHs in higher trophic levels may further intensify human exposure risk through dietary transfer. This continuum between ecological degradation and human health outcomes necessitates an integrative management model emphasizing ecosystem health as a determinant of community wellbeing [62].
Conclusions
The study revealed pronounced bioaccumulation of aliphatic and polycyclic aromatic hydrocarbons in Tilapia zillii inhabiting the Escravos River, with gill and muscle tissues showing the highest total hydrocarbon loads. The accumulation pattern reflects organ-specific exposure and lipid affinity, with strong correlations between PAH concentrations and oxidative stress biomarkers such as CYP1A, EROD, SOD, CAT, and MDA. These biochemical responses confirm the activation of detoxification and antioxidant defense pathways, indicating sublethal stress and potential genotoxicity induced by chronic hydrocarbon exposure. Monte Carlo uncertainty and sensitivity analyses further established ingestion as the dominant exposure route, with carcinogenic risk indices exceeding acceptable limits for both adults and children. Comparisons with other oil-polluted regions emphasize that the Escravos River exhibits relatively severe contamination, reinforcing its classification as a high-risk environment within the Niger Delta. The ecological implications extend beyond individual fish health to potential food web disruption and human dietary exposure risks. The mechanistic interpretation demonstrates that hydrocarbon-induced oxidative stress, cytochrome P450 activation, and lipid peroxidation constitute the underlying toxicity pathways in Tilapia zillii. Effective environmental management will require integrated remediation, stringent enforcement of effluent regulations, and continuous biomonitoring using biochemical markers. The linkage between chemical burden, biological response, and human exposure shows the urgent need for environmental-based pollution control strategies that protect both aquatic life and public health.
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