Sawe et al. 2023 — Potentially toxic metals in tilapia and catfish from Lake Manyara, Tanzania
This study measures Cu, Ni, Pb, and Zn in the edible muscle tissue of two fish species from Lake Manyara, an endorheic (no outflow) soda lake in Tanzania’s Rift Valley, at 5 landing sites in April 2022. All measured concentrations are below FAO/WHO permissible levels. Based on the highest observed Pb concentration (0.81 mg/kg dw in catfish at Mfulo wa Ng’ombe), the authors derive a maximum recommended consumption of 2.2 kg of Lake Manyara fish per week. No significant interspecies or inter-site differences were observed, indicating no detectable food-chain bioaccumulation and no localized pollution hotspots.
Key numbers
All concentrations in mg·kg⁻¹ dry weight (dw). EDXRF method.
Tilapia (Oreochromis amphimelas) overall mean:
- Ni: 0.79 mg/kg dw
- Cu: 11.29 mg/kg dw
- Zn: 40.13 mg/kg dw
- Pb: 0.64 mg/kg dw
African catfish (Clarias gariepinus) overall mean:
- Ni: 0.80 mg/kg dw
- Cu: 10.45 mg/kg dw
- Zn: 42.22 mg/kg dw
- Pb: 0.60 mg/kg dw
Site-level Pb range in tilapia: 0.55–0.77 mg/kg dw (max at Oltukai) Site-level Pb range in catfish: 0.38–0.81 mg/kg dw (max at Mfulo wa Ng’ombe)
FAO/WHO permissible levels for comparison (mg·kg⁻¹):
- Zn: 100
- Cu: 30
- Pb: 1.5 (all observed values below limit)
- Ni: no formal permissible level; WHO PTWI = 2.45 mg/person/week
Weekly consumption limit based on highest Pb (0.81 mg/kg dw in catfish): 2.2 kg/week Weekly consumption limit based on highest Ni (0.91 mg/kg dw in catfish): 2.7 kg/week
Methods (brief)
Fish muscle and skin composite samples collected from 5 landing sites in April 2022. Samples dried at 60°C for 72 h, ground, pressed into pellets with binding material. Analyzed by Energy Dispersive X-Ray Fluorescence (EDXRF) spectrometry using X-Lab Pro software. Calibration verified with certified reference material IAEA-436 (trace elements and methyl mercury in tuna fish flesh homogenate). Statistical analysis by Kruskal-Wallis test (for non-normally distributed Ni, Cu) and t-test (normally distributed Pb, Zn); p<0.05 threshold.
Limitation: Composite samples per species per site (not individual fish replicates), which limits statistical power. Results reported as dry weight; wet-weight conversion not provided. Values are total metals, not speciated. Sampling was one-time (April 2022, end of rainy season); seasonal variation not assessed. Lake Manyara is endorheic with increasing anthropogenic pressure, so current low levels may change over time.
Implications
Certification: Tilapia and catfish from Lake Manyara show Pb concentrations consistently below the FAO/WHO 1.5 mg/kg limit. Ni and Cu are also within permissible ranges. These values represent a low-to-moderate contamination baseline for freshwater fish in East African rift lakes.
Courses: Illustrates the endorheic lake risk: absence of outflow concentrates inputs over time. Current values are safe, but monitoring is warranted as the catchment becomes more intensively farmed and mined.
App: Fish muscle Pb range 0.38–0.81 mg/kg dw for East African tilapia and catfish; useful as a regional freshwater fish baseline for the app’s risk model.
Microbiome: Not addressed.